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Inhibition of Ovine Prolactin and Thyrotropin Responses to Thyrotropin-Releasing Hormone by Triiodothyronine¹

University of Idaho, Moscow 83843

Abstract

Studies were conducted with mature crossbred ewes to determine the influence of triiodothyronine (T₃) on the pituitary secretion of prolactin (PRL) and thyrotropin (TSH). Intravenous injection of T₃ (50 µg) 1 hr prior to thyrotropin releasing hormone (TRH) treatment resulted in plasma T₃ concentrations of 24 ng/ml as compared to 1.5 ng/ml in control animals. Plasma T₃ concentrations after intravenous injection of 50 µg decreased with a half life of 1 hr 39 minute. Acute T₃ treatment completely abolished (P<.01) the TSH secretion and reduced (P<.05) the PRL secretion in response to TRH. Plasma T₃ concentrations 6 hr after the final T₃ injection in chronically treated ewes (50 µg subcutaneously twice daily for 2 weeks) were 3 ng/ml as compared to 1.5 ng/ml in control ewes. This twofold elevation in plasma T₃ reduced (P<.05) both basal plasma TSH and TRH stimulation of TSH secretion. Neither basal nor TRH stimulated plasma PRL concentrations were influenced by the chronic T₃ treatment. Injection of TRH (50 µg) into control ewes resulted in a subsequent increase in endogenous plasma T₃ concentrations from 1.5 to 3.1 ng/ml. These results suggest that normal plasma concentrations of T₃ are probably not involved in physiologic regulation of PRL secretion. However, very high plasma concentrations of T₃ are capable of inhibiting the stimulation of PRL and TSH secretion by TRH, presumably at the level of the anterior pituitary.

¹ Idaho Agricultural Experiment Station Pub. No. 7543.

² The authors would like to acknowledge the technical assistance of Mr. Dan Ohlson. We are also grateful to Dr. J. G. Pierce for supplying highly purified bovine TSH for radioiodination, Dr. L. E. Richert, Jr. for purified PRL, and the National Institutes of Health for ovine PRL and TSH reference standards. Supported in part by a research grant from NIH (HD 07830) and the University of Idaho Agricultural Experiment Station. Synthetic TRH was provided by Dr. Ray Rippel, of Abbott Laboratories.