Short-Term Effects of Thyroid Hormones on the Na/H Antiport in L-6 Myoblasts: High Molecular Specificity for 3,3',5-Triiodo-L-Thyronine

Abstract

The thyroid hormones l-T3 and l-T4 were shown to activate the Na/H antiport in L-6 cells from rat skeletal muscle by a rapid, nongenomic mechanism. Under pH equilibrium conditions, a significant rise in the intracellular pH, measured by the fluorescent pH indicator 2′,7′-bis-(carboxyethyl)-5(6)-carboxyfluorescein was observed after the addition of physiological concentrations (10−10m) of either l-T3 or l-T4, but with different time courses. l-T3 at all concentrations increased the pH after a delay of 2 min, whereas l-T4 showed a concentration-dependent lag time, going from 11 min at 10−11m down to 5 min for a hormone concentration of 10−6m. The effect of l-T4 was blocked in the presence of the 5′-deiodinase inhibitor 6-n-propyl-2-thiouracil, suggesting that the difference in lag time between l-T3 and l-T4 was due to the 5′-deiodination process that transforms l-T4 into the bioactive l-T3. In short term studies (<5 min), a high molecular specificity for l-T3 was found, as l-T4, rT3, the d-isomer of T3, and the deaminated analogues were ineffective at physiological concentrations. In analogy with the results found at equilibrium, intracellular pH recovery from an acid load and set-point were increased after 2 min for l-T3 (10−9m) and after 10 min for l-T4 (10−9m). The effect of the hormones on the intracellular pH was completely blocked by the specific antiport inhibitor 5-(ethyl-N-isopropyl)amiloride. These findings suggest that thyroid hormones may play an active role in the recovery from muscular acidosis through direct stimulation of the Na/H antiport.