Sodium and Potassium balance Flashcards
Session Plan
This session has been split into 3 separate videos that detail how sodium and potassium levels are regulated and maintained in the body and how the regulation of sodium is an important determinant of extracellular fluid volume and blood pressure. They are interconnected so you will need to go through them in order for everything to make sense. The first section is on sodium regulation and we will describe the effects of sodium on blood pressure before looking at the mechanisms that regulate sodium levels. The second section will look at the how blood pressure receptors feed back to regulate sodium levels and fluid before looking at the effects of marked changes in volume on sodium balance. The last section looks at the regulation of potassium, and will consider how potassium is handled following a meal and how it is excreted. Finally we will look briefly at the effects of hypo and hyperkalemia.
Part 1. Regulation of Sodium
Osmolarity
Osmolarity is a measure of solute concentration in a solution. And by solute, I mean the number of particles. Each particle, is a solute. So for a one osmolar solution, we have one mole of dissolved particles.
One osmole is one mole of dissolved particles. When you take something like sodium chloride into consideration, you have two particles.
So one osmole (or one osmolar sodium chloride) will have one osmole. In total, it’ll be 0.5 molar sodium chloride. So osmolarity depends on the number of dissolved particles. The more dissolved particles, the greater the osmolarity. The reason this is so important to realise, is that in our bodies, our plasma osmolarity is very constant. However, we can produce very, very wide, varying osmolarities of urine.
That would be easy to explain if we had evolved pumps for water that could move water against concentration gradients. And we didn’t have semi-permeable membranes. But we do have semi-permeable membranes.
Notes from slides: As you will already know one fundamental concept in the regulation of salt and water in the body is the concept of osmolarity. This concept is central because pumps capable of moving water against what is effectively a “concentration gradient” have not evolved, and the vast bulk of biological membranes are permeable to water allowing its movement. The concept is given in the orange box where the osmolarity. The concentration of water can be described as the proportion of a solution that is water so is inversely proportional to the number of dissolved solutes (particles).
In spite of fact that biological membranes are permeable to water and that the plasma osmolarity is set within a strict range we can generate urine concentrations and you will have seen in the previous session that this is achieved by generating regions of high osmolarity in the kidney. The regulation of sodium is a key component of this.
Regulation of water and salt balance are inter-related
Water does move freely across membranes.
And what that means is that if you have increased salt in an area, water will move into that area and increase the volume.
And conversely, if you have decreased, salt water will move out, and will decrease the volume. We have a constant osmolarity.
Notes from slide: As water is the major component of our body fluids and we have a “constant osmolarity” then when the amount of salt changes the amount of water must change too and that leads to an increase in volume, and obviously the converse of this is true, a reduction in salt will lead to a reduction in water and a reduction in volume.
ECF osmolarity and volume
If we have a total number of miliosmoles in our extracellular fluid of 2900, we will have 10 litres of extracellular fluid. If we increase that number by 290 miliosmoles, or decrease that number by 290 miliosmoles, we will increase our extracellular fluid volume by 1 litre, or reduce our extracellular fluid volume by 1 litre.
Notes from the slide: So if we look at that in numbers, you have an osmolarity of approximately 290mosmol/L so if your ECF has 2900 mosmols in total you will haven an ECF volume of 10L and for each change on 290 mosmols the volume will change by 1L
Plasma Composition
Now, we have a normal plasma osmolarity of 285 to 295miliosmoles per litre. And that 285 to 295 is the sum of all the particles. Now, as you can see from this table, the main particle is sodium. The next one is chloride. But chloride will be the counter-ion to sodium in most cases. So sodium is the most prevalent and important solute in the extracellular fluid and dictates extracellular fluid volume.
Notes from slide: The major ion in the ECF is sodium which is at approximately 140mmol/L so it is going to be the most important solute in determining ECF volume, the more sodium you have the higher your ECF volume will be.
Dietary sodium and body weight
What that means is, if you change the amount of sodium in your diet, going from 100 mili-equivalents per day to 200 mili-equivalents per day, you’ll increase your total body sodium over time. As you do that, you will retain more water. You’ll also be a bit thirsty, so you’ll take in more water until you reach a plateau. Then if you reduce your sodium in your diet, back down to 100 mili-equivalents per day, you’ll lose that water because you will lose sodium from your system.
Notes from slide: We can see the effect of this effect of sodium on ECF volume if we look a the effect of the sodium content of the diet on body weight as the more sodium you eat the more water you will retain and you weight will increase
Total body sodium and blood pressure
So if you increase dietary sodium, you increase your total body sodium. If you increase your total body sodium, your osmolarity would increase. But this can’t happen because of semi-permeable membranes. So you increase water intake and you retain more of the water that you take in. As a result, you’ll end up with an increased extracellular fluid volume. But because we are basically bags, the more water we pump into that bag of a fixed size, we will increase the blood volume and will increase our blood pressure.
And if you look in the opposite direction, if you decrease your dietary sodium, you’ll decrease your total body sodium. And by the same logic, you’ll end up with decreased blood volume and pressure.
Notes from slide: This association of sodium with water retention then gives us the link between dietary sodium and blood pressure. If you increase your dietary sodium you will increase your total body sodium. This would increase osmolarity which given the way water is handled can’t happen so you retain more water and take in more water which increases your ECF fluid volume. The issue then is that you are a relatively fixed volume so the pressure in the system goes up and we see this in the increase in blood pressure.
Regulation of sodium levels
So regulating sodium levels is central to the control of blood pressure and thereby to maintaining normal physiological function, we need to be able to balance excretion and intake, balance sodium excretion under normal fluctuations in blood flow, reduce sodium excretion at times of low sodium levels increase sodium excretion in times of excess and respond to changes in blood volume
Questions are what we will discuss next.
Regulation of sodium intake
Looking very briefly at the regulation of sodium intake, under normal conditions of euvolaemia, where you have normal sodium levels, what is going on is you’re actually suppressing your sodium intake. Your desire for sodium intake. And this is done through the lateral parabrachial nucleus. A set of cells that respond to serotonin and glutamate as transmitters to suppress your basal sodium intake.
