OUABAIN-EVOKED [³H]NORADRENALINE RELEASE FROM THE RABBIT PULMONARY ARTERY IN CALCIUM-FREE SOLUTION

¹ Department of Pharmacodynamics, Semmelweis University of Medicine, H-1089 Budapest, Nagyvárad-tér 4, PO Box 370, Hungary

[³H]noradrenaline ([³H]NA) release from the isolated main pulmonary artery of the rabbit has been measured in the presence of neuronal (cocaine, 3 x 10^-5 M) and extraneuronal (corticosterone, 5 x 10^-5 M) uptake blockers. 10^-4 M ouabain significantly increased the [³H]NA release in normal external ionic environments after an initial delay (20-30 min). Excess K (23·6 mM) failed to affect the resting [³H]NA outflow, significantly inhibited the ouabain-stimulated [³H]NA release and shortened the initial delay by about 10-20 min. Higher concentration of K (47·2 mM) enhanced the outflow of [³H]NA. In the absence of external Ca and in the presence of 1 mM EGTA, 23·6 mM-K failed to exert an inhibitory action on ouabain evoked transmitter release. Higher concentrations of K, however, significantly inhibited the [³H]NA-releasing effect of ouabain without changing the resting outflow of labelled neurotransmitter. The initial delay of ouabain-evoked [³H]NA release was shortened by increasing the concentration of K. Total substitution of external Na by K (143·3 mM) increased the [³H]NA release in the absence of external Ca. After the Na gradient was re-established in Ca-free solution the release of [³H]NA was terminated. Under these conditions the NA-releasing action of ouabain was dependent on the preceding perfusing period in Na-free solution, being smaller if longer exposure time was used. When external Na was substituted by Li (137·4 mM) in Ca-free solution the [³H]NA release was dramatically increased. After Na readmission ouabain was ineffective in producing transmitter release. It is suggested that in the main pulmonary artery of the rabbit, when the electrochemical gradient of Ca is reversed, ouabain is effective in producing transmitter release if the internally stored Ca has not been completely lost. Since in Ca-free solution the ouabain-evoked [³H]NA release can be inhibited by external K it seems that the transmitter release observed is due to a Na-dependent release of intracellular Ca rather than the penetration of ouabain into the cell followed by direct inhibition of the active transport of internal store membranes.