Influence of the thyroid state on myocardial myosin in the adult pig heart.

The influence of hyper- and hypothyroidism on atrial and ventricular myosin structure and Ca2+-activated ATPase activity has been analyzed in adult mini-pigs. Whereas no difference could be demonstrated between hypo- and euthyroid ventricular myocardium, Ca2+-activated ATPase activity was significantly higher in the hyperthyroid than in the hypo- or euthyroid state. Using pyrophosphate electrophoresis and isoelectric focusing of subfragment 1 this could be ascribed to an additional ventricular myosin in the hyperthyroid myocardium. Atrial myosin ATPase activity and structure were not influenced by the thyroid state of the animals. These results present evidence that thyreotoxicosis induces an additional isomyosin in the pig ventricular myocardium, albeit to a lesser degree than in the rodent heart. The lacking difference between the hypothyroid and the euthyroid states indicates that a myosin with a lower enzymatic activity than the normal ventricular myosin is not synthesized in the heart of higher mammals.