Kinetic evaluation of carrier-mediated transport of ouabain and taurocholic acid in isolated rat hepatocytes. Evidence for independent transport systems.

Ouabain and taurocholic acid (Tch) share in common the cyclopentanophenanthrene (steroid) nucleus, and both are concentrated in the liver by hepatic sinusoidal carrier-mediated transport processes. Multiple transport systems for Tch uptake have been implicated, and Tch is an effective inhibitor of ouabain uptake. To determine if the ouabain transport system is related to transport of Tch, kinetic studies were conducted to examine the nature of cross-inhibition of ouabain and Tch. Tch was found to inhibit ouabain uptake in a competitive manner, with a Ki approximately ten times less than the Km for ouabain. However, ouabain failed to inhibit total Tch uptake in a competitive manner when added to the system at the same time as the Tch substrate. Preincubation of cells with ouabain resulted in noncompetitive inhibition of sodium-dependent Tch uptake but had no effect on sodium-independent Tch uptake. Ouabain had a weak competitive inhibitory effect on sodium-independent transport of Tch, but the Ki was approximately ten times greater than the Km for ouabain. These results demonstrate that the ouabain transport system is distinct from both the sodium-dependent and sodium-independent transport systems for Tch. Tch apparently binds competitively to the ouabain transport system but is not effectively transported across the cell membrane by this system.