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THE FATE OF BIOGENIC MONOAMINES IN PERFUSED RABBIT LUNG

1 Inactivation of β‐phenylethylamine and several of its derivatives was studied in a preparation of rabbit lung perfused with Krebs physiological medium at 37°C. Inactivation or removal of these compounds was calculated as the difference between the concentration of each amine in the perfusion medium and the effluent, collected separately from each lung. The extent of amine metabolic degradation was also measured, by column chromatography, in lung effluent. 2 With this technique the magnitude of amine removal as a function of concentration was determined and an apparent Km and Vmax of removal were calculated for each amine. 3 Percentage removal was highest with phenylethylamine (95%), and decreased, apparently in relation to increasing phenyl‐ and side chain‐hydroxylation (and therefore likely increased hydrophilicity), with 5‐hydroxytryptamine (64%), tyramine (53%), octopamine (35%), dopamine (32%) and noradrenaline (23%). 4 Inactivation of each amine could be accounted for by metabolic degradation to deaminated products, which appeared in lung effluent within 90 s of beginning amine perfusion. 5 When intrapulmonary metabolism of phenylethylamine was inhibited by simultaneous perfusion with semicarbazide (10 mM) and pargyline (10 μM), the removal rate was unaltered, establishing that uptake of the amine from the vascular space is not dependent on metabolism at least for 4 min infusions.