Modulation by D-glucose anomers of the effect of D-fructose upon 45Ca efflux from prelabelled rat pancreatic islets.

It was recently reported that alpha-D-glucose is more potent than beta-D-glucose in conferring to glucokinase positive cooperativity towards D-fructose. We have now extended pilot experiments to investigate whether a comparable situation prevails in intact rat pancreatic islets in terms of the modulation by the D-glucose anomers of the effect of D-fructose upon 45Ca efflux from prelabelled perifused islets. As expected from the effect of increasing concentrations of equilibrated D-glucose upon 45Ca efflux from the prelabelled islets, D-fructose either decreased or increased 45Ca outflow from islets perifused in the presence of either alpha- or beta-D-glucose. In all cases, the alpha-anomer of D-glucose affected more markedly than beta-D-glucose the cationic response to D-fructose. These findings indicate that the anomeric specificity of the effect of D-glucose upon D-fructose phosphorylation by glucokinase is also operative in intact islets.