VOLTAGE-INDEPENDENT CALCIUM CHANNELS MEDIATE SLOW OSCILLATIONS OF CYTOSOLIC CALCIUM THAT ARE GLUCOSE-DEPENDENT IN PANCREATIC BETA-CELLS

Pancreatic beta-cells and HIT-T15 cells exhibit oscillations of cytoso lic calcium ([Ca2+](i)) that are dependent on glucose metabolism and a ppear to trigger pulsatile insulin secretion. Significantly, differenc es in the pattern of this [Ca2+](i) oscillatory activity may have impo rtant implications for our understanding of how glucose homeostasis is achieved during the feeding and fasting states. When single beta-cell s are exposed to a stepwise increase in glucose concentration that mim ics the transition from fasting to feeding states, fast irregular osci llations of [Ca2+](i) are observed. Alternatively, when single beta-ce lls are equilibrated in a steady state concentration of glucose that m imics the fasting state, slow periodic oscillations of [Ca2+](i) are n oted. Here we report a fundamental difference in the mechanism by whic h glucose induces these two types of [Ca2+](i) oscillatory activity. I n agreement with previous studies, we substantiate a role for L-type v oltage-dependent Ca2+ channels as mediators of the fast oscillations o f [Ca2+](i) observed after a stepwise increase in glucose concentratio n. In marked contrast, we report that voltage-independent calcium chan nels (VICCs) mediate slow oscillations of [Ca2+](i) that occur when be ta-cells are equilibrated in steady state concentrations of glucose. S low [Ca2+](i) oscillations are mediated by VICCs which are pharmacolog ically and biophysically distinguishable from voltage-dependent Ca2+ c hannels that mediate fast oscillations. Specifically, slow [Ca2+](i) o scillations are blocked by extracellular La3+, but not by nifedipine, and are independent of changes in membrane potential. Measurements of membrane conductance also indicate an important role for VICCs, as dem onstrated by a steady state inward Ca2+ current that is blocked by La3 +. The steady state Ca2+ current appears to generate slow [Ca2+](i) os cillations by triggering Ca2+-induced Ca2+ release from intracellular Ca2+ stores, a process that is mimicked by extracellular application o f caffeine, a sensitizer of the ryanodine receptor/Ca2+ release channe l. Depletion of intracellular Ca2+ stores with thapsigargin stimulated Mn2+ influx, suggesting the presence of Ca2+-release-activated Ca2+ c hannels. Taken together, these observations are consistent with a role for VICCs (possibly G-type channels) and/or Ca2+-release-activated Ca 2+ channels as mediators of slow [Ca2+](i) oscillations in beta-cells. We propose that slow oscillations of [Ca2+](i) probably serve as impo rtant initiators of insulin secretion under conditions in which tight control of glucose homeostasis is necessary, as is the case during the fasting normoglycemic state.