Intestinal disappearance and portal blood appearance of amino acids in sheep

The intestinal disappearance and simultaneous arterial inflow and portal appearance of individual amino acids (AA) were studied in sheep fed closed formula, unrefined high (H.P.) and medium (M.P.) protein diets. Gut contents were sampled through four intestinal cannulae and blood was sampled through portal and arterial catheters. The amount of total and amino N that was fed decreased on passage into the duodenum but increased in the jejunum, and then again decreased steadily towards the terminal ileum. The amounts of AA passing into the duodenum were significantly higher when the H.P. rather than the M.P. diet was fed. No dietary differences in AA were noted at the ileo-cecal junction, however, meaning that greater amounts of AA disappeared from the intestine when the H.P. diet was fed. The amounts of AA appearing in portal blood were 30 to 80% of those disappearing from the intestine and were greater in sheep fed the H.P. diet. The amount of AA passing into the duodenum also significantly affected the concentrations of AA in arterial blood. Less [U-14C]glutamic acid than [U-14C]alanine, that was infused into abomasum, was detected in the digesta passing through the pylorus. The same also was true for the unlabeled free form of glutamic acid. The portal appearance of both unlabeled and labeled glutamic acid was negligible, but that of alanine was considerable. Variable amounts of [14C]citrulline, [14C]arginine, and [14C]urea were detected in the blood following the abomasal infusions of labeled glutamic acid or alanine. The portal appearance of these labeled metabolites was always negative, however, implying that they were utilized, and not formed, by gut tissues.