In Vitro Model for Transport of Solutes in Three-Phase System II: Experimental Considerations

Abstract

Key experiments were carried out that illustrated the main features of the three-phase transport theory presented earlier. These studies investigated the effects of partition coefficient and pH on transport rates and how these effects are altered by the aqueous diffusion layers adjacent to the lipid phase. The experimental setup incorporates significant improvements over previous in vitro systems: (a) stable lipid phases in which partition coefficients of solutes may be measured, and (b) a large aqueous-lipid phase volume ratio, enabling establishment of steady-state conditions. Experimental rates for a variety of conditions correlated reasonably well with computer predictions obtained by independent evaluation of physicochemical parameters in the system. Transfer rates leveled off with increasing (high) membrane permeability coefficients. Transport rates for these high permeability coefficient solutes were very dependent on stirring rates as well as on the pH of the receptor phase. The pH-transfer rate profiles generally paralleled solute titration curves, except where pH values created pseudo-two-phase conditions. The transport of a high permeability coefficient solute, n-amylpyridine, slowed the simultaneous transfer of pyridine through the system. Cumulative results indicate that the pH-partition theory is only a special case of a more general phenomenon.