Protein turnover in heart and diaphragm of rat fetus.

Fractional rate constants for protein synthesis (KS) in fetal rat heart and diaphragm were determined by the constant infusion technique. Uniformly labeled [14C]tyrosine was infused at a constant rate into the maternal jugular vein of unanesthetized unrestrained pregnant rats for 6-10 h at days 19 and 21 of gestation and KS calculated from the protein-bound to tissue-free tyrosine specific radioactivity ratio and the rate constant for attainment of plateau tyrosine specific radioactivity in fetal plasma (lambda p). Rate constants for protein degradation (KD) were calculated by subtracting fractional growth rate (KG) from KS. In both tissues, protein accretion during the most rapid phase of growth (day 19) was accomplished by greater rates of synthesis and lower rates of degradation when compared with a time of lesser growth (day 21).