In order to study the characteristics of contraluminal organic cation transport from the blood site into proximal tubular cells the stopped-flow capillary perfusion method was applied. The disappearance of N1-[H-3]methylnicotinamide (NMeN+) and [H-3]tetraethylammonium (TEA+) at different concentrations and contact times was measured and the following parameters evaluated: K(m,NMeN) = 0.54 mmol/l, J(max,NMeN) = 0.4 pmol s-1 cm-1; K(m,TEA) = 0.16 mmol/l, J(max,TEA) = 0.8 pmol s-1 cm-1. TEA+ inhibited NMeN+ transport and NMeN+ the uptake of TEA+. Thereby, the K(i) values for inhibition correspond closely to the Km values for uptake. Similar inhibitory potencies of ten organic cation against TEA+ and NMeN+ transport provide further evidence for a common transport system. Omission of HCO3-, or Na+ and addition of K+ (with or without Ba2+) reduce NMeN+ transport, while omission of K+ (with or without valinomycin) or addition of thiocyanate has no effect. Since the manoeuvres that depolarize contraluminal electrical potential difference reduce NMeN+ transport, cell-negative electrical potential difference is suggested as a driving force for contraluminal organic cation transport from the interstitium into the cell. Furthermore, the inhibitory potency (app. K(i) values) of homologous series of primary, secondary, tertiary and hydroxy amines as well as of mono- and bisquarternary ammonium compounds against NMeN+ transport was tested. The inhibitory potency increased in the sequence methyl < ethyl < propyl < butyl and primary < secondary < tertiary amines < quarternary ammonium compounds. With the amines a reversed correlation between K(i,NmeN) and the octanol/water partition coefficient (log octanol) is seen. With quarternary ammonium compounds the inhibitory potency decreases with increasing molecular size: tetrabutyl- > tetrapentyl- > tetrahexyl- > tetraheptyl > tetraoctylammonium. Introducing two OH groups into triethylamine reduces the inhibitory potency while introduction of two OH groups into diethylamine or three OH groups into triethylamine abolishes the inhibitory potency as a result of reduced hydrophobicity. With choline (trimethylethanolamine) and its analogues the reversed correlation between K(i,NMeN) and log octanol was also seen. Molecules with a similar hydrophobic moiety to those of the monoammonium compounds, but with two ammonium groups, showed only a small or no inhibitory potency against NMeN+ transport. The data indicate that (a) hydrophobic moieties are important for the interaction with the contraluminal organic cation transporter, and (b) the size of the molecule can be a limiting factor. The reduced or missing interaction of the bisquarternary compound might be caused either by the second charge and/or reduced hydrophobicity and/or too large size of a molecule.
