Factors affecting the extraneuronal inactivation of noradrenaline in cardiac and smooth muscle.

1. The relation between density of adrenergic innervation, noradrenaline (NA) accumulation (as seen with the fluorescence histochemical method) in tissues incubated in a high concentration of NA, and monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT) activities, were examined in a wide range of tissues from different species.2. Evidence was obtained to support the proposal that accumulation of NA in the non-innervated smooth muscle of the human umbilical artery and chick amnion is associated with very low activities of COMT within muscle cells.3. The wide variation in tissue accumulation of NA in adrenergically innervated muscles was confirmed. For example, in the rabbit atrium and rat vas deferens, there was high NA accumulation in vascular smooth muscle but not in other muscle cells. In the mouse vas deferens there appeared to be preferential NA accumulation in the outer longitudinal muscle in comparison with the circular muscle. In the ventricle of the rat and mouse individual muscle cells showed different degrees of accumulation of NA. Many unidentified fluorescent cells were revealed in the submucosa of the guinea-pig ureter following loading with NA. The highest activities of COMT were found in the rat vas deferens and the lowest in the rabbit vascular tissues; the highest activity of MAO was found in the guinea-pig ileum, and the lowest in the rat aorta.4. No simple relation between tissue activities of MAO and COMT and degree of NA accumulation was found. Possible directions for further investigation of the problem are discussed.