A biophysical model of passive and polarized active transport processes in Caco-2 cells: approaches to uncoupling apical and basolateral membrane events in the intact cell.

This report is aimed at the biophysical modeling of transmembrane events involving a passive diffusion and directional pumplike mechanism at the apical (AP) and basolateral (BL) membranes of cultured cell monolayers. The essence of the model is based on experimental evidences for the existence of a saturable, apically polarized transport system in Caco-2 cells for peptides which hindered apical to basolateral flux, enhanced basolateral to apical flux, and showed substrate specificity. This system was further inhibited by verapamil, suggesting some homology with P-glycoprotein, the principal mediator of drug resistance in multidrug resistant cancer cells. Preliminary evidence was also obtained suggesting an additional polarized uptake system for the same peptides in the basolateral membrane. Upon saturation and/or inhibition of the active transport mechanisms with verapamil, the peptide fluxes in apical-to-basolateral direction and the basolateral-to-apical direction converged and became controlled by the passive mechanism. Since the intent of the modeling was to provide useful templates for the design of probing experiments and to delineate and quantify mass transfer mechanisms at the AP and BL membranes and their interrelationships, theoretical equations were developed for a host of kinetic boundary conditions: (a) AP-->BL and BL-->AP transfluxes, (b) bidirectional effluxes from substrate-preloaded cells, (c) undirectional efflux across the AP or BL membrane from preloaded cells, and (d) uptake kinetics via the AP or BL membrane leading to equilibrium. Furthermore, flux expressions were reduced to membrane permeability coefficients to accommodate passive diffusion, saturation, inhibition, and directionality. The diffusional mass transport resistances of the aqueous boundary layers and microporous filter support of the cell monolayer were necessarily included.