Expression and function of pancreatic beta-cell delayed rectifier K+ channels. Role in stimulus-secretion coupling

Voltage-dependent delayed rectifier K+ channels regulate aspects of both stimulus-secretion and excitation-contraction coupling, but assigning specific roles to these channels has proved problematic. Using transgenically derived insulinoma cells (betaTC3-neo) and beta-cells purified from rodent pancreatic islets of Langerhans, we studied the expression and role of delayed rectifiers in glucose-stimulated insulin secretion. Using reverse-transcription polymerase chain reaction methods to amplify all known candidate delayed rectifier transcripts, the expression of the K+ channel gene Kv2.1 in betaTC3-neo insulinoma cells and purified rodent pancreatic beta-cells was detected and confirmed by immunoblotting in the insulinoma cells. betaTC3-neo cells were also found to express a related K+ channel, Kv3.2. Whole-cell patch clamp demonstrated the presence of delayed rectifier K+ currents inhibited by tetraethylammonium (TEA) and 4-aminopyridine, with similar Kd values to that of Kv2.1, correlating delayed rectifier gene expression with the K+ currents. The effect of these blockers on intracellular Ca2+ concentration ([Ca2+]i) was studied with fura-2 microspectrofluorimetry and imaging techniques. In the absence of glucose, exposure to TEA (1-20 mM) had minimal effects on betaTC3-neo or rodent islet [Ca2+]i, but in the presence of glucose, TEA activated large amplitude [Ca2+]i oscillations. In the insulinoma cells the TEA-induced [Ca2+]i oscillations were driven by synchronous oscillations in membrane potential, resulting in a 4-fold potentiation of insulin secretion. Activation of specific delayed rectifier K+ channels can therefore suppress stimulus-secretion coupling by damping oscillations in membrane potential and [Ca2+]i and thereby regulate secretion. These studies implicate previously uncharacterized beta-cell delayed rectifier K+ channels in the regulation of membrane repolarization, [Ca2+]i, and insulin secretion.