Reduced aerobic capacity and locomotory endurance in thyroid-deficient lizards

This study was undertaken to examine the effects of thyroid hormonal deficiency on (1) standard (SMR) and maximal (VO2max) rates of O2 consumption, (2) tissue glycolytic and oxidative capacities and (3) submaximal locomotory endurance in a lizard (Dipsosaurus dorsalis). Surgical thyroidectomy induced hypothyroidism in all animals as determined by levels of plasma thyroxine. Hypothyroid lizards had lower levels of SMR (-48%), VO2max (-16%) and citrate synthase activity in liver, heart and skeletal muscle compared to controls. There was a correlated decrease in locomotory endurance in thyroid-deficient animals. Pyruvate kinase activity (an index of glycolytic capacity) in all tissues, and myofibrillar ATPase activity (an index of contractile velocity) in white iliofibularis muscle, showed no significant changes in thyroid-deficient animals. Thyroid hormones appear to be important in ultimately establishing an animal's capacity for locomotory endurance. These findings suggest a new selective context for understanding the evolution of thyroid function.