Electrophysiology of stimulus-secretion coupling in human beta-cells

Herein, we review the applicability to human beta-cells of an electrophysiologically based hypothesis of the coupling of glucose metabolism to insulin secretion. According to this hypothesis, glucose metabolism leads to the generation of intracellular intermediates (including ATP), which leads to closure of ATP-sensitive K+ channels. Channel closure results in membrane depolarization, the onset of electrical activity, and voltage-dependent Ca2+ entry. The resultant rise in cytosolic Ca2+ leads to Ca(2+)-dependent exocytosis of insulin granules. We found that most of the published experimental evidence for human beta-cells supports this hypothesis. In addition, we present three other emerging lines of evidence in support of this hypothesis for human islet beta-cells: 1) the effects of pHi-altering maneuvers on insulin secretion and electrical activity; 2) preliminary identification of LVA and HVA single Ca2+ channel currents; and 3) validation of the feasibility of Cm measurements to track insulin granule exocytosis. On the basis of this last new line of evidence, we suggest that combinations of Cm measurements and electrical activity/membrane current measurements may help define the roles of diverse electrical activity patterns, displayed by human beta-cells, in stimulus-induced insulin secretion.