Glucocorticoid regulation of muscle branched-chain amino acid metabolism.

Branched-chain alpha-keto acid dehydrogenase (BCKAD) is a multisubunit complex regulated by phosphorylation and is considered to be rate-limiting for branched-chain amino acid (BCAA) metabolism in skeletal muscle. Glucocorticoids increase net protein degradation in muscle; associated with this increased breakdown of muscle protein is an elevated rate of BCAA oxidation. The effects of glucocorticoids on skeletal muscle BCKAD were investigated in different rat models. BCKAD was activated after glucocorticoid treatment (both acutely, within 2 h, and chronically). The amount of enzyme per muscle cell increased after 5 d of cortisone acetate treatment. Insulin administration partially blocked the acute effects of glucocorticoids on muscle BCKAD. Activation was also observed during metabolic acidosis, insulinopenic diabetes mellitus, and endotoxic shock, three conditions characterized by elevated circulating glucocorticoids, increased BCAA oxidation, and increased net protein breakdown. Activation of BCKAD may account for the increased oxidation of BCAA observed during hypercortisolemia. The sequelae of this accelerated catabolism may include increased glutamine and alanine production for gluconeogenesis and provision of ATP for muscle work.