Regulation of adenylate cyclase in rat and ovine anterior pituitary cells.

Dopamine and somatostatin inhibit release of pituitary prolactin and growth hormone ( G H ) respectively. In some species this inhibition is accompanied by lowering of cyclic A M P levels. Thus, in rat anterior pituitary membranes, dopamine can inhibit vasoactive intestinal peptidcstimulated adenylate cyclase activity 11 I and somatostatin can inhibit GH-releasing hormone (GHRH)-stimulated adenylate cyclase 121. Both factors can inhibit forskolin-stimulated rat adenylate cyclase. In ovinc anterior pituitary cells. however. dopamine and somatostatin do not inhibit cyclic A M P accumulation. Thus, Law ei a/. 131 showed that although somatostatin can inhibit GHRH-stimulated G H secretion in both rat and ovine anterior pituitary cells, it lowered cyclic A M P levcls only in the rat cells. Similarly, Ray et uI. [4] found that dopamine and somatostatin could inhibit forskolin-stimulated GH and prolactin secretion, but not cyclic A M P accumulation in ovinc pituitary cells. I t is unlikely that heterogeneity o f pituitary cells contributes t o these observations, since similar results were obtained using an enriched preparation of somatotrophs 151. T h e different responses o f ovine and rat pituitary cells could lie at the level of the inhibitory G proteins that are known t o link hormone-receptor complexes t o intracellular effector systems. We therefore investigated ( ( I ) whether the