Case 7 Flashcards
what are the three roles of the nephron?
- Filtration – takes place in glomerulus – ball of capillaries at beginning of tubule
- Selective reabsorption
- Secretion – there are some substances, like potassium and hydrogen ions, that we need to get rid of a greater rate than filtration alone with accomplish, so those can be actively transported into the tubule fluid at the later stages of the nephron, to top up whatever has been filtered
what are the functions of the kidney?
Maintenance of Extracellular Fluid Volume (ECFV) – sodium and water (therefore maintaining blood pressure) (normally amount of salt water you take in is same as what you lose – you’re in balance)
Acid-base balance regulation - therefore normally preventing acidosis/alkalosis
Excretion of metabolic waste – urea and creatinine (a waste product that comes from the normal wear and tear on muscles of the body – everyone has it in their bloodstream)
Endocrine secretion
- Renin-angiotensin system (for sodium regulation of blood pressure)
Erythropoietin (for RBC production and regulation) (centre for this because the kidneys have a very high demand for oxygen and therefore, they monitor blood oxygen levels)
Vitamin D (for calcium regulation) (calcitriol)
what is the nephron divided up into? what does each section do?
Glomerulus - filtration (renal corpuscle = production of filtrate)
Proximal Convoluted Tubule – selective reabsorption of water, ions, and all organic nutrients
Descending Limb of Loop of Henle – further selective reabsorption of water
Ascending Limb of Loop of Henle – selective reabsorption of sodium and chloride ions
Distal Convoluted Tubule – secretion of ions, acids, drugs, toxins/ variable reabsorption of water sodium and calcium ions (under hormonal control)
Collecting Duct – variable reabsorption of water and reabsorption or secretion of sodium, potassium, hydrogen and bicarbonate ions
- Papillary Duct - delivery of urine to minor calyx
what is the blood supply of the kidney like? (blood flow)
- The average cardiac output is 5 litres/min. The kidneys receive 20% of this (1 litre/min).
- The renal blood flow (RBF) is about 10-50 times greater than other the blood supply of other organs.
- RBF exceeds O2 requirements of kidneys (which reflects its function as a filter)
- RBF not regulated metabolically
what is the primary means for eliminating waste products of metabolism? what are these products?
- The kidneys are the primary means for eliminating waste products of metabolism that are no longer needed by the body.
- These products include urea (from the metabolism of amino acids), creatinine (from muscle creatine), uric acid (from nucleic acids), bilirubin (from Hb breakdown), and metabolites of various hormones.
apart from waste products of metabolism, what else is removed from the kidneys?
The kidneys also eliminate most toxins and other foreign substances that are either produced by the body or ingested, such as pesticides, drugs, and food additives.
what does the glomerulus do? what can pass through it?
- The glomerulus allows for filtration of contents of the blood into the proximal convoluted tubule (PCT).
- Proteins larger than the size of albumin can’t pass into the PCT.
what layers must fluid cross to get through glomerulus to proximal convoluted tubule?
Wall of glomerular capillary
Basement membrane
Inner layer of Bowman’s capsule
(Podocytes, Pedicels, Filtration slits)
the glomerulus provides what kind of barrier? what does it allow through?
- The glomerulus provides a size and a charge barrier.
- It allows small positive molecules through.
- Large or negatively charged molecules are repelled.
- Size and charge barrier
- Size = anything bigger than albumin cannot pass through in the normal healthy glomerulus
- Water, electrolytes and other small molecules can pass through, but albumin is the cut off barrier
- Anything bigger than that – larger proteins, red blood cells – should not get through
- Charge = a layer on the extracellular matrix called glycocalyx – it contains a number of negatively charged ions so it will repel negatively charged ions within the plasma – but encourages positive ions to come through
- The other barrier is the extracellular matrix itself – it prevents larger material passing through
what is the equation for GFR?
GFR = Kf . [P(GC) - (P(BC) + pi(GC))
what factors affect the GFR?
