INTERACTION OF RAT-KIDNEY P-GLYCOPROTEIN WITH A URINARY COMPONENT ANDVARIOUS DRUGS INCLUDING CYCLOSPORINE-A

The interaction of rat renal P-glycoprotein with various drugs and a h ydrophobic component found in rat urine was studied to gain an underst anding of both its transport function in kidney and its potential role in drug secretion and drug-induced nephrotoxicity. Rat kidney brush-b order membranes (BBM) were photolabeled with [H-3]azidopine, a calcium -channel blocker that covalently labels P-glycoprotein. P-glycoprotein was immuno-precipitated with a rabbit polyclonal antibody against the human MDR1 protein (multidrug resistance gene class 1). The amount of [H-3]azidopine incorporated into P-glycoprotein was quantitated follo wing gel electrophoresis and fluorography. Photolabeling inhibition as says were conducted with a panel of drugs known to interact with P-gly coprotein in multidrug-resistant cells. Verapamil or quinidine [half-m aximal inhibition constant (K-0.5) = 1 mu M], vinblastine (K-0.5 = 3 m u M), and doxorubicin or daunomycin (K-0.5 = 10 mu M) all blocked [H-3 ]azidopine photolabeling of renal P-glycoprotein. Of the drugs tested, the immunosuppressant drug, cyclosporin A, interacted with kidney P-g lycoprotein with the highest affinity (K-0.5 = 50 nM). However, the ca rdiac glycoside, digoxin, failed to inhibit P-glycoprotein photolabeli ng. A hydrophobic rat urine extract prepared by reverse-phase chromato graphy also blocked photolabeling of renal P-glycoprotein. Our current hypothesis is that various drugs may inhibit urinary excretion of an endogenous substrate by virtue of their ability to bind with high affi nity to P-glycoprotein. A hypothesis of drug-induced nephrotoxicity ba sed on the interaction of various compounds like cyclosporin A with P- glycoprotein is presented.