Effect of experimental dysthyroidism on the enzymatic character of the diaphragm.

This study examined the effects of long-term experimental dysthyroidism on the enzymatic character of the costal diaphragm and selected respiratory parameters. Costal diaphragms from thyroidectomized (TX), euthyroid (EU), and hyperthyroid (HT) male albino rats were used. HT was induced by subcutaneous injections of triiodothyronine on alternate days for 6 wk. Minute ventilation was greater for the HT (70%) compared with the TX rats. The enzymatic potentials of glycolysis (28%), tricarboxylic acid cycle (30%), and fatty acid oxidation (16%) were significantly increased in the HT diaphragms, whereas the potentials were lower by a similar relative extent in the TX diaphragms. The proportion of alkali-labile fibers were greater in the TX and lower in the HT diaphragm. The shifts in heart and muscle lactate dehydrogenase isoenzyme activities were consistent with the fiber type changes. These findings show that dysthyroidism modifies the overall enzymatic capacity of the diaphragm (i.e., glycolysis, tricarboxylic acid cycle, and fatty acid oxidation) along with the proportion of alkali-labile to alkali-stable fiber types. These enzymatic changes are similar to those resulting from exercise training, tracheal banding, streptozotocin diabetes, and emphysema.