Thyroxine-induced changes in metabolic rate and cytochrome oxidase activity in Thamnophis sirtalis: effects of...

The effects of nutritional status on thyroxine-induced changes in standard metabolic rate (SMR), and the activity of hepatic cytochrome oxidase were examined in the garter snake Thamnophis sirtalis. Twelve snakes were fed ad libitum, and 12 more were fed a maintenance diet, which was half as many fish per gram of body weight as that eaten by ad libitum snakes. Snakes in the first group gained weight during the 3-week treatment, while individual snakes in the second group either maintained their original weight or showed a slight loss (less than 10%). Within each diet treatment, half of the snakes received a 5-mg thyroxine (T4) pellet implant, and half received placebo implants. Plasma [T4] was unchanged by treatment. Plasma [T3] was elevated in T4-supplemented snakes fed ad libitum, but the difference was not statistically significant (P = 0.09). Standard metabolic rate and cytochrome oxidase activity at 25 degrees were increased significantly (34 and 24%, respectively) only in the T4-supplemented snakes on the ad libitum diet. Thus, T. sirtalis must be in a positive energy balance for thyroid hormones to have an effect on SMR or hepatic cytochrome oxidase activity.