| Popis: |
These studies examined the ability of ATP to stimulate transport of the organic cation tetraethylammonium (TEA) into proximal tubular brush border membrane vesicles. ATP markedly enhanced TEA uptake for 1 h or more to values severalfold above those observed in the absence of ATP. The poorly hydrolyzable analogue of ATP, AMP-PNP (adenyl-5'-yl imidodiphosphate), reduced the effect of ATP but alone did not stimulate TEA uptake. GTP and ITP also stimulated TEA uptake, whereas other nucleotides did not. ATP-stimulated TEA uptake was saturable, temperature-dependent, and markedly reduced by the organic cations amiloride, quinidine, cimetidine, and verapamil, but only modestly reduced by the organic cations N'-methylnicotinamide and choline. Some inhibitors of other transport ATPases, including N-ethylmaleimide, N,N'-dicyclohexylcarbodiimide, and oligomycin, reduced the effect of ATP, whereas ouabain, vanadate, and azide did not. 4,4'-Diisothiocyanatostilbene-2,2'-disulfonic acid also reduced TEA uptake in the presence of ATP. Vinblastine, but not actinomycin D and colchicine (all inhibitors of P-glycoprotein-mediated transport), reduced TEA uptake. The reduction of TEA transport by amiloride and cimetidine was most consistent with competitive inhibition, whereas the inhibition produced by N-ethylmaleimide and vinblastine evidently was not. ATP also stimulated uptake of N'-methylnicotinamide but not that of vinblastine. These studies have identified a previously unrecognized process by which ATP hydrolysis may directly energize the reabsorption of organic cations from the renal tubule lumen. |
