Kinetics of the TTX sensitive Na+ current in the slowly adapting lobster stretch receptor neurone

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Abstract
The kinetics of the TTX sensitive Na+ current (INa) in the slowly adapting lobster stretch receptor neurone were investigated in sub‐and near‐threshold voltage regions using electrophysiological and pharmacological techniques. In dynamic conditions INa was found to display both fast and slow reactions. These were attributed to a fast Hodgkin‐Huxley type of Na activation and inactivation, and a slow type of Na inactivation, respectively. In stationary conditions the voltage dependence of the slow Na inactivation was shifted in a depolarizing direction by increasing, and in a hyperpolarizing direction by decreasing the extracellular Ca++ concentration. From this finding as well as from its kinetic properties the slow Na inactivation was classified as a genuine gating process. The processes of fast Na activation and inactivation were too fast for a dynamic analysis with the recording technique available. An estimate of their stationary voltage dependence could however be obtained in a voltage range from about ‐80 to about ‐50 mV. The experimental findings were used for the formulation of a mathematical description of INa in the present preparation based on constant field and state transition theories.