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Factors that change distal K+ secretion :
(a) Dietary K+
increases K+ secretion, and a diet low in K+ decreases K+secretion.
■ On a high-K+ diet, intracellular K+ increases so that the driving force for K+secretion also increases.
■ On a low-K+ diet, intracellular K+ decreases so that the driving force for K+secretion decreases. Also, the α-intercalated cells are stimulated to reabsorb K+ by the H+, K+-ATPase.
■ increases K+ secretion.
■ The mechanism involves increased Na+ entry into the cells across the luminal membrane and increased pumping of Na+ out of the cells by the Na+–K+pump.
Stimulation of the Na+–K+ pump simultaneously increases K+ uptake into the principal cells, increasing the intracellular K+ concentration and the driving force for K+secretion.
Aldosterone also increases the number of luminal membrane K+
channels.
■ Hyperaldosteronism increases K+ secretion and causes hypokalemia.
■ Hypoaldosteronism decreases K+ secretion and causes hyperkalemia
■ Effectively, H+ and K+ exchange for each other across the basolateral cell
membrane.
■ Acidosis decreases K+ secretion. The blood contains excess H+; therefore, H+enters the cell across the basolateral membrane and K+ leaves the cell.
As a result, the intracellular K+ concentration and the driving force for K+secretion decrease.
■ Alkalosis increases K+ secretion. The blood contains too little H+, therefore, H+leaves the cell across the basolateral membrane and K+ enters the cell. As a result, the intracellular K+
concentration and the driving force for K+secretion increase.
■ increase K+ secretion.
■ Diuretics that increase flow rate through the distal tubule and collecting ducts (e.g., thiazide diuretics, loop diuretics) cause dilution of the luminal K+concentration, increasing the driving force for K+ secretion.
Also, as a result of increased K+ secretion, these diuretics cause hypokalemia.
K+
■ decrease K+ secretion
If used alone, they cause hyperkalemia.
■ Spironolactone is an antagonist of aldosterone; triamterene and amiloride act directly on the principal cells.
■ The most important use of the K+-sparing diuretics is in combination with thiazide or loop diuretics to offset (reduce) urinary K+losses.
■ Excess anions (e.g., HCO3
-) in the lumen cause an increase in K+ secretion by increasing the negativity of the lumen and increasing the driving force for K+secretion.