Voltage-Gated Ion Channels in Human Pancreatic β-Cells: Electrophysiological Characterization and Role in Insulin Secretion

Abstract

OBJECTIVE— To characterize the voltage-gated ion channels in human β-cells from nondiabetic donors and their role in glucose-stimulated insulin release.

RESEARCH DESIGN AND METHODS— Insulin release was measured from intact islets. Whole-cell patch-clamp experiments and measurements of cell capacitance were performed on isolated β-cells. The ion channel complement was determined by quantitative PCR.

RESULTS— Human β-cells express two types of voltage-gated K⁺ currents that flow through delayed rectifying (K_V2.1/2.2) and large-conductance Ca²⁺-activated K⁺ (BK) channels. Blockade of BK channels (using iberiotoxin) increased action potential amplitude and enhanced insulin secretion by 70%, whereas inhibition of K_V2.1/2.2 (with stromatoxin) was without stimulatory effect on electrical activity and secretion. Voltage-gated tetrodotoxin (TTX)-sensitive Na⁺ currents (Na_V1.6/1.7) contribute to the upstroke of action potentials. Inhibition of Na⁺ currents with TTX reduced glucose-stimulated (6–20 mmol/l) insulin secretion by 55–70%. Human β-cells are equipped with L- (Ca_V1.3), P/Q- (Ca_V2.1), and T- (Ca_V3.2), but not N- or R-type Ca²⁺ channels. Blockade of L-type channels abolished glucose-stimulated insulin release, while inhibition of T- and P/Q-type Ca²⁺ channels reduced glucose-induced (6 mmol/l) secretion by 60–70%. Membrane potential recordings suggest that L- and T-type Ca²⁺ channels participate in action potential generation. Blockade of P/Q-type Ca²⁺ channels suppressed exocytosis (measured as an increase in cell capacitance) by >80%, whereas inhibition of L-type Ca²⁺ channels only had a minor effect.

CONCLUSIONS— Voltage-gated T-type and L-type Ca²⁺ channels as well as Na⁺ channels participate in glucose-stimulated electrical activity and insulin secretion. Ca²⁺-activated BK channels are required for rapid membrane repolarization. Exocytosis of insulin-containing granules is principally triggered by Ca²⁺ influx through P/Q-type Ca²⁺ channels.