AJP - Regulatory, Integrative and Comparative Physiology, Vol 244, Issue 5 659-R666, Copyright © 1983 by American Physiological Society
Effects of thyroxine supplementation on metabolic rate and aerobic capacity in a lizard
H. B. John-Alder
Effects of thyroxine (T4) on standard and maximal rates of O2 consumption
and on citrate synthase activity, an index of mitochondrial oxidative
capacity, were examined in the lizard Dipsosaurus dorsalis. Two weeks of
daily injections of 200 ng T4/g body mass resulted in significant increases
in standard (+60%) and maximal (+15%) rates of O2 consumption. Citrate
synthase activity in liver and in red iliofibularis and gastrocnemius
muscles was significantly higher than in controls. Correlational analyses
indicate that intraspecific differences in standard metabolic rate may be
partially attributable to fundamental differences in tissue metabolic
capacities and that thyroxine injections lead to a general increase in
aerobic capacities. Similar increases in aerobic capacities may occur in
response to high thyroid activity during Spring in temperate-zone lizards.
The injection protocol used in this study, taken from previous authors,
results in suppression of natural thyroid secretory activity and in a 24-h
average plasma T4 level of 95 ng/ml compared with 2.5 ng/ml in controls.
