Kinetics of intramembrane charge movement and sodium current in frog node of Ranvier.

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Abstract
Intramembrane charge movement (Q) and sodium current (INa) were monitored in isolated voltage-clamped frog nodes of Ranvier. The ON charge movements (QON) for pulses from the holding potential (-100 mV) to potentials V .ltoreq. 0 mV followed single exponential time courses, whereas 2 exponentials were found for pulses to V .gtoreq. 20 mV. The voltage dependence of both QON and its time constant .tau.ON indicated that the 2 ON components resolved at V .gtoreq. 20 mV were also present, though not resolvable, for pulses to V .ltoreq. 0 mV. OFF charge movements (QOFF) monitored at various potentials were well described by single exponentials. When QOFF was monitored at -30 or -40 mV after a 200-.mu.s pulse to +20 mV and QON was monitored at the same potential using pulses directly from -100 mV, .tau.ON/.tau.OFF = 2.5 .+-. 0.3. At a set OFF potential (-90 to -70 mV), .tau.OFF first increased with increasing duration tON of the preceding pulse to a given potential (0 to +30 mV) and then decreased with further increases in tON. The declining phase of .tau.OFF followed a time course similar to that of the decline in QOFF with tON. For the same pulse protocol, the OFF time constant .tau.Na for INa also first increased with tON but then remained constant over the tON interval during which .tau.OFF and QOFF were declining. After 200- or 300-.mu.s pulses to +20, +30, or +50 mV, .tau.OFF/.tau.Na at -70 to -90 mV was 1.2 .+-. 0.1. Similar .tau.OFF/.tau.Na ratios were predicted by channel models having 3 identical charged gating particles that can rapidly and reversibly form an immobile dimer or trimer after independently crossing the membrane from their OFF to their ON locations.