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.
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