Kf = filtration coefficient (remains constant)
P(GC) = glomerular capillary hydrostatic pressure (favours filtration)
pi(GC) = glomerular capillary oncotic pressure (opposes filtration)
Oncotic pressure is a form of osmotic pressure exerted by proteins, notably albumin, in a blood vessel that pulls water into the circulatory system.
P(BC) = Bowman’s capsule hydrostatic pressure (opposes filtration)
- Shouldn’t be a protein in the Bowman’s capsule to exert an oncotic pressure so not included in equation
= 3 forces working
what is autoregulation?
the process by which the RBF and GFR are maintained despite changes in systemic pressure (blood pressure changes throughout day)
does GFR change?
not without pathology
what happens to vascular resistance when blood pressure increases? and of what? what does this do?
when the blood pressure increases, the vascular resistance of the afferent arteriole increases too
- this maintain the RBF and the GFR
- As renal arterial pressure increases, the resistance of the afferent arteriole increases (they constrict)
- Under normal circumstances, the efferent arteriole doesn’t change
- This means that glomerular capillary pressure doesn’t change as renal arterial pressure increases
- Therefore, renal blood flow does not change
- And GFR does not change as systemic and renal arterial pressure fluctuates
- Each glomerulus regulates itself and maintains its GFR at a steady level
therefore what is autoregulation what how does it occur
- the increased vascular resistance
Myogenic – vascular smooth muscle responds to stretch by vasoconstricting = narrow lumen and increase resistance – so pressure downstream is maintained
Tubuloglomerular feedback – distal tubular flow regulates vasoconstriction.
-contents of tubule are monitored and sends signal back to glomerulus to say there’s too much or too little fluid coming through
-each nephron communicates with its glomerulus and tells it how much fluid is passing through and whether flow needs to be increased or decreased
what is the macula densa?
a collection of densely packed epithelial cells at the junction of the thick ascending limb (TAL) and distal convoluted tubule (DCT)
tubuloglomerular feedback
- what does this involve
- what does this allow
- what happens
- what is indicative of what
- This process involves the macula densa.
- The macula densa is a collection of densely packed epithelial cells at the junction of the thick ascending limb (TAL) and distal convoluted tubule (DCT).
- As the TAL ascends through the renal cortex, it encounters its own glomerulus, bringing the macula densa to rest at the angle between the afferent and efferent arterioles.
- The macula densa’s position enables it to rapidly alter glomerular resistance in response to changes in the flow rate through the distal nephron.
- The macula densa uses the composition of the tubular fluid as an indicator of GFR.
- A large sodium chloride concentration is indicative of an elevated GFR.
- A low sodium chloride concentration indicates a depressed GFR.
describe the mechanism of tubuloglomerular feedback for increased GFR
• Increased arterial pressure causes increased glomerular pressure and plasma flow.
• This increases the GFR.
The plasma colloid osmotic pressure increases to limit the increased GFR. (but then increase plasma colloid osmotic pressure as more fluid has left it – becomes more concentrated)
• The increased GFR increases the tubular flow to the proximal convoluted tubule
This leads to increased reabsorption of water and ions in the proximal convoluted tubule and the loop of Henle (glomerulotubular balance)
• The increased GFR increases the tubular flow to the early distal convoluted tubule.
There is increased osmolarity of the tubular fluid (i.e. increased NaCl). (flow related increase osmolality or [NaCl] (monitoring flow by detecting osmolality or Na+ conc. or Cl- conc.))
• This is sensed by the macula densa by an apical Na-K-2Cl cotransporter (NKCC2).
- : (i) sensor mechanism (ii) transmitter (in walls of distal tubule that are adjacent to the glomerulus of the same nephron) (tubuloglomerular feedback)
• The juxtaglomerular cells in the macula densa secrete renin, which results in afferent arteriole constriction.
• This increases the preglomerular resistance, thus decreasing the GFR and keeping it maintained at a steady level. (decrease the glomerular pressure & plasma flow)
• This is known as TUBULOGLOMERULAR FEEDBACK.
what is a measurement of GFR?
