Inactivation and excretion of dopamine by the cat kidney in vivo

14C-Dopamine at a dose between 0.16 and 400 nmol per kg body weight was injected locally into the renal artery and urinary excretion of the label was followed for a period of up to 75 min. During the first renal passage the injected kidney excreted 28.2+/-8.3% (n = 8) of the activity applied. As shown by column chromatography the 14C-activity in urine was mainly present as 3,4-dihydroxyphenyl acetic acid (40%), homovanillic acid (15%) and dopamine (app. 20%). Excretion rate and the pattern of dopamine metabolites in urine was independent of the administered dose. Thus, the excretion of dopamine by the cat kidney is linked to an inactivation by the kidney enzymes MAO and COMT. From the literature it is known that in dog and chicken kidney catecholamines are not metabolized to such a large extent during renal excretion.