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 (beta TC3-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 beta TC3-neo insulinoma cells and purified rodent pancreatic beta-cells was detected and confirmed by immunoblotting in the insulinoma cells, beta TC3-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 K-d 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 beta TC3-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.