However, under conditions of sodium deprivation, you get an increased appetite for sodium that is driven by GABA and opioids (GO and eat sodium!). So we have a central mechanism for regulating sodium intake, but we also have a peripheral mechanism, and that is taste. As you know, some of our taste is based on salt. And if you have food with no salt in, it can taste fairly unpleasant. Salt is therefore appetitive when it’s present at very low concentrations in your food. But as sodium concentration increases, it becomes aversive. Sodium - very high sodium content food -can be fairly unpleasant taste.
Notes from slide: So we have both central and peripheral mechanisms to regulate sodium intake. The central mechanisms depend on a region of the brain stem the Lateral Parabroachial nucleus in which there are sets of cells that respond to different aspects of sodium balance. The normal state (in euvolaemia) is for the inhibition of Na+ intake through the activity of neurotransmitters including serotonin and glutamate. In sodium deprived states the appetite for sodium is increased through a separate set of neurotransmitters including GABA and opiods. The peripheral mechanisms controlling intake are based on taste. As you will know salt has a major effect on the taste of foods but unlike the other major tastes is bimodal so that at lower levels salt enhances the taste of food but at high concentrations it can make things taste bad (aversive)
Where is sodium reabsorbed in the nephron?
So once it’s in our bodies, given that we do not want to lose a lot of water, and losing sodium will mean losing lots of water, where do we reabsorb it in the kidney?
Well, like everything, a lot of reabsorption - the bulk of reabsorption - is done in the proximal convoluted tubule. And you’ll have been told 67-70% of your water is reabsorbed in the proximal convoluted tubule. And that’s because you reabsorb 67-70% of your sodium in that region. As the tubular fluid comes down, the thin descending limb of the loop of Henle, no sodium is reabsorbed from in this region. Sodium reabsorption occurs in the thick ascending limb. About 25% of the sodium that is filtered into the tubular filtrate is reabsorbed in this thick ascending limb. A further 5% is reabsorbed in the distal convoluted tubule. About 3% in the collecting duct. And that means you excrete less than 1% of the sodium that enters the tubular system.
But remember, these are percentages, not amounts. So if you increase your glomerular filtration rate, you will increase the amount of sodium that comes out here. If you filter 200 mils, you will get rid of 1% of 200 mils worth of sodium. But if you filter 100 mils, you’ll get rid of 1% of 100 mils worth of sodium. Well, that means changes in glomerular filtration rate should affect sodium excretion. And we need to know if that’s a good thing. Well, given that you’d need to maintain sodium to maintain your water balance. It’s not really that desirable to change based on something like glomerular filtration rate.
Notes from slide: So having taken in sodium, under normal circumstances we want to retain it to maintain our ECF osmolarity. The bulk of filtered sodium is reabsorbed in the proximal convoluted tubule (about 67% of filtered sodium). This is the same as the amount of water that is also absorbed in this region of the nephron. A lot of this reabsorption occurs through the use of sodium as a co- or counter transported ion facilitating the reabsorption of other things (e.g. glucose, amino acids, bicarbonate etc). About 25% of the total filtered sodium is taken up in the thick ascending limb of the loop of Henle as part of the counter-current mechanisms that you heard about in your last lecture primarily through the activity of the Na+/K+/Cl- triple transporter. A further 5% is taken up in the distal convoluted tubule primarily via the Na+/Cl- transporter found in this region of the tubular system. The final 3% is reabsorbed in the collecting ducts via the Na+ channel ENAC. So in the end less than 1% of the sodium that enters the tubular system is excreted.
What you will note here is that we are talking in percentages and as a consequence if GFR goes up you would expect the total amount of sodium excreted to go up. That would increase water loss and reduce blood volume. Is this a good thing?
Blood pressure renal blood flow and sodium excretion
And if we actually look at what affects glomerular filtration rate. Well over a normal range, the lower end of the range of main arterial pressures, renal plasma flow rate is proportional to arterial pressure. And glomerular filtration rate is proportional to mean arterial pressure. However, once we get past a threshold, renal plasma flow rate plateaus. And glomerular filtration rate plateaus. Because in these ranges of blood pressure, for example, if you were exercising, you don’t want to excrete more sodium than you need to.
Notes from the slide: So what might affect GFR, well as about 20% of renal plasma is filtered GFR is affected by the renal plasma flow rate. So as you increase RPF you increase GFR. Now RPF is proportional to blood pressure over a significant range of blood pressures including the normal range for mean arterial pressure. However, blood pressure can increase at times of exercise and if this relationship was maintained you would get an inappropriate level of fluid and sodium loss. So once you reach about 100mmHg RPF does not increase with increasing bp preventing this loss.
Sodium excretion and GFR
So why does that happen? Well, as glomerular filtration rate increases, the amount of sodium going through this system increases. They get to a point where there’s more sodium going through the system in a shorter period of time so that the amount of sodium that’s here will increase. Now, if you look at that in a slightly better representation, diagramative representation, that part of the distal convoluted tubule is in tight association with the glomerulus.
Notes from slide: A major mechanism contributing to this effect is a consequence of a renal feedback mechanism. As GFR increases flow rate in the PCT and the LOH will increase, the amount sodium reabsorbed increases but there will be a maximum rate dependent on the transporters present and the flow. As GFR increases there is an increase in the delivery of sodium/chloride reaching the distal nephron. Specifically in this region and if you look at a more accurate schematic of the tubular structure you see that the region of tubule exiting the LOH is in tight association with the glomerulus.
Increased tubular sodium and the Macula densa
(The macula densa is a collection of specialized epithelial cells in the distal convoluted tubule that detect sodium concentration of the fluid in the tubule.)
In the kidney, the macula densa is an area of closely packed specialized cells lining the wall of the distal tubule, at the point where the thick ascending limb of the Loop of Henle meets the distal convoluted tubule. The macula densa is the thickening where the distal tubule touches the glomerulus.
The cells of the macula densa are sensitive to the concentration of sodium chloride in the distal convoluted tubule. A decrease in sodium chloride concentration initiates a signal from the macula densa that has two effects: (1) it decreases resistance to blood flow in the afferent arterioles, which raises glomerular hydrostatic pressure and helps return the glomerular filtration rate (GFR) toward normal, and (2) it increases renin release from the juxtaglomerular cells of the afferent and efferent arterioles, which are the major storage sites for renin.[1]
As such, an increase in sodium chloride concentration would result in vasoconstriction of afferent arterioles, and reduced paracrine stimulation of juxtaglomerular cells. This demonstrates the macula densa feedback, where compensatory mechanisms act in order to return GFR to normal.
