The effects on ventricular myosin isoenzyme expression of naturally occurring 
and synthetic thyroid analogs (3,5,3'-L-triiodothyronine, 
3,5,3'-D-triiodothyronine, 3,3',5'-L- triiodothyronine , 
3,5,3'-L-triiodothyroacetic acid and 3,5-L-diiodothyronine), catecholamines and 
high carbohydrate diets have been studied in thyroidectomized and 
hypophysectomized rats. Also, the effects on myosin isoenzyme expression of 
adrenalectomy and hydrocortisone replacement have been studied in euthyroid 
animals. Myocardial CO2 production and hepatic alpha-glycerolphosphate 
dehydrogenase activity were measured to monitor the effects of these 
interventions on tissue respiration. The results indicate that there was no 
significant separation between the actions of thyroid analogs on metabolic 
parameters and myosin isoenzyme patterns. However, high carbohydrate feeding in 
hypophysectomized rats increased the isoenzyme V1 from 12% to about 36% of total 
myosin; partial replacement with 3,5,3'-L-triiodothyronine and fructose feeding 
had synergistic actions. In thyroidectomized rats, feeding a high carbohydrate 
diet increased the V1 form from undetectable levels to about 28% of total 
myosin; partial 3,5,3'-L-triiodothyronine replacement had an additive effect. 
Beta adrenergic stimulation with isoproterenol and blockade with propranolol did 
not affect myosin isoenzyme expression. Adrenalectomy in euthyroid rats caused a 
33% decrease in the V1 form and a corresponding increase in the V3 isoenzyme, 
which could be reversed by treatment with hydrocortisone. Thus, thyroid analogs 
do not selectively stimulate myosin isoenzyme expression as compared with their 
effects on energy production. Furthermore, the results suggest that the 
mechanism for regulation of cardiac myosin isoenzymes may involve a primary 
signal related to dietary carbohydrate, which is modulated by thyroid hormone, 
and possibly glucocorticoids.