Thyroid status influences in vitro thyrotropin and growth hormone responses to thyrotropin‐releasing hormone by pituitary glands of hatchling slider turtles (Pseudemys scripta elegans)

Abstract

AbstractThe effects of thyroidal feedback on thyrotropin (TSH), growth hormone (GH) and prolactin (PRL) secretion by pituitary glands, and on TSH responsiveness of thyroids in hatchling turtles, Pseudemys scripta elegans, were investigated by in vitro techniques. Pituitaries were stimulated in culture with 0, 1, 10, or 100 ng/ml thyrotropin‐releasing hormone (TRH) at intervals up to 36 hr; hormones were estimated by RIA. TSH release was elevated by all doses of TRH during the first 2 hr of stimulation (1.8 to 3.6‐fold) and secretion rates increased further during the following 4 hr, with a dose response between 1 and 10 ng/ml TRH. TSH release then returned to initial basal levels and showed the same response to TRH on the second day. GH secretion was also elevated by all doses of TRH tested. A progressive increase in basal GH secretion over time in vitro suggested that GH might normally be under inhibitory control. Medium concentrations of PRL were nondetectable at all times and thus were not measured in subsequent experiments. In vitro thyroxine (T4) secretion by thyroids from these same turtles showed a dose response when stimulated with 5–20 ng/ml purified sea turtle TSH for 8 or 20 hr.Treatment of turtles in vivo with 0.5 μg T4 at 2 hr or daily for up to 2 wk prior to sacrifice greatly reduced in vitro basal TSH secretion and the response to 10 ng/ml TRH. In contrast, several of the T4treatments significantly elevated basal GH secretion. Plasma TSH was markedly elevated after 4 wk treatment with a goitrogen, methimazole, and this was accompanied by increased basal TSH secretion and responsiveness to TRH by pituitaries in vitro; basal GH secretion was reduced but the response of GH to TRH was unaffected. The response of in vitro thyroidal T4 output to TSH was augmented in hyperthyroid animals (⩾ 1 wk T4) and was abolished by methimazole treatment. These data demonstrate a clear thyroidal feedback in the regulation of chelonian TSH secretion and are consistent with an involvement of TRH in the control of pituitary TSH and GH. While the site(s) of the inhibitory actions of T4 are unknown, changes in the functional properties of the pituitary thyrotropes are involved; the somatotropes appear to be less dependent on T4.