Renal regulation of K and Ca Flashcards
The vast majority of body potassium is located
In the cells
- is the most important intracellular ion
Potassium output
- urine: 92 mEq/d
- feces: 8 mEq/d
- total: 100 mEq/d
Partitioning of Na and K across cell membrane allows for
Polarity differences that are critical for excitable membranes and the development of action potentials
Intracellular dehydration
Due to loss or lack of intake leading to long term potassium wasting
How does urinary excretion of potassium range from 1 - over 100% of what was filtered at the glomerulus?
Tubules can reabsorb almost all of what was filtered to actually secreting extra potassium into the tubule on top of what was filtered
Plasma potassium only represents _____
A small fraction of whole body potassium
How to assess overall body potassium status
Measure renal potassium fractional excretion
- asses proportion of potassium filtered across the glomerulus compared to that contained in the urine
Causes of extracellular K moving into the cell
- increases K intake
- insulin
- aldosterone
- beta-adrenergic
- alkalosis
Causes of intracellular K moving out of the cell
- cell lysis
- strenuous exercise
- acidosis
- beta blockade
- HYPP
Insulin
Insulin surge after eating moves excess K just ingested from the ECF into the cell
____ and ____ tend to push K into cells
Insulin; alkalosis
Giving IV bicarb causes H ions to move out of the cells to maintain normal pH, and K moves _____ to maintain an electroneutral exchange of ions
Into cells
Cellular acidosis
Results in K moving out of the cell to maintain electroneutrality within the cell and reduce K uptake into the cell by decreasing the efficiency of the Na K ATPase pump
Patients that are inappetent and suffering from GI electrolyte losses are _____
In a negative potassium balance
The ______ is responsible for long term sustainable control of potassium
Kidney
Potassium excretion is determined by the sum of 3 renal processes:
- rate of filtration
- tubular K reabsorption/secretion
- tubular flow rate
Acute renal failure
Results in a decrease in GFR and can cause serious K accumulation and hyperkalemia due to a high potassium diet
What is the primary cell involved in potassium excretion?
Principal cells of the late distal and collecting tubules
What is the limiting factor of the Na K ATPase pump?
Electrolyte availability, NOT energy!
Aldosterone secretion is increased by
Increased serum potassium
- independent of angiotensin 2
- causes increased secretion of K into the distal tubule lumen by the principle cells
Where is the primary site of aldosterone?
On the principle cells of the cortical collecting tubule
Aldosterone feedback loop
Increased K intake –> increased plasma K concentration —> increased aldosterone (+) –> increased K secretion cortical collecting tubules –> increased K excretion
What happens to K in the absence of aldosterone secretion?
Renal secretion of K is impaired, leading to ECF potassium concentration to rise to high levels
What happens to K with excess aldosterone secretion?
Potassium secretion becomes greatly increased, causing potassium loss by the kidneys = hypokalemia
Does decreased aldosterone secretion due to a Na rich diet lead to K retention?
No, effects are counterbalanced by increased flow rate, which moves any filtered K downstream fast enough that there is a minimal build up of K within the lumen
An increase in distal tubular flow rate (due to volume expansion, high Na intake, etc) stimulates potassium _____
Secretion
A decrease in distal tubular flow rate (caused by sodium depletion) leads to _______
Reduced K secretion
The effect of tubular flow rate on K secretion is the distal and collecting tubules is strongly influenced by _______
Potassium intake
The principle cell apical membrane contains ________
Unregulated K channels
Intercalated type A cells are able to actively pump _____ out against a high concentration gradient
H ions
- function to reabsorb K
Acidosis results in extra H in the cells, forcing ______
K to diffuse out
- to maintain electroneutrality
- chronic acidosis results in potassium wasting and whole body potassium depeletion
Intercalated type B cells
Have the H K ATPase counter exchange pumps and H ATPase pumps in the basolateral cell membrane
- primarily responsible for maintaining pH by getting rid of excess bicarb during alkalosis
- function to secrete K
_____ of calcium is in bone
99%
Ionized divalent cation
Form of free circulating calcium
Calcium is loosely bound to ____
Albumin due to albumin’s negative charge
- bound for is not biologically active
Acidosis ____ Ca binding, while alkalosis ___ Ca-albumin binding
Decreases; increases
Number of receptors is controlled by ____
PTH
Increase in serum Ca =
Decrease in PTH
Decrease in serum Ca =
Increase in PTH
What is the name of the Ca carrier molecule located on the apical membrane of the proximal tubules?
Phosphotidyl inositides
- bind Ca avidly
- rapid turnover times
- number of receptors determined by PTH
PTH secretion is determined by state of calcium ___ or ____
Repletion or depletion
What happens when excess Ca enters the cell?
Activates proteases that can result in significant cell damage and cell death
What is another location of PTH responsive Ca absorption?
Thick ascending loop of Henle
- paracellular movement of cations predominates in this portion
Distal convoluted tubule absorbs ____ through the transcellular pathway
5-10% of calcium
Does Ca absorption occur in the medullary collecting ducts?
No
The kidney is critical in the activation of ______
Vitamin D
- need UV light if active VD is not in the diet
If you do not need more Ca, then vitamin D is ___
Not activated and is excreted
If you do need more Ca, then vitamin D is ____
Converted to active form and will activate Ca absorption in the GIT and Ca mobilizaiton out of the bone stores
Phosphates
Most diets contain phosphates in excess and reabsorption is fairly unregulated
Phosphates - short term
Transport max
- when phosphate < transport max, most is absorbed
- when phosphate > transport max, most is excreted
Phosphates - long term
Transport max can vary depending on PTH
- increase PTH decreases phosphate transport max, leads to decrease in phosphate reabsorption
