Regulation of glucagon release in mouse α-cells by KATP channels and inactivation of TTX-sensitive Na+ channels

The perforated patch whole-cell configuration of the patch-clamp technique was applied to superficial glucagon-secreting α-cells in intact mouse pancreatic islets. α-cells were distinguished from the β- and δ-cells by the presence of a large TTX-blockable Na + current, a TEA-resistant transient K + current sensitive to 4-AP (A-current) and the presence of two kinetically separable Ca 2+ current components corresponding to low- (T-type) and high-threshold (L-type) Ca 2+ channels. The T-type Ca 2+ , Na + and A-currents were subject to steady-state voltage-dependent inactivation, which was half-maximal at −45, −47 and −68 mV, respectively. Pancreatic α-cells were equipped with tolbutamide-sensitive, ATP-regulated K + (K ATP ) channels. Addition of tolbutamide (0·1 m m ) evoked a brief period of electrical activity followed by a depolarisation to a plateau of −30 mV with no regenerative electrical activity. Glucagon secretion in the absence of glucose was strongly inhibited by TTX, nifedipine and tolbutamide. When diazoxide was added in the presence of 10 m m glucose, concentrations up to 2 μ m stimulated glucagon secretion to the same extent as removal of glucose. We conclude that electrical activity and secretion in the α-cells is dependent on the generation of Na + -dependent action potentials. Glucagon secretion depends on low activity of K ATP channels to keep the membrane potential sufficiently negative to prevent voltage-dependent inactivation of voltage-gated membrane currents. Glucose may inhibit glucagon release by depolarising the α-cell with resultant inactivation of the ion channels participating in action potential generation.