A cadmium-sensitive, tetrodotoxin-resistant sodium channel in bullfrog autonomic axons.

The major component of the compound action potential of the bullfrog cardiac nerve was not blocked by TTX (10(-5) M) although the conduction velocity of these fibers was decreased by 50%. The TTX-resistant compound action potential was completely blocked by 100-300 microM CdCl2 but not by 5 mM CoCl2 or NiCl2. This cadmium-sensitive action potential was only partially reduced by removal of calcium from the medium but was abolished by replacement of sodium with sucrose, tetramethylammonium or choline. Therefore, in addition to the classical TTX-sensitive sodium channels, the parasympathetic axons of the bullfrog may also utilize a TTX-insensitive, cadmium-sensitive sodium channel.