LUMINAL TRANSPORT-SYSTEM FOR CHOLINE(-TRANSPORT SYSTEMS IN THE RAT PROXIMAL TUBULE - KINETICS, SPECIFICITY - ALKYL() IN RELATION TO THE OTHER ORGANIC CATION)ARYLAMINES, ALKYLAMINES WITH OH, O, SH, NH2, ROCO, RSCO AND H2PO4-GROUPS, METHYLAMINOSTYRYL, RHODAMINE, ACRIDINE, PHENANTHRENE AND CYANINE COMPOUNDS/

The efflux of [H-3] choline(+) from the proximal tubular lumen was mea sured by using the stop-flow microperfusion method. The 2-s efflux of [H-3] choline(+) follows kinetics with a Michaelis constant, K-m = 0.1 8 mmol . l(-1), maximal flux, J(max) = 0.43 pmol . cm(-1). s(-1) and a permeability term = 38.0 mu m(2) . s(-1). Replacement of Naf by N-met hyl-D-glucamine(+) or Li+, or a change of luminal pH do not alter chol ine(+) efflux. Replacement of Na+ by Cs+ inhibits 2-s choline(+) (0.01 mmol . l(-1)) efflux by 22% and replacement by K+ inhibits by 49%, in dicating that the electrical potential difference across the brush bor der membrane acts as driving force for choline(+) transport. Comparing the apparent luminal inhibitory constant values for choline (app. K-i ,K-l,K-choline+) With the chemical structure of inhibiting substrates, it was found that the inhibitory potency of amines with high pK(a) va lues, i.e. high basicity, and of quaternary ammonium compounds (tetrae thyl to tetrahexylammonium) increases with their hydrophobicity in a s imilar manner as was observed previously against the contraluminal N-1 -methylnicotinamide (NMeN(+)) transporter and the luminal H+/organic cation (N-methyl-4-phenylpyridinium) (MPP(+)) exchanger. Independently of their hydrophobicity, an increase in the inhibitory potency of the homologous series of aminoquinolines against the choline+ transporter was observed with increasing pK(a) values, i.e. increasing basicity, as was found previously against the two other organic cation transport ers. A third parameter influencing the interaction with the choline(+) transporter is the presence of two amino groups with high pK(a) value s or one amino group and a permanent positive charge, as is documented with the two-ring aminostyryl and rhodamine compounds, as well as thr ee-ring aminoacridine, aminophenanthrene and cyanine compounds. Thus w ith the aminostyryl, pyridinium(+), rhodamine, phenanthridium(+) and c yanine(+) dyes app.K-i,K-l,K-choline+ values of between 0.01 and 0.07 mmol . l(-1) have been found. A fourth parameter influencing the choli ne(+) transporter is the presence of an OH group on the C atom next to that bearing the N atom (as in choline(+)) or an ester-OCOR group (ac etylcholine(+), butyrylcholine(+)) or a thioester-SCOR-group (acetylth iocholine(+), butyrylthiocholine(+)); or an -OP(OH)(2)(OR) group (glyc erylphosphoryl-choline(+)), resulting in app.K-i,K-l,K-choline+ values of 0.3-1.0 mmol . l(-1) Thus the substrates for the luminal choline() transporter have general features in common with the luminal H+/orga nic cation exchanger and the contraluminal organic cation transporter, i.e. hydrophobicity and basicity. Additional parameters for interacti on are an OH (or similar) group positioned a favourable distance from the N atom or a second amino/ammonium group in multi-ring compounds.