Evidence that glucose-induced electrical activity in rat pancreatic β-cells does not require KATP channel inhibition

K-ATP-channel activity, recorded in cell-attached patches from isolated rat pancreatic beta-cells, was found to be maximally inhibited in the presence of a substimulatory concentration (5 mm) of glucose, with no further effect of higher, stimulatory glucose concentrations. K-ATP channel-independent effects of glucose on electrical activity were therefore investigated by incubating cells in the presence of a supra-maximal concentration of tolbutamide. Addition of tolbutamide (500 muM) to cells equilibrated in the absence of glucose resulted in a rapid depolarization and electrical activity followed by a gradual repolarization and disappearence of electrical activity. Repolarization was not due to desensitization of K-ATP channels to the sulfonylurea, but was probably the result of activation of another K+ conductance. The subsequent application of 16 mm glucose in the continued presence of tolbutamide depolarized the cells again, leading to renewed electrical activity. Input conductance of the cells was markedly reduced by tolbutamide, reflecting K-ATP-channel inhibition, but was not significantly affected by the addition of glucose in the presence of the drug. In cells voltage-clamped at -70 mV, addition of glucose in the presence of tolbutamide generated a noisy inward current, probably representing activation of the volume-sensitive anion channel. KATP channel-independent activation of electrical activity by glucose was inhibited by the anion channel inhibitor 4,4'-dithiocyanatostilbene-2,2'-disulphonic acid. It is concluded that the induction of electrical activity in rat pancreatic beta-cells does not require inhibition of K-ATP channels. The K-ATP channel-independent mechanism could involve, at least in part, activation by glucose of the volume-sensitive anion channel.