Energetic requirement of insulin secretion distal to calcium influx.
A number of agents that inhibit oxidative phosphorylation by different mechanisms (carbonyl cyanide mchlorophenylhydrazone [CCCP], sodium azide, oligomycin) induced an increase of cytoplasmic Ca2+ concentration ([Ca2+]i) in pancreatic beta-cells, as measured by microfluorimetry with digital imaging. All three agents are known inhibitors of insulin secretion, and the secretory response to 20 mmol/l glucose was found to be abolished in spite of elevated [Ca2+]i. Two reasons could account for this dissociation between increase of [Ca2+]i and insulin secretion: 1) the increase did not take place at a site critical for exocytosis, 2) a threshold concentration of a metabolism-derived factor like ATP exists for the induction of exocytosis. The increase of [Ca2+]i by CCCP and sodium azide involved release of Ca2+ from internal stores, whereas oligomycin induced a slow D 600-inhibitable Ca2+ influx. Because CCCP and sodium azide, but not oligomycin, decreased the mitochondrial membrane potential concomitantly with the increase of [Ca2+]i, release of Ca2+ from the mitochondria most probably plays a decisive role for the internal mobilization. A Ca2+ influx induced by 40 mmol/l K+ or 250 micromol/l tolbutamide was unimpaired in the presence of oligomycin, but oligomycin completely abolished insulin secretion in response to these agents. While CCCP and sodium azide opened ATP-sensitive K+ channels, oligomycin was virtually ineffective, although it could be shown to significantly reduce beta-cell ATP production. By comparison of the effects of different inhibitors of oxidative phosphorylation, we conclude that the initiation of exocytosis in beta-cells is particularly sensitive to a decrease of energy metabolism, more than ATP-sensitive K+ channels or voltage-dependent Ca2+ channels. Thus, any increase of [Ca2+]i in beta-cells that occurs in a situation of a decreased ATP supply is unlikely to elicit a secretory response.
Rustenbeck
I
I
Institute of Pharmacology and Toxicology, University of Göttingen, Germany.
Herrmann
C
C
Grimmsmann
T
T
D000818:Animals; D001386:Azides / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002478:Cells, Cultured; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005453:Fluorescence; D005456:Fluorescent Dyes; D016257:Fura-2; D005947:Glucose / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D008820:Mice, Obese; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism; D009840:Oligomycins / Q000494:pharmacology; D010085:Oxidative Phosphorylation / Q000187:drug effects; D018408:Patch-Clamp Techniques; D020112:Rhodamine 123; D012235:Rhodamines; D019810:Sodium Azide; D013997:Time Factors; D014475:Uncoupling Agents / Q000494:pharmacology*
D001386:Azides; D015220:Calcium Channels; D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D007328:Insulin; D009840:Oligomycins; D012235:Rhodamines; D014475:Uncoupling Agents; D020112:Rhodamine 123; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D019810:Sodium Azide; D005947:Glucose; D002118:Calcium; D016257:Fura-2