Peritubular paraquat transport in isolated renal proximal tubules

To better understand the characteristics of peritubular transport of organic cations (OCs), the uptake of the polyvalent OC dimethylbipyridinium (paraquat) and the structurally similar monovalent OC 1-methyl-4- phenylpyridinium (MPP⁺) was measured in suspensions of rabbit renal proximal tubules. Compared to the uptake of MPP⁺, the uptake of paraquat across the peritubular membrane was a low affinity, low capacity carrier-mediated process with a J(max) of 0.52 ± 0.19 nmol · mg of protein · ^-1 min^-1 and a K(m) of 162 ± 25 μM. The uptake of MPP⁺ was a carrier-mediated process with a measured J(max) and K(m) of 1.8 ± 0.09 nmol · mg of protein. ^-1min^-1 and 28 ± 8 μM, respectively. To determine whether paraquat is a substrate for the monovalent OC pathway, the effect of unlabeled MPP⁺ and tetraethylammonium (TEA) on paraquat uptake was examined. A 1 mM concentration of the monovalent OC MPP⁺ and TEA reduced the uptake of [¹⁴C]paraquat and [³H]MPP⁺ by ~ 30 and 90%, respectively, whereas 1 mM paraquat had no effect on [³H]MPP⁺ or [¹⁴C]TEA uptake. Thus, MPP⁺, but not paraquat, appears to interact with the monovalent OC transporter. On the other hand, the polyvalent OC substrates, including the polyamines putrescine and spermine, the herbicide diquat and the divalent hexamethonium bromidehydrate had no effect on either paraquat or MPP⁺ uptake. However, the synthetic polyamine methylglyoxal bis(guanyl-hydrazone)dihydrochloride (MGBG; 1 mM) reduced both paraquat and MPP⁺ uptake (by 60 and 90%, respectively). The ability of MGBG, unlike the other polyvalent substrates, to interact with paraquat transport may be related to structural similarities in the relative location of the two charged nitronium moieties in paraquat and MGBG. These observations, collectively, suggest that paraquat is transported by a novel peritubular polyvalent OC transport system which may provide an additional mechanism by which the kidney can clear potentially harmful xenobiotics from the systemic circulation.