Effect of nutrient intake on the development of the somatotropic axis and its responsiveness to GH in Holstein bull calves

We investigated the effect of increasing nutrient intake on the responsiveness of the GH/IGF-I system in calves fed a high-protein milk replacer. Fifty-four Holstein bull calves were fed one of three levels (low, medium, and high; n = 18 per treatment) of a 30% crude protein, 20% fat milk replacer to achieve target rates of gain of 0.50, 0.95, or 1.40 kg/d, respectively, for low, medium, and high. Six calves per treatment were slaughtered at approximately 65, 85, and 105 kg BW. Additionally, six calves were slaughtered at 1 d of age to provide baseline data. Plasma aliquots from blood samples collected weekly were analyzed for IGF-I, insulin, glucose, NEFA, and plasma urea nitrogen (PUN). Plasma IGF-I and insulin, measured weekly, increased (P < 0.001) with greater nutrient intake from wk 2 of life to slaughter. Plasma glucose and NEFA also increased (P < 0.05) with nutrient intake. In addition, each calf underwent a GH challenge beginning 4 d before the scheduled slaughter. Plasma from blood collected before the first GH injection and 14 and 24 h after the third injection was analyzed for IGF-I and PUN. Response to challenge, calculated as the absolute difference between the prechallenge and 14-h postchallenge plasma IGF-I concentrations, was significant in calves on all three treatments. Plasma urea nitrogen was not different among treatments as measured weekly but decreased (P < 0.001) following GH challenge in all calves. Results of ribonuclease protection assays showed increased expression of hepatic mRNA for GH receptor 1A and IGF-I with increased intake. The amounts of GH receptor and IGF-I mRNA in muscle and adipose, however, were not affected by intake. In summary, plasma IGF-I was elevated in calves with increased nutrient intake, and the elevations in plasma IGF-I following short-term administration of GH were significant in all calves by 65 kg BW. Data demonstrate that in well-managed milk-fed calves the somatotropic (GH/IGF-I) axis is functionally coordinated and sensitive to nutrient intake and GH.