Expression of Genes Involved in Energy Metabolism and Transport of Volatile Fatty Acids and Urea in Rumen Epithelium of Bulls Identified for High, Medium, and Low Residual Feed Intake

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 high (H-RFI), medium (M-RFI), and low (L-RFI) residual feed intake. Twenty-seven Nellore bulls (BW = 401 ± 42 kg) were identified for L-RFI (n = 9), M-RFI (n = 9), and H-RFI (n = 9) and finished in collective pens. 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. After 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. After 48 hours, rumen epithelium samples were placed at -80°C until total RNA isolation and qPCR analysis. Data were analyzed using SAS 9.4 with α = 0.05. Concentrations of total VFA, acetate, propionate, butyrate, valerate, iso-butyrate, iso-valerate, and acetate: propionate did not differ (P > 0.05) among treatments. The mRNA abundance of UQCR10 (P < 0.01) and NDUFB4 (trend; P = 0.07) were greater for H-RFI animals (Table 1). The mRNA abundance of genes associated with ion pumping, protein turnover, heat production, butyrate metabolism, and VFA and urea transport did not differ (P > 0.05) among treatments. These findings may indicate that L-RFI animals have lower energy expenditure in this tissue, which likely contribute for their better efficiency compared to the M-RFI and H-RFI bulls.

Table 1. Relative mRNA abundance (Arbitrary units) of target genes

Item	H-RFI	M-RFI	L-RFI	SEM	P-value
Oxidative phosphorylation
NDUFB4	1.65	1.18	1.42	0.16	0.07
UQCR10	1.54a	0.99b	1.14b	0.10	<0.01
Ion pumping
ATP1B1	1.66	1.70	1.88	0.22	0.76
ATP1B2	1.94	1.57	1.92	0.23	0.45
ATP1B3	1.84	1.55	1.82	0.21	0.56
Protein turnover
IGF1R	1.81	1.67	1.88	0.18	0.70
MKI67	1.91	1.97	2.29	0.31	0.67
UBA1	1.78	1.58	1.78	0.18	0.67
Heat production
UCP2	1.53	1.16	1.47	0.13	0.12
Butyrate metabolism
BDH1	1.81	1.61	1.57	0.17	0.55
HMGCL	1.43	1.34	1.52	0.13	0.65
VFA transport
DRA	1.82	1.53	2.06	0.23	0.28
MCT1	2.42	2.02	2.26	0.29	0.62
NHE2	1.27	1.21	1.51	0.15	0.32
Urea transport
AQP10	1.88	1.49	1.56	0.20	0.36
SLC14A1	1.74	1.46	1.86	0.22	0.42

^a,bWithin a row, different subscripts differ at P < 0.05.