Muscarinic agonists activate Ca2+ store-operated and -independent ionic currents in insulin-secreting HIT-T15 cells and mouse pancreatic beta-cells.
The neurotransmitter acetylcholine, a muscarinic receptor agonist, augments glucose-induced insulin secretion from pancreatic beta-cells by depolarizing the membrane to enhance voltage-gated Ca(2+) influx. To clarify the electrical events involved in this process, we measured ionic currents from a clonal beta-cell line (HIT-T15) and mouse pancreatic beta-cells. In whole-cell recordings, the muscarinic agonist carbachol (CCh) dose-dependently and reversibly activated a voltage-independent, nonselective current (whole-cell conductance 24 pS/pF, reversal potential of approximately -15 mV). The current, which we refer to as I(musc), was blocked by atropine, a muscarinic receptor antagonist, and SKF 96365, a nonspecific ion channel blocker. The magnitude of the current decreased by 52% when extracellular Na(+) was removed, but was not affected by changes in extracellular Ca(2+), confirming that I(musc) is a nonselective current. To determine if I(musc) activates following release of Ca(2+) from an intracellular store, we blocked intracellular IP(3) receptors with heparin. Carbachol still activated a current in the presence of heparin, demonstrating the presence of a Ca(2+) store-independent, muscarinic agonist-activated ionic current in HIT cells. However, the store-independent current was smaller and had a more positive reversal potential (approximately 0 mV) than the current activated by CCh under control conditions. This result indicates that heparin had blocked a component of I(musc), which likely activates following release of stored Ca(2+). Depleting IP(3)-sensitive calcium stores with thapsigargin also activated a non-selective, SKF 96365-blockable current in HIT cells. The properties of this putative store-operated current were similar to the component of I(musc) that was blocked by heparin, being voltage-independent and reversing near -30 mV. We conclude that I(musc) consists of store-operated and store-independent components, both of which may contribute to the depolarizing action of muscarinic agonists on pancreatic beta-cells.
Mears
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Department of Anatomy, Physiology & Genetics, Uniformed Services University of the Health Sciences, Bethesda, MD 20814, USA. dmears@usuhs.mil
Zimliki
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D000109:Acetylcholine / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism; D002217:Carbachol / Q000494:pharmacology*; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D006224:Cricetinae; D004305:Dose-Response Relationship, Drug; D006493:Heparin / Q000494:pharmacology; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007340:Insulinoma / Q000378:metabolism*; D015640:Ion Channel Gating; D007473:Ion Channels / Q000378:metabolism*; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D051379:Mice; D018721:Muscarinic Agonists / Q000494:pharmacology; D018160:Receptors, Cytoplasmic and Nuclear / Q000187:drug effects / Q000378:metabolism; D011976:Receptors, Muscarinic / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019284:Thapsigargin / Q000494:pharmacology
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007328:Insulin; D007473:Ion Channels; D018721:Muscarinic Agonists; D018160:Receptors, Cytoplasmic and Nuclear; D011976:Receptors, Muscarinic; D019284:Thapsigargin; D002217:Carbachol; D006493:Heparin; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases; D000109:Acetylcholine; D002118:Calcium