Transport of N¹-methylnicotinamide across brush border membrane vesicles from rabbit kidney

A preparation of isolated brush border membranes from the rabbit renal cortex was used to examine the characteristics of N¹-methylnicotinamide (NMN) transport in the kidney. Transport was independent of the presence of Na⁺ under Na⁺ equilibrium conditions. However, outwardly directed Na⁺ gradients stimulated NMN uptake, whereas inwardly directed Na⁺ gradients inhibited NMN uptake. Transport appeared to involve two parallel processes: one saturable with a J(max) of 5 nmol·mg protein^-1·min^-1 and an apparent K(t) of 0.6 mM, and a second that behaved like passive diffusion. Countertransport of NMN was observed when vesicles were preloaded with either NMN or another organic cation, tetraethylammonium (TEA). TEA and several structural analogues, as well as a wide variety of other organic cations and bases, were effective inhibitors of NMN uptake, though nicotinamide and p-aminohippuric acid did not interact with the uptake process. Outwardly directed proton gradients (pH 6.0 in, 7.6 out) stimulated transport, suggesting that NMN uptake may involve a countertransport of H⁺. The electrical potential difference across the vesicle membrane was manipulated using gradients of the permeant organic ion thiocyanate (SCN^-); an outwardly directed gradient of SCN^- (i.e., a depolarizing condition) stimulated uptake and produced a transient accumulation of NMN above that noted at equilibrium, whereas an inwardly directed SCN^- gradient inhibited uptake of NMN. The data can be explained by postulating the presence of an electrogenic NMN⁺-H⁺ antiporter in the rabbit luminal membrane that could play a role in organic cation secretion.