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Adenylate Cyclase in Islets of Langerhans

ISOLATION OF ISLETS AND REGULATION OF ADENYLATE CYCLASE ACTIVITY BY VARIOUS HORMONES AND AGENTS

From the ¹ From the Department of Biochemistry and Molecular Biology, University of Oklahoma School of Medicine, Oklahoma City, Oklahoma 73104, and the Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06510

An improved method for the isolation of rat pancreatic islets possessing adenylate cyclase responsive to various hormones and agents is described. The key step in this method, which yielded up to 500 islets per pancreas, involved pretreatment of rats with pilocarpine. This allowed depletion of zymogens from the exocrine pancreatic tissue and thus minimized the destruction of islet membranal structure which could otherwise occur during the collagenase digestion of the tissue.

The adenylate cyclase activity in the islet homogenates was assayed in the presence of a variety of hormones and other agents. With the exception of some insulinogenic metabolites, such as glucose, arginine, and leucine, all hormones previously shown by others to stimulate insulin secretion were found to augment the synthesis of adenosine 3',5'-monophosphate (cyclic AMP) by islet adenylate cyclase. These agents include glucagon, adrenocorticotropic hormone, secretin, pancreozymin, various prostaglandins, and -adrenergic agents. Acetylcholine, ethanol, EGTA, GTP, and GDP, also stimulated cyclic AMP production, but their insulinogenic action has not been clearly shown. Of the two hypoglycemic drugs studied, tolbutamide was found to stimulate the cyclase activity whereas phenformin was without effect. Insulin inhibited the islet adenylate cyclase activity, suggesting a possible feedback mechanism involved in regulating insulin secretion. NaF was most effective in activating the islet adenylate cyclase. None of the above-mentioned agents had any effect on the islet phosphodiesterase activity.

Based upon the above observations, it is suggested that cyclic AMP may mediate the action of certain insulin-releasing agents whereas some other agents exert their insulinogenic action via mechanisms in which cyclic AMP may not be involved.

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