Effects of harmaline on membrane excitability and ATPase activity of the crayfish giant axon.

Effects of harmaline on excitability of crayfish giant axon in relation to effects on Na,K-ATP activity were investigated and the findings were compared with relative data on ouabain. Exposure of the axon to harmaline in doses over 0.5 mM resulted in a dose-dependent reduction of the action potential. Harmaline of 2.5 mM produced a complete blockade of the action potential with a slight decrease in the resting membrane potential (5.7 mV), but no alterations of Na,K-ATPase activity. In contrast, ouabain (5 mM) produced a more pronounced decrease in the resting membrane potential (10.6 mV) and also a partial reduction in the action potential, while a significant decrease in Na,K-ATPase activity was obtained. Increasing the membrane potential to the initial level resulted in a partial recovery (46%) of the rising rate (dV/dt) of action potential. The dV/dt was exponentially decreased when the resting potential was reduced with application of KCl solution over 8 mV. These results suggest that harmaline externally applied produces a decrease in sodium conductance without affecting Na,K-ATPase activity, while ouabain inhibits Na,K-ATPase activity and, to lesser extent, decreases in sodium conductance.