The Amount of Releasable Insulin Depends on Continuous Oxidative Phosphorylation

The consensus or canonical model of glucose-stimulated insulin secretion provides that the metabolism of glucose closes KATP channels by increase of the ATP/ADP ratio and that the ensuing depolarization-induced Ca2+ influx through voltage-dependent Ca2+ channels represents the immediate signal for the onset of exocytosis. However, it has been shown earlier that the depolarization-induced secretion can be suppressed by inhibition of the oxidative phosphorylation, pointing to an energy-requiring step presumably located downstream of Ca2+ influx. Here, we have investigated the relation between oxidative phosphorylation and the insulinotropic effect of K+ depolarization to better localize the energy-requiring step. The specific inhibitor of the mitochondrial F1FO ATPase, oligomycin, concentration-dependently and time-dependently inhibited the insulin secretion elicited by a strong K+ depolarization (40 mm). Perifusion with 4 µg/mL of oligomycin for 20, 10, or 5 min prior to the K+ depolarization reduced the amount of insulin secreted from freshly isolated islets from control value to about 5% with a half-time of 1.6 min. 0.4 µg/mL of oligomycin required more time for comparable effects. Cultured islets were less susceptible to the inhibitory action of oligomycin than fresh islets, corresponding to their significantly higher ATP/ADP ratio. The perifusion with oligomycin prior to the K+ depolarization did not decrease the depolarization-elevated cytosolic Ca2+ concentration and did not affect the resting plasma membrane potential and the extent of depolarization by 40 mm KCl. In conclusion, the exocytotic machinery of the beta cell requires a continuously running oxidative phosphorylation to remain responsive to the Ca2+ signal for granule fusion.