Unidirectional fluxes in isolated splenic nerve vesicles measured by a millipore filter technique: effects of noradrenaline and competitive reversal of reserpine inhibition.

Isotopically labelled noradrenaline (NA) exchange was determined in an improved preparation of NA storage vesicles isolated from bovine splenic nerve trunk. A Millipore filter technique was used which permitted analyses of unidirectional fluxes. In a Mg-ATP supplemented medium at 30 and 20° C, vesicle NA is completely exchangeable with 0.5 to 3.0 μg l-NA/ml in the medium. The kinetics are compatible with a single exponential component of exchange, and the data suggest a fully saturated Mg-ATP complexing under these conditions. Reserpine in a concentration range between 2 × 10-8 and 2 × 10-7 M causes more pronounced inhibition of the transfer coefficient for NA influx than for efflux. However, no net loss of vesicle NA occurs because the drug inactivates a proportional amount of the readily exchangeable NA pool. Within the concentration ranges of NA and reserpine tested, virtually complete inhibition of NA exchange can be achieved and reversal of this inhibition by elevating the NA concentration in the medium can be demonstrated. The data is consistent with the suggestion that reserpine acts via transport sites in the vesicle membrane, rather than on the intravesicular NA complex per se. This membrane stabilizing concentration range of reserpine causes no obvious deleterious effects on the vesicles at the electron microscopic level.