Mechanism for drug absorption from rat-liver surface membrane: effect of dose and transport inhibitors on the pharmacokinetics of phenol red.

We examined the effect of dose and transport inhibitors on the pharmacokinetics of phenol red as a model drug after application to rat liver surface in-vivo, employing a cylindrical glass cell (i.d. 9 mm, area 0.64 cm2), to elucidate the mechanism for drug absorption from liver surface membrane. Absorption ratios of phenol red in 6 h were determined to be 91.1, 91.8 and 89.9% at a dose of 0.3, 1 and 3 mg, respectively. The AUC value for plasma concentration profile of phenol red was proportional to the dose. It is thus suggested that the absorption process of phenol red from rat liver surface does not approach saturability. The time course of the remaining amount of phenol red in the glass cell obeyed first-order kinetics at a dose of 0.3 mg, and its rate constant Ka was calculated to be 0.0069 min-1. Moreover, no significant difference was seen in the Ka value within the dose range of 0.3-3 mg, which was estimated by curve fitting of the plasma concentration profile of phenol red after application to rat liver surface in the two-compartment model with first-order absorption. 2,4-Dinitrophenol (0.3 mg) and probenecid (0.5 and 1 mg), inhibitors of metabolic energy and anion transport, respectively, had no significant effect on the pharmacokinetics of phenol red after application to rat liver surface. These data demonstrate that a specific transport mechanism such as active transport is not involved in phenol red absorption from rat liver surface membrane.