The properties of angiotensin II (AII) receptors were studied in the adrenal zona glomerulosa, and the mechanisms leading to stimulation of steroidogenesis were analyzed in isolated glomerulosa cells from the rat and bovine adrenal cortex. AII receptors were further characterized by photoaffinity labeling with a C-terminal azido AII derivative, which possessed high labeling efficiency and was applied to the analysis of receptors in several target tissues. Isolation of the photolabeled AII receptor of the bovine adrenal gland was pursued by detergent solubilization and fractionation by ion exchange, lectin-affinity, and immuno-affinity chromatography. Studies on the actions of AII revealed that the calcium channel agonist, BAY K 8644, increased basal aldosterone production and enhanced the responses to AII and K+ in a differential manner, by increasing the maximum aldosterone response to AII but not to K+. These findings suggest that voltage-sensitive calcium channels are partially operative under basal conditions and are further activated by AII and K+. Elevation of cytoplasmic calcium by AII also depends upon mobilization of intracellular calcium stores by the products of ligand-stimulated phosphoinositide turnover. Microsomal receptors for the putative mediator of calcium mobilization, inositol-1,4,5-trisphosphate (IP3) were identified in adrenal microsomes by binding studies with (32P)IP3, and show high specificity and affinity (Kd 5 nM) as well as low capacity for IP3. The generation of inositol-1,4,5-trisphosphate from phosphatidylinositol bisphosphate during AII action was extremely rapid and was accompanied by major production of IP2 and inositol-4-monophosphate as well as formation of the inactive IP3 isomer, inositol-1,3,4-trisphosphate. The finding that IP3 is rapidly degraded to Ins-4-P contrasts with the previous view that Ins-1-P is the major metabolic product, and indicates that Ins-4-P serves as a marker of polyphosphoinositide turnover. The IP3-receptor system and the activation of voltage sensitive calcium channels are the major mechanisms involved in the regulation of intracellular calcium by AII and potassium, respectively, in the adrenal zona glomerulosa cell.
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