• ‘Renal Clearance’ – volume of plasma which is cleared of substance x per unit time
- Can applied to anything that is filtered through the kidneys
- But if applied to a marker of glomerular filtration rate then it can be used to measure GFR
what is the equation for renal clearance?
(Ux) V / Px
• Ux = urinary concentration of ‘x’
• V = urine volume per unit time
• Px = plasma concentration of ‘x’
what are the features of a good marker of GFR?
Freely filtered in glomerulus – small enough to get through the glomerular capillaries
Not reabsorbed in PCT
Not secreted out of DCT
Excreted in urine
If whatever is filtered all ends up in the urine, the rate of clearance will be exactly proportional to GFR and therefore can be used to measure GFR
what are different markers of GFR? what marker of GFR is used in clinical practice? what is it affected by?
creatinine
- by-product of muscle breakdown
- affected by diet (how much protein you eat), age (older people tend to have muscle wasting), gender and ethnicity
- 51Cr-EDTA (radioactive – so not used routinely)
- 125I-iothalamate (radioactive – so not used routinely)
- 99mTc-DTPA (radioactive – so not used routinely)
- Inulin
- ‘gold standard’
- not endogenous so not used routinely in clinical situations
- have to introduce it into circulation and establish steady circulation
- takes several hours
- Cystatin C: the (clinical) future?
- used clinically
- many cases better than creatinine
- but for some reason creatinine is what’s used
sodium regulation
- how much do we reabsorb per day
- how much do we excrete per day
- what does plasma sodium concentration determine
- why better than active water transport
- what is it linked to
- We reabsorb about 1.5kg of Na+ ions a day.
- We excrete about 9g of Na+ ions a day.
- 1.5 kg salt filtered per day
- 9 g salt excreted per day
- (you excrete almost exactly what you take in in your diet)
- Vast majority of filtered sodium is reabsorbed
• Plasma [Na+] determines
Extracellular fluid volume (and therefore your blood volume and therefore blood pressure)
Arterial blood pressure
• Less “expensive” than active water transport. This is because it is easier to transport Na+ ions and allow other things (like water and glucose) to follow. This way we don’t expend excess amounts of ATP.
• Linked to most other renal transport processes e.g. glucose reabsorption.(most other things involved in the kidney gets a free ride with the movement of sodium)
- Spend energy on sodium and everything else moves passively or through secondary transport
where is sodium reabsorbed? through what?
• Proximal convoluted tubule – 67% Na+ reabsorbed (bulk – irrespective of whether you need to lose or retain sodium)
• Loop of Henlé – 25% Na+ reabsorbed
This occurs via the Na+-K+-Cl- Cotransporter (NKCC2) in the ascending loop of Henle.
• Distal convoluted tubule & collecting duct – 8% Na+ reabsorbed (fine tuning – hormonally regulated – depending on whether need to retain or lose sodium)
describe bulk sodium reabsorption
- where does this take place
Proximal convoluted tubule:
1. The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
2. This allows the Na+-H+ exchanger on the apical membrane to take up Na+ ions from the urine.
3. The anion-Cl- exchanger allows the uptake of HCO3- ions in exchange for Cl- ions.
Therefore, both Na+ ions and HCO3- ions are reabsorbed.
describe fine tuning reabsorption of sodium
Distal convoluted tubule & collecting duct:
- The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
- Aldosterone combines with a cytoplasmic receptor.
- Hormone-receptor complex initiates transcription in the nucleus.
- New protein channels (ENaC – epithelium sodium channel) and pumps are made.
- Aldosterone-induced proteins modify existing proteins.
- Result is increased Na+ reabsorption and K+ secretion.
what are the three transport protein families involved in glucose transport?
1. SLC – solute carrier family SLC5: sodium-linked cotransporters 2. SGLT SGLT1 - transports 1 glucose: 2 Na SGLT2 – transports 1 glucose: 1 Na 3. GLUT GLUT1 and GLUT2 (between these two glucose transporters, you can take up glucose from the fluid and get it back into the blood) - GLUT gene family - Facilitated diffusion
describe glucose reabsorption in the early proximal convoluted tubule
- how much reabsorption here
- how
- how glucose get to blood
- what affinity like
- what capacity like
The early proximal convoluted tubule is involved in the mass reabsorption of glucose.
