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Growth, Serum Growth Hormone, Thyroxine, Prolactin and Insulin in Calves after Thyrotropin Releasing Hormone or 3-Methyl-Thyrotropin Releasing Hormone¹

Michigan State University³, East Lansing 48824

Abstract

Holstein calves were given daily intramuscular injections (5.0 ml) of either: a) .85% NaCl (control); b) 500 µg thyrotropin releasing hormone (TRH); or c) 500 µg 3-methyl-thyrotropin releasing hormone (3-MET) beginning at 45 days of age and continuing for 90 days. At 30 to 45 days after injections commenced, calves receiving TRH began to gain more weight and consume more feed than calves receiving either saline or 3-MET. Weight gain (WG) and feed intake (FI) for the 90-day experimental period was 10% greater in TRH-injected calves than in saline-injected controls, however, feed efficiency was not affected. Injection of 3-MET did not alter weight gain or feed efficiency relative to controls. Serum collected just prior to and after injection on days 1, 15 and 60 was assayed for growth hormone (GH), prolactin (PRL), thyroxine (T₄) and insulin (INS). Both TRH and 3-MET, but not saline increased serum GH, PRL and thyroxine concentrations. Magnitude of increase in serum GH and PRL concentrations and persistency of response between days were greater for TRH than for 3-MET. The increase in serum thyroxine after TRH and 3-MET on day 15 and 60 was decreased 63 and 49%, and 51 and 77%, respectively, relative to comparable responses on day 1. Serum INS concentrations averaged for all days and times after injection were 1.5, 1.2 and 1.1 ng/ml (P<.10) for calves given TRH, 3-MET and saline, respectively. Serum INS concentration increased with age (P<.001) and time after feeding (P<.05). Average serum insulin concentration was increased by day 15 of treatment in calves receiving TRH and by day 60 of treatment in calves receiving 3-MET.

¹ Published with the approval of the Director of Agricultural Experiment Station as Journal Article No. 7546.

² Partially supported by NIH Training Grant GMO 1818 and by U.S. Public Health Service Grant No. AM-15899.

³ Department of Dairy Science.