Sulfonylurea mimics the effect of glucose in inducing large amplitude oscillations of cytoplasmic Ca2+ in pancreatic beta-cells

The effects of the insulin-releasing sulfonylurea tolbutamide on the cytoplasmic Ca2+ concentration [( Ca2+]i) in individual pancreatic beta-cells or suspensions of beta-cells were analyzed using the probe fura-2 and dual-wavelength fluorometry. Subsequent additions of 1, 10, and 100 microM tolbutamide induced a graded response, ranging from a single [Ca2+]i peak to a sustained increase. These effects depended on the presence of extracellular Ca2+ and were reversed by the hyperglycemic sulfonamide diazoxide. The responses were diminished in the presence of albumin and varied considerably between different cells. Sometimes tolbutamide triggered slow large amplitude oscillations in [Ca2+]i similar to those induced by glucose. The increase in [Ca2+]i during each tolbutamide-induced oscillation was often more rapid than for glucose-induced oscillations. Oscillations or steady state increases in [Ca2+]i induced by glucose were little influenced by tolbutamide. However, subthreshold concentrations of glucose could reactivate [Ca2+]i response to tolbutamide that had declined. Although in several ways the abilities of glucose and tolbutamide to raise [Ca2+]i were similar, the sulfonylurea lacked a [Ca2+]i-lowering component. The latter effect of glucose was so pronounced that an increase of its concentration from 3 to 20 mM caused temporary lowering of [Ca2+]i to the basal level, even during tolbutamide stimulation. The results indicate that closure of the ATP-sensitive K(+)-channels is important for the large amplitude oscillations of [Ca2+]i the appearance of which reflects the balance between entry of Ca2+ through the voltage-dependent channels and its removal from the cytoplasm.