Membrane Potential-Dependent Inactivation of Voltage-Gated Ion Channels in α-Cells Inhibits Glucagon Secretion From Human Islets

Abstract

OBJECTIVE To document the properties of the voltage-gated ion channels in human pancreatic α-cells and their role in glucagon release. RESEARCH DESIGN AND METHODS Glucagon release was measured from intact islets. [Ca2+]i was recorded in cells showing spontaneous activity at 1 mmol/l glucose. Membrane currents and potential were measured by whole-cell patch-clamping in isolated α-cells identified by immunocytochemistry. RESULTS Glucose inhibited glucagon secretion from human islets; maximal inhibition was observed at 6 mmol/l glucose. Glucagon secretion at 1 mmol/l glucose was inhibited by insulin but not by ZnCl2. Glucose remained inhibitory in the presence of ZnCl2 and after blockade of type-2 somatostatin receptors. Human α-cells are electrically active at 1 mmol/l glucose. Inhibition of KATP-channels with tolbutamide depolarized α-cells by 10 mV and reduced the action potential amplitude. Human α-cells contain heteropodatoxin-sensitive A-type K+-channels, stromatoxin-sensitive delayed rectifying K+-channels, tetrodotoxin-sensitive Na+-currents, and low-threshold T-type, isradipine-sensitive L-type, and ω-agatoxin-sensitive P/Q-type Ca2+-channels. Glucagon secretion at 1 mmol/l glucose was inhibited by 40–70% by tetrodotoxin, heteropodatoxin-2, stromatoxin, ω-agatoxin, and isradipine. The [Ca2+]i oscillations depend principally on Ca2+-influx via L-type Ca2+-channels. Capacitance measurements revealed a rapid (<50 ms) component of exocytosis. Exocytosis was negligible at voltages below −20 mV and peaked at 0 mV. Blocking P/Q-type Ca2+-currents abolished depolarization-evoked exocytosis. CONCLUSIONS Human α-cells are electrically excitable, and blockade of any ion channel involved in action potential depolarization or repolarization results in inhibition of glucagon secretion. We propose that voltage-dependent inactivation of these channels underlies the inhibition of glucagon secretion by tolbutamide and glucose.