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 cytosolic calcium ([Ca2+]i) that are dependent on glucose metabolism and appear to trigger pulsatile insulin secretion. Significantly, differences in the pattern of this [Ca2+]i oscillatory activity may have important implications for our understanding of how glucose homeostasis is achieved during the feeding and fasting states. When single beta-cells are exposed to a stepwise increase in glucose concentration that mimics the transition from fasting to feeding states, fast irregular oscillations of [Ca2+]i are observed. Alternatively, when single beta-cells are equilibrated in a steady state concentration of glucose that mimics the fasting state, slow periodic oscillations of [Ca2+]i are noted. Here we report a fundamental difference in the mechanism by which glucose induces these two types of [Ca2+]i oscillatory activity. In agreement with previous studies, we substantiate a role for L-type voltage-dependent Ca2+ channels as mediators of the fast oscillations of [Ca2+]i observed after a stepwise increase in glucose concentration. In marked contrast, we report that voltage-independent calcium channels (VICCs) mediate slow oscillations of [Ca2+]i that occur when beta-cells are equilibrated in steady state concentrations of glucose. Slow [Ca2+]i oscillations are mediated by VICCs which are pharmacologically and biophysically distinguishable from voltage-dependent Ca2+ channels that mediate fast oscillations. Specifically, slow [Ca2+]i oscillations are blocked by extracellular La3+, but not by nifedipine, and are independent of changes in membrane potential. Measurement of membrane conductance also indicate an important role for VICCs, as demonstrated by a steady state inward Ca2+ current that is blocked by La3+. The steady state Ca2+ current appears to generate slow [Ca2+]i oscillations by triggering Ca(2+)-induced Ca2+ release from intracellular Ca2+ stores, a process that is mimicked by extracellular application of caffeine, a sensitizer of the ryanodine receptor/Ca2+ release channel. Depletion of intracellular Ca2+ stores with thapsigargin stimulated Mn2+ influx, suggesting the presence of Ca(2+)-release-activated Ca2+ channels. Taken together, these observations are consistent with a role for VICCs (possibly G-type channels) and/or Ca(2+)-release-activated Ca2+ channels as mediators of slow [Ca2+]i oscillations in beta-cells. We propose that slow oscillations of [Ca2+]i probably serve as important initiators of insulin secretion under conditions in which tight control of glucose homeostasis is necessary, as is the case during the fasting normoglycemic state.
Laboratory of Molecular Endocrinology, Massachusetts General Hospital, Howard Hughes Medical Institute, Harvard Medical School, Boston 02114.
C A
CA
Leech
G G
GG
Holz
J F
JF
Habener
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism* / Q000502:physiology; D006224:Cricetinae; D003600:Cytosol / Q000378:metabolism*; D004594:Electrophysiology; D005947:Glucose / Q000502:physiology*; D007340:Insulinoma / Q000378:metabolism / Q000473:pathology; D007515:Islets of Langerhans / Q000378:metabolism*; D008297:Male; D010190:Pancreatic Neoplasms / Q000378:metabolism / Q000473:pathology; D051381:Rats; D017207:Rats, Sprague-Dawley; D014407:Tumor Cells, Cultured
D015220:Calcium Channels; D005947:Glucose; D002118:Calcium