Mechanisms of organic cation transport in kidney plasma membrane vesicles: 1. Countertransport studies.

Transport for organic cations has been described in both luminal and antiluminal membranes. The two transport systems difer from one another on the basis of kinetic parameters and countertransport. By using N1-[3H]methylnicotinamide as the indication cation, a series of organic cations (which were known to be secreted) were tested for their capacity to stimulate countertransport. The phenomenon of countertransport was symmetrical in that it was observed for both influx and efflux in both membranes. However, under appropriate conditions, certain organic cations were effective in producing "uphill" transport of N1-[3H]methylnicotinamide in the luminal membrane and the effect was limited to that membrane exclusively. Detailed analysis of countertransport in the luminal membrane showed quantitative differences in the maximal stimulation produced by the various cations: giving rise to the relationship that the concentration of organic cation which gave one-half maximal N1-[3H]methylnicotinamide countertransport approximated its affinity constant. The data were interpreted to suggest that all the members of the organic cation series tested are translocated across the luminal membrane by the same carrier but that they are moved at different rates.