A study of the thyroid-pituitary relationship in an elasmobranch, Dasyatis sabina

The pituitary-thyroid relationship has been studied in vitro in the stingray, Dasyatis sabina. The addition of mammalian TSH failed to stimulate incubated stingray thyroid glands, but mammalian TSH did stimulate teleost thyroids in parallel experiments. Homogenates of whole D. sabina pituitary significantly increased the release of thyroxine from the stingray thyroid, while homogenates of whole teleost pituitary had no effect. The greatest thyroid stimulating activity was found in the ventral lobe of the pars distalis. No seasonal or sexual differences were observed in the relationship between thyroid weight and animal weight. The ratio of total iodine to thyroxine iodine did not vary between sexes or between stimulated and unstimulated glands in vitro. It was concluded that the thyroid is associated with reproduction because (1) thyrotropic activity was located in the region known to contain gonadotrophic activity, and (2) the spontaneous rate of thyroxine release from incubated thyroid glands increased in females at the time of sexual maturation (release rates of juvenile males, adult males, and adult females were similar). There were no changes in the spontaneous rates of thyroxine release that could be correlated with seasonal changes in environmental salinity or temperature or with migratory movements