Influence of MAO A and MAO B on the inactivation of noradrenaline in the saphenous vein of the dog
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To study the termination of contractile response to exogenous NA, the oil immersion technique was used to determine the time for half-relaxation. The experiments were performed on strips with or without treatment with cocaine 10 μmol/l (to inhibit neuronal uptake) plus U-0521 100 μmol/l (to inhibit catechol-O-methyl transferase) before and after exposure to MAO inhibitors. Clorgyline, but not (−)deprenyl enhanced significantly the time for half-relaxation of the strips, whether cocaine was present or not.
To study the metabolism and accumulation of exogenous3H-NA, the strips were incubated (with or without preincubation with cocaine) with3H-NA 0.23 μmol/l and 2.3 μmol/l in the presence and in the absence of MAO inhibitors. The formation of deaminated metabolites was significantly reduced by clorgyline, but not by deprenyl.
To study the metabolism of3H-NA released by electrical stimulation, the strips were incubated with3H-NA 1.4 μmol/l. In the presence of cocaine and U-0521, field stimulation was applied during two periods of 5 min (10 Hz, 100 V, 2 ms), in the absence or presence of MAO inhibitors. Under these experimental conditions clorgyline, but not deprenyl, abolished DOPEG formation, without affecting the other metabolites.
In conclusion, only MAO A seems to be important for the inactivation of noradrenaline.
Key wordsMonoamine oxidase A and B Noradrenaline Clorgyline (−)Deprenyl Oxidative deamination Electrical stimulation
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