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Serum Prolactin and Growth Hormone Response to Thyrotropin Releasing Hormone in Prepubertal Heifers^{1, 2,}

Michigan State University³, East Lansing 48824

Abstract

Prepubertal heifers were used to examine repeatability 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 repeatability of baseline concentrations of PRL and GH (within heifer among 4 days of sampling) were .27 and .37, respectively, whereas repeatability estimates for serum hormone response areas following 10 µg TRH were .61 for PRL and .35 for GH.

The quantity of serum PRL and GH released in six heifers, as measured by area under the response curve during a 30-min interval, increased linearly (P<.01) with increases in log of the dose (5 to 100 µg) of TRH administered IV. Peak serum PRL concentrations after IV, intramuscular (IM) or subcutaneous (SC) injections of 25 µg 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 µg TRH were five, three and one-half and two times greater, respectively, than concentrations measured prior to TRH injections. These peaks were attained at 12, 14 and 22 min, respectively. In another experiment with four heifers, areas under serum PRL or GH response curves increased similarly during a 120-min sampling period after IV or SC administration of 50 µg TRH. Thus, route of administration of TRH affected peak heights and rate of increase in serum PRL and GH, but not total amount of PRL or GH released over a 120-min period.

¹ Published with the approval of the Director of the Agricultural Experiment Station as Journal Article No. 7317. This research was supported in part by U.S. Public Health Service Grant AM 15899.

² Programming and statistical advice of Dr. Roger Neitzel is appreciated. Technical assistance of Annemieke Ambrosier, William Smith and Sharon Fairchild is gratefully acknowledged.

³ Animal Reproduction Laboratory, Department of Dairy Science.