Diurnal Ambient Temperature Variations in Warm Climate Regions Affect the Serum Concentrations of Free Amino Acids in Growing Pigs

Ambient temperature (AT) changes dramatically within a 24-h period in areas where it is above the thermo neutral (TN) zone of pigs. Usually, morning-AT is close to the pig TN zone but afternoon-AT causes heat stress (HS). These changes may affect the absorption and metabolism of amino acids (AA). The serum concentration (SC) of free AA during the absorptive and post-absorptive phases reflects their absorption and cellular metabolism, respectively. Thus an experiment was conducted to analyze the SC of free AA in pigs exposed to the morning comfort-like AT or the afternoon HS conditions. Six pigs (Landrace-Hampshire-Duroc, 27.1 ±1.3 kg initial BW) were used. A thermometer was implanted subcutaneously to register body temperature (BT) at 15 min intervals; AT was also recorded during the 14-d study. Blood samples were collected on the last 3 days of the experiment, at 0600 h (preprandial and lowest AT), 1200 h (5-h postprandial and mild HS), and 1600 h (9-h postprandial and severe HS). The pigs received the same amount (1.2 kg/d) of an AA-supplemented, wheat-soybean meal diet, in two equal meals (0700 and 1900 h). The temperatures registered at 0600, 1200, and 1600 h were: AT, 30.6, 39.0, and 41.1 °C; BT, 38.3, 39.8, and 40.3 °C, respectively. The BT was highly (r=0.92) and significantly (P<0.001) correlated with AT. The SC (µM/mL) at 0600, 1200, and 1600 h were: Arg, 0.179, 0.208, 0.174; His, 0.065, 0.068, 0.063; Ile, 0.090, 0.116, 0.101; Leu, 0.135, 0.138, 0.125; Lys, 0.052, 0.129, 0.096; Met, 0.025, 0.029, 0.026; Phe, 0.078, 0.092, 0.074; Thr, 0.116, 0.142, 0.114; Trp, 0.028, 0.027, 0.026; Val, 0.205, 0.245, 0.224, respectively. Serum Ile, Lys, Met, Val, Ala, Asn, and Pro were higher (P≤0.01); Arg, Phe, Glu, and Tyr tended to be higher (P≤0.10); but Cys was lower (P<0.05) at 1200 h than at 0600 h. Lys was higher, Cys and Tyr were lower (P<0.05), and Ile and Val tended to be higher (P ≤ 0.10) at 1600 h than at 0600 h. Serum Arg, Ile, Phe, Ala, Asn, Gln, Pro, Ser, and Tyr were lower (P < 0.05), and Leu and Val tended to be lower at 1600 h than at 1200 h. These data indicate that AT directly modifies the BT, and that diurnal variations in AT differently affect the daily absorption and utilization of AA in pigs exposed to comfort-like AT and HS conditions.