ANTAGONISM OF SAXITOXIN AND TETRODOTOXIN BLOCK BY INTERNAL MONOVALENT CATIONS IN SQUID AXON

Abstract

The block of voltage-dependent sodium channels by saxitoxin (STX) and tetrodotoxin (TTX) was investigated in voltage-clamped squid giant axons internally perfused with a variety of permeant monovalent cations. Substitution of internal Na+ by either NH4+ or N2H5+ resulted in a reduction of outward current through sodium channels under control conditions. In contrast, anomalous increases in both inward and outward currents were seen for the same ions if some of the channels were blocked by STX or TTX, suggesting a relief of block by these internal cations. External NH4+ was without effect on the apparent magnitude of toxin block. Likewise, internal inorganic monovalent cations were without effect, suggesting that proton donation by NH4+ might be involved in reducing toxin block. Consistent with this hypothesis, decreases in internal pH mimicked internal perfusion with NH4+ in reducing toxin block. The interaction between internally applied protons and externally applied toxin molecules appears to be competitive, as transient increases in sodium channel current were observed during step increases in intracellular pH in the presence of a fixed STX concentration. In addition to these effects on toxin block, low internal pH produced a voltage-dependent block of sodium channels and enhanced steady-state inactivation. Elevation of external buffer capacity only marginally diminished the modulation of STX block by internal NH4+, suggesting that alkalinization of the periaxonal space and a resultant decrease in the cationic STX concentration during NH4+ perfusion may play only a minor role in the effect. These observations indicate that internal monovalent cations can exert trans-channel influences on external toxin binding sites on sodium channels.