Maximal upstroke velocity as an index of available sodium conductance. Comparison of maximal upstroke velocity and voltage clamp measurements of sodium current in rabbit Purkinje fibers.

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Abstract
We compared the maximal upstroke velocity of action potentials in short rabbit Purkinje fibers with sodium currents measured with a two-microelectrode voltage clamp. The number of sodium channels available to open during a sudden depolarization was varied either by blockade with tetrodotoxin or by inactivation with steady depolarizations. In both cases, the maximal upstroke velocity was found to be a very nonlinear measure of the number of available sodium channels. For example, 3 microM tetrodotoxin blocks 85% of the sodium channels, but reduces the maximal upstroke velocity by only 33%. Voltage clamp and upstroke velocity experiments were reconstructed with a computer model of the rabbit Purkinje fiber preparation that was closely based on experimental measurements of passive cable properties and sodium channel characteristics. The simulations indicate that our voltage clamp measurements of sodium current accurately report changes in channel availability, but they also show that the maximal upstroke velocity is a strongly nonlinear index of available sodium conductance. Most of the nonlinearity arises from the activation kinetics of the sodium channels: as the pool of available channels decreases, a greater percentage of those channels activate and contribute inward current at the time of the maximal upstroke velocity. Simulations predict that the maximal upstroke velocity-available sodium conductance relationship would still remain nonlinear at 37 degrees C or under different stimulus conditions that give uniform or continuously propagated action potentials. The nonlinearity may invalidate inferences based on earlier maximal upstroke velocity experiments: the existence of two types of sodium channels with different tetrodotoxin sensitivity, steady state voltage dependence of tetrodotoxin block, voltage range over which sodium channels inactivate, and rapid, then slow recovery of sodium channel availability following a sudden repolarization. All of these conclusions need to be reevaluated.