Carrier-mediated outward transport of noradrenaline from adrenergic varicosities

Normally, the carrier of the neuronal noradrenaline uptake mechanism (uptake1) is involved nearly exclusively in the inward transport of substrates. However, an outward transport is induced by either of two mechanisms: a. by the decrease (or reversal) of the Na+ concentration gradient across the neuronal membrane or b. by the "facilitated exchange diffusion" due to the inward transport of substrates of this carrier. In rat vasa deferentia (preloaded with 3H-noradrenaline; vesicular uptake, MAO and COMT blocked) all substrates of the carrier induced an outward transport of 3H-noradrenaline, in strict correlation with their Km for uptake1. The inward transport of these substrates increases the availability of the carrier on the inside of the neuronal membrane. However, additional factors contribute to this release: inhibition of re-uptake and co-transport of Na+ and Cl-. When vesicular uptake and MAO are intact, the strict correlation between Km and releasing effect was not seen. This is attributable to the very low axoplasmic concentration of 3H-noradrenaline under these conditions (i.e. to lack of substrate for outward transport). However, if substrates of uptake1 are also substrates of vesicular uptake, they are able to "mobilize" vesicularly stored 3H-noradrenaline and, hence, to induce substantial outward transport. These "good" releasers are identical with the "indirectly acting sympathomimetic amines" (i.e. they are tyramine-like).