Effects of L-Dopa-Induced Dopamine Accumulation on 45Ca2+ Efflux and Insulin Secretion in Isolated Rat Islets

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Abstract
Previously, the secretory granules of pancreatic .beta.-cells in several species the ability to store substantial amounts of Ca and bioactive amines, such as dopamine and serotonin. Evidence for a similar topographical localization for amine and Ca within the periphery of the granules was obtained. A possible interaction between dopamine and Ca on insulin release was investigated. Isolated rat islets were loaded with 45Ca2+ in the presence of theophylline and high glucose and then perifused in a dynamic system where radioactivity and insulin were determined in the effluent. When perifused in a bicarbonate buffer with 2 mmol/l Ca2+ and supplemented with the monoamine oxidase inhibitor pargyline, L-3,4-dihydroxyphenylalanine (L-dopa)-induced dopamine accumulation in the islets brought about a slight and transient increase in 45Ca2+ efflux. This increase was more pronounced and sustained in a Ca2+-deficient buffer or in a Ca2+-deficient buffer supplemented with ethyleneglycolbis(aminoethylether)tetraacetic acid (EGTA). Insulin release was transiently stimulated by islet dopamine accumulation in the Ca2+-deprived media, but not in a medium with 2 mmol/l Ca2+. Glucose-induced insulin release in 2 mmol/l Ca2+ was potentiated by acute dopamine accumulation. The combined effect of glucose stimulation and islet accumulation of dopamine induced a transient insulin release in the Ca2+-deprived media with and without EGTA. This release of insulin was accompanied by an increased 45Ca2+ efflux which was most pronounced in the presence of EGTA. Stimulation with glucose alone, i.e., without addition of L-dopa, tended to decrease insuln release and 45Ca2+ efflux in a Ca2+-deficient medium. No effects of L-dopa or L-dopa + glucose were encountered in a Ca2+-deficient buffer when the monoamine oxidase inhibitor pargyline was replaced by the dopa-decarboxylase inhibitor benserazide.

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