Abstract

The substrate specificity of the avian renal organic cation exchanger was examined in isolated renal brush-border membrane vesicles. Endobiotic and xenobiotic organic cations (OCs) were tested at a concentration of 100 μM for cis-inhibition of¹⁴C-tetraethylammonium (TEA)/H⁺ exchange and at 1 mM for trans-stimulation of ¹⁴C-TEA efflux. The xenobiotic cations amiloride, cimetidine, mepiperphenidol, procainamide, quinidine, quinine, and ranitidinecis-inhibited TEA uptake ≥ 80%; isoproterenol and unlabeled TEA inhibited uptake at least 30%. In contrast, the endogenous cations acetylcholine, choline, and guanidine did not inhibit TEA uptake; however, epinephrine, N¹-methylnicotinamide, serotonin, and thiamine inhibited uptake as much as 60%. Each endogenous cation, except thiamine,trans-stimulated TEA efflux, and xenobiotic cations, excluding isoproterenol and TEA, trans-inhibited TEA efflux. The data suggest that the avian renal tubule luminal OC exchanger has greater affinity for xenobiotic cations than for endobiotic cations, but greater transport capacity for endobiotics than for xenobiotics.