Contributions of saturable active secretion, passive transcellular, and paracellular diffusion to the overall transport of furosemide across adenocarcinoma (Caco-2) cells.

Furosemide permeation across Caco-2 cells was investigated to determine if previously reported directional differences in transport rates are due to a saturable, energy dependent process. In addition, studies were carried out to determine the route of permeation for this drug. By comparing apical (A) to basolateral (B) and B to A directional transport across Caco-2 cells, a saturable, nonlinear component to furosemide transport was observed. Transport in the secretory direction was fit to yield the following apparent parameters K(m) = 63 +/- 28 microM, V(max) = 436 +/- 137 pmol/cm(2)h, and P(app) = 3.7 +/- 0.9 x 10(-7) cm/s. Evidence of energy dependence was demonstrated using both metabolic inhibition, and transport against a diffusion gradient methods. Disruption of tight junctions by use of the calcium chelator, EGTA, caused a significant increase in furosemide transport (twofold and 12-fold increases in B to A and A to B, respectively) indicating the importance of the paracellular route. We conclude that furosemide secretion from Caco-2 cells is the result of saturable active transport and passive diffusion that has a significant paracellular component.