Luminal influences on potassium secretion: chloride replacement with sulfate

Electrolyte transport by the renal distal tubule of rats was studied by in vivo continuous microperfusion to determine whether replacing luminal chloride with sulfate would stimulate net potassium secretion. Results in a first series of experiments showed that replacing all Cl in the perfusion fluid with SO4 reduced net Na absorption, reversed the direction of net Cl transport from net absorption to net secretion, increased the lumen-negative transepithelial voltage, and increased net K secretion. In a second series of experiments adding NaCl to a perfusion fluid containing SO4 increased net Na absorption, reversed the direction of net Cl transport from net secretion to net absorption, decreased the transepithelial voltage, and decreased net K secretion. In a third series of experiments addition of 10(-6) M amiloride to a SO4-containing perfusion fluid reduced the transepithelial voltage but did not prevent the increase in K secretion, indicating that the changes in K secretion cannot be explained solely by changes in electrical driving forces. Since net fluid absorption was controlled, the changes in K secretion cannot be attributed to solvent drag effects. In all three series of experiments the rate of net K secretion was increased when luminal Cl concentration was reduced to less than 10 mM. The results suggest the existence of a cotransport system mediating K and Cl absorption by the distal tubule. Inhibition of K absorption by low lumen Cl may contribute to the increased net K secretion seen when nonchloride anions are present in distal fluid.