Comparison of the Effects of Rubidium and Potassium on Renin Secretion from Rat Kidney Slices*

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Abstract
It has been shown previously that both low (45 mM) concentrations of extracellular K inhibit the renin secretory rate of rat kidney slices, and that nonidentical Ca-dependent mechanisms appear to be involved. As Rb can substitute for K in many biological systems, the present experiments were designed to compare the effects of K and Rb on renin secretion of rat kidney slices. Adding either KCl or RbCl to a nominally K-free incubation medium stimulated renin secretory rate in concentration-dependent manners; secretory rate was half-maximally stimulated at approximately 1.5 mM and maximally stimulated at approximately 2–3 mM concentrations of either KCl or RbCl. Ouabain completely abolished the basal secretory rate, in either KCl- or RbCl-containing media. These results suggest that the effects of increasing KCl or RbCl in the range of 0.5–4 mM are attributable to stimulatory effects of Rb and K on Na-K-ATPase activity. Renin secretory rate was greatly inhibited by incubating kidney slices in media containing 60 mM concentrations of either KCl or RbCl. A concentration of methoxy-verapamil which completely blocked the inhibitory effects of 60 mM KCl or of 60 mM RbCl failed to antagonize the inhibitory effects of a nominally K-free medium or of media containing ouabain and either 4 mM KCl or 4 mM RbCl. Taken together with previous results, these observations suggest that Rb can substitute for K in the renin secretory process. Furthermore, they support the hypothesis that inhibitors of Na-K-ATPase, and depolarization inhibit renin secretion by Ca-dependent mechanisms which are not identical.