Mechanism of Thyroid Calorigenesis: Role of Active Sodium Transport

Abstract

The hypothesis that thyroid calorigenesis is mediated by stimulation of active Na + transport was tested by measuring the Q o 2 of liver slices and skeletal muscle (diaphragm) from thyroxine- and triiodothyronine-injected thyroidectomized and normal rats in media fortified with ouabain (10 -3 M) and/or free of Na + or K + . In both tissues, more than 90% of the increase in Q o 2 produced by injections of thyroid hormone in euthyroid rats was derived from increased energy utilization by the Na + pump. In triiodothyronine-treated thyroidectomized rats, activation of Na + transport accounted for 90% or more of the increment in Q o 2 in liver and 40% or more of the increment in diaphragm. Intracellular Na + , K + , and Cl - concentrations were measured in euthyroid and hyperthyroid liver and diaphragm. The transmembrane Na + and K + concentration differences were significantly increased in both tissues by the administration of triiodothyronine. These results indicate that thyroid hormone activates Na + extrusion and K + accumulation either by increasing the local concentration of ATP or by direct stimulation of the Na + pump.