Luminal influences on potassium secretion: Chloride, sodium, and thiazide diuretics

In the presence of Cl^-, K⁺ secretion by the distal tubule saturates with increasing luminal Na⁺ concentration. Apparent maximal K⁺ secretion is attained with luminal Na⁺ concentrations of 40 mM. The results of the present study show that lowering the Cl^- concentration of luminal fluid can increase the level of Na⁺-stimulated K⁺ secretion beyond the maximal level attained in the presence of Cl^-. The effect of lowering luminal Cl^- concentration to <10 mM on K⁺ secretion is greater with higher Na⁺ concentration. Under these conditions, chlorothiazide decreases K⁺ secretion. When chlorothiazide is present, changing the Na⁺ concentration does not affect K⁺ secretion. Because in rats a thiazide effect is attributed primarily to the distal convoluted tubule (DCT), we postulate that it is primarily DCT cells that increase K⁺ secretion when Na⁺ concentration is raised in the presence of low luminal Cl^- concentration. We propose that the rat DCT cells have both an absorptive Na⁺-Cl^- cotransport mechanism and a secretory K⁺-Cl^- cotransport mechanism in the luminal membrane that can mediate the apparent exchange of Na⁺ for K⁺.