Lecture 19- K+ homeostasis

Question 1

what functions is K+ crucial for?

Answer 1

• regulation of cell volume
• regulation of intracellular PH
• maintaining a potential diff across cell membrane - esp muscles and neurons

Question 2

What different factors alter plasma [K+]?

Answer 2

• food intake
• acid base balance - eg metabolic acidosis - increases plasma [k+] and a reduced PH
• plasma osmolality - increase in osmolality of ECF will increase plasma [K+]

Question 3

What 3 hormones are important in regulating K+ movement in and out of cells under normal conditions?

Answer 3

• Noradrenaline
• insulin
• aldosterone

Question 4

in summary, what effect does adrenaline, insulin and aldosterone have?

Answer 4

• they increase the uptake of K+ into skeletal muscle, liver, bone and rbc’s
• achieved by stimulating Na+-K+-ATPase, the NKCC2 symporter etc

Question 5

How is K+ reabsorped in the proxiaml convoluted tubule?

Answer 5

• it occurs primarily through a paracellular mechanism - ie through solvent drag
• or it can also occur through a shift in transepithelial voltage

Question 6

Describe K+ reabsorption in the TAL

Answer 6

• K+ reabsorption in the TAL is due to both transcellular movement by NKCC2 ad by paracellular movement
• and also paracellularly due to the lumen positive charge generated by apical K+ channels

Question 7

What cell is responsible in the collecting duct for K+ secretion?

Answer 7

the principal cell

Question 8

By what mechanism do the principal cells secrete K+?

Answer 8

they secrete K+ across the apical membrane passively through the ROMK channel

Question 9

what does the ROMK channel stand for?

Answer 9

renal outer medullary K+ channel

Question 10

How do changes in distal tubular lumen [k+] concentration influence the rate of K+ secretion?

Answer 10

• if there are increases in the tubular [K+], it decreases the rate of secretion of K+
• if there is a decrease in the tubular [K+], it increases the rate of secretion of K+

Question 11

What are** 3 factors** that influence K+ secretion by the cortical collecting ducts?

Answer 11

• Plasma [K+]
• aldosterone
• ADH

Question 12

Why is plasma [K+] an important determinant of K+ secretion?

Answer 12

• if the ECF [K+] is high, this conveys hyperkalemia and** stimulates the secretion of K+** within minutes via several mechanisms
• it stimulates the Na+-K+-ATPase pump to increase uptake of K+
• it increases the secretion of aldosterone by the adrenal glands
• it** increases the flow rate of tubular fluid** which stimulates secretion of K+

Question 13

How does aldosterone regulate K+ secretion?

Answer 13

• aldosterone passes through the cell membrane and binds to a mineralcortioid receptor
• the complex then acts as a transcription factor and **stimulates the production of Na+-K+-ATPase **and also ENaC channels
• it also increases the expression of K+ channels on the apical membrane

Question 13

How does aldosterone regulate K+ secretion?

Answer 13

• aldosterone passes through the cell membrane and binds to a mineralcortioid receptor
• the complex then acts as a transcription factor and **stimulates the production of Na+-K+-ATPase **and also ENaC channels
• it also increases the expression of K+ channels on the apical membrane

Question 14

What effects does ADH on K+ secretion by the principal cells?

Answer 14

• ADH stimulates the uptake of Na+ through the ENac channels across the apical membrane - this reduces the electrical potential difference across apical membrane
• ADH reduces the flow rate of tubular fluid which reduces K+ secretion

Question 15

How does flow rate affect K+ secretion?

Answer 15

• an increase in the flow of tubular fluid (eg with diuretic treatment) stimulates K+ secretion
• a decrease in the flow of tubular fluid (eg during haemorrhage or reduced blood volume) decreases K+ secretion