Expression of cationic amino acid transporters mRNA in the jejunum of high and low efficiency steers

Abstract Text: We hypothesize that small intestinal expression of nutrient transport-related genes, such as cationic AA (CAA) transporters, may contribute to differences in metabolic efficiency of finishing cattle. The objective of this study was to investigate jejunal expression of CAA transporters in steers classified as high and low efficiency based on residual feed intake (RFI). Hereford-Angus crossbred steers (yr 1, n = 59, 461 ± 4.5 kg initial BW; yr 2, n = 75, 412 ± 3.8 kg initial BW) from a single contemporary group in each year (birth through slaughter) were used. Steers were fed a finishing diet (yr 1, 11.4% CP, 2.0 Mcal NE_m/kg, 1.35 Mcal NE_g/kg; yr 2, 13.2% CP, 1.8 Mcal NE_m/kg, 1.19 Mcal NE_g/kg; DM basis) for 57 (yr 1) or 80 d (yr 2), and individual feed intake was monitored using the GrowSafe system. Residual feed intake was calculated as the difference between actual and expected feed intake of each individual, where expected intake was determined by regressing ADG and metabolic midweight on actual intake. At the end of the feeding period in each year, the 20% most efficient (low RFI, n = 8/yr) and 20% least efficient (high RFI, n = 8/yr) steers with 12^th rib fat thickness ≥ 1.02 cm were selected for slaughter between 5 and 8 d after the end of the feed intake test. Jejunal mucosa was collected and flash-frozen. Real-time RT-PCR was performed to determine jejunal expression of 7 cationic AA transporters in 4 different transport systems: cationic AA transporter-1 (CAT-1, system y⁺); ATB^0,+ (system B^0,+); b^0,+AT and rBAT (system b^0,+); and y⁺LAT1, y⁺LAT2, and 4F2hc (system y⁺L). Data were analyzed with PROC MIXED in SAS 9.2 using RFI class (high vs. low efficiency), year, and their interaction as fixed effects. Expression of y⁺LAT2 tended to be affected (P = 0.07) by the RFI class x year interaction. Low efficiency steers had greater (P = 0.05) expression of y⁺LAT2 than high efficiency steers in yr 2, whereas there was no difference (P = 0.61) in yr 1. Jejunal expression of the other 6 CAA transporters was not affected (P ≥ 0.17) by RFI class; however, expression of y⁺LAT1 was affected (P ≤ 0.01) by year. Data suggest jejunal expression of y⁺LAT2, part of the y⁺L basolateral transport system of the small intestine, may contribute to differences in metabolic efficiency of cattle.

Keywords: amino acid, feed efficiency, small intestine