BIOPHYSICAL MODEL OF PASSIVE AND POLARIZED ACTIVE-TRANSPORT PROCESSESIN CACO-5 CELLS - APPROACHES TO UNCOUPLING APICAL AND BASOLATERAL MEMBRANE EVENTS IN THE INTACT CELL

This report is aimed at the biophysical modeling of transmembrane even ts involving a passive diffusion and directional pumplike mechanism at the apical (AP) and basolateral (BL) membranes of cultured cell monol ayers. 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, e nhanced basolateral to apical flux, and showed substrate specificity. This system was further inhibited by verapamil, suggesting some homolo gy with P-glycoprotein, the principal mediator of drug resistance in m ultidrug resistant cancer cells. Preliminary evidence was also obtaine d suggesting an additional polarized uptake system for the same peptid es in the basolateral membrane. Upon saturation and/or inhibition of t he active transport mechanisms with verapamil, the peptide fluxes in a pical-to-basolateral direction and the basolateral-to-apical direction converged and became controlled by the passive mechanism. Since the i ntent of the modeling was to provide useful templates for the design o f probing experiments and to delineate and quantify mass transfer mech anisms at the AP and BL membranes and their interrelationships, theore tical equations were developed for a host of kinetic boundary conditio ns: (a) AP --> BL and BL --> AP transfluxes, (b) bidirectional effluxe s from substrate-preloaded cells, (c) undirectional efflux across the AP or BL membrane from preloaded cells, and (d) uptake kinetics via th e AP or BL membrane leading to equilibrium. Furthermore, flux expressi ons were reduced to membrane permeability coefficients to accommodate passive diffusion, saturation, inhibition, and directionality. The dif fusional mass transport resistances of the aqueous boundary layers and microporous filter support of the cell monolayer were necessarily inc luded.