Involvement of Ca2+ in the Impaired Glucose-induced Insulin Release from Islets Cultured at Low Glucose

Abstract

Islet culture at a low glucose concentration results in a progressive impairment of glucose-induced insulin release. The role of Ca2+ in this defect was studied by comparing rat islets cultured for 6 days either at 8.3 mM (control) or 2.8 mM glucose. For measurement of 45Ca content and 45Ca2+ efflux, islets were kept in the presence of 45Ca2+ throughout. In islets cultured at 8.3 mM glucose, stimulation with 16.7 mM glucose during perifusion caused a typical biphasic pattern of insulin release paralleled by an increase in the rate of 45Ca2+ efflux. Both effects of glucose were markedly reduced in islets kept at 2.8 mM glucose, despite a similar insulin content. Islet 45Ca content was reduced. Both 45Ca content and insulin release were restored when islets were kept for an additional 24 h at 8.3 mM glucose. Insulin release induced by 3-isobutyl-1-methylxanthine (IBMX) or alpha-ketoisocaproic acid was not impaired, demonstrating that there is no generalized release defect. In contrast, glyceraldehyde- or K+-induced release was decreased. In islets maintained at 2.8 mM glucose, the stimulatory effect of glucose on Ca2+ uptake and the inhibitory effect on Ca2+ efflux (in the absence of Ca2+) were found to be operative. A defect may therefore lie distal to the Ca2+ uptake step involving either the mechanism by which glucose uses cellular Ca or another step yet to be identified.

Role of single-channel stochastic noise on bursting clusters of pancreatic beta-cells

“…The lengthening of the plateau phase, in turn, gives rise to an increase in the average [Ca2"]i level. This result is particularly interesting because [Ca2+]i is implicated in the release of insulin (Scott et al, 1981;Siegel et al, 1983). The membrane potential is clearly influenced by the random opening and closing of 1,000 delayed rectifier K+ channels (see Fig.…”

Modulation of Glucose-Induced Insulin Secretion from a Rat Clonal β-Cell Line*

“…In contrast, with RINm5F cells, Clon (10 IXM) did not inhibit insulin secretion in the presence of 2.8 mM glucose, although Clon did inhibit secretion induced by 10 mM alanine (49). Qualitatively, the detrimental effect of low glucose concentration in the culture medium is similar for RIN-38 cells and normal islets (46)(47)(48). For both islets and RIN-38 cells low glucose culture impairs the secretory response to glucose, but with RIN-38 cells the glucose concentration in the medium that results in impaired secretion is shifted to very low concentrations.…”

“…Glucose-induced insulin secretion from normal islets is reduced by low extracellular concentrations of calcium (44), « 2 -adrenergic agonists (45), and culture at low glucose concentrations (46)(47)(48). With RIN-38 cells glucoseinduced secretion is not present with calcium concentrations of 0.5 mM or less, while with 2.5 mM calcium, glucose stimulates secretion.…”

Substrate Control of Insulin Release

“…Even though leucine metabolism of p cells is complex, attempts have been made to identify critical enzymes that comprise and regulate relevant pathways (88,89,145). The design of such studies was influenced by observations that prior exposure of islet tissue to high glucose desensitized P cells to stimulation by leucine in the absence of glucose.…”

“…For example, it was not determined whether insulin release due to a-ketoisocaproate was impaired following high glucose preincubation, as would be expected if reduced branched chain a-ketoacid dehydrogenase is critical in the desensitization mechanism. Others found no evidence for reduced sensitivity to a-ketoisocaproate in islets cultured for 6 days at the moderately high glucose levels shown by MacDonald to desensitize P cells to leucine (145). MacDonald and collaborators (89) discount the potential regulatory role of GDH in the desensitization process because the activity of the enzyme measured in islet cell extracts was not affected by prior glucose treatment.…”