The aim of this project is to determine the mechanisms of action of specific putative mediators of insulin secretion in isolated islets of Langerhans of the rat. The central hypothesis is that specific mediators of secretion, such as phospholipids and fatty acids, cyclic GMP and cyclic AMP, and cations including calcium and sodium, have coordinate actions in the regulation of insulin secretion.
A specific aim i s to elucidate the transduction mechanism(s) which mediate the effects of alpha-2 adrenoceptor stimulation on insulin release, and which occur distal to changes in cyclic AMP production. The effects of alpha-2 adrenoceptor agonists on total glucose utilization and pentose phosphate shunt activity, glucose transport and phosphorylation, and phospholipid turnover in isolated islets will be determined. In addition, studies with cultured Reuber hepatoma cells and RINm5F insulinoma cells, and platelets, will determine if alpha-2 adrenoceptor effects in the islet are unique or common to other tissues, and whether the effects observed in the islet occur specifically in the beta cell. Antagonism by cyclic GMP and muscarinic receptor agonists of the alpha-2 adrenoceptor inhibitory effects on secretion and carbohydrate metabolism will be investigated to determine if cyclic GMP as compared to cyclic AMP specifically affects pathways of glucose metabolism and mediates receptor stimulation. The role of Na+/H+ exchange, changes in intracellular Ca2+ levels, and arachidonic acid metabolism in alpha-2 adrenoceptor-mediated inhibition of the islet secretory response will also be investigated. Another aim of this proposal is to determine the interrelationship of lipids and cyclic nucleotides in the regulation of insulin secretion. The effects of free fatty acids, lipoxygenase- and cyclooxygenase--derived arachidonic acid metabolites, and protein kinase C activity on cyclic nucleotide biosynthesis will be determined. Conversely, effects of cyclic nucleotides on phospholipase activities will be investigated.
A third aim of this project is to determine if a specific phosphatidylcholine biosynthetic pathway contributes to a lipid pool from which arachidonic acid is mobilized to mediate the actions of beta cell stimuli. New ways of evoking insulin secretion may be determined from a better understanding of stimulus-secretion coupling mechanisms which may be responsible for impaired insulin release in diabetes mellitus.
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