Demonstration of a novel apamin-insensitive calcium-activated K⁺ channel in mouse pancreatic B cells

Abstract

The whole-cell configuration of the patchclamp technique was used to characterize the biophysical and pharmacological properties of an oscillating K⁺-current that can be induced by intracellular application of GTP[γS] in mouse pancreatic B cells (Ämmälä 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[γS]-evoked oscillations in K⁺ conductance were reversibly suppressed by a high (300 μM) concentration of carbamylcholine. By contrast, α₂-adrenoreceptor stimulation by 20 μM 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[γS] 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 μM) nor by tolbutamide (500 μM). The current evoked by internal GTP[γS] 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 (≈ 140mM) KCl solutions. The estimated singlechannel 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.