Datensatz

Zusammenfassung

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-[³H]methylnicotinamide (NMeN⁺) and [³H]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 K_m 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 HCO⁻³ , 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.