INHIBITORS OF THE MULTIXENOBIOTIC RESISTANCE MECHANISM IN NATURAL-WATERS - IN-VIVO DEMONSTRATION OF THEIR EFFECTS

Xenobiotics in water that can inhibit the multixenobiotic resistance ( MXR) defense mechanism in water organisms, the chemosensitizers, may c ause severe ecotoxicological effects. We determined their concentratio ns in polluted and unpolluted river waters and seawaters using differe nt methods. Here we present the method that enables direct determinati on of MXR-inhibitory potential in natural waters. This method measures the modulating potential of river water or seawater on the rate of rh odamine B (RB) accumulation in gills of a freshwater clam (Dreissena p olymorpha) or a marine mussel (Mytilus galloprovincialis), respectivel y. The sensitivity of the method with RB even enabled the determinatio n of MXR inhibitors by measuring their potential to modulate the rate of efflux of RB from gills of mussels. The concentrations of MXR inhib itors found by these methods (expressed in mu M of verapamil-equivalen ts) were higher in natural waters from polluted rivers or in natural s eawaters from polluted marine sites than in natural waters from unpoll uted rivers or in natural seawaters from unpolluted sites. Polluted wa ters enhanced accumulation or decreased efflux rate of RB, a good subs trate of P-glycoprotein, demonstrating that the complex mixture of che micals present in polluted waters contains MXR-inhibiting potential. I n addition, the efflux version of the method with RB has the clear adv antage of not requiring that organisms be killed and so allows repeate d use of the same individuals. In addition, it is the simplest and the most reliable method for measuring the activity of MXR in these organ isms.