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Regulatory peptide-controlled release of prolactin from GH3 pituitary tumour cells: The role of cyclic AMP
366
REGULATORY PEPTIDE-C0~ROLLED RELEASE OF PROLICTIN FROM GH 3 PITUITARY TUMOUR CELLS: THE ROLE OF CYCLIC A~,~ S Guild and A H Drummond, Department of Pharmacology, University of Glasgow, Glasgow G12 8QQ, Scotland. Prolactin release from pituitary cells is known to be regulated by a variety of peptide hormones and releasing factors, however, the mechanisms underlying this control remain unclear. GH 3 pituitary tumour cells are a clonal cell line producing prolactin and are a convenient model system for investigating the release process. The role of cyclic AI~ in hormone release was investigated: a) by measuring the cellular content of cyclic AI~ in response to peptides, and b) by studying the effect of agents known to increase cyclic AP~ levels upon hormone release. TRH (ECho = I nM) and VIP (EC50 = 0.5nM) both caused a doubling in the rate of pro[actin release from GH 3 cells. Unlike VIP, which caused a 2-fold increase in cellular cyclic A ~ content, TRH did not significantly affect cyclic AI,~ levels. Somatostatin, which produced a 5~J inhibition of prolactin release, did not alter the cyclic Ai~ content. Agents such as cholera toxin and ferskolin, which specifically stimulate adenylate cyclase, produced 30-fold and 4-fold rises in cyclic Ai~ respectively. Both substances doubled prolactin release from GH 3 cells. Isobutylmethylxanthine, a phosphediesterase inhibiter, also doubled the rate of prolactin release (EC50 = 5~M) and caused a 3-fold rise in cyclic AI~ levels at ImM. TRH and VIP most likely act through different mechanisms since, in addition to the above, their effects on prolactin release are additive. Cyclic ~g~ probably does play a role in prelactin release induced by VIP but quantitative analyses using a range of substances indicate that only a 50-75~o rise in cell cyclic A ~ levels are required to activate fully cyclic Ai~-dependent prelactin release from GH 3 cells.
SOMATOSTATIN-ANTAGONIST
: FAILURE TO ANTAGONIZE GASTRIC INHIBITION
B.H. Hirst, D.H. C oy * and B. Shaw, Department of Physiological Sciences, Medical School, University of Newcastle upon Tyne, U.K. and *Department of Medicine, Tulane University School of Medicine, New Orleans, U.S.A. Cycle [7-aminoheptanoyl-Phe-D-Trp-Lys-Thr (B l) has been described as the first competitive antagonist of somatostatiu. In rats, prior injection of this analogue completely blocked the inhibitory effects of e x ~ n o u s somatostatin on insulin, glucagon and growth hormone release. In untreated rats hepatic portal insulin and glucagon concentrations rose after 4 #g/100g of antagonist. In Nembutal-treated rats, plasma growth hormone concentrations increased after injection of 0.2 and 0.6 ~g/100g of antagonist. In the present study we investigated the pentapeptide for antagonism of somatostatin's effects on the stomach. Gastric acid and pepsin secretions were stimulated in conscious cats with pentagastrin 8 ~g/kg/h. Intravenous infusion of the pentapeptide at 2 - 50 ~g/kg/h failed to antagonize the gastric acid and pepsin inhibitory effects of somatostatin 2 or 10 ~g/kg/h. Thus the pentapeptide at molar concentrations up to 50 times greater than that of exogenous somatostatin, failed to antagonize somatostatin's inhibitory effects on the stomach in the cat. This failure further illustrates the differences between gastric somatostatin receptors compared with those in the pituitary and pancreas.
REGULATORY PEPTIDE-C0~ROLLED RELEASE OF PROLICTIN FROM GH 3 PITUITARY TUMOUR CELLS: THE ROLE OF CYCLIC A~,~ S Guild and A H Drummond, Department of Pharmacology, University of Glasgow, Glasgow G12 8QQ, Scotland. Prolactin release from pituitary cells is known to be regulated by a variety of peptide hormones and releasing factors, however, the mechanisms underlying this control remain unclear. GH 3 pituitary tumour cells are a clonal cell line producing prolactin and are a convenient model system for investigating the release process. The role of cyclic AI~ in hormone release was investigated: a) by measuring the cellular content of cyclic AI~ in response to peptides, and b) by studying the effect of agents known to increase cyclic AP~ levels upon hormone release. TRH (ECho = I nM) and VIP (EC50 = 0.5nM) both caused a doubling in the rate of pro[actin release from GH 3 cells. Unlike VIP, which caused a 2-fold increase in cellular cyclic A ~ content, TRH did not significantly affect cyclic AI,~ levels. Somatostatin, which produced a 5~J inhibition of prolactin release, did not alter the cyclic Ai~ content. Agents such as cholera toxin and ferskolin, which specifically stimulate adenylate cyclase, produced 30-fold and 4-fold rises in cyclic Ai~ respectively. Both substances doubled prolactin release from GH 3 cells. Isobutylmethylxanthine, a phosphediesterase inhibiter, also doubled the rate of prolactin release (EC50 = 5~M) and caused a 3-fold rise in cyclic AI~ levels at ImM. TRH and VIP most likely act through different mechanisms since, in addition to the above, their effects on prolactin release are additive. Cyclic ~g~ probably does play a role in prelactin release induced by VIP but quantitative analyses using a range of substances indicate that only a 50-75~o rise in cell cyclic A ~ levels are required to activate fully cyclic Ai~-dependent prelactin release from GH 3 cells.
SOMATOSTATIN-ANTAGONIST
: FAILURE TO ANTAGONIZE GASTRIC INHIBITION
B.H. Hirst, D.H. C oy * and B. Shaw, Department of Physiological Sciences, Medical School, University of Newcastle upon Tyne, U.K. and *Department of Medicine, Tulane University School of Medicine, New Orleans, U.S.A. Cycle [7-aminoheptanoyl-Phe-D-Trp-Lys-Thr (B l) has been described as the first competitive antagonist of somatostatiu. In rats, prior injection of this analogue completely blocked the inhibitory effects of e x ~ n o u s somatostatin on insulin, glucagon and growth hormone release. In untreated rats hepatic portal insulin and glucagon concentrations rose after 4 #g/100g of antagonist. In Nembutal-treated rats, plasma growth hormone concentrations increased after injection of 0.2 and 0.6 ~g/100g of antagonist. In the present study we investigated the pentapeptide for antagonism of somatostatin's effects on the stomach. Gastric acid and pepsin secretions were stimulated in conscious cats with pentagastrin 8 ~g/kg/h. Intravenous infusion of the pentapeptide at 2 - 50 ~g/kg/h failed to antagonize the gastric acid and pepsin inhibitory effects of somatostatin 2 or 10 ~g/kg/h. Thus the pentapeptide at molar concentrations up to 50 times greater than that of exogenous somatostatin, failed to antagonize somatostatin's inhibitory effects on the stomach in the cat. This failure further illustrates the differences between gastric somatostatin receptors compared with those in the pituitary and pancreas.