Separate and combined effects of ouabain and extracellular potassium on renin secretion from rat renal cortical slices

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Abstract
Renin secretion of rat renal cortical slices was measured as a function of extracellular K and ouabain [O] concentrations in the incubation medium. A sigmoid relationship was found between renin secretion and log K concentration over the range 1.0-4.0 mM. Secretion was maximal at about 2.25 mM-K and half-maximal at about 1.43 nM-K. In media containing 4.0 mM-K, O at 10-8, 10-7 and 10-6 M did not affect renin secretion. Higher O concentrations inhibited secretion. A sigmoid relationship was found between percent inhibition of secretion and log O concentration (10-6-10-3 M). Inhibition was half-maximal at 2.3 .times. 10-5 M and complete at 10-3 M-O. Lowering extracellular K concentration from 4.0 to 2.25 mM shifted the dose-effect curve of O to the left. At 2.25 mM-K, renin secretion inhibition was half-maximal at 10-5 M-O. The inhibitory effect of 2 .times. 10-5-O (2 .times. the dose for 50% inhibition) in media containing 2.25 mM-K was nearly identical to the combined effect of lowering K to 1.43 mM (the concentration required for 50% inhibition) and adding 10-5 M-O. O and low extracellular K apparently act at a common site, presumably on Na, K-ATPase activity, to inhibit renin secretion. Neither 10-3 M-O nor K-free medium inhibited renin secretion when the concentration of free Ca in the medium was lowered to < 10-8 M. Apparently, as a result of Na, K-ATPase inhibition, intracellular Na increases, intracellular Ca increases via Na-Ca exchange, provided that extracellular Ca exceeds 10-8 M, and Ca accumulation, in some unknown manner, inhibits renin secretion from rat renal cortical slices.