Effects of thyroid status on glucose cycling by isolated rat hepatocytes

The effects of alterations in thyroid status on glucose metabolism have been investigated in rat hepatocytes. Addition of 10 or 40 mmol/L glucose induced increases in respiration rate that were significantly larger in cells from hyperthyroid rats than from hypothyroid animals. The responses of hepatocytes from euthyroid rats were intermediate. In cells from hyperthyroid rats, most of the increase occurred upon addition of 10 mmol/L glucose, with only a further small stimulation resulting when glucose concentration was increased to 40 mmol/L. For a given glucose concentration, glycolytic rates, determined by measuring release of tritium from [6-3H]glucose, were comparable in all thyroid states. Studies with 10 mmol/L [2-3H]glucose showed that cycling between glucose-6-phosphate and glucose was almost twofold higher in euthyroid and hyperthyroid states as compared with the hypothyroid state, although the magnitude of the increase in cycling rate was only approximately 0.2 mumol glucose.min-1.g-1. When 40 mmol/L [2-3H]glucose was added, over 44% of the glucose that was phosphorylated to glucose-6-phosphate was cycled back to glucose, but this cycling was independent of thyroid status. Cycling between fructose-1,6-bisphosphate and fructose-6-phosphate was negligible in all thyroid states. Rates of glycogen synthesis were comparable in hypothyroid and hyperthyroid states and slightly less than in the euthyroid state. Glycolytically formed pyruvate was cycled back to glucose in hepatocytes from hypothyroid, euthyroid, and hyperthyroid rats. During a 60-minute incubation period, cycling to glucose in the presence of 10 mmol/L or 40 mmol/L glucose was up to twofold higher in cells from euthyroid and hyperthyroid rats than in hepatocytes from hypothyroid animals. The measured increases in cycling rates induced by thyroid hormone were small and in theory could have been satisfied by a much smaller increase in respiration rate than was observed.