Glucocorticoid effects on peptide-chain initiation in skeletal muscle and heart

Treatment of normal rats with cortisone acetate for 5 days reduced the rate of synthesis of skeletal muscle protein 55% as measured in the perfused rat hemicorpus. Loss of tissue RNA accounted for 15% of this decrease; the remaining reduction resulted from a block in peptide-chain initiation that developed in vivo. In contrast, protein synthesis in perfused heart muscle was not affected by steroid treatment, nor was tissue RNA content or peptide-chain initiation altered. eIF-2-like activity in postribosomal supernatants from psoas was reduced 35% by cortisone treatment. This difference was not abolished by overnight dialysis and was not due to accelerated rates of deacylation of Met-tRNA(f)(Met), or to changes in nucleotide levels during incubation. No difference in activity was observed in supernatants from hearts of normal and cortisone-treated animals. Changes in RNA content in skeletal muscle, but not in heart, of hormone-treated rats suggested a relationship between initiation factor activity and RNA content. In other experiments, ribosomal subunits accumulated in psoas muscles of rats treated 4 h with dexamethasone, whereas tissue RNA content and Met-tRNA(f)(Met) binding activity remained unchanged. These results suggested that adrenocortical steroids may reduce skeletal muscle protein synthesis in part by regulating the activity of factors involved in peptide initiation and that factor activity may be linked to tissue RNA content.