Release of Thyrotropin by the Rat Pituitary Glandin Vitro

The release of thyrotropin in vitro by the rat pituitary gland was studied. Significant quantities were released from single glands for up to 5 consecutive 1-hr periods, the rate of release varying with the preceding state of thyroid-pituitary interaction. When glands were obtained from rats given 0.066 % thyroid USP in the diet, the rate was low, in association with a low level of thyrotropin in the serum and in the pituitary. The thyrotropin content of pituitaries from rats given 0.1% propylthiouracil in the diet was similar to that of control rats fed Remington-type diet, but the release rate was low, despite high concentrations in the serum. Hypothalamic extract (2 hypothalamic equivalents/ pituitary) stimulated the release of thyrotropin. An apparent seasonal variation in the release rates, unrelated to recognized environmental conditions, was noted. Approximately the same quantity of thyrotropin was released per incubation period, whether this lasted 15 min or 1 hr, and, as a result, a local negative feedback influence of thyrotropin on the pituitary was postulated. Several hypotheses are advanced to explain the unexpected findings in pituitaries from propylthiouracil-treated rats. (Endocrinology78: 699, 1966)