Serum prolactin and growth hormone response to thyrotropin releasing hormone in prepubertal heifers.

Prepubertal heifers were used to examine repeatabil i ty of prolactin (PRL) and growth hormone (GH) responses to thyrotropin releasing hormone (TRH) stimulation, and to determine effects of various doses and routes of administration of TRH on serum PRL and GH concentrations. Baseline concentrations of PRL and GH prior to intravenous (IV) injection of TRH in eight heifers were correlated (r = .64 and .60, respectively) with magnitude of. the post-TRH response. Correlation coefficients for repeatabil i ty of baseline concentrations of PRL and GH (within heifer among 4 days of sampling) were . 27 a n d . 37, respectively, whereas repeatabil i ty estimates for serum hormone response areas following 10/2g TRH were .61 for PRL and .35 for GH. The quant i ty of serum PRL and GH released in six heifers, as measured by area under the response curve during a 30-rain interval, increased linearly (P<.01) with increases in log of the dose (5 to 100 b/g) of TRH administered IV. Peak serum PRL concentrations after IV, intramuscular (IM) or subcutaneous (SC) injections of 25 /ag TRH into six heifers were eight, five and five times greater, respectively, than PRL concentrations before TRH. These peaks were attained at 9, 22 and 22 min, respectively. Maximal serum GH concentrations following IV, IM or SC administration of 25/2g TRH were five, three and one-half and two times greater, respectively, than concentrations measured prior to TRH injections. These peaks were