Intestinal absorption-partition relationships: A tentative functional nonlinear model

Abstract

Models and equations designed to elucidate passive intestinal absorption mechanisms by analysis of the relationship between the absorption rate constant (k_a)and either the partition coefficients (P)or a related partition constant for homologous series of substances, are reviewed. Classical nonlinear physical models, such as those which assume the existence of a nonstirred layer or equilibrium extraction, predict sigmoidal or hyperbolic relationships between k_a and P,whereas other models, which regard the membrane as a heterogeneous multicompartment system, predict parabolic or bilinear relationships between log k_aand log P.In the present paper, an alternative model is proposed, which incorporates the Wagner-Sedman equilibrium extraction model together with the existence of pores, which play a fundamental role for compounds below 250in molecular weight. Several apparently contradictory absorption-partition literature data are shown to be highly consistent with the tentative model proposed.