Studies on the reliability of a bihyperbolic functional absorption model. I. Ring-substituted anilines.

The present study reviews and checks the rearranged master lines of the functional bihyperbolic absorption model proposed by Plá-Delfina and Moreno, by examining the correlations between absorption rate constants experimentally found in the small intestine and in the colon of the rat, and two types of partition constants for a series of ring-substituted anilines, of low to medium molecular weight. Evidence is given which demonstrates the reliability of the bihyperbolic equation for the small intestine data, showing the importance of the type of substitution in the absorption rate of compounds; for the colonic data, the collapsed, monohyperbolic form of this equation (i.e., the Wagner-Sedman equilibrium model) applies, independent of the nature and position of the group substituents in the aniline molecule. This means that, as the bihyperbolic model equation predicts, aqueous pore diffusion is a crucial factor, as important as membrane permeation, in absorption in the small intestine, whereas in the colon only this latter route is operative. The perspectives opened by the application of the model to gastrointestinal absorption studies are also discussed.