Response of growth hormone to various doses of growth hormone releasing factor and thyrotropin releasing hormone administered separately and in combination to dairy calves

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Abstract
Doses of growth hormone releasing factor (GRF) and thyrotropin releasing hormone (TRH) and combinations of these were administered by intravenous injection to six calves aged 155 ± 3 days and weighing 136 ± 16 kg. Injections were at 09:00, 12:00 and 15:00 h on 4 days, and doses were 0, 15, 30 and 60 pmol GRF kg−1 and 0, 275, 550 and 1100 pmol TRH kg−1, with GRF plus TRH at all combinations of these doses. Response of serum growth hormone (GH) was measured as the mean at 5, 10, 15 and 20 min following injection (PEAK) and the area under the curve during 0–60 min (AUC). The correlation between PEAK and AUC was 0.98. The variation in PEAK was related to GH prior to injection and to PEAK 3 h earlier. Separate multiplicative effects for each secretagogue were fitted, with the effects related to the logarithm of dose. Doubling the dose increased PEAK by 1.46-fold following GRF (P < 0.05) and 1.25-fold following TRH (P < 0.05). There was no evidence that the results for either secretagogue were affected by the presence or absence of the other. This multiplicative model provides a description of the synergy between these secretagogues. Key words: GH-release, GRF, TRH, calves, dose response

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