Resting membrane potential depends on the concentration gradient of what electrolyte?
potassium
This ion has a major role in the excitability of nerve and muscle tissue…
potassium
What is ECF potassium concentration a function of?
amount of K in body
distribution between ICF and ECF
What percent of potassium is located inside the cells?
98%
What three hormones exert control on potassium?
Epinephrine
Insulin
Aldosterone
Epinephrine’s activation of alpha-1 receptors causes what, which could lead to what condition?
shift of potassium out of cells
hyperkalemia
Insulin has what two effects on potassium?
increase uptake
stimulates sodium-potassium ATPase
Aldosterone has what three effects on potassium distribution?
increases potassium uptake to tubule
increases potassium excretion
stimulates sodium-potassium ATPase
Acidoses causes potassium to move where?
out of cells
Alkalosis causes potassium to move where?
into cells
Does transport of potassium in the PT and LOH change with changes in total body potassium?
no
Where does physiological regulation of potassium occur?
DT and CD
The magnitude of potassium secretion depends on the size of what?
electrochemical gradient across luminal membrane
The below factors have what effect on potassium secretion?
Increased dietary intake
Aldosterone presence
Alkalosis
High urine flow rate
Increased sodium to distal nephron
increase
What causes the secretion of potassium when sodium loads in the distal nephron increase?
stimulates sodium-potassium ATPase
intracellular potassium increases, and is then secreted
Which drugs cause an increased delivery of sodium to distal nephron, and therefore can lead to hypokalemia?
loop diuretics
thiazides
Which two diuretics are potassium sparing?
spironolactone and amiloride
What stimulates aldosterone?
high plasma potassium levels
How does aldosterone promote potassium secretion?
stimulates basolateral sodium-potassium ATPase
Increases luminal membrane permeability to potassium
Aldosterone increases potassium secretion in the DT from ____ to _____ %
10-50%
Aldosterone increases potassium secretion in the CD from _______ to ______ %
5-30%
In which cell type does potassium secretion (as regulated by aldosterone) occur?
principal cells
Aldosterone stimulates the upregulation of which two channels in the principal cells?
Apical potassium channels
sodium potassium ATPases
Aldosterone causes increased tubular reabsorption of _______ and increased tubular secretion of ______
reabsorption of sodium
Secretion of potassium
What two factors cause an increase in aldosterone secretion?
increased AT II
Increased plasma potassium
A low potassium diet causes what homeostatic response?
decreased excretion
In a low potassium diet, what percent of filtered potassium is reabsorbed and where?
87% reabsorbed in the PT and LOH
The PT and LOH reabsorb 87% of filtered potassium in low potassium conditions. Where is the remaining potassium reabosorbed?
distal nephron (DT/CD)
Potassium reabsorption in the distal nephron occurs in what cell type?
alpha intercalated cells
By what mechanism do alpha intercalated cells reabsorb potassium?
via hydrogen-potassium exchange at apical membrane
Potassium diffuses across basolateral membrane
What percent of calcium store is in the ECF?
0.1%
Of the 2.5 mmol/L of plasma calcium, what percent is ionized and biologically active?
What percent is bound to proteins?
What percent is complexed to anions?
50% active
40% protein bound
10% complexed
What percent of plasma calcium is filtered, and why?
60% filtered because 40% is bound to protein
Calcium reabsorption in the nephron occurs everywhere except…
the descending limb of LOH
What percent of filtered calcium ends up in the urine?
1%
Rank the sites of calcium reabsorption from greatest to least.
PT > Thick Ascending > DT > CD
What percent of calcium is reabsorbed at the proximal tubule?
67%
What percent of calcium is reabsorbed in the thick ascending limb?
25%
What percent of calcium is reabsorbed in the dT?
5-10%
What percent of calcium is reabsorbed in the collecting duct?
< 5%
What decreases the reabsorption of calcium in the thick ascending limb?
Furosemide
What increases the reabsorption of calcium in the distal tubule?
thiazide diuretics
Name this site of calcium reabsorption:
Passive
coupled/directly related to sodium uptake
Reabsorbed via paracellular route
Proximal tubule
In the PT, when sodium uptake is high, calcium uptake will be _______
proportionally increased
Name this site of calcium reabsorption:
Reabsorption driven by positive lumen potential
enters via paracellular pathway
loop diuretics interfere with calcium reabsorption
Thick ascending limb
What is responsible for generating the positive lumen potential that drives calcium reabsorption in the thick ascending limb of LOH?
Diffusion of potassium back into lumen after transport by NKCC2
Name this site of calcium reabsorption:
enters via calcium channels across the apical membrane
Crosses basolateral membrane via calcium ATPase and sodium-calcium exchange
Reabsorption stimulated by PTH (cAMP mediated)
Distal tubule
By what mechanism do thiazide diuretics increase calcium reabsorption?
decreases of ICF sodium increase the inward activity of the sodium-calcium exchanger
sodium pumped in, calcium pumped out
Sodium and calcium in the distal tubule have what relationship?
inverse due to sodium-calcium exchanger
What percent of plasma PO4 is bound?
10%
What role does PO4 have in the urine?
acts as a buffer
What percent of PO4 is filterable at the glomerulus?
90-95%
What percent of filtered PO4 is reabsorbed and where?
85% reabsorbed in convoluted or straight proximal tubule
What percent of filtered PO4 ends up excreted?
15%
Increases in what hormone will decrease PT potassium reabsorption and therefore increase excretion?
PTH
What relationship does PO4 have with calcium in the presence of PTH?
inverse relationship.
increased PTH = Decreased PO4 reabsorption, increased calcium reabsorption