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    • To support maximal growth and metabolic function

    2022-02-15

    To support maximal growth and metabolic function of major organs, glucose and ML324 are essential. We assessed the plasma levels of glucose and amino acids in LBW and NBW piglets at birth and at day 7 postpartum. Blood glucose levels were influenced by birthweight and age. In this study, the levels were lower in LBW piglets than in NBW piglets at day 0. At day 7, blood glucose increased in the LBW group to reach normal range. In newborn piglets, plasma glucose levels depend solely on lactose from the sows’ colostrum and milk. The amount of colostrum intake correlates positively with their birthweight (Devillers et al., 2011, Quesnel et al., 2012, Ferrari et al., 2014); it is also essential for piglets’ thermoregulation (Lepine et al., 1991, Berthon et al., 1993). Hypoglycemia in LBW piglets in our study is thus in agreement with previous reports (Herpin et al., 2002, Caldara et al., 2014). The circulating levels of glucose and amino acids increase at day 7 postpartum due to intestinal maturation. Obviously, LBW piglets catch up with NBW piglets with time. Surprisingly, plasma concentrations of free amino acids did not differ between LBW and NBW piglets on both day 0 and day 7 despite the difference in body weight. The levels of essential and conditionally essential amino acids (arginine, histidine, isoleucine, leucine, methionine, proline, threonine, tryptophan and tyrosine) increased with age in piglets. Although at day 0, most amino acids were lower in LBW piglets compared to NBW piglets, by day 7 of age, in which the differences were not obvious. At day 0, the plasma levels of arginine, leucine, isoleucine, and proline were lower in LBW piglets than in NBW piglets. These amino acids are needed for protein accretion for rapid postnatal growth and to stimulate the secretion of growth hormone and insulin (Flynn et al., 2000, Flynn et al., 2002, Nair and Short, 2005). Therefore, the reduced levels of these amino acids might be a factor in restricting development and growth in neonatal pig (Yin et al., 2011). Arginine is synthesized from glutamine by enterocytes from 0- to 7-day-old pigs (Wu et al., 2009). Wu and Knabe (1995) described that arginine synthesis in NBW pigs is 3–4 times greater at day 0–2 than at day 7. In our study, the increased plasma concentration of arginine in LBW piglets at day 7 could promote growth. The increase in plasma arginine might be enhanced endogenous synthesis to compensate for arginine deficiency in sow’s milk (Wu et al., 2010). The increase in plasma leucine and isoleucine in LBW piglets at day 7 could be due to decreased catabolism of these amino acids in skeleton muscle and other tissues. Chen et al. (2009) reported that leucine and isoleucine are actively metabolized in the enterocytes of 0- to 21-day-old-pigs. Recently, Zhang et al. (2017) reported novel functions of branched-chain amino acids, particularly isoleucine. In both NBW and LBW piglets, plasma proline at day 7 was higher than at day 0. Proline is catabolized in enterocytes to yield arginine in neonates (Wu, 1997). Enteral intake of proline from sow’s milk is effective in ameliorating arginine deficiency in neonatal pigs (Wu et al., 1994). Thus, it seems that during the first week of life, the LBW piglets have mechanisms to ameliorate the limitations to absorb glucose and amino acids, arising from limited supply of these nutrients in order to optimize growth and development; this might involve upregulation of selective transporters in the intestine. Birthweight had no influence on small intestinal development (Wiyaporn et al., 2013, Huygelen et al., 2014, Huygelen et al., 2015). Wiyaporn et al. (2013) found that the small intestine of LBW piglets had potential for compensatory enhancement of intestinal function. Rezaei et al. (2011) observed that milk consumption per kg of body weight did not differ between LBW and NBW piglets during the first week of age. They also observed that surviving LBW piglets were more efficient in utilizing milk nutrients for growth than larger litter mates. During the suckling period, absorption of glucose, amino acids and peptides via several transporters occurs mainly in the proximal region of the small intestine (Puchal and Buddington, 1992, Nosworthy et al., 2013). Here we further clarified this phenomenon by assessing the expressions of SGLT1, GLUT2, LAT2, b0,+AT and PepT1 in small intestine at day 0 and 7 postpartum. Unexpectedly, despite the differences of the levels of circulating nutrients or age in birthweight, the expression of these transporters did not differ. The expression however varied along the different regions in intestine. Several mechanisms might be responsible for this phenomenon (Cherbuy et al., 1997, Zuo et al., 2010, Yang et al., 2012). While b0,+AT mRNA expression was higher in the ileum than in other segments, GLUT2 mRNA expression was higher in the duodenum/jejunum than in the ileum. Similarly, PepT1 expression was higher in the proximal parts of the intestine than in distal parts (Thongsong et al., 2016). Surprisingly, PepT1 mRNA did not change from birth to day 7. PepT1 is solely responsible for the absorption di- and tripeptides (Ganapathy et al., 2008, Ganapathy, 2012).