PLASMA CONCENTRATION OF THYROTROPIN-RELEASING-HORMONE, THYROTROPIN, PROLACTIN, AND GROWTH-HORMONE DURING 5-DAY OSMOTIC PUMP INFUSION OF THYROTROPIN-RELEASING-HORMONE

Pituitary TSH, PRL, and GH responses to 5-day infusion of TRH using Alzet osmotic minipumps (Alza Corp.) were investigated in two trials to examine comparative hormonal response to continuous stimulation. In each trial, eight (four control and four treatment) ovariectomized ewes were used. The TRH dose infused was 500 μg/day (trial 1) and 100 μg/day (trial 2). Infusion of TRH with osmotic pumps increased mean plasma TRH to 637.8 pg/ml in comparison to 35.2 pg/ml in the untreated control animals in trial 1 and 60.0 vs 9.4 pg/ml in trial 2. On day 1 of trial 1, there was an initial stimulation of mean TSH levels to 12.2 ng/ml, which declined to 4.5 ng/ml during the collection period. A mean concentration of 7.9 ng/ml was observed on day 3. The TSH level on day 5 was 13.4 ng/ml, as compared to 1.6 ng/ml in the control animals throughout trial 1. In trial 2, a similar pattern of TSH secretion was observed. The concentration on day 1 declined from 10.2 to 6.1 ng/ml. The mean concentration on days 3 and 5 ere 9.7 and 7.8 ng/ml, respectively, compared to a mean level of 4.3 ng/ml in the control animals. A significant PRL response to exogenous TRH was observed only in trial 1. On day 1 in this study, PRL response declined from an initial peak of 100.9 to 43.7 ng/ml throughout the collection period. There was no renewal of PRL response on days 3 and 5 as was observed with TSH. No response was observed in GH secretory patterns in trial 1 with the largest dose of TRH; therefore, GH was not measured in the second trial. Infused TRH may stimulate an initial release of preformed stores of TSH, while elevated plasma levels seen on days 3 and 5 of infusion may reflect increased synthesis and release of TSH. Infusion of TRH in a sufficient dose results in an initial stimulation of release of pituitary pools of PRL but does not seem to stimulate synthesis of immunoreactive PRL for those pools. The inability to stimulate plasma PRL with a dose of TRH which clearly elevated TSH levels implies endogenous TRH does not serve as the PRL-releasing factor. © 1979 by The Endocrine Society.