Involvement of Ca2+ in the impaired glucose-induced insulin release from islets cultured at low glucose.
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.
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D046508:Culture Techniques; D005947:Glucose / Q000494:pharmacology*; D005985:Glyceraldehyde / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D009994:Osmolar Concentration; D051381:Rats; D011919:Rats, Inbred Strains; D013997:Time Factors
D007328:Insulin; D005985:Glyceraldehyde; D005947:Glucose; D002118:Calcium