Characterization of a Ca2+ Release-activated Nonselective Cation Current Regulating Membrane Potential and [Ca2+] i Oscillations in Transgenically Derived β-Cells

• Published in: The Journal of biological chemistry 1998-04, Vol.273 (17), p.10402
• Main Authors: Michael W. Roe, Jennings F. Worley III, Feng Qian, Natalia Tamarina, Anshu A. Mittal, Flora Dralyuk, Nathaniel T. Blair, Robert J. Mertz, Louis H. Philipson, Iain D. Dukes
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.273.17.10402

Although stimulation of insulin secretion by glucose is regulated by coupled oscillations of membrane potential and intracellular Ca 2+ ([Ca 2+ ] i ), the membrane events regulating these oscillations are incompletely understood. In the presence of glucose and tetraethylammonium, transgenically derived β-cells (βTC3-neo) exhibit coupled voltage and [Ca 2+ ] i oscillations strikingly similar to those observed in normal islets in response to glucose. Using these cells as a model system, we investigated the membrane conductance underlying these oscillations. Alterations in delayed rectifier or Ca 2+ -activated K + channels were excluded as a source of the conductance oscillations, as they are completely blocked by tetraethylammonium. ATP-sensitive K + channels were also excluded, since the ATP-sensitive K + channel blocker tolbutamide substituted for glucose in inducing [Ca 2+ ] i oscillations. Thapsigargin, which depletes intracellular Ca 2+ stores, and maitotoxin, an activator of nonselective cation channels, both converted the glucose-dependent [Ca 2+ ] i oscillations into a sustained elevation. On the other hand, both SKF 96365, a blocker of Ca 2+ store-operated channels, and external Na + removal suppressed the glucose-stimulated [Ca 2+ ] i oscillations. Maitotoxin activated a nonselective cation current in βTC3 cells that was attenuated by removal of extracellular Na + and by SKF 96365, in the same manner to a current activated in mouse β-cells following depletion of intracellular Ca 2+ stores. Currents similar to these are produced by the mammalian trp -related channels, a gene family that includes Ca 2+ store-operated channels and inositol 1,4,5-trisphosphate-activated channels. We found several of the trp family genes were expressed in βTC3 cells by reverse transcriptase polymerase chain reaction using specific primers, but by Northern blot analysis, mtrp-4 was the predominant message expressed. We conclude that a conductance underlying glucose-stimulated oscillations in β-cells is provided by a Ca 2+ store depletion-activated nonselective cation current, which is plausibly encoded by homologs of trp genes.