Effect of thyroid hormone on protein turnover in cultured cardiac myocytes

Since systemic actions of thyroid hormone increase cardiac work, direct effects of T3 on myocardial protein turnover may be obscured in the intact animal. For this reason, the effects of T3 on synthesis and degradation of cellular protein were measured in replicate cultures of cardiac myocytes obtained from chick embryos. During the first 3 days of exposure, 10(-8) M T3 increased the fractional rate of protein synthesis 10% to 16% and the fractional rate of cell growth 20% to 40% with no change in protein degradation. During the fourth and fifth days of 10(-8) M T3 exposure, fractional synthesis rates in T3 cultures increased 15% to 19% but fractional degradation rates also increased 17% to 29% so that growth rates in T3 cultures fell to control levels. Similar changes in myocardial protein turnover have occurred in response to T3 treatment in intact animals. T3 treatment caused a disproportionately large increase in the rate of myosin heavy chain turnover when compared to total cellular protein and actin. This may be related to the change in amounts of myocardial isomyosins occurring in response to thyroid hormone treatment.