The release of renin is an essential component of the renin–angiotensin–aldosterone system (RAAS), which regulates blood pressureand volume.
notes from lecturer:
And if we look at that in a slightly different way. You can see that this is the distal convoluted tubule, actually in contact with cells around the glomerulus. This juxtaglomerular apparatus contains a specialised set of cells called the macula densa. These cells, the macula densa, in response to high sodium levels in the tubular fluid, increase their uptake of sodium and chloride via the sodium-chloride-potassium triple transporter. That causes these cells to release adenosine. The adenosine triggers the extraglomerular messangial cells to interact with the smooth muscle cells, causing them to contract. By causing these cells to contract, you reduce the flow into the glomerulus. So you reduce the glomerular filtration rate.
The other thing that happens is this release of adenosine does lead to a reduction in renin. But because this is over a relatively short period of time, because you’re getting short term contraction, because you got more sodium, so you get less sodium into the system, it’s not going to affect your overall renin production over a long period of time. So what we get is a reduction in perfusion pressure, and so glomerular filtration rate.
Notes from slide: The structure of this region is shown schematically here with the proximal part of the distal convoluted tubule separated from the glomerulus by a set of specialised cells both extraglomerular mesangial cells and juxta glomerular cells. As more Na/Cl is delivered to the distal convoluted tubule, the amount of sodium and chloride transported by the cells of the macula densa increases. Above a threshold value the cells start to release adenosine and ATP and these activate receptors in the extraglomerular mesangial cells causing 2 effects. 1. the reduction in renin production (which will be a long term response) but more directly stimulating the contraction of the smooth muscle cells of the afferent arteriole. This leads to a reduction in the RPF and a reduction in the perfusion pressure. Tubular flow rate therefore responds to short term changes in blood pressure preventing the loss of sodium and fluid due to increased activity.
What happens when you need to retain sodium or water?
That’s fine, but that’s a short term regulatory system.
So we need to know what happens if you need to retain sodium, or because of the effect of sodium on water, if you need to retain water.
Notes from slide: So that is a way of controlling things when the physiology is relatively constant, but what about if you need to retain sodium. Low sodium would lead to low plasma volume because of excess loss of water, similarly if you lose plasma volume it is important to retain sodium so that you can retain water. As a result the mechanisms of responding to low sodium or low volume have similar mechanisms and the main processes will include both reducing sodium and water excretion and constricting the vasculature to reduce the volume of the system and increase the pressure in it.
Best way to retain sodium is to filter less
Think Macula Densa mechanisms: ie less nacl in dct so decrease resistance of aa to increase gfr so this increases and increase renin
Think Systemic/long term ie less nacl in blood so increase sympathetic to increase resistance of aa to decrease gfr so less filtered and less excreted and increase renin to increase sodium reabsorption.
Well, one way to think about the whole tubular system is that, what we do, is we throw away 20% of everything into our tubular fluid. And then we try and grab as much back as we need. So one way to retain sodium and retain water is to filter less. Because if we filter less, less will come out of this end. We do that by reducing the pressure gradient across here. If we lower the pressure at this end, we’ll get more flowing through. We’ll have a lower filtration pressure and we will get better retention of sodium, and also of water.
Notes from slide: One way of looking at the whole renal system is that we throw away 20% of everything and then get back as much of what we wanted as we can so that we don’t lose it. It therefore stands to reason that the best way to reduce loss of sodium and water is to reduce glomerular filtration. This can be achieved by reducing the filtration pressure across the bowmans capsule which itself can be achieved by constricting the afferent arteriole more than the efferent arteriole or relaxing the efferent arteriole more than the afferent arteriole.
Controlling sodium excretion
So what factors affect that system? Well if we need to increase sodium reabsorption or retention, one system that functions here is sympathetic activity. Sympathetic activity actually directly stimulates (or contracts) the smooth muscle cells of the afferent arteriole. It also stimulates sodium uptake by the cells of the proximal convoluted tubule.
Furthermore, it stimulates the cells of the juxtaglomerular apparatus to produce renin. And that renin will lead to the production of Angiotensin 2. Angiotensin 2 will also stimulate the cells of the proximal convoluted tubule to take up sodium. Angiotensin 2 will also stimulate the adrenal glands to produce aldosterone, which stimulates uptake of sodium in the distal parts of the distal convoluted tubule, and in the collecting duct. And finally, if we have very low sodium in this tubular fluid, that itself will stimulate the production of renin. And so the production of angiotensin 2 and contribute to this system.
Counteracting that, if we need to decrease sodium reabsorption, we have atrial naturetic peptide, which acts as a vasodilator. It reduces sodium reuptake in the proximal convoluted tubules, the distal convoluted tubule, and the collecting duct. And it suppresses the production of renin by the juxtaglomerular apparatus. So you can see by these mechanisms we can regulate the amount of sodium that’s taken up, the amount of fluid that is going into the system. So the amount of water that’s going to be reabsorbed.
Notes from slide: A reduction in perfusion pressure is therefore one consequence of sympathetic stimulation which directly promotes contraction of the smooth muscle in the afferent arteriole. However, the resulting reduction in GFR isn’t the only mechanism by which sympathetic stimulation reduces sodium loss as this stimulation also increases the uptake of sodium by the cells in the proximal convoluted tubule increasing the activity of the sodium proton exchanger in a mechanism that may rely on an intra-renal renin-angiotensin system. Sympathetic stimulation also activates the production of renin by the juxtaglomerular cells which results in . The second mechanism is the reduction in sodium reaching the distal tubule (measured at the JGA). This leads to a reduction in the production of adenosine and a reduction in the inhibition of renin production by the juxtaglomerular cells. From the previous look at tubular glomerular feedback you may expect this also to cause relaxation of the smooth muscle and so an increase in RPF and thereby GFR. However, under the circumstances we are looking at here the sympathetic activity overrides any effect of the extraglomerular cells on the smooth muscle of the afferent arteriole and we have an overall contraction of the smooth muscle and reduced GFR. The production of renin leads to an increase in angiotensin II. AII not only increases vascular resistance but also stimulates the uptake of sodium by the PCT and stimulates the production of aldosterone which stimulates sodium uptake in the distal convoluted tubule and collecting duct.
