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

摘要

OBJECTIVE-To document the properties of the voltage-gated ion channels in human pancreatic α-cells and their role in glucagon release.rnRESEARCH DESIGN AND METHODS-Glucagon release was measured from intact islets. [Ca~(2+)]_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 a-cells identified by immunocytochemistry.rnRESULTS-Glucose inhibited glucagon secretion from human islets; maximal inhibition was observed at 6 mmol/1 glucose. Glucagon secretion at 1 mmol/1 glucose was inhibited by insulin but not by ZnCl_2. Glucose remained inhibitory in the presence of ZnCl_2 and after blockade of type-2 somatostatin receptors. Human α-cells are electrically active at 1 mmol/1 glucose. Inhibition of K_(ATP)-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, stroma-toxin-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 Ca~(2+)-channels. Glucagon secretion at 1 mmol/l glucose was inhibited by 40-70% by tetrodotoxin, heteropodatoxin-2, stromatoxin, ω-agatoxin, and isradipine. The [Ca~(2+)]_i oscillations depend principally on Ca~(2+)-influx via L-type Ca~(2+) -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 Ca~(2+)-currents abolished depolarization-evoked exocytosis.rnCONCLUSIONS-Human a-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.