Somatostatin is being increasingly recognized as a family of peptides, widely distributed in many tissues and subserving important regulatory functions on many body processes including the control of islet hormone secretion and nutrient metabolism. The long-term objective of this proposal is to make further progress towards an understanding of the biosynthesis, secretion, mechanism of action, and functional significance of somatostatin in the islet physiologically and in diabetes. Using an in vitro model of islet cells in monolayer culture, sequence specific antisera, and chromatographic analyses (affinity, gel, and high performance liquid chromatography), the biosynthetic relationships between the main molecular forms (prosomatostatin (Pro-S), somatostatin-14 (S-14), somatostatin-28 (S-28), and S-28 (1-12) will be delineated and in particular the question of whether A-14 synthesis proceeds via two separate pathways via sequential breakdown of Pro-S greater than S-28 greater than S-14 + S-28(1-12), or through direct processing of Pro-S greater than S-14, resolved. Electron microscopic (EM) immunocytochemistry will be used to determine whether these pathways exist in different islet D-cell populations. The factors and the mechanisms regulating biosynthesis will be investigated by determination of biosynthetic rates for somatostatin and islet somatostatin mRNA levels. It is hoped to elucidate whether the secretion of these forms is differentially regulated by D-cell secretagogues. With the help of newly developed techniques (1) quantitative EM autoradiography (QEMA), and (2) the use of cysteamine + specific antisera against S-14 and S-28 to product functional islet somatostatin deficiency, separate receptors for S-14 and S-28 will be identified and their relative physiological roles within the islet with respect to insulin and glucagon, determined. Both QEMA and direct binding studies with islet cell membranes will be used to study the regulation of somatostatin receptors to resolve whether somatostatin regulates its own receptors in the islet as it appears to do in some other tissues. The hepatic metabolism of the different molecular species will be characterized, using the isolated perfused liver and isolated hepatocytes and Kupffer cells. Finally, the question of disturbances of somatostatin secretion, metabolism and possible biosynthesis in diabetes will be pursued using the spontaneously diabetic BB rat model.
