The effects of ruthenium red (RuR) were tested on the membrane currents of internally perfused, voltage-clamped nerve cell bodies from the snail Limnea stagnalis. Bath application of nanomolar concentrations of RuR produces a prolonged Na current that decays approximately 40 times slower than the normal Na current in these cells. The relationship between the reversal potential for the prolonged Na current and the intracellular concentration of Na+ agrees well with the constant-field equation, assuming a small permeability for Cs+. Because a strong correlation was found between the magnitude of the normal Na current and that of the prolonged Na current, it is concluded that the prolonged Na current flows through the normal Na channels. This conclusion is supported by the similar selectivities, voltage dependencies, and tetrodotoxin (TTX) sensitivities of these two currents. This action of RuR to slow the inactivation of the Na channel was not observed at concentrations below 1 nM, but was complete at 10 nM. When the concentration of RuR is increased to 0.1 mM, the Ca current in these cells is blocked; but at this high concentration RuR also reduces the outward voltage-dependent currents and resting membrane resistance. Therefore, RuR is not a good Ca blocker because of its lack of specificity. However, its action of slowing Na current inactivation is very specific and could prove to be useful in studying the inactivation of the Na channel.

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