Molecular model for sodium conductance and calcium transport in the squid axon.

A molecular and biochemically plausible model for the excitation process of the sodium pore is suggested. From basic arguments it is concluded that the sodium pore exists in at least three states: the resting state, the sodium conducting state, and the refractory state. They are connected to form a cyclic process. A specification of the different states is given. It is suggested that inactivation of the sodium pore results from a conformational change, which is caused by the transport of a calcium ion through the membrane. The transport carrier is the sodium pore. This assumption can explain the observed calcium influx during stimulation, and the effect of Ca on the rate of inactivation and on the rate, at which sodium conductance shuts off upon repolarization. It cannot give a quantitative explanation for the effect of Ca on the rate of rise, peak sodium conductance, and steady state inactivation. These asects are successfully described by the surface potential hypothesis, which has been published recently. It is concluded, that a combination of both theories gives a rather complete description of the sodium pore. The Ca transport model is discussed quantitatively and in great detail.