Lack of Thyroid Hormone Effect on Activation Energy of NaK-ATPase

In order to differentiate whether activation of NaK-ATPase in thyroid thermogenesis is due to increased numbers of active 'sodium pump' units or due to a change in the kinetics of the enzyme, the effect of T3 on activation energy (Ea) of NaK-ATPase was determined in rat liver, kidney and brain. Injection of T3 produced significant increases in the specific activity of NaK-ATPase in liver and kidney but not in brain homogenates. T3 injections produced no significant change in the Ea of NaK-ATPase in any of the three tissues. The data are compatible with the hypothesis that thyroid stimulation of the sodium pump is brought about by an increase in the number of active pump units.