1. 1. The spontaneous [3H]-release has been measured from the isolated main pulmonary artery of the rabbit preloaded with [3H]noradrenaline in the presence of uptake blockers (cocaine, 3 × 10-5 M; corticosterone, 5 × 10-5 M). 2. 2. The Ca-ionophore A-23187 (3 × 10-7-3 × 10-5 M) increased the outflow of [3H] by a concentration dependent manner. 3. 3. Inhibition of Na+-pump by removal of K+ from the external medium also increased the release of labelled noradrenaline. 4. 4. In the absence of external K+, the applied A-23187 (3 × 10-6 M; EC50) further increased the release of [3H]. 5. 5. Reactivation of Na+-pump by readmission of K+ (5.9 mM) to the external medium abolished the [3H]-release which had previously been increased in "K+-free" solution. 6. 6. The reactivated Na+-pump significantly inhibited the transmitter releasing action of A-23187. 7. 7. This latter was antagonized by an increase of external Ca2+ (7.5 mM). 8. 8. It is concluded that the reactivated Na+-pump caused re-establishment of Na+-gradient is capable to counteract the Ca-ionophore facilitated Ca2+-influx and release from internal stores, which can be antagonized by excess Ca2+.
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