The Vasopressin-sensitive Adenylate Cyclase of the Rat Kidney: EFFECT OF ADRENALECTOMY AND CORTICOSTEROIDS ON HORMONAL RECEPTOR-ENZYME COUPLING

Abstract

Abstract A subcellular fraction prepared from rat kidney medulla contained vasopressin-sensitive adenylate cyclase and vasopressin binding sites. Vasopressin stimulation resulted in an increase in the maximal velocity of the reaction with no change in the apparent Km for ATP. A maximum activation ratio (5 to 10) was obtained at low Mg2+ concentration (0.75 mm) and pH 7.4. The apparent Km for vasopressin was (1 to 7 x 10-8 m). Vasopressin binding sites (0.3 pmole per mg of protein) can be identified with the hormonal receptors involved in adenylate cyclase activation on the basis of the two following criteria: (a) saturation of receptor sites and adenylate cyclase activation occurred in the same range of hormone concentrations and (b) a good correlation was observed between the relative potencies of unlabeled [8-arginine]vasopressin, [8-lysine]vasopressin, and oxytocin as inhibitors of [3H]vasopressin binding and as activators of the adenylate cyclase. Adrenalectomy reduced the adenylate cyclase stimulation that was induced by vasopressin. It did not modify the basal enzyme activity or its activation by parathyroid hormone and sodium fluoride. The small reduction in vasopressinbinding capacity observed after adrenalectomy cannot account for the reduction in enzyme activation, an observation which suggests an impairment in the efficacy of receptor-enzyme coupling. Aldosterone treatment of adrenalectomized rats restored the hormone-binding capacity to control level but did not correct the receptor-enzyme coupling defect. Dexamethasone enhanced coupling efficiency in adrenalectomized rats and to a lesser extent in control animals. Dexamethasone was as active as aldosterone in restoring the hormone-binding capacity of adrenalectomized rats but had no effect in normal animals. Corticosterone was inactive under our experimental conditions. Neither adrenalectomy nor corticosteroid treatment modified the apparent Km of vasopressin for its receptor. Aldosterone and dexamethasone were ineffective when added in vitro to the membrane preparation. It is suggested that adrenal steroids exert a dual action on the vasopressin-sensitive adenylate cyclase of the kidney: (a) modulation of the number of receptor sites; this effect can be elicited by mineralo and glucocorticoids (aldosterone and dexamethasone) and (b) control of synthesis of a component enhancing the receptor-enzyme coupling efficiency. In normal animals the receptor-enzyme coupling did not appear to be maximally stimulated.