The Mechanism of the Calorigenic Action of Thyroid Hormone : Stimulation of Na(+) + K(+)-activated adenosinetriphosphatase activity

In an earlier study, we proposed that thyroid hormone stimulation of energy utilization by the Na(+) pump mediates the calorigenic response. In this study, the effects of triiodothyronine (T(3)) on total oxygen consumption (Q (O(O2))), the ouabain-sensitive oxygen consumption [Q (O(O2))(t)], and NaK-ATPase in liver, kidney, and cerebrum were measured. In liver, ∼90% of the increase in Q (O(O2)) produced by T(3) in either thyroidectomized or euthyroid rats was attributable to the increase in Q (O(O2))(t). In kidney, the increase in Q (O(O2))(t) accounted for 29% of the increase in Q (O(O2)) in thyroidectomized and 46% of the increase in Q (O(O2)) in euthyroid rats. There was no demonstrable effect of T(3) in euthyroid rats on Q (O(O2)) or Q (O(O2))(t) of cerebral slices. The effects of T(3) on NaK-ATPase activity in homogenates were as follows: In liver +81% from euthyroid rats and +54% from hypothyroid rats. In kidney, +21% from euthyroid rats and +69% from hypothyroid rats. T(3) in euthyroid rats produced no significant changes in NaK-ATPase or Mg-ATPase activity of cerebral homogenates. Liver plasma membrane fractions showed a 69% increase in NaK-ATPase and no significant changes in either Mg-ATPase or 5'-nucleotidase activities after T(3) injection. These results indicate that thyroid hormones stimulate NaK-ATPase activity differentially. This effect may account, at least in part, for the calorigenic effects of these hormones.