Ouabain-evoked norepinephrine release from intact rat sympathetic neurons: evidence for carrier-mediated release

Inhibition of the Na,K-ATPase is known to cause transmitter release from many neurons. The mechanism of [3H]norepinephrine release was examined in primary cultures of sympathetic neurons. Ouabain caused [3H]norepinephrine release at concentrations that produced 80 to 90% inhibition of Na,K-ATPase activity. The effect of ouabain was compared with the effects of high K+ and the Ca2+ ionophore A23187. [3H]Norepinephrine release elicited by depolarization with high extracellular K+ was dependent on extracellular Ca2+, was unaffected by tetrodotoxin, was potentiated by reducing extracellular Na+, and was potentiated by the norepinephrine uptake inhibitor desipramine. These are the results expected if high K+ causes release by exocytosis, and if blockade of the Na+-dependent norepinephrine uptake system increases the net measurable release of the transmitter. The Ca2+ ionophore A23187 caused [3H]norepinephrine release that was not dependent on extracellular Ca2+ but which was like the release elicited by high K+ in other respects. Release elicited by ouabain was independent of extracellular Ca2+, was delayed for several hours by tetrodotoxin, was inhibited by reducing the concentration of extracellular Na+, and was inhibited by desipramine. These results suggest that the measured increase in transmitter release is secondary to a rise in the concentration of intracellular Na+. The data are consistent with the hypothesis that at high levels of pump inhibition, ouabain elicits nonvesicular [3H]norepinephrine release through reversal of the Na+-dependent plasma membrane carrier.