Cation Proton Antiporter Research Articles

Organic cation transport in human renal brush-border membrane vesicles

The renal proximal tubule is responsible for the active elimination of organic cations. Studies in brush-border membrane vesicles (BBMV) suggest that active organic cation transport is mediated by an organic cation-proton antiporter. The goals of this study were to determine whether this transporter is expressed in human kidney and to elucidate its characteristics. Transport of the organic cations N1-[3H]methylnicotinamide (NMN) and [14C]tetraethylammonium (TEA) was determined by rapid filtration in BBMV of donor human kidneys. The uptake of NMN and TEA was driven against a concentration gradient by an outwardly directed proton gradient. NMN uptake was inhibited by the organic cations TEA, NMN, quinine, and cimetidine, but was not affected by p-aminohippuric acid, cephalexin, and polyamines. The electroneutral transport of NMN was characterized by a Km of 0.44 +/- 0.07 mM and a Vmax of 24.4 +/- 15.2 pmol.mg protein-1.s-1. The rate of proton efflux from BBMV increased in the presence of an inwardly directed TEA gradient. Preloading BBMV with NMN or TEA trans-stimulated the initial rate of uptake of TEA. Therefore the human proximal tubule expresses an organic cation-proton antiporter in the brush-border membrane. The antiporter is similar to that described in other mammalian species but shows some differences in its transport characteristics.

Evidence for an organic cation-proton antiport system in brush-border membranes isolated from the human term placenta.

Uptake of guanidine, an endogenous organic cation, into brush-border membrane vesicles isolated from human term placentas was investigated. Initial uptake rates were manyfold greater in the presence of an outward-directed H+ gradient ([pH]o greater than [pH]i) than in the absence of a H+ gradient ([pH]o = [pH]i). Guanidine was transiently accumulated inside the vesicles against a concentration gradient in the presence of the H+ gradient. The H+ gradient-dependent stimulation of guanidine uptake was not due to a H+-diffusion potential because an ionophore (valinomycin or carbonylcyanide p-trifluoromethoxyphenylhydrazone)-induced inside-negative membrane potential failed to stimulate the uptake. In addition, uphill transport of guanidine could be demonstrated even in voltage-clamped membrane vesicles. The H+ gradient-dependent uptake of guanidine was inhibited by many exogenous as well as endogenous organic cations (cis-inhibition) but not by cationic amino acids. The presence of unlabeled guanidine inside the vesicles stimulated the uptake of labeled guanidine (trans-stimulation). These data provide evidence for the presence of an organic cation-proton antiporter in human placental brush-border membranes. Kinetic analysis of guanidine uptake demonstrated that the uptake occurred via two saturable, carrier-mediated transport systems, one being a high affinity, low capacity type and the other a low affinity, high capacity type. Studies on the effects of various cations on the organic cation-proton antiporter and the Na+-H+ exchanger revealed that these two transport systems are distinct.