Fig. 1.

Schemes depicting the uptake and efflux protocol and the mechanistic model of [³H]TCA disposition in SCH. (A) Uptake and efflux studies were conducted in the presence of standard (+Ca²⁺) Hanks’ balanced salt solution (Std HBSS). Tight junctions remained sealed throughout the study period. (B) Tight junctions remained open throughout the study period by preincubating with Ca²⁺-free HBSS, then performing an uptake phase in standard HBSS to provide relief from the removal of Ca²⁺, followed by a brief wash and efflux in Ca²⁺-free HBSS. In the uptake and efflux protocols, the dashed box represents preincubation with 10 µM TGZ in standard HBSS for TGZ-treated groups. Gray shading represents inclusion of the substrate, 1 µM [³H]TCA, in standard HBSS during the uptake phase. Black shading represents 1-minute wash followed by 10- (human SCH) or 15-minute (rat SCH) efflux phase in standard or Ca²⁺-free HBSS. In the model schemes, X, V, and C denote the mass of TCA, compartmental volume, and TCA concentration, respectively. Subscripts on mass, volume, and concentration terms denote the corresponding compartment in the model scheme. Superscripts represent the presence (+, intact tight junctions; cells + bile) and absence (−, modulated tight junctions; cells) of Ca²⁺ in the preincubation and efflux buffer. CL_uptake, CL_BL, and CL_Bile represent clearance values for uptake from buffer into hepatocytes, efflux from hepatocytes into buffer, and canalicular excretion from hepatocytes, respectively. K_Flux represents the first-order rate constant for flux from bile networks into buffer.