Somatostatin inhibits growth hormone-releasing factor-stimulated adenylate cyclase activity in GH, cells

GH3 cells were used to study the effect of rat growth hormone-releasing factor on adenylate cyclase activity and its interaction with somatostatin. Rat GRF stimulates adenylate cyclase activity (ED5 0 = 6 X 10(-8) M) and somatostatin-14 inhibits this GRF-stimulated activity in a non-competitive manner without affecting the basal enzyme levels. Inhibition by somatostatin-14 is observed at concentrations as low as 10(-11) M and the half-maximal effect is obtained with 10(-8) M. Somatostatin-28 is more potent than SS-14 and has an ED5 0 of 3 X 10(-11) M. VIP is more active than rat GRF in stimulating adenylate cyclase activation. We conclude that in GH3 cells rat GRF behaves as a partial VIP agonist by interacting with VIP-preferring receptors and its effects are inhibited by somatostatin.