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Flashcards in regulation of ECF potassium Deck (19):
1

Importance of K concentration in ECF

determines the resting membrane potential of all excitable cells

2

Normal potassium concentration in ECF

3.5 to 5.0mM

3

Normal range of K filtration and excretion each day

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

4

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

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

5

Mechanism of K reabsorption in proximal tubule

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

6

Mechanism of K reabsorption in Loop of Henle

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

7

What determines the amount of K excretion

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

8

Mechanism for K secretion and location

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

9

List two feedback mechanisms for potassium secretion regulation

mass action effects and hormonal regulation via aldosterone

10

Describe Mass action effects in K secretion regulation

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

11

Describe hormonal regulation of K secretion

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

12

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

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

13

Effects of tubular flow on K secretion

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.

14

Discuss loop diuretics and tubular flow

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.

15

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

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

16

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

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.

17

how does alkalosis affect K secretion

Alkalosis increases K secretion and tends to produce hypokalemia.

18

Pathway for generation of hypokalemia by alkalosis

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

19

How does acidosis affect K secretion

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