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.

Supported by U.S. National Health Service International Fellowships # TW-01779, # HL-06285 and the Bay Area Heart Association.