Effects of divergent selection for residual feed intake on nitrogen metabolism and lysine utilization in growing pigs.

A study was conducted to evaluate the effects of divergent genetic selection for residual feed intake (RFI) on nitrogen (N) metabolism and lysine utilization in growing pigs. Twenty-four gilts (body weight [BW] 66 ± 5 kg) were selected from generation nine of the low RFI (LRFI; n = 12) and high RFI (HRFI; n = 12) Iowa State University Yorkshire RFI selection lines. Six pigs from each genetic line were assigned to each of two levels of lysine intake: 70% and 100% of estimated requirements based on the potential of each genetic line for protein deposition (PD) and feed intake. For all diets, lysine was first limiting among amino acids. Using isotope tracer, N-balance, and nutrient digestibility evaluation approaches, whole-body N metabolism and the efficiency of lysine utilization were determined for each treatment group. No significant interaction effects of line and diet on dietary N or gross energy digestibility, PD, and the efficiency of lysine utilization for PD were observed. The line did not have a significant effect on PD and digestibility of dietary N and GE. An increase in lysine intake improved N retention in both lines (from 15.0 to 19.6 g/d, SE 1.44, in LRFI pigs; and from 16.9 to 19.8 g/d, SE 1.67, in HRFI pigs; P < 0.01). At the low lysine intakes and when lysine clearly limited PD, the efficiency of using available lysine intake (above maintenance requirements) for PD was 80% and 91% (SE 4.6) for the LRFI and HRFI pigs, respectively (P = 0.006). There were no significant effects of line or of the line by diet interaction on N flux, protein synthesis, and protein degradation. Lysine intake significantly increased (P < 0.05) N flux (from 119 to 150, SE 8.7 g/d), protein synthesis (from 99 to 117, SE 10.6 g of N/d), and protein degradation (from 85 to 100, SE 6.6 g of N/d). The protein synthesis-to-retention ratio tended to be higher in the LRFI line compared with the HRFI line (6.5 vs. 5.8 SE 0.62; P = 0.06), indicating a tendency for the lower efficiency of PD in this group. Collectively, these results indicate that genetic selection for low RFI is not associated with improvements in lysine utilization efficiency, protein turnover, and nutrient digestibility.

Immune system stimulation induced by porcine reproductive and respiratory syndrome virus alters plasma free amino acid flux and dietary nitrogen utilization in starter pigs1.

Changes in plasma free amino acid (AA) flux reflect the modification of AA metabolism in different metabolic states. Infectious diseases repartition AA away from protein retention toward processes involved in immune defense, thus impacting AA utilization in pigs. The current study sought to evaluate the effects of disease induced by a live pathogen on plasma free AA flux and whole-body nitrogen (N) utilization. Twenty gilts (BW 9.4 ± 0.9 kg) were surgically catheterized into the jugular vein, individually housed in metabolism crates, and feed-restricted (550 g/d). Intramuscular inoculation of a live field strain of porcine reproductive and respiratory syndrome virus (PRRSV) was used to induce disease. Whole-body N-balance was conducted across 3 d both before PRRSV inoculation (PRRSV-) and also after PRRSV inoculation (PRRSV+). At the end of each N-balance period, a bolus dose of a labeled [U-13C, U-15N]-AA mixture (Ile, Leu, Lys, Met, Phe, Thr, Trp, Val, and Gln) was infused intravenously, followed by serial blood collection for measurement of isotopic enrichment. A double exponential model was fitted with plasma enrichment data for each pig and each AA, and equation parameters were used to estimate plasma free AA flux and pool size. Apparent ileal digestibility (AID) of dietary N was determined using the slaughter technique and an indigestible marker. Blood chemistry, hematology, body temperature, and serum viremia indicated that PRRSV induced effective immune response in pigs (P < 0.05). Challenge with PRRSV reduced the AID of N (P < 0.05), but had no effect on apparent total tract digestibility of dietary energy (P = 0.12). Plasma flux (µmol/kg BW/h) for Met and Thr was increased by PRRSV infection (P < 0.05). A strong tendency of increased Val flux was observed in PRRSV+ pigs (P = 0.06). Infection with PRRSV increased the pool size for Lys, Met, Thr, Trp, Leu, Val, and Gln (P < 0.05). Collectively, these results suggest that PRRSV alters the utilization of dietary N and AA flux, as well as pool size, in growing pigs. The increase in Thr and Met flux in PRRSV+ pigs may be associated with enhanced utilization of these AA for the synthesis of immune system metabolites and increased catabolism of these AA. Thus, dietary Met, Thr, and Val requirements may increase in pigs infected with PRRSV, relative to the requirements for other AA.

Immune system stimulation induced by Escherichia coli lipopolysaccharide alters plasma free amino acid flux and dietary nitrogen utilization in growing pigs.

Changes in plasma free AA flux reflect the modification of AA metabolism in different metabolic states. Immune system stimulation (ISS) in growing pigs may redistribute AA from protein retention towards processes involved in the immune response, thus impacting AA utilization. The aim of the current study was to evaluate the effect of ISS on whole-body nitrogen (N) utilization and the kinetics of plasma free AA. Ten gilts (BW 9.4 ± 1.1 kg) were surgically fitted with jugular vein catheters, individually housed in metabolism crates, and feed-restricted (550 g/d). Repeated intramuscular injections of increasing amounts of Escherichia coli lipopolysaccharide (LPS) were used to induce ISS (30 and 36 µg/kg BW, given 48 h apart). Whole-body N-balance was determined for 3-d before ISS (ISS-) and 3-d during ISS (ISS+). At the end of each N-balance period, a bolus dose of labeled [U-13C, U-15N]-AA mixture (Ile, Leu, Lys, Met, Phe, Thr, Trp, Val, and Gln) was infused intravenously, followed by serial blood collection for determination of isotopic enrichment. A double exponential model was fitted with plasma enrichment data for each pig and each AA, and equation parameters were used to estimate plasma-free AA flux and pool size. Apparent ileal digestibility (AID) of N was determined using the slaughter technique and an indigestible marker. Blood samples were collected before and 76-h after the initiation of ISS and assayed for hematology and blood chemistry. Body temperature (BT) was monitored during the course of study. Blood chemistry, hematology, and BT results indicated that LPS induced effective ISS in pigs (P < 0.05). ISS tended to reduce N retention (P = 0.09) and the N retention-to-N intake ratio (P = 0.08). Apparent total tract digestibility of dietary energy and AID of N were reduced by ISS (P < 0.05). Plasma flux (µmol/kg BW/h) for Ile and Phe was reduced by ISS (P < 0.05). Strong tendencies for decreased Lys flux and N retention were observed in ISS pigs (P < 0.10). ISS increased the pool size for Leu but reduced the pool size for Ile (P < 0.05). Collectively, these results suggest that ISS alters the utilization of dietary N and AA flux, as well as pool size in growing pigs. The decrease in Lys, Phe, and Ile flux during ISS may be attributed to a reduction in whole-body protein synthesis or decreased catabolism of these AA. Relative to other AA, dietary Lys, Phe, and Ile requirements may decrease in ISS pigs.