Increased protein synthesis and degradation in the dog heart during thyroxine administration.

Mongrel dogs were injected with 1-thyroxine (0.5 mg/kg body weight) for 7 or 14 days and fractional synthesis rates of total, sarcoplasmic and myofibrillar protein were measured in the left ventricle using continuous intravenous infusion of [14C]tyrosine. The synthesis rate of total protein was increased from 7.2% to 9.2% per day and 10.9% per day after 7 and 14 days respectively. The normally lower synthesis rate of myofibrillar protein was stimulated proportionately more than that of sarcoplasmic protein, increasing from 6.0% to 11.7% per day over 14 days, while that of sarcoplasmic protein increased from 10.3% to 13.1% per day. There was a significant increase in left ventricular RNA concentration and decrease in collagen, concentration. Left ventricular weight/body weight was 17% greater than normal after 7 days but the lack of further change in the second week suggests that, after the early development of hypertrophy, protein degradation was also increased, along with the markedly increased synthesis rate.