Secretion of adrenocorticotropin (ACTH) by AtT-20 pituitary tumor cells is under multi-hormonal control. Somatostatin (SRIF) inhibited ACTH secretion stimulated by cAMP-dependent and -independent pathways. SRIF inhibition of both adenylate cyclase and Ca++ entry via voltage-dependent channels was mediated by GTP-regulatory (G) proteins. In addition, G proteins mediated SRIF inhibition of ACTH release by phorbol esters (activation of C kinase) and by mechanisms distal to both the activation of cAMP-dependent kinase and Ca++ entry. Radioligand binding studies revealed a single class of SRIF receptor whose affinity was regulated by Na+ and GTP. These data suggest that inhibitory G proteins mediate both SRIF action and receptor desensitization through multiple transduction mechanisms. GTP-regulatory proteins, in particular transducin, were shown to mediate light-induced stimulation of phospholipase A2 and C (PLA2, PLC) in rod outer segments (ROS) of bovine retina. Studies using G protein-specific agents, cholera toxin and pertussis toxin, in both light and dark-adapted ROS suggested a dual role for G proteins in both activation and inhibition of PLA2 and PLC. In retina from Xenopus laevis, both somatostatin and dopamine inhibited the circadian rise in N-acetyl transferase (NAT), the rate limiting enzyme in melatonin synthesis. These effects appear mediated by G proteins through the inhibition of adenylate cyclase. In addition a role for arachidonic acid (AA) or AA metabolites in the regulation of NAT activity was found. Alpha1-adrenergic activation of PLA2 and PLC was studied in FRTL-5 rat thyroid cells. Alpha1-Agonists increase PGE2 formation from AA which stimulates cell growth. Both AA and inositolphosphate release were stimulated by GTP analogues, but only AA release was inhibited by pertussis toxin. This suggests that Alpha1-receptors can couple to two distinct G proteins in these cells.
