Paraquat 2+/H + exchange in isolated renal brush-border membrane vesicles

Abstract

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 14C-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 s uptakes of 230 μM 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 μM paraquat and by 5 min produced a transient intravesicular accumulation of paraquat that exceeded equilibrium (2 h) uptake by 45%. The presence of 200 μM paraquat in the extravesicular solution competitively inhibited H-gradient-stimulated transport of 14C-TEA in renal BBMV, increasing the apparent K t for TEA transport from 169 μM to 379 μM, 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 μM (depending upon the method of estimation). Significantly, the K t for MPP +/H exchange is 12 μM, 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.