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Abstract

1. The net uptake of noradrenaline occurring in the presence of ATP as well as the adenosine triphosphatase activity of purified membranes isolated from bovine adrenomedullary storage vesicles reach maximal values at pH 7.4. However, the steep pH-dependent enhancement of the enzymatic activity between pH 5.0 and 6.2 does not correspond to any catecholamine uptake. This indicates that the membrane contains two ATPases of which only a small fraction is involved in the catecholamine transport. 2. In the presence of ATP the uptake of noradrenaline by membrane particles is concentration-dependent, but not the release of noradrenaline occurring in the absence of ATP. A concentration ratio of about 1:140 is reached during ATP-dependent catecholamine uptake. 3. The uptake of noradrenaline in the presence of ATP is to some extent stereospecific; (−)-noradrenaline is taken up preferentially. Both influx and efflux are different for the two isomers. The catecholamine release occurring in the absence of ATP is not stereospecific. The results obtained with the isolated membrane support the concept that the decisive step in catecholamine accumulation is the enzymatic, energy-requiring, process that transports catecholamine across the membrane.