Abstract: In cattle, dietary protein is gradually degraded into peptide-bound amino acids (PBAAs), free amino acids (FAAs), and ultimately into ammonia by the rumen microbes. Both PBAA and FAA are milk protein precursors, and the rumen and small intestines are the main sites where such precursors are produced and absorbed. This work was designed to investigate the expression of the peptide transporter PepT1 and the AA transporters ASCT2, y⁺LAT1, and ATB^0,+, and the concentrations of PBAA, FAA, and soluble protein in the rumen, omasum, and duodenum of dairy cows. Tissues and digesta were collected from six healthy Chinese Holstein dairy cows immediately after the animals were slaughtered. The expression of transporters was analyzed by real-time quantitative polymerase chain reaction (PCR). The FAA concentration was assessed using an amino acid (AA) analyzer, PBAA concentration by quantification of AA before and after acid-hydrolysis by 6 mol/L HCl, and soluble protein concentration by quantification of the bicinchoninic acid content. The results showed that the relative abundance of mRNA of the transporters and the soluble non-ammonia nitrogen (SNAN) concentration of each fraction were greater in the duodenum than in the rumen or omasum. These results indicate that the duodenum is the predominant location within the nonmesenteric digestive tract for producing milk protein precursors. In addition, PBAA was the largest component of SNAN in the digesta from the rumen, omasum, and duodenum. In conclusion, the duodenum has the greatest concentrations of SNAN and PBAA, and the greatest potential for absorption of SNAN in the form of PBAA in the nonmesenteric gastrointestinal tissues of dairy cows.

十二指肠在奶牛非肠系膜系统中吸收可溶性非氨态氮的潜在作用

[1]Bröer, S., 2008. Amino acid transport across mammalian intestinal and renal epithelia. Physiol. Rev., 88(1):249-286.

[2]Chen, G., Russell, J.B., Sniffen, C.J., 1987. A procedure for measuring peptides in rumen fluid and evidence that peptide uptake can be a rate-limiting step in ruminal protein degradation. J. Dairy Sci., 70(6):1211-1219.

[3]Choi, C.W., Choi, C.B., 2003. Flow of soluble non-ammonia nitrogen in the liquid phase of digesta entering the omasum of dairy cows given grass silage based diets. Asian Australas. J. Anim. Sci., 16(10):1460-1468.

[4]Choi, C.W., Vanhatalo, A., Huhtanen, P., 2002a. Concentration and estimated flow of soluble non-ammonia nitrogen entering the omasum of dairy cows as influenced by different protein supplements. Agric. Food Sci. Finland, 11(2):79-91.

[5]Choi, C.W., Ahvenjӓrvi, S., Vanhatalo, A., et al., 2002b. Quantitation of the flow of soluble non-ammonia nitrogen entering the omasal canal of dairy cows fed grass silage based diets. Anim. Feed Sci. Tech., 96(3-4):203-220.

[6]Choi, C.W., Vanhatalo, A., Ahvenjärvi, S., et al., 2002c. Effects of several protein supplements on flow of soluble non-ammonia nitrogen from the forestomach and milk production in dairy cows. Anim. Feed Sci. Tech., 102(1-4):15-33.

[7]Clark, J.H., Klusmeyer, T.H., Cameron, M.R., 1992. Symposium: nitrogen metabolism and amino acid nutrition in dairy cattle microbial protein synthesis and flows of nitrogen fractions to the duodenum of dairy cows. J. Dairy Sci., 75(8):2304-2323.

[8]Gilbert, E.R., Wong, E.A., Webb, K.E.Jr., 2008. Board-invited review: peptide absorption and utilization: implications for animal nutrition and health. J. Anim. Sci., 86(9):2135-2155.

[9]Liao, S.F., Vanzant, E.S., Boling, J.A., et al., 2008. Identification and expression pattern of cationic amino acid transporter-1 mRNA in small intestinal epithelia of Angus steers at four production stages. J. Anim. Sci., 86(3):620-631.

[10]Liao, S.F., Vanzant, E.S., Harmon, D.L., et al., 2009. Ruminal and abomasal starch hydrolysate infusions selectively decrease the expression of cationic amino acid transporter mRNA by small intestinal epithelial of forage-fed beef steers. J. Dairy Sci., 92(3):1124-1135.

