Cytoplasmic Ca2+ in glucagon-producing pancreatic α-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 α-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 β-cells the generation of [Ca2+]i oscillations in the α-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 mMl-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 α-amino-isobutyric acid.l-glycine induced sustained increase of [Ca2+]i also when the oscillatory activity was suppressed by exposing the α-cells to 20 mM glucose in the presence of insulin. The observation that carbachol induces a [Ca2+]i response in isolated α-cells calls for reconsideration of current ideas that muscarinic stimulation of glucagon release is an indirect effect mediated by adjacent β-cells.