The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
This allows the Na+-glucose SGLT2 cotransporter on the apical membrane to take up Na+ and sodium from the urine.
Next, the GLUT2 protein allows passage of glucose from inside the cell into the blood.
These have a low-affinity but high capacity because there is a lot of glucose available and these transport proteins allow for mass reabsorption of glucose.
- SGLT2 (take glucose and sodium in from tubule)
- GLUT2 (glucose moves out into blood)
= both low-affinity but high-capacity – grabs all glucose passing by and chucks it back into blood
describe glucose reabsorption in the late proximal convoluted tubule
- how much reabsorption
- how
- how glucose get to blood
- what affinity like
- what capacity like
The late proximal convoluted tubule is involved in the fine reabsorption of glucose.
The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
This allows the Na+-glucose SGLT1 cotransporter on the apical membrane to take up Na+ and sodium from the urine.
Next, the GLUT1 protein allows passage of glucose from inside the cell into the blood.
These have a high-affinity but low capacity because glucose has already been mass absorbed and so there is less glucose left in the tubular fluid. These transport proteins allow for fine-tuning reabsorption of glucose.
- SGLT1 (take glucose and sodium in from tubule)
- GLUT 1 (glucose moves out into blood)
= both high-affinity but low-capacity – a lot less glucose present but they are able to mop up remaining amount of glucose because they have a high affinity for it - So, by the time the fluid leaves the proximal tubule there should be no glucose left in the fluid – it reabsorbs all of filtered glucose under normal circumstances
glucose excretion
- what is fasting glucose level
- what is GFR
- what is filtered glucose rate
- what is transport maximum
- what happens when Tm is reached
- what happens to rest of glucose
- Fasting glucose ~ 5 mmol/L and GFR = 125 ml/min
- Filtered glucose = 5 x 0.125 = 0.63 mmol/min
- Transport maximum (Tm) ~ 1.25 mmol/min Plasma glucose ~ 10 mmol/L
- This graph shows that once the Tm is reached, no more glucose can be reabsorbed.
- The excess glucose must be excreted (the point of intersection of the lines).
- So, you can normally reabsorb all the glucose that you filter
- But there is a limit to how much glucose they can transport (this is known as transport maximum) – problem in people with diabetes
- Not until you get to a plasma glucose of about 10 mmol/L or higher that you are filtering more than you can reabsorb
- Under normal circumstances, the amount of glucose filtered matches the amount reabsorbed
- Only when exceeds the amount that you can reabsorb that you get excretion
- Relationship not perfectly linear because not every nephron has the same transporting capacity
how is the kidney a source of glucose? how much of all glucose in the body? what happens to it? what happens in diabetes?
• The kidney itself is a source of glucose via gluconeogenesis.
• The kidney makes around 20% of all glucose in the body, but it then breaks it back down.
- 300% increase in diabetes
what does water reabsorption occur?
in the descending loop of Henle
• The longer the loop of Henle, the greater the amount of water that is reabsorbed.
if you need to reabsorb more water, what is this mediated by?
the effect of ADH/ Vasopressin
how does ADH lead to water reabsoption? what is reabsorption dependent on?
• ADH inserts aquaporins (AQP2) in the apical membrane of the cells in the late distal convoluted tubule and the cells of the collecting tubule.
This allows water to be reabsorbed from the cells back into the body.
The water will only flow through these channels in the presence of an osmotic gradient caused by Na+ ions.
which aquaporins are already present on the basolateral membrane of these cells?
AQP3/4
what is the effect of aldosterone?
allows reabsorption of water via ENaC
what is the descending limb impermeable to? why?
what is the ascending limb impermeable to? why?
- thin descending limb is permeable to water; impermeable to solutes
- thick ascending limb is impermeable to water; active solute transport
how does concentration change through the nephron? what is the maximum gradient in the ascending limb?