[11]Livak, K.J., Schmittgen, T.D., 2001. Analysis of relative gene expression data using real-time quantitative PCR and the 2^−ΔΔCT method. Methods, 25(4):402-408.

[12]Matthews, J.C., Webb, K.E.Jr., 1995. Absorption of L-carnosine, L-methionine, and L-methionylglycine by isolated sheep ruminal and omasal epithelial tissue. J. Anim. Sci., 73(11):3464-3475.

[13]Matthews, J.C., Wong, E.A., Bender, P.K., et al., 1996. Demonstration and characterization of dipeptide transport system activity in sheep omasal epithelium by expression of mRNA in Xenopus laevis oocytes. J. Anim. Sci., 74(7):1720-1727.

[14]McCollum, M.Q., Vazquez-Anon, M., Dibner, J.J., et al., 2000. Absorption of 2-hydroxy-4-(methylthio) butanoic acid by isolated sheep ruminal and omasal epithelia. J. Anim. Sci., 78(4):1078-1083.

[15]McDougall, E.I., 1948. Studies on ruminant saliva: the composition and output of sheep saliva. J. Biochem., 43(1):99-109.

[16]Oh, Y.K., Kim, J.H., Kim, K.H., et al., 2008. Effects of level and degradability of dietary protein on ruminal fermentation and concentrations of soluble non-ammonia nitrogen in ruminal and omasal digesta of Hanwoo steers. Asian Australas. J. Anim. Sci., 21(3):392-403.

[17]Piepenbrink, M.S., Schingoethe, D.J., 1998. Ruminal degradation, amino acid composition, and estimated intestinal digestibilities of four protein supplements. J. Dairy Sci., 81(2):454-461.

[18]Rémond, D., Bernard, L., Savary-Auzeloux, I., et al., 2009. Partitioning of nutrient net fluxes across the portal-drained viscera in sheep fed twice daily: effect of dietary protein degradability. Br. J. Nutr., 102(03):370-381.

[19]Reynal, S.M., Ipharraguerre, I.R., Liñeiro, M., et al., 2007. Omasal flow of soluble proteins, peptides, and free amino acids in dairy cows fed diets supplemented with proteins of varying ruminal degradabilities. J. Dairy Sci., 90(4):1887-1903.

[20]Schmittgen, T.D., Livak, K.J., 2008. Analyzing real-time PCR data by the comparative C_T method. Nat. Protoc., 3(6):1101-1108.

[21]Tagari, H., Webb, K.E.Jr., Theurer, B., et al., 2008. Mammary uptake, portal-drained visceral flux, and hepatic metabolism of free and peptide-bound amino acids in cows fed steam-flaked or dry-rolled sorghum grain diets. J. Dairy Sci., 91(2):679-697.

[22]Taghizadeh, A., Mesgaran, M.D., Valizadeh, R., et al., 2005. Digestion of feed amino acids in the rumen and intestine of steers measured using a mobile nylon bag technique. J. Dairy Sci., 88(5):1807-1814.

[23]Volden, H., Mydland, L.T., Olaisen, V., 2002. Apparent ruminal degradation and rumen escape of soluble nitrogen fractions in grass and grass silage administered intraruminally to lactating dairy cows. J. Anim. Sci., 80(10):2704-2716.

[24]Webb, K.E.Jr., 1990. Intestinal absorption of protein hydrolysis products: a review. J. Anim. Sci., 68(9):3011-3022.

[25]Webb, K.E.Jr., Dirienzo, D.B., Matthews, J.C., 1993. Recent development in gastrointestinal absorption and tissue utilization of peptides: a review. J. Dairy Sci., 76(1):351-361.

[26]Xu, Q.B., Wu, Y.M., Liu, H.Y., et al., 2014. Establishment and characterization of an omasal epithelial cell model derived from dairy calves for the study of small peptide absorption. PLoS ONE, 9(3):e88993.

[27]Zhao, J.W., Zhao, S.G., Sun, P., et al., 2012. The influence factors of synthesis and utilization in cow milk protein source precursor. Chin. Anim. Husb. Vet. Med., 39:78-81 (in Chinese).

[28]Zhu, W., Tang, C.H., Sun, X.P., et al., 2013. Rumen microbial protein synthesis and milk performance in lactating dairy cows fed the fortified corn stover diet in comparison with alfalfa diet. J. Anim. Physiol. Anim. Nutr., 12(5):633-639.