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

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Abstract
1 The role of ejecting and retaining currents in determining the kinetics of the release of [14C]-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.