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 α-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.