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Bovine Serum Hormone Concentrations after Thyroprotein and Thyrotropin Releasing Hormone¹

Michigan State University, East Lansing 48824 and University of Idaho, Moscow 83843

Abstract

Twenty-four Holstein heifers were assigned randomly to one of four treatment groups to be given: 1) thyroprotein (TP) for 6 days; 2) casein for 6 days; 3) TP for 27 days; or 4) casein for 27 days. Thyrotropin releasing hormone (TRH, 33.3 µg/100 kg BW) was given at 1030 and 1330 hr on day 6 or 33 to heifers treated for 6 or 27 days, respectively.

Prior to TRH treatment, serum thyroxine (T₄) averaged 133.8 and 82.3 ng/ml in heifers given TP or casein for 6 days and 24.0 and 82.2 ng/ml six days after feeding TP or casein for 27 days. Six hours following TRH, average serum T₄ concentration had increased (35 ng/ml) in heifers given casein for 6 or 27 days, and 4.8 ng/ml in heifers given TP for 27 days, but remained unchanged in heifers fed TP for 6 days.

Basal serum thyrotropin (TSH), growth hormone (GH) and prolactin (PRL) concentrations were not affected by TP treatments, averaging 6.7 ng TSH/ml, 13.8 ng GH/ml, and 7.0 ng PRL/ml. Following TRH injection, TSH concentrations were increased to 26 ng/ml in heifers given casein for 6 or 27 days and to 20 ng/ml in heifers given thyroprotein for 27 days. However, TSH was unchanged after TRH given on the 6th day of TP feeding. Serum GH and PRL concentrations (ng/ml) were increased by TRH to 36 and 100, respectively, and responses were independent of TP treatments.

Magnitude of GH and PRL increases after the first TRH injection were greater than after the second. In addition, increase in serum PRL concentration after TRH was greater in heavier-older than in lighter-younger heifers.

We conclude that 1) changing serum T₄ concentration does not alter basal serum TSH, GH or PRL concentrations; 2) increased T₄ in serum inhibits TSH release by TRH but under our experimental conditions low serum T₄ concentrations did not influence TSH release by TRH; 3) serum T₄ concentrations do not alter the magnitude of increase in serum GH or PRL following TRH; 4) there is a reduction in the magnitude of increase in GH and PRL concentration following a second TRH challenge as compared with the first which is independent of serum T₄ concentrations; and 5) PRL release by TRH is greater in heavier-older than in lighter-younger heifers.

¹ Published with the approval of the Michigan Experimental Station as Journal Article No. 7572. This research was supported in part by NIH Grant No. AM 15899.

² Department of Dairy Science, Michigan State University.

³ Department of Animal Industries, University of Idaho.

⁴ The authors would like to thank Eileen Bostwick and Tammy Ager for their technical assistance, Dr. Roger Neitzel for statistical advice, and Sue Post for secretarial assistance.