QUANTIFICATION OF GLUCOSE CYCLING AND THE EXTENT OF EQUILIBRATION OF GLUCOSE-6-PHOSPHATE WITH FRUCTOSE 6-PHOSPHATE IN ISLETS FROM OB/OB MICE

Pancreatic islets from fed and fasted obese hyperglycaemic (ob/ob) mice were incubated with [1-C-14]glucose at 5.5 mM and 16.7 mM, [1-C-14]mannose at 16.7 mM, and (H2O)-H-3. Yields of (CO2)-C-14 and C-14-labelled lactate, and amounts of C-14 from [1-C-14]mannose incorporated into glucose and of H-3 bound to C-2 of glucose, were measured. Glucose utilization was determined from yields of (H2O)-H-3 from [5-H-3]glucose. From the results using C-14, 32-43% of the hexoses phosphorylated to hexose 6-phosphate were estimated to have been dephosphorylated, i.e. cycled. Estimates of hexose cycling from H-3 incorporation into glucose were similar. Differences in the ratios of C-14 yields in CO2 and lactate indicated incomplete isotopic equilibration between glucose 6-phosphate and fructose 6-phosphate, making the estimates of cycling semiquantitative. In the fasted state, a larger percentage of the hexose utilized went to lactate than in the fed state. Thus conversion of mannose into glucose in islets indicates the occurrence of glucose cycling in islets. Yields of C-14 from [1-C-14]mannose, compared with from [1-C-14]glucose, provide an approach for quantifying the extent of this cycling. These data provide further evidence for extensive glucose cycling occurring in ob/ob islets in both the fed and the fasted state.