Sodium channel gating currents. Origin of the rising phase.

There has been some uncertainty in the past as to the origin of the rising phase of the gating current. We present evidence here that proves that the gating current does not have a rising phase and that the observed rising phase is due to an uncompensated series resistance in the Frankenhaeuser-Hodgkin (F-H) space. When a squid giant axon is bathed in a solution that is 10-20% hyperosmotic with respect to the internal solution, the rising phase of the gating current is eliminated. In parallel with this, a component of the capacity transient (time constant, 20 microseconds) is reduced so that the capacity transient now appears to be closer to a single fast (5-10 microseconds) component. These changes in the capacity transient and gating current occur without altering the amount of charge moved in either. This indicates that the charge is simply redistributed in time. The gating current without a rising phase can still be immobilized by inactivation. Supporting evidence is provided by measuring the accumulation and washout of K+ from the F-H space. It was found that K+ washes out 35% faster when the axon is bathed in hyperosmotic solution. It was estimated that the F-H space thickness (theta) increased 2.5 +/- 0.4-fold (mean +/- SEM) in hyperosmotic solution. Similarly, K+ accumulation in the F-H space was decreased, leading to an estimate of a 5 +/- 1.4-fold increase in theta in hyperosmotic solution. These results are consistent with the simple structural model presented.