These mechanisms are all opposed by the activity of atrial naturietic peptide which promotes dilation of the afferent arteriole inhibits renin release and reduces the uptake of sodium in the PCT, DCT and CT
Volume expansion and contraction
So if we look at a condition where we have low sodium, we’ll end up with a low fluid volume, and that willgive us beta 1 sympathetic activity that will stimulate, that will cause contraction of the afferent arterial, and reduce glomerular filtration pressure. It will also stimulate the production of renin, causing the cleavage of angiotensinogen to angiotensin 1. Angiotensin one will go up. ACE then cleave angiotensin 1 into angiotensin 2, which will stimulate the zona glomerulosa of the adrenal glands to produce aldosterone. Angiotensin 2 will also stimulate vasoconstriction, promote vasoconstriction and sodium reabsorption. Aldosterone will increase sodium reabsorption. So we will reabsorb more sodium and reduce our water output. If we have high sodium, we will have a higher fluid volume. So we will have a higher blood pressure and that’ll suppress the beta 1 sympathetic activity and cause the production of atrial naturetic peptide. That will reduce renin, reducing angiotensin one, reduce angiotensin 2, reduce aldosterone, promote vasodilation and reduce sodium and water reabsorption.
Notes from slide: So we end up with the following scenarios
If blood volume falls or sodium levels are low and we need to promote sodium retention we see
Increased sympathetic activity leading to a reduction in GFR and reduced delivery of sodium and water to the nephron, and increased renin production, renin converts angiotensinogen into angiotensin I which is then converted into angiotensin II in circulation. AII promotes renal NaCl reabsorption and water reabsorption to reduce volume loss and causes vasoconstriction to increase blood pressure. It also causes the release of aldosterone which also causes the reabsorption of sodium and thereby the reabsorption of water. If sodium levels are high you see an increase in blood pressure. This causes a reduction in sympathetic activity so the opposite response to those described above. It also causes the release of ANP which further reduces sodium and water retention and reduces blood pressure by causing relaxation of the renal arteriolar SMCs and some in the periphery as well in particular those in skeletal muscle.
Aldosterone synthesis
So what is Aldosterone? Well, Aldosterone is a steroid hormone. It’s synthesised and released in response to A2 from the zona glomerulosa of the adrenal cortex. It also stimulated in response to a decrease in blood pressure by baroreceptors. What the Angiotensin 2 does is promote the synthesis of aldosterone synthase. And aldosterone synthase causes the last two steps (enzymatic steps) in the production of Aldosterone from cholesterol. Aldosterone is then released and can go and have its function in the kidney.
Notes from slide: The steroid aldosterone is an important part of this system and is one of the steroids synthesised by the adrenal gland. Steroid synthesis in the adrenals is regionalised and the part of the adrenals required for aldosterone production is the zona glomerulosa. Release occurs in response to angiotensin II in part due to an increase in the activity of aldosterone synthetase which is required for the last 2 steps of aldosterone synthesis. Aldosterone synthesis can also be increased in response to a decrease in blood pressure
Aldosterone function
In the kidney, aldosterone stimulates sodium reabsorption, it controls this reabsorption normally about 35 grams of sodium per day. In response to increased sodium, it also causes increased potassium secretion, so activation of sodium uptake here causes more sodium to be pumped out at this end, more potassium to come in, And so we get more potassium coming out and it promotes hydrogen ion secretion here. So it will lead to hypokalaemic alkalosis if Aldosterone is in excess.
Notes from slide: The overall effects of aldosterone are to increase sodium reabsorption and also increases potassium secretion and proton excretion so an excess of aldosterone can lead to hypokalaemic alkalosis. Aldosterone increase in K+ secretion is a consequence of the increased sodium reabsorption. The change in voltage promotes an indirect stimulation of proton secretion but there are also direct effects of aldosterone on the secretion of protons via alterations in the expression of anion exchanges as H+-ATPases
How does Aldosterone work?
As I said, it’s a steroid hormone, steroid hormones are lipid soluble, they’ll pass through the cell membrane. Once inside the cell, Aldosterone binds to the mineralocorticoid receptor. This is a steroid hormone receptor. And like many of these, sits inside the cytoplasm, normally bound to a protein called HSP 90.
Once the aldosterone binds, the HSP 90 gets removed. Now instead of being a monomer, the mineralocorticoid receptor will dimerise. That allows it to translocate into the nucleus, bind to DNA, where it stimulates the production of mRNAs for genes that are under its control.
Notes from slide: So how does aldosterone cause these effects.
Aldosterone is a steroid hormone so crosses cell membrane where it binds to the mineralocorticoid receptor. In the absence of aldosterone this receptor is a monomer bound to HSP90 and kept in the cytoplasm. On binding the steroid, the MR loses its association with HSP90 and dimerises. It then translocates into the nucleus where it binds to DNA in the promoter region of target genes and stimulates their expression.
Aldosterone in the cortical collecting Duct
Very important amongst those genes are the sodium channel itself - the epithelial sodium channel - and the sodium potassium ATPase. And in addition to those two proteins, which then go to their respective membranes, regulatory proteins that stimulate the activity of these two transporters and channels are active. They are expressed as well. So not only do you get more sodium channel, you get more active sodium channel in response to the production of aldosterone.
Notes from slide: Important target genes include the ENaC (epithelial sodium channel) the sodium potassium ATPase and sets of regulatory proteins. This co-ordinates an increase in the number of sodium transporters and their activity thereby increasing sodium reabsorption
Diseases of Aldosterone synthesis/secretion
Like many systems, this can go wrong so we can have diseases of aldosterone synthesis and secretion. Too little aldosterone (hypoaldosteronism). We get reduced reabsorption of sodium in the distal nephron. That leads to increased loss of sodium. Increased loss of sodium will be associated with increased loss of water. So our extracellular fluid volume will fall. Other systems designed to cause the retention of water will be activated and the retention of water and sodium will be activated. But because we have too much sodium in system. We will still excrete more water than we need to, so we will get a low blood pressure, low blood pressure will bring dizziness, the increased reduction of salt will cause salt cravings and the lower salt concentration will cause palpitations because of the changing membrane potential.
