Effects of glucocorticoids on peptide chain initiation in heart and skeletal muscle.

Treatment of rats for 5 days with cortisone acetate reduced synthesis of skeletal muscle protein by 56% compared to untreated controls, but had no effect on protein synthesis in heart. The reduction in synthesis in skeletal muscle was accounted for by both a loss of tissue RNA and development of a block in peptide-chain initiation. Activity of an eIF-2-like initiation factor decreased in psoas muscle of hormone-treated rats in proportion to the loss of RNA. Peptide-chain initiation, RNA content, and initiation factor activity were unaffected in heart muscle. In skeletal muscle of rats treated for 4 hr with dexamethasone, peptide-chain initiation was inhibited, whereas tissue RNA content and initiation factor activity were unchanged. These experiments suggested that total activity of eIF-2 did not always correlate with the rate of initiation, but that there did appear to be a relationship between initiation factor activity and tissue RNA content. Purification of eIF-2 from bovine heart muscle was undertaken in order to directly investigate the mechanism by which glucocorticoids modify eIF-2-activity and control peptide-chain initiation.