PCT = 300 mOsm/kg
bottom of loop of Henle = 1200 mOsm/kg
DCT = 100 mOsm/kg
leaving the CD = 1200 mOsm/kg
(maximum gradient in ascending limb = 200 mOsm/kg)
where does calcium and magnesium ion reabsorption take place?
• PCT & Loop of Henlé
91% Ca2+ reabsorbed – paracellular route (passive reabsorption)
89% Mg2+ reabsorbed – paracellular route (passive reabsorption)
• DCT
3-7% Ca2+ reabsorption
5-6% Mg2+ reabsorption
how are calcium ions reabsorbed in the PCT and loop of Henle?
passively reabsorbed (paracellular route)
how are calcium ions reabsorbed in the DCT?
- what through apical
- what in cell
- what through basolateral
using transport proteins
using transport proteins
• Ca2+ ions enter the cell via TRPV5 transport protein channels.
• Because Ca2+ is an intracellular signalling molecule, we can’t have free Ca2+ in the cell as it would trigger other signalling pathways.
• Therefore, Ca2+ needs to be chaperoned from the apical membrane to the basolateral membrane where they would then enter blood.
• Ca2+ ions bind to an intracellular protein called Calbindin-D28K.
• This allows Ca2+ ions to move to the basolateral membrane.
• Once at the basolateral membrane, these ions can exit the cell into the bloodstream via two transport proteins: (1) Na+/Ca2+ exchanger [NCX1] and (2) plasma-membrane-calcium-ATPase-pump [PMCA1b].
- Calcium channel (TRPV5) in apical membrane – bring calcium in
- Calbindin-D28K – binding protein in cell
- Calcium pump – PMCA1b – principal route of calcium moving into blood – energy requiring – pumping out against a gradient
- Ca2+/Na+ exchanger – NCX1 – calcium out into blood and sodium in
what can TRPV5 (calcium channel) be regulated by?
Parathyroid hormone (PTH)
Vitamin D – the kidneys activate vitamin D which stimulates TRPV5 in the DCT
Sex hormones
Klotho (higher up in tubule) – this is a protein that’s associated with longevity.
- Channel in apical membrane is opened by the above
how is Mg2+ absorbed in the DCT? what stimulated by?
• Mg2+ reabsorption is less known.
• There is a ROMK potassium channel on the apical membrane of these cells.
• This causes the movement of K+ ions out of the cell and into the tubular fluid.
• This makes the tubular fluid positively charged.
• This favours the movement of Mg2+ ions into the cell via TRPM6 transport protein channels.
This is activated by epidermal growth factor.
• There is a Mg2+ exchanger on the apical membrane but we don’t know what Mg2+ is exchanged for.
- Lot less known about this
- Magnesium channel – TRPM6 – apical
- Buffer protein?
- Mg2+ exchanger in basolateral?
- Potassium leaks out in potassium channel in apical
- This makes tubular fluid more positive
- Which makes Mg2+ ions move from tubule into cell
- Magnesium channel (TRPM6) can be opened by epidermal growth factor
we absorb potassium from our diet. what happens to it next? how taken up into cells?
- It enters the ECF.
* It is taken up by cells by a Na+-K+ pump.
what is the Na+/K+ pump activated by?
insulin
how much of the potassium ions are taken up by the cells and how much remains in the ECF?
98% of the potassium ions is taken up by the cells and 2% remains in the ECF.
where are K+ ions reabsorbed?
- Proximal Convoluted Tubule takes up 65% of K+ ions.
- Loop of Henlé takes up 25% of K+ ions.
- Distal Convoluted Tubule & collecting duct have variable K+ reabsorption and secretion. Here, you either get net reabsorption or net secretion.
where is K+ excreted? and what percentage where?
- 92% of the potassium is excreted by the kidneys.
* 8% of the potassium is excreted by the colon.
why does potassium need to be carefully regulated? what can hyper and hypokalaemia lead to?
- Potassium needs to be carefully regulated because both hypokalaemia and hyperkalaemia can be fatal.
- Hypolkalaemia causes excess hyperpolarisation which leads to paralysis and so death.
