Intra- and extraneuronal metabolism of 5-hydroxytryptamine in the isolated saphenous vein of the dog

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Abstract
The uptake and metabolism of 3H-5-hydroxytryptamine (5-HT) in the isolated saphenous vein of the dog was studied, as well as the sensitivity to 5-HT and its termination of action (as determined by oil immersion experiments). After exposure of the vein to 0.23 μmol/15-HT for 30 min, about one half of the 5-HT removed was accumulated in the tissue, the other half appearing in the form of metabolites. Cocaine, chlorimipramine or denervation reduced the accumulation, but not the metabolism, of 5-HT; hydrocortisone reduced both accumulation and metabolism. Hydrocortisone plus cocaine (or phenoxybenzamine alone) caused a greater reduction of accumulation and metabolism than hydrocortisone or cocaine alone. Pargyline caused an increase in accumulation and virtually abolished metabolism; amezinium also abolished metabolism, but reduced accumulation (the same effects as caused by cocaine plus pargyline). Cocaine (12 μmol/l) caused supersensitivity to 5-HT and a prolongation of the half-time for relaxation in oil; hydrocortisone (40 μmol/l) had no effect on sensitivity, but prolonged somewhat the half-time for relaxation in oil, showing supra-additive effects with cocaine. 5-HT, taken up into adrenergic nerve terminals, was released by electrical stimulation; no appreciable reduction in fractional release occurred during repeated stimulations. Exposure to cocaine prior to incubation with 5-HT prevented the neuronal uptake of 5-HT. Compartmental analysis, done with the help of washout curves of strips pretreated with pargyline and loaded with 3H-5-HT, shows that 3H-5-HT distributed into 3 compartments plus a bound fraction. The latter (and probably compartment III) is of neuronal nature; compartments II and I appear to be of extraneuronal and extracellular nature, respectively. Strips from animals pretreated with reserpine and exposed to pargyline and cocaine were used for the study of the kinetics of the extraneuronal uptake of 5-HT. A saturable and a non-saturable component were found to be present. The saturable component of this uptake had a Km of 391 μmol/l and a Vmax of 45 nmol · g−1 · min−1; the k value of the non-saturable component was 0.022 · min−1. It is concluded that 5-HT behaves in a very similar way to biogenic catecholamines: it is taken up into adrenergic nerve terminals by a cocaine-sensitive mechanism and partially incorporated into vesicles, from where it may be released by electrical stimulation, thus behaving as a co-transmitter; it is also subject to uptake into extraneuronal cells, through corticosteroid-sensitive and corticosteroid-resistant mechanisms. After uptake of any kind, metabolism by monoamine oxidase of type A may occur.