Catecholamines

Fate of Tissue CatecholaminesTissue norepinephrine may be released from sympathetic-nerve endings unchanged, or it may first be transformed within the nerve by the enzyme monoamine oxidase (MAO) (Fig. 12). Small amounts of "free" or "active" material are released spontaneously; more is liberated after sympathetic-nerve impulses.56,79 A tiny fraction of this "free" norepinephrine probably interacts with nearby receptor sites and has physiologic effects; the remainder is rapidly inactivated by O-methylation or rebinding within the nerve ending, or is washed away from its site of action by the circulation (Fig. 12).84 Most of the norepinephrine synthesized by the sympathetic-nerve endings never . . .