BISUBSTRATES - SUBSTANCES THAT INTERACT WITH RENAL CONTRALUMINAL ORGANIC ANION AND ORGANIC CATION-TRANSPORT SYSTEMS .1. AMINES, PIPERIDINES, PIPERAZINES, AZEPINES, PYRIDINES, QUINOLINES, IMIDAZOLES, THIAZOLES,GUANIDINES AND HYDRAZINES

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 clas ses or organic substrate against contraluminal NMeN and PAH transport were determined. Organic ''anion'' and organic ''cation'' transport ar e in inverted commas because neither transporter sees the degree of io nization in bulk solution, and they also accept nonionizable substrate s [Ullrich KJ, Rumrich G (1992) Pflugers Arch 421:286-288]. Amines mus t be sufficiently hydrophobic (phenylethylamine, piperidine, piperazin e) in order to interact with NMeN transport. Additional Cl, Br, NO2 or other electronegative groups render them inhibitory towards PAH trans port also. Such bisubstrate amines were identified as follows: metoclo pramide, bromopride, diphenhydramine, bromodiphenhydramine, verapamil, citalopram, ketamine, mefloquine, ipsapirone, buspirone, trazodone, H 7 and trifluoperazine. Imidazole analogues interact with both transpor ters if they bear sufficiently hydrophobic alkyl or aryl groups or ele ctronegative side-groups. Bisubstrate imidazole analogues are tinidazo le, pilocarpine, clonidine, azidoclonidine and cimetidine. Pyridines a nd thiazoles interact with the NMeN transporter if they have an additi onal 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 seve ral electronegative side-groups such as guanfacine, amiloride, benzyla miloride and ranitidine, interact with both transporters. Amongst the phenylhydrazines only 4-bromophenylhydrazine interacts with the NMeN t ransporter and 4-nitrophenylhydrazine with both transporters. Quinolin e (isoquinoline) and its amino and hydroxy analogues interact with bot h transporters, their pK(a) values correlate directly with the affinit y to the NMeN transporter and reciprocally with their affinity to the PAH transporter. In experiments with labelled substrates only the suff iciently hydrophilic cimetidine, amiloride and ranitidine show a satur able transport, which can be inhibited by probenecid (apalcillin) and tetraethylammonium in an additive manner. The highly hydrophobic subst rates verapamil, citalopram, imipramine diltiazem and clonidine enter the cell very fast in an unsaturable and uninhibitable manner, apparen tly in the undissociated form, since N-methyl-4-phenylpyridinium, whic h - disregarding its ionization - is similarly hydrophobic, shows a tr ansport behaviour similar to that of tetraethylammonium [Ullrich et al . (1991) Pflugers Arch 419:84-92]. Ethidium bromide and dimidium bromi de, which have a permanent cationic quaternary nitrogen and two suffic iently electronegative NH2 groups, also interact with both transporter s. The data indicate that a molecule qualifies as a bisubstrate if it carries both the essentials for organic anion (PAH) transport: hydroph obicity, sufficient acidity or electron-attracting O, OH, Cl, Br, NO2 groups, plus the essentials for organic cation transport: hydrophobici ty, sufficient basicity or electron-donating N-containing groups. The nitrogen atoms in the N-containing molecules quinoline (pK(a)4.9), iso quinoline (pk(a)5. 4) and benzylpyridine (pK(a)5.13) are of such low b asicity that they apparently can also interact with the PAH transporte r. Apparent hydrophobicity (disregarding ionization) determines intera ction wi