The mechanism of the 3H-noradrenaline releasing effect of various substrates of uptake1: multifactorial induction of outward transport |
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Authors: | A Langeloh H Bönisch U Trendelenburg |
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Institution: | (1) Institut für Pharmakologie und Toxikologie der Universität Würzburg, Versbacher Strasse 9, D-8700 Würzburg, Federal Republic of Germany |
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Abstract: | Summary The mechanism of action of indirectly acting sympathomimetic amines was studied in the rat vas deferens, after inhibition of vesicular uptake (by reserpine), of MAO (by pargyline) and of COMT (by U-0521). 1. K
m-values for the neuronal uptake of 12 substrates were determined as the IC50 of the unlabelled substrate inhibiting the initial rate of neuronal uptake of 0.2 mol/l 3H-(–)-noradrenaline. The IC50 ranged from 0.35 mol/l (for (+)-amphetamine) to 44.3 mol/l (for 5-HT). The V
max (determined for 8 substrates) was substrate-dependent. 2. Tissues were loaded with 0.2 mol/l 3H-(–)-noradrenaline and then washed out with amine-free solution. All 12 substrates of uptake1, induced an outward transport of 3H-noradrenaline, and equieffective concentrations were positively correlated with K
m. Moreover, the EC50 for release greatly exceeded K
m. It is proposed that this discrepancy between EC50 and K
m is indicative of the fact that at least four factors (each one in strict dependence on K
m) contribute to the initiation of outward transport of 3H-noradrenaline: a) the appearance of the carrier on the inside of the axonal membrane (facilitated exchange diffusion), b) the co-transport of Na+, c) the co-transport of Cl– (both lowering the K
m for 3H-noradrenaline at the inside carrier), and d) inhibition of the re-uptake of released 3H-noradrenaline (through competition for the outside carrier). 3. At least for amezinium, V
max. appears to limit the maximum rate of outward transport. 4. For some substrates (especially for the highly lipophilic ones) bell-shaped concentration-release curves were obtained. Apparently, inward diffusion of the substrates can lead to partial saturation of the inside carrier. Moreover, if release is expressed as a FRL (fractional rate of loss), loading with 37 mol/l 3H-(–)-noradrenaline decreased the releasing effect of various substrates. In this case the inside carrier appears to be partially saturated by the high axoplasmic concentration of 3H-noradrenaline. 5. Very high concentrations (especially of highly lipophilic substrates) were able to induce an additional intraneuronal release mechanism, presumably by increasing the pH inside storage vesicles.Abbreviations COMT
catechol-O-methyl transferase
- DOMAA
dihydroxymandelic acid
- DOPEG
dihydroxyphenylglycol
- FRL
fractional rate of loss (rate of efflux/tissue tritium content)
- 5-HT
5-hydroxytryptamine
- MAO
monoamine oxidase
- OM-fraction
sum of all O-methylated metabolites of noradrenaline, deaminated or not
This study was supported by the Deutsche Forschungsgemeinschaft (Bo 521, Tr 96 and SFB 176). Some of the results were presented to the German Pharmacological Society (Langeloh 1986)A. L. was the recipient of a fellowship of the Humboldt-Foundation
Send offprint requests to: U. Trendelenburg |
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Keywords: | Adrenergic nerve endings Outward transport of noradrenaline Uptake1 Indirectly acting sympathomimetic amines Release of noradrenaline |
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