Association of body weight gain with muscle, fat, and liver expression levels of growth hormone receptor, insulin-like growth factor I, and beta-adrenergic receptor mRNAs in steers.

The physiological basis of feed efficiency is unclear. Administration of GH or beta-adrenergic agonists improves feed efficiency in various animals. The objective of this study was to test the hypothesis that more efficient cattle have greater expression of GH receptor (GHR) or beta-adrenergic receptor (ADRB) mRNA in skeletal muscle, fat, and liver, the major target tissues of GH and beta-adrenergic agonists. Fifty Angus steers were fed a finishing diet for 75 d to determine residual feed intake (RFI). Carcass measures, skeletal muscle, subcutaneous fat, and liver samples were collected from the top 10 high-RFI steers and top 10 low-RFI steers at slaughter. Abundances of GHR, insulin-like growth factor I (IGF1), IGF1 receptor (IGF1R), beta-1 adrenergic receptor (ADRB1), ADRB2, and ADRB3 mRNAs were quantified by real-time reverse transcription-PCR. Low-RFI steers consumed 11% less dry matter intake than high-RFI steers (P = 0.004). Low- and high-RFI steers, however, did not differ in ADG or other growth or carcass measures. Low-RFI steers had a tendency to have smaller birth weights than high-RFI steers (P = 0.089). The expression levels of GHR, IGF1, IGF1R, ADRB1, ADRB2, and ADRB3 mRNAs in muscle, fat, and liver were neither different (P > 0.1) between high- and low-RFI steers nor correlated (P > 0.1) with RFI. These results do not support our original hypothesis. However, the expression levels of GHR, IGF1, and IGF1R mRNAs in muscle and fat were positively correlated with ADG (r = 0.52 to 0.65, P = 0.002 to 0.02), whereas the expression levels of GHR mRNA (r = -0.50, P = 0.03) and IGF1 mRNA (r = -0.47, P = 0.04) in the liver were negatively correlated with ADG. These results suggest that the GHR, IGF1, and IGF1R mRNA expression levels in the muscle and fat have a positive effect, whereas the GHR and IGF1 mRNA expression levels in the liver have a negative effect on postweaning body weight gain in cattle.