Effects of D-glucose during preincubation of pancreatic islets with 45Ca upon the subsequent stimulation of 45Ca outflow by hypoglycemic sulfonylureas and K+.

Nonnutrient secretagogues, such as gliclazide or a high extracellular concentration of K+, are known to stimulate 45Ca outflow from prelabelled pancreatic islets. The present study aimed at characterizing the dose-related effect of D-glucose during preincubation of the islets with 45Ca upon the magnitude of the subsequent increment in 45Ca efflux evoked by these nonnutrient secretagogues in the prelabelled islets. Such an increment was much greater in islets first incubated with D-glucose than in islets prelabelled in the absence of exogenous nutrient. The effect of D-glucose was already evident at a concentration of 5.6 mM but a further increase in 45Ca outflow was seen in islets preincubated at higher glucose concentrations. The effect of D-glucose was not attributable to a higher net uptake of Ca2+ in response to stimulation by a given nonnutrient secretagogue. Instead, the present data suggest that glucose, in addition to stimulating Ca2+ inflow into the islet cells, favours the sequestration of the cation by intracellular organelles.