Altered intracellular Ca2+ regulation in pancreatic acinar cells from acute streptozotocin-induced diabetic rats

We investigated intracellular Ca2+ regulation in pancreatic acinar cells from rats with diabetes induced by a single injection of streptozotocin (80 mg/kg). Experiments were performed 2 days and 7 days after the injection of streptozotocin. The density of muscarinic receptors, measured by [H-3]N-methyl scopolamine binding, was unchanged in 2-day-diabetic rats, but was significantly increased in 7-day-diabetic rats. The percentage of high affinity receptors (R(H)) and low affinity receptors (R(L)) determined from the competitive curves with [H-3]N-methyl scopolamine and carbachol was not change in 2-day-diabetic rats compared to controls, whereas 7-day-diabetic rats showed a decrease in %R(H) and an increase in %R(L). The carbachol-evoked initial peak of intracellular Ca2+ concentration ([Ca2+](i)) was increased in 2-day-diabetic rats and decreased in 7-day-diabetic rats, compared to controls. In the carbachol-induced sustained phase in [Ca2+](i), the response in 7-day-diabetic rats was significantly decreased; however, there was no difference between controls and 2-day-diabetic rats. Carbachol (100 mu M)-induced [H-3]inositol 1,3,4-trisphosphate generation was significantly lower in diabetic rats than in the controls. The addition of inositol 1,4,5-trisphosphate (1,4,5-IP3) significantly increased Ca-45(2+) release from saponin-permeabilized cells in 2-day-diabetic rats, but did not do so in 7-day-diabetic rats. Ca2+ refilling into the intracellular stores, determined by second cholecystokinin-8 (10 nM) stimulation after 10 mu M carbachol stimulation, was increased in 2-day-diabetic rats and decreased in 7-day-diabetic rats. These observations indicate that the alterations in intracellular Ca2+ regulation accompanied by changes in transmembrane signaling occur in the earlier stage of the diabetic state. The findings also suggest that the increase in the carbachol-evoked [Ca2+](i) peak in 2-day-diabetic rats is related predominantly to the higher sensitivity of 1,4,5-IP3-sensitive Ca2+ stores and the increase in the capacity of Ca2+ refilling in these animals, whereas the reduction in the [Ca2+](i) peak in 7-day-diabetic rats appears to be related to the essential decrease in receptor-mediated 1,4,5-IP3 generation and the decrease in Ca2+ refilling capacity.