Catecholamine Metabolism in the Dog: Comparison of Intravenously and Intraventricularly Administered [¹⁴c]dopamine and [³h]norepinephrine

Abstract —The urinary excretion of labelled metabolites was measured in dogs which had been injected intravenously or intraventricularly with [ 3 H]norepinephrine or [ 14 C]dopamine. [ 3 H]Norepinephrine injected by either route produced more labelled 3‐methoxy‐4‐hydroxy‐phenylglycol than 3‐methoxy‐4‐hydroxymandelic acid, as did [ 14 C]dopamine after intravenous administration. In contrast, following the intraventricular injection of [ 14 C]dopamine, more [ 14 C]3‐methoxy‐4‐hydroxymandelic acid was formed than [ 14 C]3‐methoxy‐4‐hydroxyphenylglycol. These observations suggest that the metabolism of exogenously‐administered and endogenously‐formed norepinephrine may proceed through different routes and that the predominant metabolite of norepinephrine in canine brain may be 3‐methoxy‐4‐hydroxymandelic acid rather than 3‐methoxy‐4‐hydroxyphenylglycol.