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  • 1
    ISSN: 1432-2013
    Keywords: Transport kinetics ; Distribution ratio ; Driving forces ; Hydrophobicity plot ; Choline ; Acetylcholine
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract 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 N 1-[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.
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