Cytoplasmic Ca2+ in glucagon-producing pancreatic alpha-cells exposed to carbachol and agents affecting Na+ fluxes

The cytoplasmic Ca2+ concentration ([Ca2+](i)) was measured with dual wavelength fluorometry in glucagon-producing mouse pancreatic alpha-cells loaded with the indicator fura-2. Spontaneous rhythmic activity in terms of slow oscillations from a basal level was observed at 3 mM glucose. Like in the insulin-secreting beta-cells the generation of [Ca2+](i) oscillations in the alpha-cells was affected by the activity of the Na/K pump. Blocking the pump with ouabain resulted in an initial rise of [Ca2+](i) followed by gradual return to the basal level. The oscillations were transformed into sustained elevation of [Ca2+](i) by 10 mM L-glycine, which is cotransported with Na+. A similar but less pronounced effect was obtained when Na+ was cotransported with 10 mM of the nonmetabolizable amino acid alpha-amino-isobutyric acid. L-glycine induced sustained increase of [Ca2+](i) also when the oscillatory activity was suppressed by exposing the alpha-cells to 20 mM glucose in the presence of insulin. The observation that carbachol induces a [Ca2+](i) response in isolated alpha-cells calls for reconsideration of current ideas that muscarinic stimulation of glucagon release is an indirect effect mediated by adjacent beta-cells.