Effects of diet forage:concentrate ratio and mesenteric vein L-alanine infusion on net VFA metabolism by portal-drained viscera (PDV) and liver were determined in four multicatheterized Hereford x Angus heifers (443 kg BW) fed in equal meals every 2 h pelleted, isonitrogenous diets containing 75% alfalfa or 75% corn:soybean meal (SBM) in a balanced single reversal of diets with 6 wk for adaptation. In addition, effects of L-alanine infusion on net metabolism of individual amino acids (AA) by PDV and liver was determined when heifers were fed 75% corn:SBM. Four measurements of blood flow (p-aminohippurate dilution) and net flux of VFA and AA (venous-arterial difference times blood flow) were obtained at 30-min intervals immediately before beginning and ending a 24-h mesenteric vein infusion of L-alanine at 75 mmol/h. Net PDV absorption of acetate (P < .02), propionate (P < .02), and total VFA (P < .02) was lower when 75% corn:SBM was fed, reflecting increased carbohydrate digestion postruminally and only partially balanced by increases in net PDV glucose absorption reported previously. In addition, net PDV absorption of n-butyrate was lower when 75% alfalfa was fed. Excluding acetate, net liver removal of VFA was 83 to 108% of their net PDV release. L-Alanine infusion did not affect net PDV or liver flux of VFA but decreased net total splanchnic (PDV plus liver) release of propionate (P < .07) and n-butyrate (P < .03) and decreased (P < .10) net PDV release and liver removal of glycine. In spite of the increase in liver L-alanine removal, the ratio of glucose release to precursor removal remained remarkably constant across the liver when L-alanine was infused, emphasizing the tight control of liver glucose release and the dynamic interplay of precursor supply and removal in the maintenance of liver carbon balance.
Alanine Amino Acids