Role of voltage-dependent ionic currents in coupling glucose stimulation to insulin secretion in canine pancreatic islet B-Cells

Summary

Glucose-induced electrical activity in canine pancreatic islet B cells is distinct from that in rodent islets, though both display Ca²⁺-dependent insulin secretion. Rodent islet B cells undergo regular bursts of Ca²⁺-dependent action potentials, while canine islet B cells generate isolated Na⁺-dependent action potentials which often give way to a plateau depolarization. Here we present evidence to reconcile the species difference in electrical activity with the similarity of Ca²⁺ dependence of secretion. (i) In canine B cells increasing glucose concentrations produce membrane depolarization and increasing frequency of Na_o-dependent action potentials until a background membrane potential (∼-40mV) is reached where Na⁺ currents are inactivated. (ii) Voltage-dependent Ca²⁺ currents are present which are activated over the voltage excursion of the action potential (−50 to +20 mV) and inactivate slowly, (over seconds) in the range of the plateau depolarization (−40 to −25 mV). Hence, they are available to contribute to both phases of depolarization. (iii) Tetrodotoxin (TTX) reduces by half an early transient phase of glucosestimulated insulin secretion but not a subsequent prolonged plateau phase. The transient phase of secretion often corresponds well in time to the period of initial high frequency action potential activity. These latter results suggest that in canine B cells voltagedependent Na⁺ and Ca²⁺ currents mediate biphasic glucose-induced insulin secretion. The early train of Na⁺-dependent action potentials, by transiently activating Ca²⁺ channels and allowing pulsatile Ca²⁺ entry, may promote an early transient phase of insulin secretion. The subsequent sustained plateau depolarization, by allowing sustained Ca²⁺ entry, may permit steady insulin release.