REGULATION OF K+EXCRETION

Regulation of K+ excretion ECF K+ is normally regulated precisely at  4.2mEg/L, rarely ± 0.3mEg/L, Only  2% of total potassium is in ECF  59mEg. Daily intake 50 –200mEg/day. Maintenance of K+ balance depend primarily on excretion by the kidneys 5 – 10% of K+ intake excretion in feces.

Roles of K Major ion determining the resting membrane electrical potential, which in turn, limits and opposes K efflux Changes in K concentrations (particularly in the ECF) have marked effects on cell excitability (heart, brain, nerve,muscle). Major intracellular osmotically active cation and participates in cell (intracellular) volume regulation (exits with Cl when cells swell).

Roles of K 4. Critical for enzyme activities and for cell division and growth 5. Intracellular K participates in acid base regulation through exchange for extracellular H and by influencing the rate of renal ammonium production

Regulation of K+ excretion K+ excretion is determined by Rate of K+ filtration Rate of reabsorption 65% in proximal tubules. 25 –30 in the loop of Henle Rate of secretion

Regulation of K+ excretion The most important site for regulation of K+ excretion are the distal tubules and cortical collecting tubules. K+ can be reabsorbed or secreted depending on the body’s need. With high K+ intake the required extra excretion is achieved almost entirely by increasing secretion of K+ into the distal and collecting tubules.

Regulation of K+ excretion With reduced K+ intake, the rate of secretion is decreased With extreme reductions in the K+ intake there is net reabsorption of K+ in the distal segments of the nephron.

K+ secretion in the late distal tubules and cortical collecting tubules Cells present Principal cell ( 90% of Epithelia cell in these regions). Intercalated cell.

Mechanism Na+ - K+ATPase pump in the basolateral membrane of the cell move K+ to the interior of the cell. Passive diffusion of K+ from the interior of the cell into the tubular fluid. Luminal membrane of Principal cell is highly permeable to K+ Special channel that are specifically permeable to K+

Mechanism Renal ISF principal cells Tubular lumen Na+ Na+ ATP K+ K+ ---------->K+

Mechanism Intercalated cells reabsorb K+ during K+ depletion. H+ -K+ATPase transport mechanism in the luminal membrane. K+diffuses through the basolateral membrane of the cell into the ISF

Factors that regulate K+ secretion by principal cells Stimulants Increased ECF [k+] Effect is especially pronounced when ECF [K+]rises above  4.1mEg/L Increased Aldosterone Increased tubular flow rate. Inhibitors Increased [H+]

Mechanism  ECF [K+] stimulates Na+ - K+ATPase pump  concentration gradient between the renal ISF to the interior of the epithelia cell Reduce back leakage of K+ from inside the cells through basolateral membrane  aldostrone secretion. Stimulates Na+ - K+ATPase pump Increase permeability of the luminal membrane for K+

Increased tubular flow rate Occurs with volume expansion,  Na+ intake, diuretics Stimulates K+ secretion With increased tubular flow rate, the secreted K+ is continuously flushed down the tubule This counterbalances the effect of decreased aldostorone secretion caused by  Na+ intake or volume expansion.

mechanism  K+ intake  Plasma [K+]  Aldosterone  K+ secretion by principal cells  K+ excretion

mechanism Stimulates aldosterone secretion Increases number of Na+/K+ATPases Increasing distal k + delivery

 Na+ intake Aldosterone  GFR  Proximal tubular Na+ reabsorption  Distal tubular flow rate K+ secretion by principal cells Unchanged K+ excretion

Likewise with low Na+ intake there is also little change in K+ excretion because of counterbalancing effects of increased aldosterone secretion and decreased tubular flow rate on K+ excretion

Increased H+ concentration in ECF Acute acidosis reduces K+ secretion where decreased [H+] increases secretion. Reduces Na+ -K+ATPase pump activity. Prolonged increase in [H+] increases urinary K+ excretion. Inhibits proximal tubular NaCl and water reabsorption, overriding the inhibitory effect of H+ on Na+ - K+ATPase pump.