P-glycoprotein in the catfish intestine: inducibility by xenobiotics and functional properties.

The p-glycoprotein (pgp)-mediated multixenobiotic resistance (MXR) mechanism of aquatic animals has been associated with protection against pollution. Recent studies in mammals suggest that intestinal pgp may modulate intestinal bioavailability of dietary xenobiotics. In order to further delineate this mechanism in the catfish, these studies: (1) examined the pgp-related distribution in the intestine and liver of catfish, (2) evaluated the MXR response following exposure to various dietary xenobiotics and a prototypic pgp inducer and (3) evaluated pgp functional activity in membrane vesicles, using prototypic substrates and inhibitors. For this purpose, catfish were exposed in vivo to the pgp inducer vincristine (VIN), and the xenobiotics beta-naphthoflavone (BNF), benzo[a]pyrene (BaP), and 3,4,3',4'-tetrachlorobiphenyl (TCB). Membrane vesicles, prepared from liver and intestine (proximal and distal sections) of control and exposed catfish, were subjected to SDS PAGE, Western Blot, and detection with the pgp C219 monoclonal antibody. Transport activity was evaluated in vitro using the pgp substrate [3H]vinblastine (VBL), and the pgp inhibitor verapamil (VP). Immunoblot studies demonstrated a pgp-related protein of approximately 170 kDa in the intestine and liver of catfish. This protein appears to be very susceptible to degradation, and was present in higher levels in the liver, in comparison to the intestine, where regional differences were not observed. Dietary exposure to the pgp substrate VIN, or the xenobiotics BNF, BaP, and TCB, did not appear to affect pgp-related reactivity. Transport studies with VBL indicate that the pgp-related protein of the catfish intestine displays classic pgp-mediated multidrug resistance (MDR) characteristics, such as energy-dependency, and sensitivity to VP. These studies suggest that the pgp-related protein in the catfish intestine and liver is not only immunochemically, but also functionally related to the mammalian MDR. Moreover, the results presented indicate that pgp-related reactivity and transport in intestinal vesicles of catfish may be influenced by factors including method sensitivity, sample collection, sample preparation, and immunoblot conditions.