SPONTANEOUS [ 3 H]NORADRENALINE RELEASE FROM THE MAIN PULMONARY ARTERY OF THE RABBIT INDUCED BY SODIUM‐PUMP INHIBITION

Inhibition of Na pump either by ouabain (10 −4 M) or by K removal increased the [ 3 H]noradrenaline ([ 3 H]NA) release from the isolated main pulmonary artery of the rabbit in the presence of neuronal (cocaine, 3 x 10 −5 M) and extraneuronal (corticosterone, 5 x 1O −5 M) uptake blockers. The ouabain‐evoked [ 3 H]NA release began after a delay of about 30 min and peaked after 66 min of ouabain application. Both times were shortened by omission of K from the external medium. About 90% of ouabain‐evoked [ 3 H]NA release proved to be external Ca concentration ([Ca] o ) dependent and the peak effect was delayed by about 80 min in Ca‐free (+1 mM EGTA) solution. In the presence of external Ca (2·5 mM) the [ 3 H]NA‐releasing effect of ‘K‐free’ treatment was much less pronounced than that of 10 ‐4 M ouabain, the initial delay in transmitter release was shorter (10‐15 min) and the peak effect developed earlier (at 42 min). On readmission of K the [ 3 H]NA release recovered quickly to the original value. Ca removal did not antagonize the transmitter release observed in K‐free solution, but the peak release was delayed by about 90 min. A low concentration of ouabain (10‐5 M) failed to produce transmitter release in the presence of normal external K, but markedly increased the release in K‐free solution. The release was much bigger than the sum of their separate effects, and the rate of rise was faster than when 10–4 M ouabain was applied in normal solution. Excess Ca (7·5; 15 mM) inhibited the [ 3 H]NA release observed in K‐free solution. 7·5 mM‐Ca also delayed the transmitter‐releasing action of 10 −4 M ouabain, an effect antagonized by omission of K from the external medium. The mitochondrial uncoupler carbonyl cyanide m‐chlorophenylhydrazone (CCCP, 10 −5 M) significantly increased the [ 3 H]NA release in Ca‐free, 1 mM EGTA‐containing solution, and enhanced the effects of ouabain (10 −4 M). The Ca ionophore A23187 (10 −5 M) also significantly increased the [ 3 H]NA release in the absence of external Ca and in the presence of 1 mM EGTA. Again, in the presence of A23187 the effects of 10 −4 M ouabain in releasing neurotransmitter were enhanced. When CCCP and A23187 were applied together in Ca‐free, EGTA solution the [ 3 H]NA releasing action of ouabain was still apparent. Veratridine (10 −4 M) enhanced the transmitter release in the absence of external Ca in a tetrodotoxin (TTX)‐sensitive manner. After veratridine treatment the action of ouabain was totally abolished. However, in the presence of TTX (10 −7 and 3 x 10 −7 M) which by itself significantly inhibited the veratridine‐evoked transmitter release the action of ouabain persisted. It is suggested that excess Ca, like external K, inhibits the transmitter‐releasing effect of Na‐pump inhibition, furthermore that in the absence of external Ca the ouabain‐evoked transmitter release is the result of Ca release from internal stores rather than Na‐pump inhibition per se .