Rate-limiting step in the metabolism of polar and non-polar monoamines in lung and liver

1 Uptake of the non-hydroxylated amines, [14C]-tryptamine and [14C]-benzylamine in rat lung, infused through the pulmonary circulation, was not saturable over the concentration range 2.5-1,000 microM. 2 The kinetic constants for deamination of a variety of hydroxylated and non-hydroxylated monoamines in liver, perfused via the portal circulation, with monoamine oxidase activity in homogenates of liver were similar. 3 In lung, uptake of both [14C]-tryptamine and [14C]-benzylamine was inhibited by the monoamine oxidase inhibitor deprenyl and competition occurred between tryptamine, benzylamine and beta-phenylethylamine for uptake. 4 These results indicate that tryptamine and benzylamine metabolism in lung is not limited by uptake, unlike that of the hydroxylated amines 5-hydroxytryptamine and noradrenaline and that uptake resembles that of beta-phenylethylamine in lung. 5 the selectivity of the lung in handling monoamines is not shown by the liver, suggesting that lung has a specific role in clearing certain biogenic monoamines.