DEMONSTRATION OF A NOVEL APAMIN-INSENSITIVE CALCIUM-ACTIVATED K+ CHANNEL IN MOUSE PANCREATIC B-CELLS

The whole-cell configuration of the patch-clamp technique was used to characterize the biophysical and pharmacological properties of an oscillating K+-current that can be induced by intracellular application of GTP[gammaS] in mouse pancreatic B cells (Ammala et al. 1991). These K+ conductance changes are evoked by periodic increases in the cytoplasmic Ca2+ concentration ([Ca2+]i) and transiently repolarize the B cell, thus inhibiting action-potential firing and giving rise to a bursting pattern. GTP[gammaS]-evoked oscillations in K+ conductance were reversibly suppressed by a high (300 muM) concentration of carbamylcholine. By contrast, alpha2-adrenoreceptor stimulation by 20 muM clonidine did not interfere with the oscillatory behaviour but evoked a small sustained outward current. At 0 mV membrane potential, the oscillating K+-current elicited by GTP[gammaS] was highly sensitive to extracellular tetraethylammonium (TEA; 70% block by 1 mM). The TEA-resistant component, which carried approximately 80% of the current at -40 mV, was affected neither by apamin (1 muM) nor by tolbutamide (500 muM). The current evoked by internal GTP[gammaS] was highly selective for K+, as demonstrated by a 51-mV change in the reversal potential for a sevenfold change in [K+]o. Stationary fluctuation analysis indicated a unitary conductance of 0.5 PS when measured with symmetric (almost-equal-to 140 mM) KCl solutions. The estimated single-channel conductance with physiological ionic gradients is 0.1 pS. The results indicate the existence of a novel Ca2+-gated K+ conductance in pancreatic B cells. Activation of this K+ current may contribute to the generation of the oscillatory electrical activity characterizing the B cell at intermediate glucose concentrations.