Characterization of the Electrogenic Sodium Channel from Rat Brain Membranes Using Neurotoxin-Dependent22Na Uptake

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Abstract
The Na channel was studied in osmotically sensitive membrane preparations from rat brain and in innervated and chronically denervated rat soleus and extensor digitorum longus muscles. The experiment was undertaken to define a set of parameters for Na channel function at the subcellular level which could be used as a measure of retention of channel integrity upon subsequent isolation of the channel. Various neurotoxins and drugs were employed to control the permeability of the brain membranes to 22Na and the Na-conductance properties of the muscles. Batrachotoxin (ED50 = 0.2 .mu.M), veratridine (ED50 = 1 .mu.M) or grayanotoxin I (ED50 = 30 .mu.M) stimulated 22Na uptake in brain membranes is inhibited in an apparently uncompetitive manner by the Na channel blocking agents tetrodotoxin and saxitoxin, in a simple competitive manner by Ca2+, and in a partial or allosteric competitive manner by lidocaine and procaine. This 22Na uptake assay, which can be equated to a measure of equilibrium toxin binding, shows dependence on the concentration of the membranes and is sensitive to pH, temperature, ionic strength and the ionic composition of the media. Parallel biophysical studies on Na channels in rat muscle show that the properties of the Na channel are similarly affected by these agents.