Secretion‐stimulating and secretion‐inhibiting hormones stimulate high‐affinity pertussis‐toxin‐sensitive GTPases in membranes of a pituitary cell line

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Abstract
Different peptide hormones influence hormone secretion in pituitary cells by diverse second messenger systems. Recent data indicate that luteinizing‐hormone‐releasing hormone (LHRH) stimulates and somatostatin inhibits voltage‐dependent Ca2+ channels of GH3 cells via pertussis‐toxin‐sensitive mechanisms [Rosenthal et al. (1988) EMBO J. 7, 1627–1633]. In other pituitary cell lines, somatostatin has been shown to cause a pertussis‐toxin‐sensitive decrease in adenylate cyclase activity, and LHRH and thyrotropin‐releasing hormone (TRH) stimulate phosphoinositol lipid hydrolysis in a pertussis‐toxin‐independent manner. Whether stimulation of Ca2+ influx by TRH is affected by pertussis toxin is not known. In order to ellucidate which of the hormone receptors interact with pertussis‐toxin‐sensitive and ‐insensitive G‐proteins, we measured the effects of LHRH, somatostatin and TRH on high‐affinity GTPases in membranes of GH3 cells. In control membranes, both LHRH and TRH stimulated the high‐affinity GTPases by 20% somatostain by 25%. Maximal hormone effects were observed at a concentration of about 1 μM. Pretreatment of cells with pertussis toxin abolished pertussis‐toxin‐catalyzed [32P]ADP‐ribosylation of 39–40‐kDa proteins in subsequently prepared membranes and reduced basal GTPase activity. The toxin also reduced by more than half the increases in GTPase activity induced by LHRH and TRH; stimulation of GTPase by somatostatin was completely suppressed. Stimulation of adenylate cyclase by vasoactive intestinal peptide (VIP) was not impaired by pretreatment of cells with pertusis toxin. Somatostatin but not LHRH and TRH decreased forskolin‐stimulated adenylate cyclase activity. The results suggest that the activated receptors for LHRH and TRH act via pertussis‐toxin‐sensitive and ‐insensitive G‐proteins, whereas effects of somatostatin are exclusively mediated by pertussis toxin‐sensitive G‐proteins.