Amino acid flux across the gastrointestinal tract and liver of calves.

Fluxes of peptide and of free AA in plasma and blood cells were quantified across the gastrointestinal tract and liver of six growing Holstein steers (136 kg). Calves were fed hourly, and blood was obtained simultaneously from the aorta, protal vein, and hepatic vein cannulas on 1 d between d 9 and 12 postsurgery and again following 72 h without feed. Blood flow was determined by p-aminohippuric acid infusion. Peptide AA accounted for the greatest concentration of AA in arterial blood of all calves. There was a net flux of free AA in plasma across the tract, and lack of feed reduced the magnitude of the flux. Glutamine and glutamate were the only free AA in plasma with a negative flux across the tract in fed calves. Alanine accounted for over 14% of the total tract flux of free AA in plasma of fed calves. Hepatic removal of free AA in plasma increased after feed deprivation. Flux of free AA in blood cells was negative across the tract of fed calves but to a lesser extent in unfed calves. The hepatic flux of free AA in blood cells offset the negative tract flux, thus resulting in no net splanchnic output of free AA in blood cells by calves. Peptide AA flux across the tract was 2.5 times (fed) to 7.2 times (unfed) greater than free AA flux. Net splanchnic output of peptide AA was 7 times the magnitude (738 vs. 92 g/d) of the net splanchnic output of free AA in plasma. Peptides with molecular mass from 500 to 1500 Da accounted for the largest flux of peptide AA across the tract. Quantitatively, peptides appeared to be involved extensively in interorgan transport and may be a substantial form of absorbed AA in calves.