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A B S T R A C T Aldosterone receptors from rat kidney slices were utilized in a competitive binding technique to analyze the contribution of various steroids to plasma "mineralocorticoid" activity and to assess their possible role in hypertension. To consider simultaneously the plasma binding, steroids were incubated with slices in undiluted plasma; competitor activities for [3H]aldosterone binding were aldosterone, 100%; deoxycorticosterone, 16.2%; cortisol, 0.4%; and 18-hydroxy-deoxycorticosterone and 18-hydroxy-corticosterone, 0.1%. These steroids were more active in buffer than plasma, suggesting that they bind to plasma and that this reduces their receptor binding. Analysis of the competition data suggests that at normal plasma concentrations, aldosterone occupies the receptors to a major extent, cortisol occupies some of the receptors, and deoxycorticosterone and 18-hydroxydeoxycorticosterone contribute little to receptor occupancy. Two steroids implicated in low-renin essential hypertension, 16i#-hydroxy-dehydroepiandrosterone and 16-oxoandrostenediol, did not have significant competitor activity.Competitor activity in plasmas from normal subjects taken at 12 noon (upright) was greater than that in those taken at 8 a.m. (supine). Since the 12 noon samThis work has been published in abstract form: 1975.

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