- Hyperkalaemia causes excess depolarisation which also leads to paralysis and so death.
- Hypokalaemia -> hyperpolarisation -> paralysis -> death
- Hyperkalaemia -> depolarisation -> paralysis -> death
potassium regulation in the PCT
• On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions. (from blood) - high blood glucose concentration in glomerulus
- Glucose Tm exceeded
- Glucose present in tubular fluid in collecting duct
- > 1200 mOsm/kg
- = osmotic diuresis -> thirst
- These can leave via a K+ channel on the basolateral membrane.
- The net effect is the recycling of K+ ions.
- On the apical membrane there is a K+ channel.
- This allows the outflow of K+ ions, into the urine.
- As the tubular fluid continues through the PCT, there is a gain in the charge of the fluid because of the secretion of positively charged ions into the tubular fluid.
- This causes K+ ions to diffuse through the tight junctions between cells, back into the blood (down an electrochemical gradient).
- This is unregulated.
- In this way, most of the K+ ions are secreted into the tubular fluid, however, some diffuse back into blood.
- Na+/K+-ATPase pump in basolateral side (blood)
- Potassium ion channel in apical side going into lumen of proximal tubule
- K+ ions makes the fluid go from negative to positive charge
- This then favours the movement of potassium through a paracellular route -> into extracellular fluid where it is reabsorbed into the blood
- Pretty much passive reabsorption – due to build up of positive charge – pushes the ions out
potassium regulation in the (thick) ascending limb of loop of Henle
- On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions.
- These can leave via a K+ channel on the basolateral membrane.
- The net effect is the recycling of K+ ions.
- On the apical membrane there are two K+ transport proteins.
- The NKCC2 is a sodium-potassium- 2 chlorine cotransporter.
- It allows the entry of potassium ions into the cell.
- The ROMK2 potassium channel causes an outflow of K+ ions.
- This causes recycling of K+ ions on the apical membrane, similar to that on the basolateral membrane.
- The net movement of potassium ions favours the influx via NKCC2.
- Na+/K+-ATPase transporter in basolateral side (lowers intracellular sodium which causes sodium to move in through the next transporter)
- NKCC2/SLC12A1 (K+/Na+/2Cl- transporter – all moving into cell across apical membrane) – sodium and potassium reabsorption (potassium hitches a ride)
- ROMK2 – apical membrane potassium channel (potassium recycling) (potassium into lumen)
- Potassium channel on basolateral membrane (potassium can then be reabsorbed into blood)
- Both potassium channels (basolateral and apical membranes – both potassium cell leaving cell) help reabsorption of potassium through NKCC2
what two types of cells does the collecting duct have? which are the vast majority?
- principal (vast majority is this)
2. intercalated
what are principal cells involved in?
the secretion of K+ ions into urine
what are intercalated cells involved in?
the reabsorption of K+ ions from the urine
describe potassium secretion in principal cells
- what does aldosterone do? what stimulates aldosterone release?
On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions.
There is another K+ channel that allows the uptake of K+ ions form the blood into the cell.
On the apical membrane, there is a K+/Cl- cotransporter which causes the secretion of both K+ and Cl- ions into the urine.
There is also a K+ channel (ROMK1) that allows secretion of K+ ions.
Principal cells:
- Na+/K+-ATPase (K+ in) and K+ (out but can do either direction) channel in basolateral membrane
- Na+ channel (sodium in) and K+ channel (potassium out) (ROMk1 and 3) and Cl-/K+ cotransporter (both out) in the apical membrane
- Due to potassium channel on both apical and basolateral side we have a route for getting rid of (apical) and a route for retaining potassium (basolateral)
- Aldosterone can be activated: opens sodium channel, activate potassium channel on apical membrane, and reverse the direction the potassium is going in on basolateral membrane (so the channel brings potassium into the cell instead of going into blood)
- Hyperkalaemia can stimulate aldosterone release
what can the ROMK1 channel be stimulated by?
by aldosterone or high plasma K+ (hyperkalaemia)
describe potassium reabsorption in the intercalated cells
On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions.
