Increasing the hydrogen ion concentration of the bathing medium reversibly depresses
the sodium permeability of voltage-clamped frog nerves. The depression depends on
membrane voltage: changing from pH 7 to pH 5 causes a 60% reduction in sodium permeability
at +20 mV, but only a 20% reduction at +180 mV. This voltage-dependent block of sodium
channels by hydrogen ions is explained by assuming that hydrogen ions enter the open
sodium channel and bind there, preventing sodium ion passage. The voltage dependence
arises because the binding site is assumed to lie far enough across the membrane for
bound ions to be affected by part of the potential difference across the membrane.
Equations are derived for the general case where the blocking ion enters the channel
from either side of the membrane. For H+ ion blockage, a simpler model, in which H+ enters the channel only from the bathing medium, is found to be sufficient. The dissociation
constant of H+ ions from the channel site, 3.9 x 10-6 M (pKa 5.4), is like that of a carboxylic acid. From the voltage dependence of the block,
this acid site is about one-quarter of the way across the membrane potential from
the outside. In addition to blocking as described by the model, hydrogen ions also
shift the responses of sodium channel "gates" to voltage, probably by altering the
surface potential of the nerve. Evidence for voltage-dependent blockage by calcium
ions is also presented.