Blockade of sodium and potassium channels in the node of Ranvier by ajmaline and N-propyl ajmaline

Abstract

The inhibition of sodium and potassium currents in frog myelinated fibres by ajmaline (AM) and its quaternary derivative, N-propyl ajmaline (NPA), depends on voltage-clamp pulses and the state of channel gating mechanisms. The permanently charged NPA and protonated AM interact only (or mainly) with open channels, while unprotonated AM affects preferently inactivated Na channels. Inhibition of Na currents by NPA and AM does not depend on the current direction and Na ion concentration in external or internal media. In contrast only the outward potassium currents can be blocked by NPA and AM; the inward potassium currents in high K+ ions external media are resistant to the blocking action of these drugs. The voltage dependence of ionic current inhibition by charged drugs suggests the location of their binding sites in the inner mouths of Na and K channels. Judging by the kinetics of current restoration after cessation of pulsing, the drug-binding site complex is much more stable in Na than in potassium channels. Batrachotoxin and aconitine, unlike veratridine and sea anemone toxin, decrease greatly the affinity of Na channel binding sites to NPA and AM. The effects of NPA and AM are compared with those of local anesthetics and other amine blocking drugs.