Developmental changes in the myosin composition of guinea pig ventricular muscle. Relation to thyroid state and mechanical properties

In a variety of mammalian species, thyroid hormone regulates the contractile properties of the heart as well as the expression of the α and β heavy chains of myosin. We have previously shown that the plasma levels of thyroid hormone reach a peak immediately after birth in guinea pigs and decline with maturation. We therefore studied age-related changes in the expression of the myosin heavy chains in the guinea pig ventricle in relation to the ventricular mechanical properties and the levels of thyroid hormone. The composition of the myosin heavy chains was characterized by gel electrophoresis and immunoblotting. Anti-β-chain antibody stained equally myosins from newborns (0-5 days) and adults (75-90 days), while anti-α- chain positively decorated only the myosins of euthyroid newborns or of hyperthyroid adults, but not myosins of embryos, hypothyroid newborns or hypothyroid adults. Myosin of euthyroid adults was faintly stained by anti- α-chain. The alterations in the composition of myosin corresponded with the 'thyroid state' of these groups. The plasma levels of total T₃ were 24.3 ± 2.7, 9.04 ± 1.2* and 139.0 ± 9.3* ng/dl (mean ± SEM) in the euthyroid, hypothyroid and hyperthyroid adults, respectively. In euthyroid and hypothyroid newborns, the plasma levels of T₃ were 56.5 ± 11.9 and 26.5 ± 9.8* ng/dl, respectively. Within each age group the thyroid state corresponded with maximal twitch tension (T(max)), rates of development of tension and relaxation, time to peak tension and rate of activation. Euthyroid newborns were relatively 'hyperthyroid' compared to euthyroid adults, and this difference in the thyroid state corresponded to differences in the rate of activation, time to peak tension and twitch duration.