Integration of amino acid metabolism in sheep: effects of fasting and acidosis

Abstract

Net fluxes of 25 plasma amino acids were measured across 4 major vascular beds (portal-drained viscera, liver, kidneys and hindquarters) in 9 fed, 8 fasted and 10 acidotic adult sheep. In fed animals, the liver extracted most amino acids at approximately the same rates as those that were added by the portal-drained viscera. This hepatic removal was maintained even after 3 days of fasting when portal addition of amino acids had nearly ceased. This phenomenon was solely due to increased amino acid release by both kidneys and muscle tissue. The glucogenic amino acids, alanine, glutamine and glycine, were always removed by the liver and released by the hindquarters even in fed animals. Contrarily, glutamate was released by the liver and, together with serine, removed by the hindquarters. Of the urea cycle amino acids, arginine was always removed by the liver, and citrulline and ornithine were released. Oppositely, arginine was released by the kidneys and hindquarters, but citrulline and ornithine were removed. Unlike nonruminants, the branched-chain amino acids were always removed by the liver and, during fasting, were released by the kidneys and hindquarters. Acidosis increased the hepatic removal of amino acids solely because of increased portal addition to the blood. In acidosis, alanine was diverted away from the kidneys and toward the liver, but, oppositely, glutamine was diverted away from the liver to the kidneys. Hepatic amino acid metabolism is probably continuous and peripheral tissues may act secondarily to maintain homeostasis. Only a selected few amino acids serve as quantitatively important nitrogen and carbon carriers. These include alanine and the amino acid couplets, glutamine-glutamate, glycine-serine and arginine-ornithine.