|dc.description.abstract||Immune system stimulation (ISS) results in metabolic alterations characterized by distinct changes in protein and amino acid (AA) metabolism that impact the pigs’ productivity and efficiency of nutrient utilization for growth. This occurs, in part, by repartitioning AA away from body protein deposition (PD) towards process involved in an immune response, thus impacting AA requirements both quantitatively and qualitatively. Furthermore, the metabolism of threonine (Thr) during ISS has gained attention due to its role in the synthesis of Thr-rich immune system metabolites, such as immunoglobulins, acute phase proteins and, in particular, intestinal mucins. The research herein was conducted to evaluate the effect of ISS on the aspects of protein metabolism and AA utilization in the pig.
In experiment 1 (n=10; BW 9.4 ± 1.1 kg), a stable isotopic tracer technique was used to evaluate the impact of Escherichia coli Lipopolysaccharide (LPS) induced-ISS on plasma free AA (Lys, Met, Thr, Trp, Ile, Leu, Val, Phe, Gln) flux as an indicator of AA utilization in growing pigs during the fed state using. In this study, Blood chemistry, hematology, and BT results indicated that LPS induced effective ISS in pigs (P<0.05). Immune system stimulation tended to reduce N retention (P=0.09) and the N retention-to-N intake ratio (P=0.08). Apparent ileal digestibility of N and apparent total tract digestibility of dietary energy were reduced by ISS (P<0.05). Plasma flux (µmol/kg BW/h) for Ile and Phe was reduced by ISS (P<0.05). A strong tendency of decreased Lys flux was observed in ISS pigs (P=0.08). Immune system stimulation increased the pool size for Leu (P<0.05) but reduced the pool size for Ile (P<0.05). In experiment 2 (n=20; BW 9.4 ± 0.9 kg), using the same stable isotopic tracer technique, the effects of porcine reproductive and respiratory syndrome virus (PRRSv) infection on PD and AA metabolism was evaluated. Blood chemistry, hematology, BT, and serum viral load results indicated that PRRSv inoculation induced effective ISS in pigs (P<0.05). Immune system stimulation significantly reduced the ADFI by 21.7 ± 4.58 g/kg BW/d (P=0.01). Immune system stimulation had no effect on N retention (P=0.99) and the N retention-to-N intake ratio (P=0.85) after controlling for ADFI. Apparent ileal digestibility of N was reduced by ISS (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 ISS (P<0.05). A strong tendency of increased Val flux was observed in ISS pigs (P=0.08). Immune system stimulation increased the pool size for Ile, Leu, Lys, Phe, and Val (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 increase in Thr and Met flux in PRRSv-infected pigs may be associated with enhanced utilization of Met and Thr for the synthesis of immune system metabolites and increased catabolism of these AA. Thus, dietary Met and Thr requirements may increase in health challenged pigs, relative to the requirements for other AA.
The observed 1.5 times increase in plasma Thr flux in PRRSv challenged pigs, suggested an increased metabolic demand for Thr during ISS. Therefore, in experiment 3 (n=39; initial BW 32 ± 2.1 kg) the impact of ISS on dietary Thr requirements in growing pigs was evaluated using the LPS model of ISS. Based on the relationship between standardized ileal digestible Thr intake and whole-body PD, the marginal efficiency of SID Thr utilization for PD was not affected by ISS. However, ISS substantially increased the extrapolated maintenance SID Thr requirements, represented by the intercept at zero PD (ISS- vs. ISS+, −11.2 vs. −56.3 SE 14.8; P < 0.05).
In experiment 4 (n=12; initial BW 31 ± 4.8 kg), the impact of immune system stimulation (ISS) on whole-body nitrogen metabolism and muscle fiber characteristics was evaluated in growing pigs. Immune system stimulation significantly reduced whole-body protein synthesis (10.77 vs. 7.12 g N/kg BW0.60/d; SE 0.44), degradation (9.41 vs. 6.64 g N/kg BW0.60/d; SE 0.04), and retention (1.38 vs. 0.58 g N/kg BW0.60/d; SE0.08) and increased the protein synthesis to retention ratio (7.61 vs. 12.26; SE 1.4) in ISS+ pigs relative to ISS- pigs (P<0.01). Furthermore, ISS showed a general decrease in skeletal muscle nuclei and fiber cross-sectional area and a shift from myosin heavy chain (MHC)-IIX towards MHC-I fibers (P<0.05).||