Notes from slide: As with all things it is possible to have diseases directly associated with aldosterone synthesis and secretion caused genetically (very rare) or environmentally some medications including non-steroidal anti-inflammatories, some pathologies lead poisoning, diabetes and kidney disease
Hypoaldosteronism has reduced reabsoroption of sodium in the distal nephron and increased urinary sodium loss. This reduces water retention and causes the ECF volume to fall, increasing renin, AII and ADH. Symptoms include dizziness, low BP salt craving and palpitations
Diseases of Aldosterone synthesis/secretion
The other side of the coin, hyperaldosteronism. We can get increased reabsorption of sodium in the distal nephron, reduced urinary loss of sodium. So an increase in total sodium. That will lead to increased extracellular fluid volume. So hypertension. That’ll reduce renin, angiotensin and ADH production and increase ANP and BNP production. But again, because we have very little sodium coming out, we’ll be reabsorbing lots of water. That’ll lead to a high blood pressure, muscle weakness. Because we think we have insufficient water in the system, we’ll have thirst and because we are drinking more water, we will need to get rid of more water. And that leads to polyuria.
Notes from slide: Hyperaldosteronism caused by a range of things including adrenal tumors, increases sodium retention leading to high blood pressure, muscle weakness polyuria and thirst. The poly uria may seem counter-intuitive but is a consequence of the thirst and increased drinking
Liddle’s Syndrome
And there’s a syndrome called Liddle’s syndrome, which looks a bit like hyperaldosteronism, but with normal or low aldosterone levels. So this is an inherited disease of high blood pressure. The cause of this is a mutation in this sodium channel. So this sodium channel could be regulated by the proteins I’ve told you. So it can be turned on or off, its activity can be increased or decreased. If the channel is always in an ‘on’ state, you’ll get increased reabsorption of sodium. So you’ll get increased sodium retention, increased water retention and hypertension.
Notes from slide: Genetics can also lead to a condition that has the same phenotype as hyperaldosteronism but normal to low levels of aldosterone and that is called Liddle’s syndrome. This an inherited disease of high blood pressure in which there are mutations to the aldosterone activated sodium channel that increase overall ENaC activity causing increased sodium retention and hypertension. There are a number of mutations but one main one alters re-internalisation and degradation of the channel and others change the opening time of the channel increasing the likelihood of it being open.
Baroreceptors
So how do we measure these? Well, we use baroreceptors to measure blood pressure. And on the low pressure side of the cardiovascular system, we have baroreceptors in the atria, the right ventricle, the pulmonary vasculature. And on the high pressure side, we have them in the carotid sinus in the aortic arch, and in the juxtaglomerular apparatus.
Notes from slide: Blood pressure is monitored by a range of systems both on the high pressure and low pressure sides of the cardiovascular system. On the low pressure side we have baroreceptors in the atria, right ventricle and in the pulmonary vasculature, and on the high pressure side there are receptors in the carotid sinus, aortic arch and in the juxtaglomerular apparatus
Detecting changes in blood pressure
In response to low pressure, on the low pressure side, we get a reduction in baroreceptor firing. We get signalling through afferent fibres to the brain stem that leads to sympathetic activity. And the release of ADH, which you’ve seen will promote water retention. Whereas in response to high pressure, we get atrial stretch leading to the release of ANP and BNP, and so greater water loss. (ANP stimulates vasodilation of the afferent arteriole of glomerulus: this results in increased renal blood flow and an increase in glomerular filtration rate. Increased glomerular filtration, coupled with inhibition of reabsorption, results in increases in excretion of water and urine volume - diuresis!) The high pressure side responds to low pressure again, with a reduction in baroreceptor firing, signalling again through the afferent fibres to the brain stem leading to sympathetic activity, and ADH release, but also signalling in the juxtaglomerular apparatus suppress renin release.
Notes on slide: In response to low pressure baroreceptors on both the low and high pressure sides send signals through the afferent fibres to the brainstem. This occurs as a result of a reduction in the firing of the receptors which tonically supress sympathetic activity. The renal baroreceptors also suppress the release of renin so that a reduction in blood pressure reduces baroreceptor firing and increases renin production. The low pressure side also responds to high pressure by promoting the production and release of natriuretic peptides ANP and BNP. These are synthesised in response to atrial stretch
Arial Natriuretic Peptide (ANP)
Atrial natriuretic peptide is a small peptide, made it in the atria. It’s released, as I just said, in response to atrial stretch. What it does is it binds to a receptor that is a guanylyl cyclase. And that guanylyl cyclase causes the conversion of GTP to cyclic GMP and we get activation of protein kinase G. And so the cellular responses in response to that. The cellular responses we’re particularly interested in are vasodilation in the renal blood vessels, as well as other systemic blood vessels, an inhibition of sodium reabsorption in the proximal convoluted tubules and in the collecting ducts and an inhibition of renin and aldosterone production. So a reduction in blood pressure through both basic dilation and a reduction in sodium reabsorption.
Notes from slide: ANP is released in response to atrial stretch and circulates in blood when it binds to its receptor it stimulates the production of cyclic GMP and activates protein kinase G leading to vasodilatation to reduce blood pressure and inhibits the reabsorption of sodium in the PCT and collecting ducts. It also inhibits the release of renin and aldosterone
Volume expansion
So in response to an expansion of volume, we’ll get a reduction in sympathetic activity leading to reduced sodium, re-uptake in the proximal convoluted tubule, a reduction in renin production. So an end reduction in aldosterone and angiotensin production leading to increased sodium excretion, because we’re not reabsorbing as much. We also see this effect of an increase in pay ANP and BNP affecting GFR and sodium reabsorption. So again, promoting sodium excretion.
Notes from slide: So just as with increased sodium if you increase blood volume you need to increase sodium and water excretion so a volume expansion leads to reduced sympathetic activity, afferent arteriolar dilation and increased GFR (throwing away more water and sodium). It will also reduce sodium uptake in the PCT and reduce renin- angiotensin and aldosterone levels to reducing sodium reuptake in the PCT, DCT and CT. With reduced sodium reabsorption there is reduced water reabsorption so increased excretion of both. ANP release compliments these effects suppressing renin increasing GFR and inhibiting sodium reuptake in the CT. It will also suppress the release of ADH. Combined these responses cause a reduction in volume
Volume Contraction
In response to a contraction in blood volume, we’ll get the opposite. We’ll get an increase in sympathetic activity, stimulating sodium reabsorption in the proximal convoluted tubule, increasing renin release, increasing angiotensin 1, increasing A2 and aldosterone, stimulating sodium reabsorption and therefore water reabsorption in the collecting ducts. So we’ll get a reduction in sodium excretion. We also see activity in the brain increasing ADH production. Again, promoting water absorption by inserting more aquaporins into the epithelial wall in the collecting ducts.
