Protein synthesis in the early stages of cardiac hypertrophy

Cardiac hypertrophy, induced in rats by either tri-iodothyronine or isoproterenol, administered daily for 7 days, was monitored using several parameters. Both treatments increased RNA concentrations 24 hr after the first injection, while heart weight increased following 2 injections to 46% above control after 7 days. Cardiac protein synthetic activity, as determined by the rate of peptidyl-puromycin formation, was increased by both tri-iodothyronine and isoproterenol 24 hr after a single injection, implying an increase in the number of functional ribosomes. RNA activity (the rate of peptidyl-puromycin formation per unit RNA) remained constant, suggesting that neither accelerated rates of initiation or translation nor increased activation of pre-existing, non-translating ribosomes was involved in the observed increase in protein synthetic activity. In contrast, constant infusion of [14C] tyrosine indicated no change in protein synthetic rate 24 hr after a single tri-iodothyronine injection and decreased protein synthetic rate after isoproterenol injection. It is concluded that the use of [3H]puromycin to estimate protein synthetic activity may be a more sensitive procedure for detecting early changes in protein synthesis in cardiac hypertrophy than constant isotope infusion, owing to the problems associated with determining the precise precursor pool for protein synthesis in this latter method.