Kinetics of the release of noradrenaline from micropipettes: interaction between ejecting and retaining currents.

1. The role of ejecting and retaining currents in determining the kinetics of the release of [(14)C]-noradrenaline (NA) from micropipettes of the type used in microelectrophoresis experiments has been investigated by the liquid scintillation counting technique.2. In the absence of any electrophoretic current a constant rate of release of NA was established.3. All retaining currents examined gradually reduced the spontaneous release to zero. Higher retaining currents abolished spontaneous release more quickly.4. A linear relationship was identified between the rate of electrophoretic release of NA and the intensity of the ejecting current. The mean transport number of NA was found to be 0.17.5. All retaining currents studied reduced the amount of NA released during a subsequent application of an ejecting current. This was due to a prolongation of the time necessary to establish a steady-state rate of release. The magnitude of this effect was related to both the intensity and the duration of application of the retaining current.6. The results are discussed in terms of a theoretical model of ion movements within the tip of the micropipette.