Notes from slide: If the volume is reduced the opposite occurs increased sympathetic activity renin/angiotensin aldosterone production and increased AVP expression. These increase sodium reuptake, water retention and prevent further loss of volume.
Water reabsorption requires an osmotic gradient
Well, if you go back to how water is reabsorbed, we reabsorb water because we can generate this hyperosmolar region. And we get this gradient of osmolarity throughout the medulla. And we end up matching the hyperosmolarity here with hyperosmolarity in the tubular system because we can take water out of the tubular fluid, into the hyperosmolar region. So if we increase the osmolarity of the tubular fluid, by increasing the amount of sodium in it, we’ll reduce the gradient across this membrane, so we’ll reduce the amount of water we can reabsorb. So the more sodium, in fact the more solute that arrives in this later part of the tubular system, the less water we can reabsorb.
Notes from slide: As you know water reabsorption requires an osmotic gradient as we have not evolved pumps for water so we generate a gradient of interstitial osmolarity through the renal medulla. If the osmolarity of the fluid in the tubule is the same as that in the interstitium there will be no net movement of water onto the interstitium. So if you reduce the osmolarity of the interstitium you will reduce water reabsorption similarly if you increase the osmolarity of the tubular fluid you will reduce the difference between the tubular fluid and interstitium and so reduce water reabsorption
Increased sodium excretion reduces ECF volume
As sodium levels determine our extracellular fluid volume and reducing extracellular fluid volume reduces blood pressure, if we reduce sodium reabsorption, we’ll reduce our total sodium levels. If we reduce those we’ll reduce our extracellular fluid volume and we’ll reduce our blood pressure. And this we can take advantage of, to reduce blood pressure by providing diuretics.
Notes from slide: As increasing sodium excretion reduces ECF volume and thereby reduces blood pressure, increasing sodium excretion has become a major method to reduce blood pressure through the use of diuretics
Renin Angiotensin System: ACE Inhibitors
The first diuretic I want to talk about is ACE inhibitors. ACE inhibitors have a relatively complex set, or have a set of effects. So the ACE inhibitor itself reduces angiotensin production. Reduced angiotensin 2 will directly lead to vasodilation because we’ve reduced the effects of A2 that are contracting the blood vessel. That will increase our vascular volume and in itself will lead to a reduction in blood pressure. But as you know, angiotensin has additional effects. It has direct effects on sodium re-uptake in the proximal, convoluted tubule, and that will lead to an increase in sodium in the distal part of the nephron. That will reduce the gradient across the tubular fluid and into the interstitium. That’ll lead to a reduction in water reabsorption. It also has adrenal effects.
It leads to reduced aldosterone production because we’ve got less A2, we’ll get less aldosterone. That’ll again reduce the uptake of sodium in the cortical collecting duct. And will increase the amount of sodium in the very distal parts of the collecting duct of the nephron. And those are the regions with the highest interstitial osmolarity. So we’ll again reduce water reabsorption because of the reduction in this osmotic gradient across the tubular wall. And the reduction in water volume, water reabsorption, will reduce our water volume, our extracellular fluid volume, and that will lead to a reduction in blood pressure.
Notes from slide: The first diuretic in the armoury for most cases in the UK is an ACE inhibitor. These work by reducing the production of angiotensin II from angiotensin I. The reduction in AII leads to vascular effects (vasodilation) increasing the volume of the vascular tree so reducing blood pressure but also has diuretic effects reducing sodium uptake in the PCT. Reduced aldosterone also reduces sodium uptake in the CCT. Both of these changes in sodium handling increase the amount of sodium in the distal nephron and will reduce the osmotic difference between the tubular fluid and interstitium so reduce water reabsorption and thereby reduce blood pressure
Other diuretics
ACE inhibitors aren’t the only diuretics we can use. And in fact, we have diuretics that don’t necessarily interfere directly with the sodium system. And diuretics working different parts of the nephron, dependent on what their targets are. So we have osmotic diuretics where we put in something that doesn’t get reabsorbed. And if it doesn’t get reabsorbed in this region, then it will increase the osmolarity in this region and we’ll get less water absorption in the proximal convoluted tube. As that is the region of most reabsorption, that is the region where we’ll see the greatest effect. We can also have carbonic anhydrase inhibitors that work in this region because the enzyme they block (carbonic anhydrase) is most active again in this region.
Loop diuretics work on the thick, ascending limb of the loop of Henle.
Thiazide diuretics work in the distal convoluted tubule.
And potassium sparing diuretics, work in the collecting duct.
Notes from slide: ACE inhibitors aren’t the only way to increase the osmolarity of the distal nephron and so increase water loss. Indeed increasing sodium in the distal nephron isn’t the only way of increasing osmolarity in the distal nephron. Each diuretic works in a specific region of the nephron depending on where its target system is. Conceptually the simplest way of increasing osmolarity in the distal nephron is by adding something that cannot be reabsorbed as the water is removed from the tubular fluid the concentration of the non-reabsorbed substance increases and the osmolarity increases above what would normally occur under those conditions so reducing water uptake. One such diuretic is mannitol which is given as an infusion.
Carbonic anhydrase inhibitors
So carbonic anhydrase inhibitors work because they block this enzyme carbonic anhydrase. And this is involved in the reabsorption of sodium. Because it’s involved in the reabsorption of bicarbonate through CO2. So if we block carbonic anhydrase activity, we no longer turn bicarbonate into CO2. We no longer get the CO2 coming back in and we no longer get this, we also inhibit this carbon anhydrase activity. So these protons aren’t produced. If these protons aren’t produced, this sodium proton exchange activity will reduce. And so the amount of sodium that comes in will go down, the amount of sodium that comes out will go down.
The net reabsorption of sodium will go down. So we’ll get a reduction of sodium uptake in the proximal convoluted tubule, increased sodium in the distal nephron, a reduction in water reabsorption, and we’ll also get a reduction in the acidity of the urine because got more bicarbonate going through and less proton’s being pumped out here.
