Effects of deuterium oxide on the rate and dissociation constants for saxitoxin and tetrodotoxin action. Voltage-clamp studies on frog myelinated nerve.

Abstract

Actions of tetrodotoxin (TTX) and saxitoxin (STX) in normal water and in deuterium oxide (D2O) were studied in frog myelinated nerve. Substitution of D2O for H2O in normal Ringer''s solution had no effect on the potency of TTX in blocking action potentials, but increased the potency of STX by .apprx. 50%. Under voltage clamp, the steady-state inhibition of Na currents by 1 nM STX was doubled in D2O as a result of a halving of the rate of dissociation of STX from the Na channel; the rate of block by STX was not measurably changed by D2O. Steady-state inhibition or the on- or off-rate constants of TTX were not changed by D2O substitution. Isotopic effects on STX binding were observed < 10 min after the toxin was added to D2O, eliminating the possibility that slow-exchange (t1/2 [half life] > 10 h) H-binding sites on STX were involved. Results were consistent with a hypothesis that attributed receptor-toxin stabilization to isotopic changes of H-bonding; this interpretation suggested that H-bonds contributed more to the binding of STX than to that of TTX at the Na channel.