The Metabolic Fate of Levodopa in Man and Animals

SUMMARY 1. Administration of levodopa in man and rats resulted in a large increase in the daily excretion in the urine of dopamine and its metabolites, but not of noradrenaline. 2. In the rat administration of levodopa substantially increased dopamine concentrations in the heart and brainstem, but noradrenaline concentrations were decreased. 3. It is suggested that administration of levodopa leads to dopamine production in concentrations high enough to saturate intraneuronal uptake, so that the main uptake is extraneuronal, hence the major products are dopamine and its metabolites. 4. The lack of evidence of increased noradrenaline synthesis in the presence of diminished tissue levels of noradrenaline suggests that either the intraneuronal vesicular uptake or the β-hydroxylation of dopamine may be an important regulating step in the synthesis of noradrenaline.