Characterization of somatostatin receptors in the rat adrenal glomerulosa zone.

Specific receptors for somatostatin have been identified and characterized in the rat adrenal glomerulosa zone in vivo and in vitro by binding studies with [125I]iodo-Tyr-somatostatin. In adult rats, the injection of [125I]iodo-Tyr-somatostatin was followed by rapid uptake of the labeled peptide in several tissues. The highest uptake was in the adrenal capsule, with a tissue to blood ratio of 4.5, followed by kidney, anterior pituitary, and liver with tissue to blood ratios of 2.0, 1.8, and 1.4, respectively. In vitro binding studies with adrenal capsular particles were performed at 16 C in the presence of bacitracin and thimerosal to reduce tracer degradation. Under these conditions, binding of [125I]iodo-Tyr-somatostatin to capsular membrane-rich fractions reached a steady state within 30-40 min and remained at a plateau for 120 min. Dissociation of bound somatostatin from its adrenal receptors was also rapid, with an initial half-time of 5 min. Equilibrium binding of somatostatin to adrenal capsular particles was saturable and of high affinity, with an association constant (Ka) of 1.5 x 10(10) M-1. The receptor-binding activities of several somatostatin analogs were consistent with their potencies as inhibitors of angiotensin II-stimulated aldosterone production in adrenal capsular cells and with their known biological activities upon GH release. These findings demonstrate that high affinity receptors with structural and biological specificities for somatostatin are present in the adrenal glomerulosa zone. Such receptors serve as the regulatory sites through which somatostatin inhibits the action of angiotensin II upon aldosterone production in the adrenal glomerulosa cell.