Renal regulation of ECF potassium

Question 1

3 rules for potassium regulation

Answer 1

1) all filtered K+ is obligatorily reabsorbed so regulated K+ secretion in fine tuning segments determines K+ excretion and ECF potassium balance
2) variable secretion of K+ accounts for homeostasis

E = F - R + S

E = S because what is filtered is reabsorbed (prox tubule and loop)

Question 2

bulk of filtered K+ (80%) reabsorbed in __

reabsorption of K+ is ___ and ___ driven by the ___

because it is freely permeable in tight junctions, it is swept along with ___ movement to the ___ side of the epithelium

Answer 2

1) proximal tubule
2) paracellular and passive; driven by the bulk flow of water through the tight junctions
3) water movement to the serosal side of epithelium

drives Na+ into serosal, Cl- follows and water follows through tight junction that sweeps along K+

Question 3

1) in the loop of henle, K+ movement is largely ___, first using the ___ cotransporter at the apical membrane to gain access in a ___ transport process
2) K+ runs down its ____ through a ___ to the serosal/basolateral side
3) loop of henle accounts for ___ % of filtered load reabsorption

Answer 3

1) transcellular using Na/K/2Cl; gain access in secondary active transport process
2) electrochemical gradient through a K+ channel
3) 10-15%

Question 4

secretion of K+ occurs in the ___ of the fine tuning segments (distal tubule/collecting duct)

Answer 4

principal cells (single cilia)

Question 5

Secretory process has two parts

opposite of ___ transport

Answer 5

1) basolateral entry of K+ into cell via Na/K atpase
2) apical secretion of K+ into the tubular lumen

coupled to Na+ transport (in aldosterone sensitive cells)

______

reverse Na+ transport

balanced by charge neutrality from Na+ movement in opposite direction

Question 6

2 steps in secretion

Answer 6

1) Na/K atpase to pump K across basolateral membrane into cell (Na out, K in)
2) K+ ion flow passively along electrochem gradient through K+ channel in apical membrane into lumen
3) excreted in urine

Question 7

Feedback mechanism of incr K+ ingestion

mass action route

Answer 7

1) incr K+ ingestion
2) incr K+ in ECF
3) incr basolateral pumping of K+ into cell (MASS ACTION)
4) incr intracellular K+
5) incr driving force for apical K+ movement into lumen (urine)
6) incr K+ secretion and excretion

Question 8

feedback mechanism of incr K+ ingestion

hormonal route (aldosterone in adrenal cortex)

Answer 8

1) incr K+ ingestion into ECF
2) incr stim of adrenal cortex cells
3) incr aldosterone syntehsis
4) incr # Na/K atpase in basolateral and incr in apical Na+ and K+ channels
5) incr basolateral pumping of K+ into cell and MASS ACTION ROUTE

incr permeability of lumenal membrane to K+

6) incr K+ secretion and excretion

Question 9

What is the flow effect on K+ movement

Answer 9

drives passive movement of K+ through potassium channels from cell to lumen (incr permeability of lumenal membrane and incr driving force into lumen)

Question 10

if you have slow tubular flow, how does that affect movement of K+

Answer 10

movement of K+ into lumen is slowed

more time for K+ to build up in tubular fluid before washed away by flow downstream

as lumenal K+ builds up, electrochem gradient for secretion decr (because close to electromchem equilibrium for K+)

secretion decr

Question 11

if you have incr tubular flow what happens to K+ mvoement

Answer 11

K+ will better wash away newly secreted K+ before it builds up

keeps electrochem gradient high

K+ secreted is washed away by fast tubular flow so secretion rate remains elevated

Question 12

what happens with loop diuretic on tubular flow

Answer 12

1) block Na/K/2Cl
2) inhibit NaCl excretion, inhibit H2O reabsorption
3) incr tubular flow because more H2O stays in tubule
4) incr K+ secretion and incr K+ excretion –> K+ wasting

also diuretic incr K+ excretion to 150% rate (10x faster rate of excretion than just inhib Na/K/2Cl due to incr tubular flow)