On the apical membrane, there is a K+/H+ exchanger which causes the reabsorption of K+ ions.
There is also another H+ channel which causes the outflow of H+ ions.
Intercalated cells:
- K+/H+ cotransporter in apical membrane (potassium in, hydrogen out)
- H+ pump in apical membrane (channel) (out)
- If we have acidosis or low plasma K+ conc., the cotransporter kicks in – the system in the principal cell that loses potassium becomes less dominant
- Na+/K+-ATPase in basolateral membrane
what activates the intercalated cells?
acidosis or hypokalaemia
what are the different sources of acid? what happens to it?
• Normally, there is around 15,000mmol of CO2 produced per day. This is ‘potential acid’, but it usually isn’t a problem as it is efficiently excreted by the lungs.
• Metabolism also produces ~40 mmol H+ per day (‘non-volatile acids’: sulphuric, phosphoric, organic acids).
• There is also a net uptake of ~30 mmol H+ per day by GI tract
• So the kidney has to:
Excrete ~70 mmol H+ per day
Reabsorb all the filtered HCO3 -
when excess H+ ions excreted in the urine, what needs to happen and why?
- Excess H+ ions in the urine can cause the urine to become very acidic (pH = 1.3), thus painful.
- Therefore, in the urine, the H+ ions needs to be buffered.
what do carbonic anhydrases contain?
Zn
which carbonic anhydrases are there? what do they do?
• These are enzymes that contain Zn.
• There are at least 16 isoforms, but two important isoforms reside in the kidneys:
CA II – soluble cytoplasmic (found freely dispersed in the cytoplasm)
CA IV – extracellular, linked to cell membrane (by a GPI anchor)
• They catalyse the hydration of CO2. But, this is what actually happens…
• Carbonic anhydrase (CA) catalyses the second reaction (CO2 + OH-…)
H2O H+ + OH-
CO2 + OH- HCO3- (this reaction)
May see this shorted as H2O + CO2 -> H2CO3 -> H+ + HCO3- (but this isn’t really what’s happening)
reasbsorption of filtered HCO3- (general overview)
- HCO3- in the tubular fluid is converted to CO2 and OH- ions as a result of CA IV.
- The CO2 diffuses into the cell.
- The OH- ions combine with the H+ ions in the tubular fluid to form water.
- This water diffuses into the cell via osmosis.
- Once in the cell, the water breaks down again into H+ and OH- ions.
- The CO2 combines with the OH- (via CA II) ions to from HCO3- ions.
- At the basolateral membrane, the HCO3- ions exit the cell and enter the blood.
- The net movement of HCO3- ions is from the tubular fluid into the blood.
- This process uses a lot of ATP.
- H+ secretion at apical membrane reclaims HCO3- -> CO2 + H2O (carbonic anhydrase IV (CAIV) – apical membrane)
- CO2 can freely diffuse across apical cell membrane
- In cell, CO2 + H2O -> H+ + HCO3- (CAII)
- H+ recycled and resecreted into lumen
- HCO3- extruded at basolateral membrane
- Net transfer of HCO3- from lumen to interstitium/blood
what are the sites of HCO3- reabsorption?
• HCO3- ions are reabsorbed throughout the nephron:
PCT = 80% (same as reabsorption of NaCl and water)
Thick ascending loop of Henle (TAL) = 10%
DCT = 6%
Collecting duct = 4%
how much HCO3- is excreted?
Less than 0.01% is excreted.
describe how HCO3- is reasborbed in the PCT.
what happens to everything else?
- HCO3- in the tubular fluid is converted to CO2 and OH- ions as a result of CA IV.
- The CO2 diffuses into the cell.
- The OH- ions combine with the H+ ions in the tubular fluid to form water.
- This water diffuses into the cell via osmosis.
- Once in the cell, the water breaks down again into H+ and OH- ions.
- The CO2 combines with the OH- (via CA II) ions to from HCO3- ions.
- At the basolateral membrane, the HCO3- ions exit the cell via a ‘kidney variant of Na+/ HCO3- cotransporter’ (kNBCe1) and enter the blood.
