METABOLISM OF GLUCOSE IN HYPER-THYROID AND HYPO-THYROID RATS INVIVO - GLUCOSE-TURNOVER VALUES AND FUTILE-CYCLE ACTIVITIES OBTAINED WITH C-14-LABELED AND H-3-LABELED GLUCOSE

A trace amount of glucose labelled with 14C uniformly and with 3H at position 2, 3 or 6 was injected intravenously into starved rats to measure the turnover rate of blood glucose. Reliable estimates were made based on the semilogarithmic plot of specific radioactivity of the glucose contained in whole blood samples taken from the tail vein. Glucose turned over more rapidly in hyperthyroid and more slowly in hypothyroid than in euthyroid rats. The percentage contribution of glucose recycling (determined from the difference in replacement rates between [U-14C]glucose and [6-3H]glucose) to the glucose utilization increased on induction of hyperthyroidism. Futile cycles between glucose and glucose 6-phosphate (determined from the difference between replacement rates of [2-3H]glucose and [6-3H]glucose) were activated and inactivated by induction of hyperthyroid and hypothyroid states respectively. The hepatic content of glycogen was much lower in hyper- and hypo-thyroid than in euthyroid rats. The enhanced glucose production in hyperthyroid rats resulted from not only activation of hepatic gluconeogenesis but also diversion of the final product of gluconeogenesis from liver glycogen to blood glucose. In hypothyroidism, the inhibition of gluconeogenesis led to suppression of both glucose production and glycogenesis in the liver.