CHARACTERIZATION OF A CA2-ACTIVATED NONSELECTIVE CATION CURRENT REGULATING MEMBRANE-POTENTIAL AND [CA2+](I) OSCILLATIONS IN TRANSGENICALLY DERIVED BETA-CELLS( RELEASE)

Although stimulation of insulin secretion by glucose is regulated by c oupled oscillations of membrane potential and intracellular Ca2+ ([Ca2 +](i)), the membrane events regulating these oscillations are incomple tely understood. In the presence of glucose and tetraethylammonium, tr ansgenically derived beta-cells (beta TC3-neo) exhibit coupled voltage and [Ca2+](i) oscillations strikingly similar to those observed in no rmal islets in response to glucose. Using these cells as a model syste m, we investigated the membrane conductance underlying these oscillati ons. Alterations in delayed rectifier or Ca2+-activated K+ channels we re excluded as a source of the conductance oscillations, as they are c ompletely blocked by tetraethylammonium. ATP-sensitive K+ channels wer e also excluded, since the ATP-sensitive K+ channel blocker tolbutamid e substituted for glucose in inducing [Ca2+](i) oscillations. Thapsiga rgin, which depletes intracellular Ca2+ stores, and maitotoxin, an act ivator of nonselective cation channels, both converted the glucose-dep endent [Ca2+](i) oscillations into a sustained elevation. On the other hand, both SKF 96365, a blocker of Ca2+ store operated channels, and external Na+ removal suppressed the glucose-stimulated [Ca2+](i) oscil lations. Maitotoxin activated a nonselective cation current in beta TC 3 cells that was attenuated by removal of extracellular Na+ and by SKF 96365, in the same manner to a current activated in mouse beta-cells following depletion of intracellular Ca2+ stores. Currents similar to these are produced by the mammalian trp-related channels, a gene famil y that includes Ca2+ store-operated channels and inositol 1,4,5-trisph osphate-activated channels. We found several of the trp family genes w ere expressed in beta TC3 cells by reverse transcriptase polymerase ch ain reaction using specific primers, but by Northern blot analysis, mt rp-4 was the predominant message expressed. We conclude that a conduct ance underlying glucose-stimulated oscillations in beta-cells is provi ded by a Ca2+ store depletion-activated nonselective cation current, w hich is plausibly encoded by homologs of trp genes.