Proton-coupled organic cation transport in renal brush-border membrane vesicles

We had previously proposed that organic cations are transported across the brush-border membrane in the canine kidney by a H+ exchange (or antiport) system (Holohan, P.D. and Ross, C.R. (1981) J. Pharmacol. Exp. Ther. 216, 294-298). In the present report, we demonstrate that in brush-border membrane vesicles the transport of organic cations is chemically coupled to the countertransport of protons, by showing that the uphill or concentrative transport of a prototypic organic cation, N1-methylnicotinamide (NMN), is chemically coupled to the flow of protons down their chemical gradient. In a reciprocal manner, the concentrative transport of protons is coupled to the counterflow of organic cations down their concentration gradient. The transport of organic cations is monitored by measuring [3H]NMN while the transport of protons is monitored by measuring changes in acridine orange absorbance. The functional significance of the coupling is that a proton gradient lowers the Km and increases the Vmax for NMN transport.