1. Tissue composition and skeletal muscle cathepsin D (EC 188.8.131.52) activity weremeasured in wether lambs treated with trenbolone acetate (TBA) and oestradiol-17β (Oe) in combination and female lambs treated with TBA or zeranol. Muscle and liver protein fractional synthesis rates and plasma leucine flux were measured in the female lambs.
2. Male castrate lambs treated with TBA plus Oe showed increased growth rate, improvedfood conversion efficiency, decreased muscle RNA concentration and decreased total cathepsin D activity in muscle.
3. Female lambs treated with TBA or zeranol showed increased weight gain, improved food conversion efficiency, decreased muscle RNA and DNA concentrations and decreased free cathepsin D activity in muscle. Mixed muscle protein fractional synthesis rate was decreasedafter TBA treatment. Plasma leucine flux, not corrected for oxidation or food intake, wasnot increased by TBA or zeranol treatment.
4. Treatment of female lambs with TBA or zeranol caused increased growth rate. This increased growth rate is probably due in part to decreased muscle protein degradation, since evidence was obtained that muscle protein synthesis is decreased by TBA and zeranol treatment.