The mechanism of the respiratory and transport effect of thyroid hormone on small intestinal mucosa.

Abstract

The hypothesis that thyroid hormone thermogenesis is mediated via active Na+ transport was examined in rat small intestinal mucosa. Total oxygen consumption (QO2) and Na+ — dependent oxygen consumption [QO2(t)] were measured in intestinal segments stripped of muscularis. The activities of (Na+ + K+) — activated adenosine — triphosphatase (NaK-ATPase) and Mg++ − activated ATPase (Mg-ATPase) were measured in crude plasma membrane preparations of mucosal cell scrapings. Euthyroid and thyroidectomized rats were injected with either triodothyronine (T3), 50 μg/100 g body wt., or diluent on alternate days for a total of three doses. In the transition from hypothyroid to the euthyroid state, T3 evoked a 23% increase in QO2, a 41% increase in QO2(t), and a 72% increase in NaK-ATPase activity. The increase in QO2(t) accounted for 55% of the increase in QO2. In the transition from the euthyroid to the hyperthyroid state, QO2 activity increased 15%, QO2(t) 12% and NaK-ATPase 26%. The increase in QO2(t) accounted for 30% of the increase in QO2. Thyroid mediated increase in NaK-ATPase activity could result either from an increase in the activity of a fixed number of enzyme sites or an increase in the number of enzyme sites per cell.