Influence of terbutaline on TRH-induced prolactin and thyrotropin release in normal subjects

To investigate whether terbutaline (T) influences the release of prolactin (Prl) and/or thyrotropin (TSH) from the anterior pituitary, 25 micrograms thyrotropin-releasing hormone (TRH) was injected iv in 7 normal subjects who were pre-treated orally with either T or placebo. The TRH-induced Prl response, as reflected by the Prl incremental area, was more pronounced after priming with placebo (2071 +/- 606) than after T (1391 +/- 434; P less than 0.05). In contrast, the TRH-elicited TSH responses did not differ significantly after the two pre-treatments. When TRH was given to 8 additional individuals on iv background infusion of either T or saline, the Prl response was significantly larger during saline (2123 +/- 354) than during T infusion (1540 +/- 235; P less than 0.01), whereas the TSH responses were of similar magnitudes. Six subjects, given 25 micrograms TRH iv on background infusion of T or saline, were also given propranolol orally before commencement of the T infusion and placebo before the saline infusion. This resulted in similar Prl responses and also in similar TSH responses, during the two background treatments. The results imply that oral as well as iv administration of T has inhibitory influence on human lactotrophs, but lacks measurable effect on thyrotrophs.