Intestinal absorption-partition relationships: a tentative functional nonlinear model.

Models and equations designed to elucidate passive intestinal absorption mechanisms by analysis of the relationship between the absorption rate constant (ka) 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 ka and P, whereas other models, which regard the membrane as a heterogeneous multicompartment system, predict parabolic or bilinear relationships between log ka and 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 250 in molecular weight. Several apparently contradictory absorption-partition literature data are shown to be highly consistent with the tentative model proposed.