Modulation by thyroid hormone of myosin light chain phosphorylation and aquaporin 5 protein expression in intact lung.

Myosin light chain kinase (MLCK) may play a key role in cellular contraction, paracellular permeability and lung water homeostasis. In vitro, thyroid hormones (THs) potently inhibit MLCK activation and, hence, MLC phosphorylation. Whether similar effect is exerted by THs in in vivo systems is not known. Therefore, we investigated the effects of hypothyroid (HO) and hyperthyroid (HR) states on the level of phospho-MLC, aquaporin 5 (AQP5) protein expression, and water holding capacity in the rat lung. Alterations in thyroid state were induced by adding methimazole or levothyroxine (L-T4) to animals' drinking water. Serum TH concentration and thyroid gland histomorphology were assessed to verify the onset of the thyroid state. Lung phospho-MLC and AQP5 proteins were assessed by Western blotting and immunohistochemistry. Lung extravascular water content was estimated by the tissue wet weight-to-dry weight (W/D) ratio. The HO state induced significant increases in the expression of lung phospho-MLC and AQP5 proteins. In contrast, the HR state caused moderate decreases in lung phospho-MLC and AQP5 proteins. While lung water holding capacity was significantly increased in HO animals, it was significantly reduced in HR animals. The data of this study show that THs are able to modulate MLC phosphorylation in in vivo systems. Besides, they suggest that the circulating level of THs may alter lung fluid balance not only through expression of water channels but also through regulation of cellular contraction and paracellular permeability.