A comparative study of the role played by some inactivation pathways in the disposition of the transmitter in the rabbit aorta and the saphenous vein of the dog.

The aim of this study was to determine the relative importance of neuronal uptake, catechol-O-methyltransferase (COMT) and diffusion in the disposition of endogenous nor-adrenaline (NA) in the rabbit aorta and dog saphenous vein. Helical strips of dog saphenous vein and rabbit aorta were stimulated transmurally, relaxation occurring either in oil or in Krebs-Henseleit solution. The rate of inactivation of NA by various mechanisms was assessed from the decrease in the rate of relaxation after blockade of one or more of these processes. Cocaine was used to inhibit neuronal uptake and COMT was blocked with 3,4-dihydroxy-2-methyl propiophenone. Diffusion out of the tissue was prevented by replacing the Krebs-Henseleit solution in the muscle bath with mineral oil. Neuronal uptake was the primary mechanism of inactivation in the saphenous vein whereas diffusion and COMT appear to be more important in the aorta. The differences in the relative importance of the inactivation mechanisms (neuronal uptake, O-methyltransferase and diffusion) existing between these two vessels are attributed primarily to differing density and distribution of the adrenergic innervation.