Mechanism of organic cation transport in rabbit renal basolateral membrane vesicles

P. P. Sokol and T. D. McKinney Department of Medicine, Indiana University School of Medicine, Indianapolis. The transports of [3H]tetraethylammonium (TEA), [3H]procainamide (PCA), and N1-[3H]methylnicotinamide (NMN) were studied in rabbit renal basolateral membrane vesicles (BLMVs) by use of a rapid filtration assay. All three compounds exhibited a similar uptake profile into the BLMVs and reached equilibrium by 1 h. In the presence of valinomycin, a K+ ionophore and K+ gradients (in to out), an inside-negative potential difference (PD) was generated that stimulated the uptake of TEA, PCA, and NMN by 1.9-, 1.9-, and 2.1-fold, respectively. The effect of PD could be blocked by the organic cation transport inhibitor mepiperphenidol. An inside-negative PD was also generated by a pH gradient (inside acidic). An overshoot of TEA uptake was produced, which was blocked by a valinomycin voltage clamp. Counterflow studies revealed that 1 mM TEA was capable of trans-stimulating 50 microM [3H]TEA uptake and producing a peak overshoot of nearly three times the equilibrium value, which was not abolished in the presence of a valinomycin voltage clamp or a gramicidin pH clamp. When an inside-negative PD was imposed on 1 mM TEA-loaded BLMVs, the uptake of [3H]TEA was 33% less. In contrast, neither NMN nor PCA produced a trans-stimulation of [3H]NMN or [3H]PCA transport, respectively. In addition, the effect of several organic cations on the TEA-TEA exchange mechanism was studied. Mepiperphenidol, PCA, choline, cimetidine, and NMN all demonstrated cis inhibition (82, 81, 58, 51, and 20%, respectively). Arginine, a basic amino acid, and probenecid, an organic anion transport inhibitor, had no effect. Choline was capable of trans-stimulating TEA uptake.