Effects of L-dopa-induced dopamine accumulation on 45Ca2+ efflux and insulin secretion in isolated rat islets

It has previously been demonstrated in several species that the secretory granules of pancreatic beta-cells have the ability to store substantial amounts of calcium and bioactive amines, such as dopamine and serotonin. Furthermore, evidence for a similar topographical localization for amine and calcium within the periphery of the granules has been obtained. In the present study, a possible interaction between dopamine and calcium 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 insulin 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. The results are interpreted as being a consequence of a complex interaction between the accumulated dopamine and a pool of Ca2+ mainly confined to the secretory granules. This interaction could be followed by a transient increase in cytosolic Ca2+ and a subsequent efflux of Ca2+ out of the cell, eventually accompanied by insulin release. Increasing the cytosolic Ca2+ by acute dopamine accumulation makes the cell more sensitive to a concomitant stimulation with glucose, and the release of insulin is triggered. A long-term dopamine accumulation. On the other hand, might diminish the granular Ca2+ pool to such a level where insulin release is inhibited after stimulation with certain secretagogues.