Expression of key genes of the somatotropic axis in longissimus dorsi muscle of beef heifers phenotypically divergent for residual feed intake.

This study evaluated the effect of residual feed intake (RFI) on the expression of constituent genes of the somatotropic axis in skeletal muscle across 2 diverse dietary regimes. Beef heifers (n=86; initial BW=191.8 kg; SD=37) fed a low-forage (LF) total mixed ration comprising 70:30 concentrate:corn silage (11.0 MJ ME/kg DM) were ranked on RFI. The 10 greatest- (high-RFI) and 10 lowest- (low-RFI) ranking animals were selected for the current study. Biopsies of the LM were harvested at the end of LF dietary period and again after a 6-wk period during which heifers were offered a high-forage grass-silage-only diet (9.7 MJ ME/kg DM). Real-time PCR was used to quantify mRNA transcripts of 11 genes including IGF-1, IGF-2, their receptors (IGF-1R and IGF-2R), 6 IGFBP, and GH receptor (GHR). There was no evidence of a RFI phenotype×diet interaction (P>0.10) for any of the genes examined. An effect (P=0.02) of RFI phenotype was evident for the abundance of GHR mRNA, with twofold greater expression detected in the low-RFI compared with the high-RFI phenotype. Additionally, mRNA expression of IGF-1R was upregulated (1.7-fold; P=0.04) for the low-RFI compared with high-RFI heifers. Residual feed intake was negatively associated with IGF-1R (r=-0.41; P<0.05) and GHR (r=-0.50; P<0.05) mRNA. Moderate negative correlation coefficients were also observed between feed conversion ratio (F:G) and gene expression levels for IGF-1R (r=-0.61; P<0.01) as well as for GHR (r=-0.49; P<0.05). Moreover, associations were detected between DMI with IGF-1R (r=-0.42; P=0.07) and IGF-2R (r=0.40; P=0.07). The IGF-1R mRNA was positively correlated with IGF-1 (r=0.34; P<0.05) and IGF-2 (r=0.71; P<0.001) mRNA. Associations between IGF-1R and IGF-2R with IGFBP5 and GHR were positive (ranging from, r=0.32 to 0.49). These data suggest that components of the somatotropic axis expressed within muscle tissue may be involved in the regulation of feed efficiency in beef cattle. This effect is apparently consistent across contrasting dietary regimens.