Blocking of the sodium channel by external Tris in the squid giant axon

Abstract

The macroscopic current carried by Na + ions was recorded in voltage-clamped squid giant axons that were dialysed with a mixture of CsF and NaF, and bathed in chloride solutions in which the sodium current (1 Na ) was reduced by partly blocking the sodium channels with tetrodotoxin, or by replacing four fifths of the Na + either with choline or with tris buffer at pH 7.3. The permeability coefficient (TNa fast) for the fast-inactivating current was unchanged on substitution of choline for sodium in the bathing solution, but in the high-Tris bathing solution, TNa fagt was reduced in a voltage-dependent manner, so that the permeability relative to normal rose from 0.2 at —40 m V to 0.8 at +100 mV. The coefficie10t TNa non for the non-inactivating current behaved in a similar way, except that the permeability ratio fell between test potentials of —40 and + 10 mV instead of rising. The blocking effect of Tris was unaffected by temperature, but there was some interaction with that of internal tetramethylammonium ions.