Summary
After oral administration of 3H-p-octopamine (8 mg≙300 μCi) more 3H-activity (93% of the dose) is excreted in the urine within 24 h than after intravenous infusion (2 mg≙300 μCi) over 2.5 h (82% of the dose). This proves that p-octopamine is absorbed quantitatively in man. The absorption proceeds rapidly, peak plasma levels are reached between 30 and 60 min.
The only metabolic pathways for p-octopamine are deamination and conjugation. The predominant step is oxidative deamination by monoamine oxidase (MAO) to p-hydroxymandelic acid. This acid represents 2/3 of the urinary 3H-activity after both routes of admistration.
A quantitative difference is seen in the fraction of free p-octopamine which equals the amount of conjugated amine after infusion but is only 1/20 after oral administration. This indicates a higher total clearance after an oral dose which consequently explains the diminished efficacy on blood pressure after this route.
Hydroxylation to catecholamines was not found.
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Hengstmann, J.H., Konen, W., Konen, C. et al. The physiological disposition of p-octopamine in man. Naunyn-Schmiedeberg's Arch. Pharmacol. 283, 93–106 (1974). https://doi.org/10.1007/BF00500148
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DOI: https://doi.org/10.1007/BF00500148