Nellore bulls (Bos taurus indicus) with high residual feed intake have increased the expression of genes involved in oxidative phosphorylation in rumen epithelium

The objective of this study was to investigate the expression of genes involved in energy metabolism and volatile fatty acids (VFA) and urea transport in rumen epithelium of bulls with different feed efficiency based on residual feed intake (RFI). Twenty-seven Nellore bulls (age = 22.5 +/- 0.8 mo; initial body weight = 401 +/- 42 kg) were classified as either Low, Medium, and High (n = 9 per group) RFI and finished in individual pens. During the finishing phase animals were fed the same diet formulated to meet the requirements of 1.3 kg daily gain with a target finish weight of 550 kg. At the end of the trial, bulls were slaughtered and ruminal fluid was collected for further VFA analyses. Samples of rumen epithelium from ventral sac were rapidly excised, rinsed with phosphate buffered saline solution (pH = 7.04) and immediately immerged in RNA later solution within 2 mL tubes. After 48 h, rumen epithelium samples were placed at - 80 degrees C until total RNA isolation and qPCR analysis. Data were analyzed using SAS 9.4 with a = 0.05; trends were determined as 0.10 > P > 0.05. The concentrations of total VFA, acetate, propionate, butyrate, valerate, isobutyrate, isovalerate, and acetate: propionate ratio did not differ (P > 0.05) among RFI groups. The mRNA abundance of UQCR10 (P < 0.01) and NDUFB4 (trend; P = 0.07) were greatest for High RFI animals. However, the mRNA abundance of ATP5H, COX10, and SDHF2 did not differ (P > 0.05) among treatments. No differences (P > 0.05) were observed for mRNA abundance of genes associated with ion pumping (ATP1B1, ATP1B2, ATP1B3), protein turnover (IGF1R, MKI67, UBA1), heat production (UCP2), butyrate metabolism (BDH1, BDH2, HMGCL, HMGS1), and VFA (CA7, DRA, NHE2, MCT1, and PAT1) and urea transport (AQP3, AQP7, AQP8, AQP10, and SLC14A1) in rumen epithelium of bulls classified as High, Medium, and Low RFI. These findings may indicate that more efficient animals (Low and Medium RFI) have lower energy expenditure in this tissue, which likely contribute for their better efficiency compared to the less efficient (High RFI) bulls.