MECHANISMS OF SECRETION AND RETENTION OF NE IN SYNAPTIC VESICLES IN AXOPLASM

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This laboratory has studied the uptake and retention of norepinephrine (NE) in synaptic vesicles in the axoplasm (in situ) of adrenergic nerve terminals in vitro. When slices of rat heart ventricle are incubated in a Na+-deprived (12.5 to 25muM Na, choline) Krebs-bicarbonate medium (Ch+-Ca++), synaptic vesicles and axolemma appear to establish a new morphological unit which can secrete stored NE and recapture NE in the medium. This laboratory was among the first to recognize that electron transport could be involved in retention. Secretion is mediated by the amine pump in the axolemma and recapture is mediated by the amine pump in the vesicle membrane. In this preparation, the latter is dependent upon exogenous ATP. Retention as well as recapture are inhibited by Mg++-ATPase inhibitors, K+-H+ exchange ionophores, H+ ionophores and reserpine, which blocks the NE pump in the vesicle membrane. These inhibitors are all known to block uptake dependent upon H+ transport energized by the activity of Mg++-ATPase in the vesicle membrane. Furthermore, uptake and retention are prevented by ammonia. It was concluded that uptake and retention in vesicles in situ are energized by a pH gradient, delta pH, across the vesicle membrane (Bogdanski, 1982,1983,1986,1988). These findings are physiologically significant. The same inhibitors prevent the retention of NE in incubated terminals in KRB. Mg++ and Mn++ block the voltage dependent entry of Ca++ in nerve endings (Blaustein). Mg++ blocks Ca++ entry and NE depletion from vesicles in situ. Mn++ blocks amine secretion specifically. The excretion of deaminated metabolites is not inhibited. It would appear that the Ca++ serves a similar function in secretion in vivo and in vesicles in situ. Secretion is not mediated by exocytosis. The fusion of vesicle to axollemma is controlled by Na+. Terminals in Ch+-CA++ do not respond to ATP if Na+ is completely omitted from the medium. Lithium inhibits the Na+ dependency.