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Notes: Abstract In order to test what chemical structure is required for a substrate to interact not only with the contraluminal organic anion (p-aminohippurate, PAH) transporter, but also with the organic cation (N 1-methylnicotinamide, NMeN, or tetraethylammonium, TEA) transporter, the stop-flow peritubular capillary perfusion method was applied and app. K i values were evaluated. Zwitterionic hydrophobic dipeptides not only interact with PAH but also with NMeN transport although with lower inhibitory potency (K i,PAH=0.2–1.4; K i,NMeN 614 mmol/l). Amongst the zwitterionic cephalosporins, which all inhibit PAH transport, the amino cephalosporin analogue cefadroxil was identified to interact also with NMeN transport (K i,PAH = 3.0, K i,NMeN=11.2 mmol/l). All Zwitterionic naphthyridine and oxochinoline gyrase inhibitors tested inhibit NMeN transport with app. K i,NMeN values between 1.2 mmol/l and 4.7 mmol/l; the naphthyridine analogues show a good inhibitory potency against PAH transport (K i,PAH ≈ 0.4 mmol/l), the piperazine-containing quinolone analogues have a moderate inhibitory potency (K i,PAH=1.1–2.5 mmol/l) and the piperazine-containing pipemidic acid did not inhibit PAH transport at all. Zwitterionic thiazolidine carboxylate phosphamides also interact with both transporters (app. K i,PAH ≈ 3.0; app. K i,NMeN ≈ 18.0 mmol/l). The nonionizable oxo- and hydroxy-group-containing corticosteroid hormones also interact with the two transporters. (a) An OH group in position 21 is necessary for interaction with the PAH transporter, but not for interaction with the TEA transporter. (b) Introduction of an OH group in position 17α abolishes interaction with the TEA transporter, but has different effects with the PAH transporter. (c) Introduction of an OH group in position 6 abolishes interaction with both, the PAH and the TEA transporter. (d) A change of the side-group in position 11 of corticosterone from -OH to -H to=O enhances interaction with the PAH transporter but has no effect on the interaction with the TEA transporter. Nonionizable 4- or 5-androstene analogues inhibit both transporters with app. K i between 0.16 mmol/l and 0.64 mmol/l, if the steroids are soluble in a concentration greater than 1 mmol/l. Nonionizable oxazaphosphorins with more than one chloroethyl group interact with the PAH transporter with app. K i between 0.84mmol/l and 4.9mmol/l and with the NMeN transporter with app. K i between 3.2 mmol/l and 18.7 mmol/l. Thus a substrate interacts with both transporters if it is sufficiently hydrophobic, possesses acidic and/or electron-attracting plus basic and/or electron-donating groups, or possesses several electron-attracting nonionizable groups (O, OH, Cl). A certain spatial arrangement of the interacting groups seems to be necessary.

Notes: Abstract In order to evaluate whether N-containing substrates interact with the organic “anion” (p-aminohippurate, PAH) or only with the organic “cation” (N 1-methylnicotinamide, NMeN) transport system or with both, the stop-flow peritubular capillary microperfusion method was applied in the rat kidney in situ and the apparent K i values of several classes or organic substrate against contraluminal NMeN and PAH transport were determined. Organic “anion” and organic “cation” transport are in inverted commas because neither transporter sees the degree of ionization in bulk solution, and they also accept nonionizable substrates [Ullrich KJ, Rumrich G (1992) Pflügers Arch 421:286–288]. Amines must be sufficiently hydrophobic (phenylethylamine, piperidine, piperazine) in order to interact with NMeN transport. Additional Cl, Br, NO2 or other electronegative groups render them inhibitory towards PAH transport also. Such bisubstrate amines were identified as follows: metoclopramide, bromopride, diphenhydramine, bromodiphenhydramine, verapamil, citalopram, ketamine, mefloquine, ipsapirone, buspirone, trazodone, H7 and trifluoperazine. Imidazole analogues interact with both transporters if they bear sufficiently hydrophobic alkyl or aryl groups or electronegative sidegroups. Bisubstrate imidazole analogues are tinidazole, pilocarpine, clonidine, azidoclonidine and cimetidine. Pyridines and thiazoles interact with the NMeN transporter if they have an additional ring-attached NH2 group. Again with an additional Cl, Br, or NO2 group the aminopyridines and aminothiazoles also become inhibitors for the PAH transporter. Amongst the guanidines only substances with several electronegative side-groups such as guanfacine, amiloride, benzylamiloride and ranitidine, interact with both transporters. Amongst the phenylhydrazines only 4-bromophenylhydrazine interacts with the NMeN transporter and 4-nitrophenylhydrazine with both transporters. Quinoline (isoquinoline) and its amino and hydroxy analogues interact with both transporters, their pKa values correlate directly with the affinity to the NMeN transporter and reciprocally with their affinity to the PAH transporter. In experiments with labelled substrates only the sufficiently hydrophilic cimetidine, amiloride and ranitidine show a saturable transport, which can be inhibited by probenecid (apalcillin) and tetraethylammonium in an additive manner. The highly hydrophobic substrates verapamil, citalopram, imipramine, diltiazem and clonidine enter the cell very fast in an unsaturable and uninhibitable manner, apparently in the undissociated form, since N-methyl-4-phenylpyridinium, which — disregarding its ionization — is similarly hydrophobic, shows a transport behaviour similar to that of tetraethylammonium [Ullrich et al. (1991) Pflügers Arch 419:84–92]. Ethidium bromide and dimidium bromide, which have a permanent cationic quaternary nitrogen and two sufficiently electronegative NH2 groups, also interact with both transporters. The data indicate that a molecule qualifies as a bisubstrate if it carries both the essentials for organic anion (PAH) transport: hydrophobicity, sufficient acidity or electron-attracting O, OH, Cl, Br, NO2 groups, plus the essentials for organic cation transport: hydrophobicity, sufficient basicity or electron-donating N-containing groups. The nitrogen atoms in the N-containing molecules quinoline (pK a 4.9), isoquinoline (pK a 5.4) and benzylpyridine (pK a 5.13) are of such low basicity that they apparently can also interact with the PAH transporter. Apparent hydrophobicity (disregarding ionization) determines interaction with the transporters, while real hydrophobicity [log (octanol distribution values)] determines the diffusion through the lipid bilayer of the cell membrane.