Heterogeneity of external surface charges near sodium channels in the nodal membrane of frog nerve.

The conductance gamma and the number of No of Na channels in the nodal membrane of frog nerve fibres were determined from ensemble average values of the Na current and the variance of Na current fluctuations. Replacement of extracellular Cl- by NO3- shifts the voltage dependencies of all Na gating parameters towards more negative voltages, reduces gamma by a factor of 0.84 and hardly changes the number No of channels not blocked by 8 nM TTX. Adding 0.1 mM LaCl3 to the extracellular solution shifts the voltage dependencies of all Na gating parameters towards more positive voltages, reduces gamma by a factor of 0.75 and hardly changes the number No of channels not blocked by 8 nM TTX. It is concluded that changes of the external surface potential induced by Cl-, NO3- replacement do not alter the local Na+ concentration in the outer mouth of the Na channel and hardly affect the TTX binding to toxin receptors. Surface potential changes by addition of LaCl3 also have no clear effect on TTX binding. The reduction of gamma in 0.1 mM LaCl3 is probably due to a direct interaction of La3+ with Na channels. Our results suggest a heterogeneous distribution of external fixed surface charges in the outer mouth of the Na channel, at the TTX binding site and near the Na channel gates.