Role of high-voltage-activated calcium channels in glucose-regulated -cell calcium homeostasis and insulin release

Taylor, James T., Luping Huang, Brian M. Keyser, Hean Zhuang, Craig W. Clarkson, and Ming Li. Role of high-voltage-activated calcium channels in glucose-regulated -cell calcium homeostasis and insulin release. Am J Physiol Endocrinol Metab 289: E900–E908, 2005. First published June 14, 2005; doi:10.1152/ajpendo.00101.2005.—Highvoltage-activated (HVA) calcium channels are known to be the primary source of calcium for glucose-stimulated insulin secretion. However, few studies have investigated how these channels can be regulated by chronically elevated levels of glucose. In the present study, we determined the level of expression of the four major HVA calcium channels (N-type, P/Q-type, LC-type, and LD-type) in rat pancreatic -cells. Using quantitative real-time PCR (QRT-PCR), we found the expression of all four HVA genes in rat insulinoma cells (INS-1) and in primary isolated rat islet cells. We then determined the role of each channel in insulin secretion by using channel-selective antagonists. Insulin secretion analysis revealed that Nand L-type channels are both involved in immediate glucose-induced insulin secretion. However, L-type was preferentially coupled to secretion at later time points. P/Q-type channels were not found to play a role in insulin secretion at any stage. It was also found that long-term exposure to elevated glucose increases basal calcium in these cells. Interestingly, chronically elevated glucose decreased the mRNA expression of the channels involved with insulin secretion and diminished the level of stimulated calcium influx in these cells. Using whole cell patch clamp, we found that Nand L-type channel currents increase gradually subsequent to lower intracellular calcium perfusion, suggesting that these channels may be regulated by glucoseinduced changes in calcium.