Notes from slide: A second class of drug that work in the PCT are inhibitors of carbonic anhydrase. As carbonic anhydrase activity leads to sodium uptake and proton export in the PCT inhibiting carbonic anhydrase inhibits sodium uptake here leading to higher levels of sodium in the distal nephron and so a reduced difference between the tubular and interstitial osmolarity so reduces water reabsorption
Loop diuretics (furosemide)
Loop diuretics include furosemide. These work by blocking the triple transporter, the sodium, chloride, potassium triple transporter. And again, if we reduce sodium uptake in the loop of Henle( in the thick ascending limb of the loop of Henle), more sodium will reach the distal nephron and that’ll reduce that osmotic gradient across the tubular wall and into the interstitium. It’ll mean we will re-absorbed, much less water.
Notes from slide: Loop diuretics target the triple transporter in the ascending limb of the loop of Henle. Therefore these drugs inhibit sodium uptake here leading to higher levels of sodium in the distal nephron as well as a reduction in the osmolarity of the interstitial fluid. These together reduce the difference between the tubular and interstitial osmolarity so reduce water reabsorption.
Thiazides
Thiazide diuretics work by blocking this sodium chloride uptake transporter. So this is in the distal convoluted tubule, and so we reduce sodium uptake. And because we reduce sodium uptake we increase sodium in the distal nephron and thereby by the same mechanism, we reduce water reabsorption. We have a diuretic effect. We reduce blood pressure.
But one other effect of these thiazide diuretics is to increase calcium reabsorption. The reason they increase calcium reabsorption is because if you block sodium reuptake but you don’t effect this sodium potassium ATPase activity, you reduce the amount of sodium that’s in the intracellular fluid. So you increase the gradient across this membrane. Now you have these sodium, calcium, anti porters on the baso-lateral side of the distal convoluted tubule cells. And those are required for calcium absorption. If there’s less sodium in here, the potential for sodium to cross this membrane increases, then they can do it at the expense of moving calcium out. So we can pump more calcium out of these cells. If we pump more calcium out, we reduce the intracellular calcium concentration. We increase the gradient across this membrane. So we increase the net flux of calcium in, because we have stopped this sodium chloride transporter.
Notes from slide: Diuretics that work in the DCT are the thiazides. These target the sodium chloride transporter reducing sodium uptake and increasing tubular sodium concentration. One side effect of these drugs is an increase in Calcium reabsorption. The reason for this is that in this art of the nephron, calcium is reabsorbed across the lumen membrane down its concentration gradient that is generated by the activity of the sodium calcium exchanger. Blocking the entry of sodium into the cell through the sodium calcium exchanger whilst the sodium potassium exchanger still functions means that the return of sodium into the cell via the sodium calcium exchanger is increased. This reduces the calcium concentration in the cell and therefore increases the potential for calcium to be removed from the tubular fluid.
Potassium sparing diuretics
These are inhibitors aldosterone function. They bind to the mineralocorticoid receptor, and they block its function. Things like spironolactone (p for potassium sparing diuretic). As we said before, aldosterone stimulates the production of this sodium channel. And this sodium potassium ATPase to increase sodium uptake. So if we block sodium re uptake, we will reduce the amount of this and the activity of this uptake system, so we’ll reduce sodium re-uptake in the distal nephron. They are potassium sparing, because if we have sodium re-uptake, as the sodium comes in, the potassium will go out. More sodium is going across here. More potassium is coming into here. More potassium will go out.
Notes from slide: The overall effects of aldosterone are to increase sodium reabsorption and also increases potassium secretion and proton excretion so an excess of aldosterone can lead to hypokalaemic alkalosis. Therefore inhibiting the activity of aldosterone reduces sodium reabsorption and the excretion of potassium.
Part 3. Potassium balance
Potassium regulation
In the last 2 talks we looked at the regulation of sodium balance as it is the major component of the extracellular fluid and an important determinant of extracellular fluid volume with the impact that has on blood pressure. We will now look at the regulation of the other major cation potassium. Potassium is the major intracellular ion with an intracellular concentration of 150mmol/L. The extracellular concentration is much lower at 3-5mmol/L. This high intracellular concentration is maintained by the activity of the sodium potassium ATPase.
From your lectures in the first year on cell biology and from your knowledge of the neural and cardiovascular system you will know that this difference in potassium is an important contributor to the membrane potential. Consequently extracellular potassium levels have effects on excitable cells with high potassium causing depolarisation leading to action potentials and in the heart arrhythmias. Arrhythmias (in this case asystole) can also occur in the presence of low extracellular potassium levels
Notes:
Potassium is the main intracellular ion at about 150 millimoles per litre. Our extracellular potassium, however, is much, much lower at only three to five millimoles per litre. This difference in a charged particle is a major contributor to the membrane potential. So extracellular potassium has major effects on excitable membranes, examples being nerve and muscle. A high potassium, extracellularly will depolarise the membranes, getting action potentials and heart. A low potassium will make depolarisation more difficult, again, leading to heart arrhythmias and asystole.
Dietary potassium
However, as potassium is a major intracellular ion, that is, but is lower in the extracellular fluid, when we eat cellular material, we will be eating potassium. So potassium is present in most, if not all foods, especially unprocessed foods. So we have the meal that will lead to passive plasma potassium absorption, that’ll increase our plasma potassium concentration. What we then need to do is to reduce that plasma potassium concentration and then we do that by taking the potassium up into the tissue. Fortunately, that uptake is stimulated by insulin, something that we will produce after the meal. It’s also stimulated by aldosterone and adrenaline.
Notes from slide: As potassium levels in the ECF need to be kept low, the first thing to consider is the effect of potassium in the diet. Potassium is found in most/ all foods (especially those that are not processed). Therefore after you have eaten your plasma potassium will increase and needs to be brought down. At the same time as you eat your plasma insulin will increase. Insulin increases plasma uptake of potassium indirectly.
Immediate response to dietary K+
The way that insulin stimulates this increase in potassium uptake is indirect because insulin actually stimulates the sodium proton exchanger. In stimulating the activity of this exchanger, we increased sodium coming into tissue cells. Once the sodium is in, we get an increase in sodium concentration, which means it’s got to be reduced and that’s done through this sodium, potassium ATPase. So the activity of the sodium potassium ATPase increases in response to the increase in intracellular sodium and that brings more potassium into the cell.
Notes from slide: Insulin stimulates the activity of the sodium proton exchanger which increases intracellular sodium. This increase in intracellular sodium activates the sodium potassium ATPase increasing potassium uptake.
