Abstract
The pharmacological effect of a drug depends primarily on its concentration at the place of action. This is the plasma concentration for many drugs, but not for diuretics, which have their main site of action on the luminal side of the renal tubules. They reach this site partly by filtration. However, since most diuretics are to a large extent bound to plasma albumin (Knauf et al. 1992) and are therefore not available for filtration, transtubular secretion of the substances plays a pivotal role. In general, transtubular transport of xenobiotics takes place in the proximal tubule, where several transport systems are involved (Fig. 1): a contraluminal transport system for hydrophobic organic anions (para-aminohippurate, PAH) (Ullrich et al. 1987a,b, 1988, 1989, 1991a), luminal and contraluminal transport systems for hydrophobic organic cations (N1-methylnicotinamide, NMeN+; tetraethylammonium, TEA+; N-methyl-4-phenylpyridinium, MPP+; choline+) (David et al. 1995; Ullrich et al. 1991b, 1992a; Ullrich and Rumrich), contraluminal and luminal transport systems for dicarboxylates (methylsuccinate, succinate) (Sheridan et al. 1983; Ullrich et al. 1984), and contraluminal and luminal transport systems for sulfate (David and Ullrich 1992; Ullrich et al. 1985a,b,c). The specificities of these transport systems have also been evaluated.
