Thyroid thermogenesis. Relationships between Na+ dependent respiration and Na++K+ adenosine triphosphatase activity in rat skeletal muscle

Abstract

The effect of thyroid status on Q(O2), Q(O2)(t) and NaK-ATPase activity was examined in rat skeletal muscle. Q(O2)(t) (i.e. Na+-transport dependent respiration) was examined with ouabain or Na+-free media supplemented with K+. In contrast to the effects of ouabain on ion composition, intracellular K+ was maintained at about 125 meq/liter, and intracellular Na+ was almost nil in the Na+-free media. The estimates of Q(O2)(t) were independent of the considerable differences in tissue ion concentrations. The increase in Q(O2)(t) accounted for 47% of the increase in Q(O2) in the transition from the hypothyroid to the euthyroid state and 84% of the increase in the transition from the euthyroid to the hyperthyroid state. Surgical thyroidectomy lowered NaK-ATPase activity of the microsomal fraction (expressed per milligram protein) 32%; injections of triiodothyronine (T3) increased this activity 75% in initially hypothyroid rats and 26% in initially euthyroid rats. Thyroidectomy was attended by significant falls in serum Ca and Pi concentrations. Administration of T3 resulted in further declines in serum Ca and marked increases in serum Pi concentrations. Similar effects were seen in 131I-treated rats, but the magnitude of the declines in serum Ca were less. The effects of T3 on Q(O2), Q(O2)(t), and NaK-ATPase activity of skeletal muscle were indistinguishable in the 131I ablated and surgically thyroidectomized rats. In thyroidectomized or euthyroid rats given repeated doses of T3, Q(O2)(t) and NaK-ATPase activity increased proportionately. In thyroidectomized rats injected with single doses of T3 either 10, 50 or 250 μg/100 g body weight, Q(O2)(t) increased linearly with NaK-ATPase activity. The kinetics of NaK-ATPase activity were assessed with an ATP regenerating system. T3 elicited a significant increase in V(max) with no change in K(m) for ATP.