The Influence of Corticosterone and Glucagon on Metabolic Recovery from Exhaustive Exercise in the Desert IguanaDipsosaurus dorsalis

The skeletal muscles of ectothermic vertebrates possess an elevated glyconeogenic capacity that is responsible for a major portion of lactate removal and glycogen resynthesis following exercise. In lizards, changes in plasma hormone levels and the influence of differing hormone levels on muscle metabolism postexercise are poorly understood. We measured the effects of 5 min of exhaustive exercise on plasma levels of glucagon and corticosterone in the desert iguanaDipsosaurus dorsalis.We also determined the extent to which these hormones influence, or are influenced by, postexercise plasma lactate concentrations postexercise. Exercise resulted in the accumulation of 20 mMblood lactate, while plasma glucose levels remained stable throughout 90 min of recovery. Plasma glucagon was elevated sevenfold during 5 min of exercise and returned to resting levels within 45 min of recovery. Glucagon stimulated lactate incorporation into glycogen in isolated red muscle fiber bundles. Plasma corticosterone was also elevated to three times normal resting values, but only after 45 min of recovery. Blocking corticosterone elevation with metyrapone did not alter the kinetics of plasma lactate removal. In lizards, the dramatic rise in plasma glucagon occurs at the same time as previously reported elevated skeletal muscle glyconeogenesis and elevated glucagon stimulates lactate removalin vitro,strongly suggesting a role for glucagon in postexercise skeletal muscle metabolism.