Regulation of ECF K levels

Question 1

In the prox tub. what stim the reabsorption of K+ from the lumen to the serosa?

Answer 1

K+ is paracellular and passive thru tight jxns. Driven by bulk flow of water.

Question 2

In the LOH, what stim the reabsorption of K+ from the lumen to the serosa?

Answer 2

K+ is transcellular, using the Na/K/2Cl cotransporter at the apical membrane via active transport

K+ then runs down its electrochemical gradient on the basolateral surface.

(this is where ~10-15% of R of occurs)

Question 3

Where is the remaining K+ reabsorbed after 95% of it has been Reabsorbed before the fine tuning/obligatory segments ?

Answer 3

By the principal cells in the fine tuning segments

K+ SECRETION also occurs in this stage

Question 4

In the fine tuning segments, how does K+ get secreted?

Answer 4

Basically like Na+ generic transport but in reverse

1. basolateral entry of K+ into cell from serosal side via Na/K antiporter pump
2. passive apical secretion of K into lumen

Question 5

Mass action effect of K+ ingestion

Answer 5

↑ K+ ingestion →
↑ ECF [K+] →
↑ (faster!) basolateral pumping of [K+] into cell (mass action) →
↑ intracellular [K+] →
↑ driving force for apical K+ movement →

pumping rate change is proportional to change in K+ concentration: only good for lg [K+], not small

Question 6

Hormonal regulation via aldosterone works parallel with the mass action effect of K+, which transporters does it act on?

Answer 6

1. Na/K atpase pump - basolateral
2. Apical Na+ channels
3. Apical K+ channel

• net effect is strong enhancement of K+ secretion by aldosterone

Question 7

Na+ reabsorption is usually coupled with water reabsorption, does this have a stimulatory or inhibitory effect on K+ secretion?

Answer 7

very strong inhibitory - offsets the stim effects of Na+ charge movement

Question 8

Primary Hyperaldosteronism usually causes a decrease/increase K+ secretion? What does it do to ECF volume and MAP?

Answer 8

Increase K + secretion and loss

Expands ECF V and increases MAP

Question 9

Alkalosis (increase ECF pH) does what to K+ ECF levels?

Answer 9

Alkalosis shifts K+ into the cells from the ECF, thus lowering ECF levels →
increase intracellular K+ →
increase driving F for apical K+ secretion→
Pee out more K+ (increase secretion) →
inappropriately = hypokalemia

*note that alkalosis also stimulates K+ apical channels, making them more permeable to K+

Question 10

Effects of acidosis on K+ ECF levels

Answer 10

It depends

mile acidosis- it shifts K+ into ECF, and inhibits apical K+ channels, leading to hyperkalemia but…

• severe acidosis inhibits transporters involved in Na+ reabsorption → less water reabsorption → slower tubular flow → more K+ secretion
• hence acidosis effects on K+ are unpredictable depending on severity of acidosis

Question 11

Feedback pathways that regulate ECF K levels

Answer 11

1. mass action effect (passive feedback loop)

2. hormonal regulation