Ventricular myosin from young and adult animals with respect to the thyroid state

Studies were conducted to analyze the effect of the thyroid hormone on ventricular myosin during ontogenesis of mice, rats and rabbits. Hypothyroidism was induced in mice and rats by administering propylthiouracyl in drinking water. Rabbits were made hyperthyroid by chronic administration of thyroxine. The change in the thyroid state of rats and rabbits influenced young and adult animals differently depending on whether V1 or V3 was the major ventricular isomyosin form present. Measurements of Ca2+-ATPase activity of myosins from young and old control animals and from animals with changed thyroid state showed that hypothyroidism in rats is associated with a greater decrease of myosin ATPase in young rats which contain V1 isomyosin only, when compared with old rats which contain a preponderance of V3 isomyosin and less of the V1 form. In rabbits, ATPase activity of ventricular myosin was more elevated after thyroxine administration in adult rabbits, which contain V3 isomyosin only, than in young rabbits in which myosin consists of V1 and V3 isomyosins. Ventricular myosins of young and adult mice did not differ in their ATPase activity and the treatment of mice with propylthiouracyl had only slight effect on myosin ATPase. It can be concluded based on these results that the hypothesis concerning hypothyroidism inducing transformation of V1 into V3 isomyosin does not hold generally.