Energetic Requirement of Insulin Secretion Distal to Calcium Influx

Abstract

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.

Dual actions of the metabolic inhibitor, sodium azide on KATP channel currents in the rat CRI‐G1 insulinoma cell line

“…Another possible explanation for the lack of effect of rotenone and oligomycin on cell‐attached K ATP channel activity is that these compounds are less effective at depleting intracellular levels of ATP than sodium azide. However, this seems unlikely as rotenone, oligomycin and sodium azide all produce comparable reductions in intracellular ATP levels ( Niki et al ., 1989 ; Rustenbeck et al ., 1997 ; Gribble et al ., 1997a ). It should be noted that recent evidence indicates that oligomycin fails to activate K ATP channels in pancreatic beta cells even though it significantly reduces ATP production ( Rustenbeck et al ., 1997 ).…”

“…However, this seems unlikely as rotenone, oligomycin and sodium azide all produce comparable reductions in intracellular ATP levels ( Niki et al ., 1989 ; Rustenbeck et al ., 1997 ; Gribble et al ., 1997a ). It should be noted that recent evidence indicates that oligomycin fails to activate K ATP channels in pancreatic beta cells even though it significantly reduces ATP production ( Rustenbeck et al ., 1997 ). Alternatively the metabolic source and/or site of ATP production may be important in CRI‐G1 insulinoma cells.…”

Imidazolines and the Pancreatic B‐Cell: Actions and Binding Sites

“…The actions of imidazolines on pancreatic islets or single pancreatic B‐cells were investigated using the following techniques: measurement of insulin secretion of perifused mouse islets by radioimmunoassay, measurement of K ATP channel activity and membrane potential by patch‐clamping of single B‐cells, and measurement of cytoplasmic Ca 2+ concentration by microfluorimetry of single islet cells (refs. 12 and 21 and references therein).…”

“…Apparently, there are sites additional to K ATP channels where energy metabolism influences insulin secretion. 12 In addition, electrophysiologic experiments in which membrane capacitance was measured as an indicator of insulin secretion suggested that sulfonylureas may induce secretion by mechanisms other than closure of K ATP channels. Activation of protein kinase C was reported to play a decisive role, whereas Ca 2+ influx due to K ATP channel closure appeared only to be responsible for a minor part of the enhanced rate of secretion.…”

BBT improves glucose homeostasis by ameliorating β-cell dysfunction in type 2 diabetic mice

“…2C is essential for the BBT-potentiated GSIS As reported, besides the cAMP/PKA pathway, Ca 2C -influxrelated signaling also functions potently in insulin secretion (Rustenbeck et al 1997 (Gomez et al 2002)) and thapsigargin (an inhibitor of…”