Intestinal absorption pathway of γ-aminobutyric acid in rat small intestine

Intestinal absorption of γ-aminobutyric acid (GABA), as a model compound for γ-aminoacids, has not been extensively studied from the kinetic viewpoint. Since data from our laboratory suggested that some competition arises between intestinal absorption of β-alanine and GABA and since our intent was to maintain the aqueous stagnant diffusion layer in order to approach absorption tests to in vivo physiological conditions, a rat jejunum in situ study was undertaken in order to gain an insight into the mechanism of GABA absorption. In the present paper, results from assays using isotonic perfusion solutions with starting GABA concentrations ranging from 1 to 50 mM are reported. They show that the intestinal absorption of the γ-aminoacid can be apparently described as a specialized transport mechanism which obeys Michaelis-Menten and first-order kinetics. Parameter values found were V_m = 13.99 ± 2.37 mM h^?1, K_m = 3.87 ± 0.63 mM, and k_a(passive) = 0.362 ± 0.120 h^?1. Through the perfusion of 5 mM β-alanine solutions containing variable concentrations of GABA (from 5 to 50 mM), a partially competitive inhibition of β-alanine absorption was apparently characterized.