Concentration of soluble non-ammonia nitrogen and related transporter expression in non-mesenteric gastrointestines of dairy cows

Abstract Text: Dietary protein is gradually degraded from peptide-bound amino acids (PBAA) into free amino acids (FAA) and ultimately into ammonia by the rumen microbes. Both PBAA and FAA are milk protein precursor, and the rumen and small intestines are the main sites where milk protein precursor is produced and absorbed. This work was designed to investigate the concentrations of PBAA, FAA and soluble protein and the expression of peptide transporter (PepT-1) and amino acid transporters in the rumen, omasum and duodenum of dairy cows. The digesta and tissues were collected from six healthy Holstein dairy cows (BW= 643 ± 67.4 kg and milk yield = 13.4± 2.05 kg/d) immediately after the animals were slaughtered. The FAA was assessed by amino acid (AA) analyzer, PBAA was by quantification of AA before and after acid-hydrolysis by 6M HCl, and soluble protein was by bicinchoninic acid. Concentrations of FAA, PBAA and soluble protein are shown in Table 1. Concentration of all portions of non-ammonia nitrogen was the highest in the duodenum. The PBAA was the largest component of soluble non-ammonia nitrogen in all the digesta from rumen, omasum and duodenum, indicating that peptides may be the main form absorbed in these sites. Abundance of PepT-1 mRNA was consistent with the concentrations of PBAA from rumen to duodenum. Expression of all genes was greater in the duodenum than in the rumen and omasum, suggesting that the duodenum is the major nonmesenteric site where peptides are transported and absorbed.

 Table 1. Concentrations of soluble non-ammonia nitrogen (mg N/100ml) in rumen, omasum and duodenum (n=6)

	Rumen	Omasum	Duodenum	SEM	P-values
Free amino acid	22.4B	49.9B	189.8A	24.5	<0.01
Peptide-bound amino acids	40.0B	73.3B	285.5A	35.8	<0.01
Soluble protein	15.1B	34.6B	168.4A	16.4	<0.01
Total	77.5B	157.7B	643.6A	65.8	<0.01
Peptide-bound/Total (%)	51.6	46.5	44.4

^A,B Values with different letters differ significantly (P < 0.01).

Keywords: gastrointestine; milk protein precursor; peptide-bound amino acids