Lecture 37 4/12/24 Flashcards
Why is precise control of ECF potassium necessary?
many cell functions are sensitive to changes in ECF potassium concentration
Where is potassium contained within the body?
-98% within the cells
-2% in the ECF
Why is it important that the ECF is rapidly rid of ingested potassium?
daily potassium intake is often high enough to cause severe, life-threatening hyperkalemia if the potassium is not removed and/or stored in the cells
What does potassium homeostasis depend on?
-excretion of potassium by the kidneys
-control of potassium distribution between the extracellular and intracellular compartments
How does insulin affect the distribution of potassium between the cells and the ECF?
insulin stimulates Na+/K+ ATPase activity, which transports potassium into the cells
How do catecholamines affect the distribution of potassium between the cells and the ECF?
catecholamines activate beta2-adrenergic receptors which stimulate Na+/K+ ATPases, causing transport of potassium into the cells
How does alkalosis affect the distribution of potassium between the cells and the ECF?
-causes activation of Na+/H+ exchanger, which leads to an increase in intracellular Na+
-blocks the H+/K+ exchanger to prevent K+ from leaving the cell
-increased intracellular Na+ activates the Na+/K+ ATPase to bring K+ into the cell
How does increased potassium intake affect the distribution of potassium between the cells and the ECF?
stimulates the secretion of aldosterone, which increases cell potassium uptake
How does metabolic acidosis affect the distribution of potassium between the cells and the ECF?
-blocks the Na+/H+ exchanger; decreased intracellular Na+ prevents Na+/K+ ATPase activity
-activates the H+/K+ exchanger
-both lead to increased ECF potassium conc.
Why is cell destruction associated with hyperkalemia?
cell destruction releases stored potassium into the ECF
Renal potassium excretion is determined by the sum of which three processes?
-rate of potassium filtration
-rate of potassium reabsorption by the tubules
-rate of potassium secretion by the tubules
Where does potassium reabsorption occur?
-65% in the proximal tubule
-25-30% in the LOH (active co-transport with Na and Cl)
-variable rates in collecting tubules and ducts
Which cells in the late distal and collecting tubules can reabsorb potassium when potassium intake is low?
type A intercalated cells
Which mechanisms are thought to be behind type A intercalated cell reabsorption of potassium?
-H+/K+ ATPase transport
-diffusion through potassium membrane channels
Which regions of the nephron excrete extra potassium when intake is high?
distal and collecting tubules
What are the steps involved in secretion of potassium from the blood into the tubular lumen?
-uptake from the interstitium into the cell via Na+/K+ pump
-passive diffusion of K+ from the cell interior into the tubular fluid
Which channels are present in the principal cell membranes that allow for rapid potassium diffusion?
-renal outer medullary potassium (ROMK) channels
-high-conductance, “big” potassium (BK) channels
Which steps are involved in type B intercalated cells excreting extra potassium during high intake?
-potassium pumped into cells via H+/K+ ATPase
-diffusion of potassium into the tubular lumen
Which factors stimulate potassium secretion?
-increased ECF K+ concentration
-increased aldosterone
-increased tubular flow rate
What does increased ECF K+ lead to increases in?
-principal cell activity/Na+/K+ ATPase activity
-BK and ROMK channels
-K+ gradient from the renal interstitium
-aldosterone
How does aldosterone lead to increased K+ excretion?
-aldosterone stimulates active reabsorption of Na+, which is mediated by Na+/K+ pumps
-aldosterone also increases the number of potassium channels in cells
In addition to increased excretion, how else does aldosterone lower the ECF conc. of K+?
by increasing cellular uptake of potassium
How does a high-volume flow rate increase potassium secretion?
-causes secreted potassium to be continuously flushed down the tubule, minimizing tubular K+ conc. and increasing net secretion
-increases number of BK channels in the luminal membrane
Why is it important that high sodium intake increases distal flow rate?
-increased distal flow rate increases potassium secretion
-this counterbalances the decrease in potassium secretion that occurs due to aldosterone, which also accompanies high sodium intake
-counterbalance ensures that potassium excretion remains constant even though sodium intake changes
