Stimulus-Secretion Coupling in β-Cells of Transplantable Human Islets of Langerhans: Evidence for a Critical Role for Ca²⁺ Entry

With human islets isolated for transplantation, we examined the applicability to humans of a metabolic fuel hypothesis of glucose transduction and a Ca²⁺ hypothesis of depolarization-secretion coupling, both previously proposed for rodent islet β-cells. We report that several features of human β-cell physiology are well accounted for by these hypotheses. With whole-islet perifusion, we demonstrated that insulin secretion induced by glucose, tolbutamide, or elevated K⁺ is dependent on extracellular Ca²⁺. Insulin release induced by these secretagogues is enhanced by the dihydropyridine Ca²⁺ channel agonist BAYk8644 and depressed by the dihydropyridine Ca²⁺-channel antagonist nifedipine. All of the aforementioned secretagogues provoke increases in cytosolic free Ca²⁺, which are dependent on extracellular Ca²⁺ and are altered by the dihydropyridine drugs. Individual β-cells in the islet display diminished resting membrane conductance, graded depolarization, and complex electrical patterns, including bursts of action potentials in response to stimulatory concentrations of glucose or tolbutamide. Individual islet β-cells display voltage-dependent Ca²⁺ currents that are activated at membrane potentials traversed during the excursion of the action potential. In most cells, the Ca²⁺ currents are enhanced by BAYk8644 and depressed by nifedipine at concentrations that have parallel effects on secretagogue-induced increases in cytosolic Ca²⁺ and insulin secretion. These survey studies should provide the basis for more detailed investigations of the relationship of voltage-dependent ionic currents to electrical activity patterns and of electrical activity patterns to granule exocytosis in single human β-cells.