Regulation of glucagon release in mouse alpha-cells by K-ATP channels and inactivation of TTX-sensitive Na+ channels

1. The perforated patch whole-cell configuration of the patch-clamp techniq ue was applied to superficial glucagon-secreting alpha -cells in intact mou se pancreatic islets. 2. alpha -cells were distinguished from the beta- and delta -cells by the p resence of a large TTX-blockable Na+ current, a TEA-resistant transient Kcurrent sensitive to 4-AP (A-current) and the presence of two kinetically s eparable Ca2+ current components corresponding to low(T-type) and high-thre shold (L-type) Ca2+ channels. 3. The T-type Ca2+, Na+ and A-currents were subject to steady-state voltage -dependent inactivation, which was half-maximal at -45, -47 and -68 mV, res pectively. 4. Pancreatic alpha -cells were equipped with tolbutamide-sensitive, ATP-re gulated K+ (K-ATP) channels. Addition of tolbutamide (0.1. mM) evoked a bri ef period of electrical activity followed by a depolarisation to a plateau of -30 mV with no regenerative electrical activity. 5. Glucagon secretion in the absence of glucose was strongly inhibited by T TX, nifedipine and tolbutamide. When diazoxide was added in the presence of 10 mM glucose, concentrations up to 2 muM stimulated glucagon secretion to the same extent as removal of glucose. 6. We conclude that electrical activity and secretion in the alpha -cells i s dependent on the generation of Na+-dependent action potentials. Glucagon secretion depends on low activity of K-ATP channels to keep the membrane po tential sufficiently negative to prevent voltage-dependent inactivation of voltage-gated membrane currents. Glucose may inhibit glucagon release by de polarising the alpha -cell with resultant inactivation of the ion channels participating in action potential generation.