Thyroid regulation of resting metabolic rate and intermediary metabolic enzymes in a lizard (Sceloporus occidentalis).

This study investigates the effects of physiological increments in plasma thyroxine (T4) at three levels of biological organization in thyroid-intact and thyroidectomized captive western fence lizards, Sceloporus occidentalis. Two doses of T4-loaded pellets elevated plasma T4 in thyroid-intact lizards from 4.8 +/- 0.47 to 10.7 +/- 2.25 and 20.4 +/- 5.77 ng/ml (mean +/- SE). Surgical thyroidectomy reduced T4 to 1.8 +/- 0.23 ng/ml, and subsequent T4 pellet implantation raised T4 to 14.8 +/- 4.30 ng/ml. Minimal resting metabolic rate (= standard metabolic rate; SMR), a common organismal metric of thyroid perturbation, was reduced 31% (P less than 0.0001) by thyroidectomy and was restored by T4 replacement but was not stimulated by T4 supplementation in thyroid-intact lizards. In T4-replaced, thyroidectomized lizards, SMR was significantly correlated with plasma T4 (r2 = 0.626, P = 0.003, n = 11). At the organ level, liver mass was not changed by any treatment; heart mass was decreased by thyroid deficiency and restored by T4 replacement. At the molecular level, citrate synthase activity was significantly reduced by thyroidectomy and was returned to control levels by T4 replacement in liver and skeletal muscle (gastrocnemius) but was not changed in cardiac muscle. Citrate synthase was not affected in any tissue by T4 supplementation in thyroid-intact lizards. Pyruvate kinase activity was not affected by any of the treatments in any of the tissues. Cytosolic alpha-glycerophosphate dehydrogenase was significantly reduced in liver by all treatments and in skeletal muscle by T4 replacement after thyroidectomy. These results indicate that SMR and cardiac muscle mass in lizards are dependent on normal thyroid function and are expressed maximally in euthyroid animals. The stimulatory effect of T4 on SMR in thyroid-intact lizards, which has been reported previously by several investigators, is a nonphysiological response to pharmacological T4 levels, at least in these captive lizards. Molecular responses are tissue and enzyme dependent and cannot be generalized. Pellet implantation is an effective means of inducing physiological increments in plasma T4 and should replace previously used injection protocols. This new method can be used in capture-recapture experiments involving field-active lizards.