Influence of chronic thyrotropin-releasing hormone injections on secretion of prolactin, thyrotropin and growth hormone and on growth rate in wether lambs.

Abstract

In a series of experiments, wether lambs (beginning weight 20 to 30 kg) were chronically injected (subcutaneoulsy) with synthetic thyro-tropin-releasing hromone (TRH). The objectives were to examine pituitary prolactin (PRL), thyrotropin (TSH) and growth hormone (GH) secretion in response to chronic TRH stimulation and subsequent treatment effects on growth rate and feed efficiency. The injection of TRH twice daily for 2 weeks (50 µg/injection) increased (P<.01) plasma concentrations of PRL and TSH but not GH on days 1, 7 and 14 of treatment. There was no difference between days 1, 7 or 14 in TSH or PRL secretion in response to TRH stimulation. Treatment with TRH was associated with a nonsignificant increase in nitrogen retention. Once weekly injections of TRH (1 mg) for 9 to 12 weeks increased the concentration of PRL and TSH in plasma on days 1, 30 and 60 of treatment. Plasma GH concentrations were higher (P<.05) in TRH treated lambs on day 1 and 30, but not on day 60. Chronic TRH treatment was associated with average daily gains which were 1.3 to 18.4% above saline injected controls. These differences, however, were not significant. In the final experiment, TRH (1 mg once weekly) treated lambs grew at a faster rate (.10>P>.05), were on feed 6 days less (P<.05) than controls, and their carcasses contained less (P<.05) marbling suggesting that TRH treatment did produce a significant growth effect which was associated with less deposition of intramuscular fat. These results demonstrate that PRL and TSH secretions were chronically elevated during chronic TRH injections into wether lambs. Secretion of GH in response to TRH was variable. The data also suggest that, using appropriate dose and method of administration, TRH may be a potential growth stimulant in lambs.