Potassium in the kidney
With potassium in our glomerular filtrate, we’ve got to either reabsorb it or or get rid of it. And sometimes we need to reabsorb it, sometimes we need to excrete it. But we need to retain some. So under normal conditions, or if we could increase potassium intake, the glomerular filtrate will contain potassium and about 67 percent of that will be reabsorbed in the proximal convoluted tubule. Another 20 percent will be reabsorbed in the thick ascending limb of the Loop of Henle. And that’s through the sodium potassium chloride triple transporter. Then in the latter parts of the nephron, in the Distal convoluted tubule, then of course the collecting duct, we get potassium secretion. So, up to 50 percent of the potassium will be secreted into distal convoluted tubule, and up to 30 percent in the cortical collecting duct, resulting in somewhere between 15 and 80 percent of the potassium being excreted, 15 to 80 percent of the potassium that is in the glomerular filtrate being excreted.
If we have potassium depleted, you find that actually we reabsorb potassium in these regions. Three percent of it being absorbed in the distal convoluted tubule, and nine percent in the cortical collecting duct. This potassium secretion is stimulated by higher plasma, potassium levels, increased aldosterone, increased tubular flow rate and increased plasma pH.
Notes from slide: Like sodium and water about 67% of filtered potassium is reabsorbed in the PCT with a further 20% reabsorbed in the loop of Henle irrespective of plasma potassium. However, in the latter parts of the nephron the handling of potassium depends on a range of factors including plasma potassium such that the amount of potassium excreted is between 1 and 80% of the initial load. In conditions of potassium depletion further potassium reabsorption occurs in the DCT and CT with 3% in the DCT and 9% in the CT. In normal or high potassium levels potassium is secreted with a number of factors affecting it including plasma potassium, aldosterone tubular flow rate and plasma pH.
Potassium secretion by the principal cells
The increased potassium secretion increase in response to increased plasma potassium come to the increase in the activity of the sodium potassium ATPase. We have more potassium to come in more and we will end up with more potassium inside the cells, and more potassium going out. And we’ll also have an effect on the membrane potential, which will help to stimulate that potassium secretion.
Notes from slide: Increased plasma potassium leads to increased activity of the sodium potassium ATPase and reduce return of potassium into the plasma so increased potassium excretion We have discussed the effects of aldosterone on the secretion of potassium so I won’t describe them again
Tubular Flow and K+ excretion
In response to tubular flow, the cells of the, distal cells have primary cilia. As we get an increase in flow, these cilia stimulate PDK1, which increases calcium concentrations in cell. Which stimulates the activity of the openness of the potassium channels, allowing potassium to move out of the cell because it’s being pumped in by the sodium potassium ATPase. It’ll just come out of the cell in this direction.
Notes from slide: Tubular flow regulates potassium by activating cilia that activate PDK1. This increases Ca++ in the cell which stimulates the opening of potassium channels on the apical membrane. The mechanism by which increased plasma pH increases potassium secretion is not simple and involves a number of different potential pathways
Hypokalemia
Now, obviously, these systems don’t work perfectly, and we end up with, we can end up with hypokalaemia, we can end up with hyperkalaemia.
Hypokalaemia is one of those common electrolyte balances, and we’ve seen in up to 20 percent of hospitalised patients. It can result from either inadequate dietary intake and that would be too much processed food. It can be a result of the use of diuretics because flow, tubular flow rate stimulates the secretion of potassium.
And if you use a diuretic, you will increase tubular flow rate. Surreptitious vomiting leads to reduced intake. Diarrhoea can also lead to reduced intake. And there are also conditions that lead to increased potassium loss, hypokalemia, so genetic conditions like Gitelman’s Syndrome, which is actually a mutation in the sodium chloride transporter in the distant nephron, still leads to increased potassium loss.
Notes from the slide: Disturbances of potassium balance include hypokalemia which is one of the most common electrolyte imbalances. It can be caused by inadequate intake, increased tubular flow rate due to the use of diuretics, non-renal excretion via vomiting or diarrhoea or genetics
Hyperkalemia
Hyperkalaemia is also common, but in a smaller percentage of patients, one to 10 percent of hospitalised patients, it seen in response to potassium sparing diuretics, where instead of having the increased flow early on, we’ve actually got the sparing effect of by blocking the role, the effects of Aldosterone. ACE inhibitors can also lead to hyperkalaemia. It’s often seen in the elderly. It’s often seen in severe diabetes and can also be seen in kidney disease.
Notes from slide: Hyperkalemia is also seen in a significant number of patients and can occur in response to the use of potassium sparing diuretics, ACE inhibitors in the elderly, severe diabetes (insulin resistance) and in kidney disease
Session review
a) Identify A.
b) What is the effect of increased A on GFR?
c) What is B and how does it change in response to volume expansion?
d) How do the levels of renin change?
e) What is C?
a) ANP (or BNP).
b) It increases the GFR (by relaxing the SMCs in the afferent arteriole).
c) Angiotensin I and it reduces.
d) Renin levels go down (because of the reduction in sympathetic activity).
e) ADH OR vasopressin.
f) What proteins does C affect to alter water reabsorption and how does it affect these proteins?
g) Identify D?
h) How does Angiotensin II affect the production of D?
i) How does D affect water reabsorption?
j) The excretion of which major ions/molecules is altered to correct the volume expansion and how is it affected?
f) C causes the aquaporins in the cells of the collecting duct to translocate from the cytoplasm to the cell wall increasing the permeability to water (in this context there is a reduction in C so a reduction in water permeability).
g) Aldosterone.
h) It causes an increase in the expression of aldosterone synthase (which is required for the last 2 steps of the conversion of cholesterol to aldosterone).
i) D increases water reabsorption by increasing the permeability of the principal cells to sodium (for information but not required for the answer it increases the expression of ENaC, the Na/KATPase and proteins that increase their activity so increases Na reabsorption).
j) Sodium and water and their reabsorption is reduced to correct the volume expansion.
(my answer: Higher fluid volume, so we will have a higher blood pressure and that’ll suppress the beta 1 sympathetic activity and cause the production of atrial naturetic peptide. That will reduce renin, reducing angiotensin one, reduce angiotensin 2, reduce aldosterone, promote vasodilation and reduce sodium and water reabsorption. So Increased sodium excretion.)
