Absorption kinetics for leucine, cycloleucine and alpha-aminoisobutyric acid by rat small intestine in vivo

Abstract

The intestinal absorption kinetics of three neutral amino acids, leucine, cycloleucine and alpha-aminoisobutyric acid, has been studied in rat jejunum in vivo, with luminal perfusion during successive periods, by measuring the passive component and the active transport.The mass-transfer coefficients of the passive process, are similar for the three amino acids and increase with the perfusion rate.The transport component, obtained from the difference between total absorption and passive diffusion, shows saturation kinetics and also increases with the perfusion rate.The apparent Michaelis constants, Km, and the maximal transport rates for the three amino acids have been determined.The Km values are greater than those reported for in vitro studies, a result imputable to greater thickness of the unstirred layers in vivo and to the unequal signification of the constant in both conditions.Passive flux has proved to be an important component for in vivo absorption, even at low substrate concentrations (1-5 mM), so that its evaluation cannot be neglected for the calculation ot the kinetic constants of the mediated transport.