Abstract

Objective: Ca²⁺-regulated K⁺ channels are involved in numerous Ca²⁺-dependent signaling pathways. In this study we investigate whether the Ca²⁺-activated K⁺ channel of intermediate conductance SK4 (KCa3.1, IK1) plays a physiological role in pancreatic beta-cell function.

Research Design and Methods: Glucose tolerance and insulin sensitivity were determined in wildtype (WT) or SK4 knock-out (SK4-KO) mice. Electrophysiological experiments were performed with the patch-clamp technique. The cytosolic Ca²⁺ concentration ([Ca²⁺]_c) was determined by fura-2 fluorescence. Insulin release was assessed by radioimmunoassay and SK4 protein was detected by western blot analysis.

Results: SK4-KO mice showed improved glucose tolerance whereas insulin sensitivity was not altered. The animals were not hypoglycemic. Isolated SK4-KO beta-cells stimulated with 15 mM glucose had an increased Ca²⁺ action potential frequency and single action potentials were broadened. These alterations were coupled to increased [Ca²⁺]_c. In addition, glucose responsiveness of membrane potential, [Ca²⁺]_c and insulin secretion was shifted to lower glucose concentrations. SK4 protein was expressed in WT islets. An increase in K⁺ currents and concomitant membrane hyperpolarization could be evoked in WT beta-cells by the SK4 channel opener DCEBIO (100 μM). Accordingly, the SK4 channel blocker TRAM-34 (1 μM) partly inhibited K_Ca currents and induced electrical activity at a threshold glucose concentration. In stimulated WT beta-cells TRAM-34 further increased [Ca²⁺]_c and broadened action potentials similar to those seen in SK4-KO beta-cells. SK4 channels were found to substantially contribute to K_slow.

Conclusions: SK4 channels are involved in beta-cell stimulus-secretion coupling. Deficiency of SK4 current induces elevated beta-cell responsiveness and coincides with improved glucose tolerance in vivo. Therefore pharmacologic modulation of these channels might provide an interesting approach for the development of novel insulinotropic drugs.