Myosin isoenzyme distribution and Ca+2-activated myosin ATPase activity in the rat heart is influenced by fructose feeding and triiodothyronine.

Studies were conducted to determine if the level of cardiac Ca+2-activated myosin ATPase activity and ventricular myosin isoenzyme distribution are influenced by both T3 administration and fructose feeding. Previous studies have shown that in the cardiac ventricle of hypothyroid rats, only myosin V3 is present, and the Ca+2-activated myosin ATPase activity is markedly decreased. Hypothyroid [thyroidectomized (Tx)] rats were fed a diet containing 60% fructose or a regular diet (47% complex carbohydrates) for 4 weeks. Fructose feeding of hypothyroid rats led to a significant increase in Ca+2-activated myosin ATPase activity (Tx regular diet, 0.33 +/- 0.02 mumol Pi/mg protein X min; Tx fructose diet, 0.54 +/- 0.04 mumol Pi/mg protein X min). In addition, myosin V1 was detectable in the heart of fructose-fed Tx rats, but was absent in Tx rats on the regular diet. To determine if fructose had an effect of similar magnitude in animals of different thyroid states, Tx rats were injected with 0.075, 0.150, 0.225, and 0.300 micrograms T3/100 g BW daily and placed on fructose or regular diets. The fructose-induced increase in Ca+2-myosin ATPase activity was between 24-27% in Tx rats receiving 0-0.15 micrograms T3/100 g BW daily. In animals receiving 0.225 and 0.300 micrograms T3/100 g BW daily, fructose feeding did not induce a significant increase in myosin ATPase activity. This is due to the fact that the Ca+2-activated myosin ATPase activities of euthyroid and hyperthyroid animals are not significantly different from each other. In hypothyroid rats receiving a 60% glucose diet, Ca+2-myosin ATPase activity showed a significant 20% increase above the value in regular diet-fed Tx rats. Fructose- and glucose-induced changes in Ca+2-myosin ATPase activity occurred in the absence of changes in thyroid hormone or insulin levels. Our findings may indicate that cardiac carbohydrate consumption influences the predominance of ventricular myosin isoenzymes in the rat heart.