The hypothesis, based on studies in A6 epithelia in culture and in man and rat, is that corticosterone (B) and cortisol (F) stimulate renal transepthelial active Na+ transport (urinary Na+ retention, in vivo) by a low affinity (Type IV) receptor mechanism distinct from Type I (mineralocorticoid) or Type II (glucocorticoid) receptors. The agonists are the 6 beta-OH derivatives, 6 beta- OH-B and 6 beta-OH-F, made in the effector cells by a steroid inducible microsomal P450. This mechanism would be activated in states of high circulating glucocorticoid concentration (stress, Cushing's syndrome) or at normal concentration of glucocorticoids, if renal 6 beta-hydroxylation is accelerated. Our long term goals are 1) to determine to what extent this unique receptor system is operative in the mammalian kidney and examine its' role in the genesis of essential hypertension and in other states with defects in Na+ retention and 2) characterize and study the regulation of both the receptor and the 6 beta- hydroxylase (6 beta-OHase) using contemporary molecular biological techniques.
Our specific aims are: 1. Determine whether F induced active Na+ transport stimulation in A6 cells is mediated by Type IV nuclear receptor activation by the target cell metabolite 6 beta-OH-F; test the hypothesis that F and B mediate Na+ retention in the rat by such a low affinity mechanism; evaluate the role of alteration of 6 beta-OHase activity in changing the sensitivity of Type IV receptor activation to the extracellular concentration of F and B. 2. Test the hypotheses, in normal man, that the Na+ retention and the hypertensinogenic effect of high dose F is not mediated by mineralo-and glucocorticoid receptor activation, respectively; evaluate a subset of essential hypertensives with evidence of enhanced 6 beta-hydroxylation of F, as to their level of P450p activity, BP response to F administration, salt sensitivity of BP, ability to excrete a saline load and renin-angiotensin status. 3. Characterize the cyrtosolic form of the type IV receptor in A6 cells and rat in terms of physical properties and relative affinities for steroids; develop an assay for the corticosteroid 6 beta-OHase in the rat kidney and examine its zonal distribution. 4. Identify the unknown polar metabolite of B found in rat urine and produced by A6 cells; assess its' role in the complex effects of B in A6 cells; examine the possibility that the 6 beta-OH derivative of a glucocorticoid antagonist (RU486) is a selective Type IV receptor antagonist. 5. Establish a cDNA library from A6 cells; select cDNA probes for the 6 beta-OHase using existent antibodies; use these probes for the study of the molecular details of the corticosteroid mechanism of induction of the enzyme.
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