Effect of heat stress on the serum concentrations of free amino acids and some of their metabolites in growing pigs

Exposure to heat stress (HS) may affect the intestinal epithelia of pigs, resulting in impaired digestive and absorptive capacity. The serum concentration (SC) of free AA in pigs can be used as indicators of their availability. This study was conducted with 12 crossbred (Landrace x Hampshire x Duroc) pigs (29.0 +/- 2.8 kg initial BW) distributed into 2 groups to analyze the SC of free AA and some AA metabolites in pigs exposed to HS conditions. The treatments were pigs housed under natural HS conditions in a room with no ambient temperature control (23.6 to 37.6 degrees C, HS) and pigs housed at thermoneutral conditions (24 +/- 2 degrees C), feed restricted to a level similar to that of their HS counterparts. All pigs received a wheat-soybean meal diet. Blood samples were collected at both the absorptive (2.5 h after a meal) and postabsorptive (10.0 h after a meal) phase. At the absorptive phase, the SC of free Arg, Leu, Lys, Phe, Thr, and Trp were lower (P < 0.05) and the SC of His, Val, Ala, Pro, Ser, and Tyr tended to be lower (P < 0.10) in HS pigs. At the postabsorptive phase, the SC of free Arg, His, Met, Asn, Gln, and Tyr were higher (P < 0.05) but the SC of Ala was lower (P < 0.01) and the SC of Pro tended to be lower (P < 0.10) in HS pigs. The absorptive SC of carnosine, ornithine (Orn), and Tau were lower (P < 0.05) and of citrulline (Cit), cystathionine, and urea tended to be lower (P < 0.10) in HS pigs. The postabsorptive SC of 3-methyl-His, homo-Cys, OH-Lys, and OH-Pro increased (P = 0.05) and of Cit tended to increase (P = 0.10) but that of carnosine and sarcosine (P < 0.05) decreased in HS pigs. The results of this study show a marked and differential effect of HS on the SC of AA. These data indicate that HS negatively affects the digestive and absorptive capacity of pigs and that the metabolism of some AA is modified in pigs to counteract the negative effects of the HS.