Amino Acid Flux Across the Gastrointestinal Tract and Liver of Calves

Abstract

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.

Glucose, alpha-amino nitrogen, and amino acid exchange across the hindlimb in young double-muscled type bulls maintained at two growth rates

“…The significant increase in uptakes of serine and lysine found in the present experiment also provides support to the results of Ballard et aI. (1976) and Bergen and Heitmann (1978) Purser (1970) Koeln et al 1993). Moreover, peptides play a role in transport of amino acids by the gut (Fenaris et al 1988) and might concern NEAA more than EAA (Hirst 1993 …”

Flux of amino acids and peptides across the portal vein-drained tissue of pigs

“…However, flux of amino acids into and out of the red blood cells have been reported across the gastrointestinal and liver tissues of dogs (Elwyn et al, 1968), rats (Boullin et al, 1973;Galibois et al, 1991) and sheep (Heitmann and Bergman, 1980). Moreover, the flux of amino acids across the gastrointestinal tissue was substantially higher in the form of plasma peptides than that of the plasma and red blood cell pools of the portal vein blood of fed and unfed calves (Koeln et al, 1993). The relationship between gastrointestinal flux of amino acids and the different blood transport pools has not been reported in pigs.…”

“…On the other hand, total concentration of amino acids in the arterial red blood cells of fed compared to unfed pigs suggest that this blood pool also has a role in the transport of free amino acids that has been reported for other animals. For instance, transport of amino acids in red blood cells have been reported in cattle (Hanigan et al, 1991;Koeln et al, 1993), dogs (Elwyn, 1966;Elwyn et al, 1968), sheep (Young and Ellory, 1977;Heitmann and Bergman, 1980) and humans (Felig et al, 1973). Moreover, the difference in flux of glutamate and glutamine in the red blood cells of the fed compared to unfed pigs suggests that this blood pool has a specific counter current role to that of plasma in transporting amino acids destine for deamination to a-ketoglutarate and subsequent glucose synthesis in the liver.…”

“…The flow of whole blood and plasma in the portal vein were calculated by the indicator-dilution method, using PAH as blood flow marker and the equations of Yen and Killefer (1987). Flux of amino acids across the portal vein-drained tissue were calculated as the difference between the respective concentrations in the portal vein and carotid artery blood pools, multiplied by the portal vein blood flow rate as described by Koeln et al (1993).…”

“…Difference in the amino acid concentrations of deproteinized whole blood and the plasma hydrolysates from that of deproteinized plasma represented values for the red blood cell and plasma peptide pools, respectively. Concentration of amino acids was reported on a whole blood basis with values for the plasma and plasma peptide pools corrected for a 20% content of plasma in the packed cell volume according to Koeln et al (1993).…”