Leucine induces initial lowering of cytoplasmic Ca2+ in pancreatic beta-cells without concomitant inhibition of insulin release

The early effects of glucose and leucine on cytoplasmic Ca2+ and insulin release were compared in suspensions of cells prepared by dispersal of the beta-cell-rich pancreatic islets of ob/ob-mice. Adequate temporal resolution was achieved by continuously recording the 340/380 nm fluorescence excitation ratio from cells loaded with the Ca2+ indicator fura-2 and measuring insulin in the perifusate from cells mixed with polyacrylamide beads. Raising the glucose concentration from 3 to 20 mM resulted in concomitant reductions of cytoplasmic Ca2+ and insulin release during the first minute. Whereas 10 mM leucine was as efficient as glucose in inducing temporary lowering of cytoplasmic Ca2+, this amino acid did not depress insulin release. It is concluded that the initial decrease of cytoplasmic Ca2+ is a phenomenon coupled to stimulation of the metabolism. The leucine-induced lowering of Ca2+ may essentially reflect changes in cytoplasmic pools other than in a peripheral one regulating insulin release.