A thyroid hormone antagonist has not been previously described. A number of thyroid hormone analogues have been shown to compete with ["25Iltriiodothyronine (1125I1T3) for bind-ing to the intranuclear thyroid hormone receptor and to have agonist activity proportional to their affinities for the receptors. We report that the benzofuran amiodarone acts as a competitive antagonist to thyroid hormone action as defined by its dose-dependent ability to (a) bind to the thyroid hormone receptor and (b) inhibit T3-induced increases in growth hormone mRNA levels in a cultured rat pituitary cell line, GC cells. Like T3 itself, amiodarone also decreases transport of 112511T3 across GC cell membranes. An analysis of the amiodarone structure suggests that this compound has certain similarities to T3. These findings hold promise for the development of other thyroid hormone antagonists for clinical use and for understanding thyroid hormone action.

Plasma corticosterone levels of female lizards (Anolis carolinensis) were sampled at 4‐h intervals before and after a photothermal regimen known to stimulate ovarian growth in January and February, respectively. Female A. carolinensis have a diurnal cycle in plasma corticosterone with a daily peak at 1700 h in lizards with unstimulated ovaries exposed to short photophase and low temperature (12L 20°C:12D 20°C). Stimulation of ovarian growth by increasing the temperature and lengthening the photophase (14L 32°C:10D 20°C; 80% relatively humidity) caused an 8‐hr shift in the phase of the corticosterone peak to 0900 h. In lizards exposed to stimulatory temperature and photoperiod but also to low humidity (<35%) corticosterone exhibited a biphasic rhythm, with peaks at both 0900 and 1700 h. These results suggest that conditions which stimulate reproductive activity also cause a phase shift in the daily rhythm of plasma corticosterone. The results also suggest that chronic low humidity during these stimulatory conditions creates a biphasic rhythm of plasma corticosterone in this species. Therefore, a natural condition, low humidity, may be a chronic stressor of the lizard Anolis carolinensis.

No abstract

The aim of this study was to determine the timing of adenohypophysial activation during metamorphosis of the tiger salamander, Ambystoma tigrinum. It consisted of two parts: 1) determination of plasma thyroid hormone concentrations and analysis of thyroid gland histology as a function of metamorphic stage and 2) analysis of the time-course of uptake of 125I by the thyroids during metamorphosis as an indicator of endogenous thyrotropin (TSH) levels. Significant increases in both triiodothyronine (T3) and thyroxine (T4) first were evident at the onset of metamorphic climax (stage II). Maximum levels of both hormones were not observed, however, until the completion of gill resorption (stage VII). No changes in thyroid histology were observed that could be unambiguously related to metamorphic transformation. The thyroids accumulated 125I in a slow but linear fashion in premetamorphic larvae (stage I). However, uptake exhibited a rapid peak during early climax (stage II), before maximum concentrations of thyroid hormones were observed. In addition, uptake was maintained above premetamorphic levels at stage VII, in conjunction with maximum levels of T4 and T3. Captivity alone produced a small but significant increase in plasma concentrations of T3. It produced no significant effect on either thyroid histology or uptake of 125I. These results indicate that adenohypophysial activation occurs rapidly and is maximal at the onset of metamorphic climax.

The aim of this study was to develop a set of practical metamorphic stages for the tiger salamander Ambystoma tigrinum that would 1) provide an early and quantifiable marker of the onset of metamorphosis and 2) not be dependent on knowledge of maximal values of gill length or tail height. Twenty morphological parameters were obtained from Ambystoma tigrinum larvae as they progressed through laboratory-induced metamorphosis (21 degrees C, continuous light). These parameters included measures of body weight, gill length, and tail height. Initiation or duration of metamorphosis could not be predicted when parameters were expressed relative to the number of days larvae were held in laboratory conditions. Early phases of metamorphosis were characterized by regression of the lower tail fin. Selected morphological parameters were expressed relative to the day on which each subject exhibited complete regression of the lower fin. This technique made it possible to order these parameters into a progressive series of metamorphic phases. Seven stages of metamorphosis were derived from this series, ranging from larvae (stage I) to complete regression of the gills (stage VII). Application of these stages does not require prior knowledge of maximal (initial) values of gill length or tail height.