Comparison of the effects of thyroxine and triiodothyronine on protein turnover and apoptosis in primary chick muscle cell cultures

Primary chick muscle cells were treated with physiological level of thyroxine (T-4) or triiodothyronine (T-3) to examine the effects of the hormones on growth, protein turnover, and apoptosis of the cells. Creatine kinase activity, as an index of differentiation, was increased by both T-4 and T-3. Even when the conversion from T-4 to T-3 was blocked by iopanoic acid, T-4 increased creatine kinase activity. The rate of protein degradation estimated from [H-3] tyrosine release was increased by T-3 but not by T-4. DNA cleavage and fragmentation, as indices of apoptosis, were induced by T-3 but not by T-4. These results show that T-4 stimulates cell differentiation but not protein degradation and apoptosis in primary chick muscle cells, while all events are stimulated by T-3. (C) 1998 Academic Press.