DEVELOPMENT 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 contr actile properties of the heart as well as the expression of the alpha and beta heavy chains of myosin. We have previously shown that the pla sma levels of thyroid hormone reach a peak immediately after birth in guinea pigs and decline with maturation. We therefore studied age-rela ted changes in the expression of the myosin heavy chains in the guinea pig ventricle in relation to the ventricular mechanical properties an d the levels of thyroid hormone. The composition of the myosin heavy c hains was characterized by gel electrophoresis and immunoblotting. Ant i-beta-chain antibody stained equally myosins from newborns (0-5 days) and adults (75-90 days), while anti-alpha-chain positively decorated only the myosins of euthyroid newborns or of hyperthyroid adults, but not myosins of embryos, hypothyroid newborns or hypothyroid adults. My osin of euthyroid adults was faintly stained by anti-alpha-chain. The alterations in the composition of myosin corresponded with the ''thyro id 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 eu thyroid hypothyroid and hyperthyroid adults, respectively. In euthyroi d 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 thyr oid state corresponded with maximal twitch tension (T(max)), rates of development of tension and relaxation, time to peak tension and rate o f activation. Euthyroid newborns were relatively ''hyperthyroid'' comp ared to euthyroid adults, and this difference in the thyroid state cor responded to differences in the rate of activation, time to peak tensi on and twitch duration. p < 0.01 compared with euthyroid.