regulation of ECF potassium

Question 1

Importance of K concentration in ECF

Answer 1

determines the resting membrane potential of all excitable cells

Question 2

Normal potassium concentration in ECF

Answer 2

3.5 to 5.0mM

Question 3

Normal range of K filtration and excretion each day

Answer 3

About 30 grams are filtered each day, and 0-45 grams are excreted per day

Question 4

Where does K reabsorption occur and what percentage of filtered K load is reabsorbed?

Answer 4

In proximal tubule, 80% of filtered load is reabsorbed. In loop of Henle, 10-15% of filtered load is reabsorbed. The remaining is reabsorbed by principal cells in fine tuning segments

Question 5

Mechanism of K reabsorption in proximal tubule

Answer 5

passive, paracellular movement of K occurs through the tight junctions driven by bulk flow of water, also through the tight junctions

Question 6

Mechanism of K reabsorption in Loop of Henle

Answer 6

Transcellular: Na/K/2Cl co-transporter moves K from lume into cell via a secondary active transport process. . Potassium then passively runs down its electrochemical gradient through a K+ channel in the basolateral membrane to the serosal domain

Question 7

What determines the amount of K excretion

Answer 7

Since all of the filtered K is obligatorily reabsorbed, K secretion in fine tuning segments determines K excretion and K balance in ECF

Question 8

Mechanism for K secretion and location

Answer 8

Occurs in principal cells of fine tuning segments. 1) Na+/K+-ATPase exchanger pumps Na out of the cell while pumping K into the cell via the basolateral membrane. 2) K flows passively down its electrochemical gradient through a K channel in the apical membrane into the lumen where it is excreted in the urine

Question 9

List two feedback mechanisms for potassium secretion regulation

Answer 9

mass action effects and hormonal regulation via aldosterone

Question 10

Describe Mass action effects in K secretion regulation

Answer 10

increased K ingestion > increased K in ECF > increased action of basolateral Na/K pump (mass action effect) > increased intracellular K > increased driving force for apical K movement into lumen > increased K secretion > increased K excretion

Question 11

Describe hormonal regulation of K secretion

Answer 11

Increased K in ECF > stimulation of adrenal cortical cells (zona glomerulosa cells) > increased aldosterone synthesis and secretion. Aldosterone increases K secretion

Question 12

Describe step 1 and step 2 effects of Aldosterone on K secretion

Answer 12

step 1) Aldosterone increases the number of Na/K/ATPase pumps on the basolateral surface, which increases the rate of K entry and increases the intracellular concentration of K (this enhances step 2). Step 2) Aldosterone increases number of apical Na channels, so as more Na flows passively into the cell, K is secreted out of the cell into the lumen to balance charge. Also, Aldosterone increases the number of apical K channels making this K flow in step 2 easier

Question 13

Effects of tubular flow on K secretion

Answer 13

With slow tubular flow, K builds up in the tubular fluid and as K conc rises, the electrochemical gradient for subsequent K secretion decreses, slowing secretion downstream. With fast tubular flow, K does not build up as fast so the electrochemical gradient stays relatively high and K secretion is maintained at a higher rate.

Question 14

Discuss loop diuretics and tubular flow

Answer 14

Loop diuretics Inhibit the ascending limb Na/K/2Cl co-transporter so that Na/K/2Cl are not reabsorbed. This decreases the tonicity of the interstitium, so water stays in the descending loop instead of being reabsorbed. This increases tubular flow, and will also increase K secretion even more by maintaining a large electrochemical gradient for K.

Question 15

Discuss the balance btw Aldosterone induced K secretion and tubular flow reduced K secretion

Answer 15

Elevated aldosterone increases number of apical Na and K channels. As Na flows into the cell, K flows out of the cell to balance charge. However, as Na flows into the cell, water follows it and this can decrease the tubular volume which can reduce tubular flow rate. A reduced tubular flow can then decrease K secretion because of the accumulation of K lowering the electrochemical gradient. WHichever process dominates determines whether overall K secretion increases or decreases

Question 16

Is K secretion increased or decreased in primary hyperaldosteronism? Cardiac insufficiency?

Answer 16

Primary hyperaldosteronism: K secretion is increased overall. Increased MAP and decreased ECF volume causes an increased filtration that keeps tubular flow normal to elevated. Thus the effects of the aldosterone will increase K secretion overall. Cardiac insufficiency: a lowered MAP stimulates aldosterone production, but overall K secretion is decreased b/c reduced tubular flow decreases K secretion more than the effects of increased K secretion from aldosterone.

Question 17

how does alkalosis affect K secretion

Answer 17

Alkalosis increases K secretion and tends to produce hypokalemia.

Question 18

Pathway for generation of hypokalemia by alkalosis

Answer 18

increased ECF pH (alkalosis) > shift of K into all cells from ECF > decreased K in ECF causes hypokalemia > increased intracellular K enhances electrochemical gradient for K secretion via step 2 from tubular cells > increased K secretion due to increased apical permeability to K > increased K excretion > hypokalemia

Question 19

How does acidosis affect K secretion

Answer 19

acidosis causes a shift of K from cells to ECF and inhibits the apical K channels, which would cause a decrease in K secretion. However, severe acidosis also inhibits the transporters involved in sodium reabsorption. Since this transport drives water reabsorption throughout the tubule, its inhibition increases tubular fluid flow, hence potassium secretion would also increase via the flow effect. Final outcome depends on the proportion of both of these