Interaction of P-glycoprotein with a hydrophobic component of rat urine.

The presence of an endogenous P-glycoprotein substrate in rat urine was examined by testing the ability of a hydrophobic extract to reverse multidrug resistance in CHO cells and to inhibit [3H]azidopine photolabelling. The accumulation of several hydrophobic drugs and dyes, known to be transported by P-glycoprotein, was dramatically enhanced in multidrug-resistant CHO cells (CHRC5) by a component contained in a hydrophobic extract prepared from rat urine by octadecyl (C18) reverse phase chromatography. The biological action of this urinary component involves a direct interaction with P-glycoprotein since it blocked photolabelling of the protein with [3H]azidopine. The effective concentration of the substance required to enhance drug accumulation and inhibit photolabelling was similar and within the range of its urinary content. These results suggest that a hydrophobic substance in urine may be an endogenous substrate of kidney P-glycoprotein.