- The H+ ions are secreted from the cell via a H+-ATPase pump and via a Na+/H+ exchanger (NHE3).
- The net movement of HCO3- ions is from the tubular fluid into the blood.
- This process uses a lot of ATP.
- Acid secreted across apical membrane into lumen
- NHE3 (sodium-hydrogen exchanger) (very common to have exchangers using sodium) (sodium is moving along its concentration gradient and hydrogen ions moving against concentration gradient (secondary active transport?))
- H+-ATPase – primary active transport into lumen
- H+ ions combine with bicarbonate (CAIV) -> CO2 + H2O
- CO2 -> diffuse into cell -> break back down into H+ and bicarbonate
- In PCT, transporter that’s responsible for transporting HCO3- across basolateral membrane = kNBCe1 (kidney sodium-bicarbonate cotransporter) – 3HCO3- and 1Na+ -> both transporter out of cell (unusual for having Na+ moving out of cell in cotransport because that’s against sodium’s natural gradient) (however it is able to function like this due to its unusual stoichiometry – moving three negative charges and one positive charge in the same direction – net movement of charge = 2 negative = transporter is electrogenic – this net movement of charge allows sodium to move in unusual direction)
• NHE3 is dominant in proximal tubule (majority of H+ secreted is by NHE3 rather than H+-ATPase)
• large capacity (the transporter – trying to reabsorb 80% of bicarbonate and to do this we need to secrete acid) but limited gradient generation (only down to pH 6-ish in the lumen (if we just had the NHE3 transporter))
• V-type (vacuolar) H+-ATPase can generate a bigger gradient (down to pH 4 or 5) (this becomes really important in the collecting duct)
• more important later in the tubule where lumen is more acidic
• 1:3 stoichiometry of kNBCe1 makes it electrogenic
• allows HCO3- efflux from the cell because of extra drive from membrane potential
• unusual to have Na+ leaving the cell on a cotransporter
NHE3 = apical membrane
kNBCe1 = basolateral membrane
where is the NHE3 dominant? what is capacity like? why?
in PCT (Na+/H+ exchanger)
- The NHE3 is dominant in proximal tubule. It has a large capacity but limited gradient generation (from 7.35 to 6 pH). This is because the Na+ ions move over a small gradient and so only small amounts of H+ are exchanged (secreted), therefore the pH only decreases by a small amount.
- V-type (vacuolar) H+-ATPase can generate a bigger gradient (from 7.35 to 4/5 pH).
what makes the kNBCe1 electrogenic? what does this mean for efflux?
• 1:3 stoichiometry of kNBCe1 makes it electrogenic.
It allows HCO3 - efflux from the cell because of extra drive from membrane potential.
This is because there is a net efflux of 2- charge. This gives the protein the extra drive.
what is proximal renal tubular acidosis?
- Rare autosomal-recessive disease
- Impaired HCO3- reabsorption in PCT
- Severe metabolic acidosis (PCT responsible for absorbing 80% of bicarbonate)
- Not treatable by HCO3- supplementation (80% of bicarbonate is reabsorbed in PCT)
- Attributed to mutations in kNBCe1 (reabsorbing 3 bicarbonate ions)
- Ocular abnormalities too because of kNBCe1 and pBNCe1 expression there too
how are HCO3- ions transported at the basolateral membrane in the thick ascending limb and DCT?
- The transport protein for HCO3- ions at the basolateral membrane differs to that in the PCT.
- Here, it is an anion-exchanger (AE2).
- It exchanges HCO3- ions for Cl- ions.
- same processes but slightly different combination of transporters
- H+ generated from CO2 + H2O as usual (CAII)
- Luminal CAIV less important (slower here)
- Still have Na-H exchanger (NHE3) and H+-ATPase but there is a different transporter on the basolateral membrane
- No kNBCe1 here, just AE2 (carbonate-chloride exchanger) for HCO3- exit at the basolateral membrane
- 1 bicarbonate ion out of cell for 1 chloride ion into cell
