PARAQUAT(2+)/H+ EXCHANGE IN ISOLATED RENAL BRUSH-BORDER MEMBRANE-VESICLES

The mechanism(s) by which paraquat (1, 1'-dimethyl-4,4'-bipyridinium), a divalent organic cation (OC) and proximal tubule nephrotoxicant, crosses renal cell membranes is unclear. The structurally related monovalent OC, 1-methyl-4-phenylpyridinium (MPP(+)), crosses the renal brush border via OC/H+ exchange using the same pathway by which tetraethylammonium (TEA) is transported. We examined whether paraquat shares the TEA(MPP(+))/H+ exchanger by examining C-14-paraquat transport in rabbit renal BBMV. Compared to a pH equilibrium condition (pH 7.5(in):7.5(0)), an H-gradient (pH 6(in):7.5(0)) stimulated the 5 s and 60 s uptakes of 230 mu M paraquat by 51% and 108%, respectively, and this stimulation was blocked by both 20 mM unlabeled paraquat and TEA. Pre-loading BBMV with 2 mM unlabeled TEA (under conditions of pH equilibrium) stimulated by 3-fold the 60 s uptake of 120 mu M paraquat and by 5 min produced a transient intravesicular accumulation of paraquat that exceeded equilibrium (2 h) uptake by 45%. The presence of 200 mu M paraquat in the extravesicular solution competitively inhibited H-gradient-stimulated transport of C-14-TEA in renal BBMV, increasing the apparent K-t for TEA transport from 169 mu M to 379 mu M, without significantly influencing the J(max) (16.0 vs. 15.4 nmol mg(-1) min(-1)). The calculated K-i for paraquat (presumably equal to its K-t for transport) after transport was between 160 and 220 mu M (depending upon the method of estimation). Significantly, the K-t for MPP(+)/H exchange is 12 mu M, suggesting that the affinity of the exchanger is profoundly influenced by the presence on paraquat of a second positive charge. We conclude that renal transport of paraquat involves the OC/H+ exchanger of proximal cell luminal membranes and that this pathway may play a role in the renal secretion of polyvalent organic cations.