2) 25% NaCl in asecending limb drives out water from descending limb and distal tubule/collecting duct (ADH area) keeping interstitium hypertonic

Question 13

How does K+ movement change if you applied lasix

Answer 13

block Na/2Cl/K transporter in descending limb of loop of henle

decr hypertonicity of interstitim (less Na/K/Cl move out)

less water reabosrbed

incr tubular flow

incr K+ excretion

Question 14

how does alkalsosis affect K+ level and secretion

Answer 14

alkalosis –> hypokalemia

1) incr ECF pH (decr [H+]; as H+ leave, induce K+ to enter

loss of H+ from cells due to lower driving force for H+ into cells –> (H+ normally block Na+ channel) –> incr K+ permeability of apical K+ channels for K+ excretion

2) shift K+ from ECF into all cells
3) incr driving force for apical secretion and excretion

incr passive entry of K+ into principal cells

incr gradient for K+ excretion in Step 2 (across lumen) –> incr K+ excretion (hypokalemia)

4) decr K+ in ECF (hypokalemia)
5) also incr permeability of K+ in distal tubule and collecting duct on apical, incr K+ secretion

Question 15

how does incr ECF pH affect K+ level

Answer 15

1) shift of K+ into cells
2) decr K+ concentration of ECF –> hypokalemia

incr K+ channel permeability because less H+ inhib apical K+ channels –> incr K+ secretion

3) incr intracellular K+
4) incr driving force for apical K+ secretion into lumen (incr permeability)
5) incr K+ secretion and excretion beyond needed for ECF balance–> hypokalemia

Question 16

what does mass action mean

what is the impact of mass action on K+ regulation

Answer 16

Mass action = passive process driven by chemical kinetics

only good for K+ excess –> good for large change but as K+ return to normal, delay when chemical process slows

Question 17

does hypokalemia cause alkalosis?

Answer 17

1) give patient loop diuretic
2) patient becomes hypokalemic
3) also become alkalotic

Question 18

Normal filtration K+ per day

Normal excretion K+ per day

Answer 18

30 g K+ filtered

0-45 g K+ excreted

Question 19

Reabsorption occurs where?

all of filtered potassium is obligatory reabsorbed, hence potassium secretion in fine tuning segments determines K+ excretion and ECF K+ balance

Answer 19

via principal cells of distal tubule/collecting duct

Question 20

K+ is a ___ for Na+/K+ ATPase

Answer 20

required cofactor

binding of K+ to pump is rate limiting step in exchange pump

more K+ in serosa more quickly pumped into cell

Question 21

Mass action effect is most effective when?

If you have lower K+, what problem

Answer 21

slows down

problem with membrane excitability –> cardiac arrhythmias

Question 22

Effect of aldosterone on K+ (3)

Answer 22

1) incr Na/K atpase
2) incr Na+ entry on apical to incr K+ entry into cell
3) incr K+ on apical to allow for more passive diffusion

Question 24

Effect of primary hyperaldosteronism on tubular flow

Answer 24

aldosterone= incr K+ secretion and incr Na reabsorption, thereby decr tubular flow

Question 25

Primary vs. Secondary hyperaldosteronism

Answer 25

Primary = incr K+ secretion because incr ECF volume, incr MAP, incr filtration (tubular flow normal or incr)

Secondary = cardiac insufficiency, decr MAP, (still incr aldosterone due to RAAS), decr K+ secretion becasue decr GFR, decr filtration flow, decr tubular flow

Question 26

alkalosis incr ___, tends to produce ___

Answer 26

incr K+ secretion, hypokalemia

Question 27

Effect of low acidosis (mild acidosis) on K+ levels

Effect of severe acidosis on K+ levels

Answer 27

1) shift K+ from cells to ECF
2) apical K+ channels inhib by incr H+
3) decr K+ secretion

____

inhibited Na reabsorption and inhib water reabs

incr tubular flow, incr K+ secretion, hypokalemia