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Down-Regulation of Prolactin Secretion in Men during Continuous Thyrotropin-Releasing Hormone Infusion: Evidence for Induction of Pituitary Desensitization by Continuous TRH Administration

Endocrinology Unit, Neuroendocrinology Section, Department of Internal Medicine and Department of Pharmacology, Catania Medical School, University of Catania, Catania, Italy

Address correspondence and requests for reprints to: Dr. R. D'Agata, Unitá Endocrina, Istituto di Patologia Medica I, Ospedale Garibaldi, 95124 Catania, Italy.

TRH was administered as a 5-h constant rate iv infusion (5 µg/min) to seven healthy adult men. Serum samples were collected at regular intervals for measurement of PRL, TSH, and T₃.

Serum levels of PRL during TRH infusion increased sharply to maximum level by 40 min, and then, despite continued TRH stimulation, PRL levels declined gradually to a plateau value after 100 min. No further rise in serum PRL was observed when a bolus of 200 µg TRH was administered to three subjects after 240 min ofinfusion. Conversely, an iv bolus of sulpiride (25 mg), a dopaminergic antagonist, given to four subjects after 240 min, brought about a marked increase in serum PRL values abovethe plateau level.

These results are consistent with the interpretation that down-regulation in PRL secretion which follows the initial peak of response most likely represents pituitary desensitization to TRH.

During the infusion serum TSH increases in two phases. A first phase of secretion was observed by 40 min followed by a plateau, with a second phase of increase occurring between 80–180 min. (J Clin Endocrinol Metab 56: 904, 1983)

Received September 20, 1982.

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