Characteristics of sodium and calcium conductance changes produced by membrane depolarization in an Aplysia neurone

1. The time course and voltage dependence of Na and Ca conductance changes produced by depolarization of the soma of the neurone R15 in the abdominal ganglion of Aplysia juliana were examined at temperatures of 10--14 degrees C. 2. During a maintained depolarization, Na currents turned on then decayed (inactivated). Inactivation was exponential with time constant tauh. Activation (after correction for inactivation) was reasonably well described by the expression G'Na(t) = G'Na (infinity) (1 - exp [-t/taum])3 over a wide range of potentials. 3. taum and tauh were both voltage dependent. In the range -20 to +40 mV, taum varied from 5 to 0.5 msec and tauh from 25 to 8 msec (13.5 degrees C). Steady-state Na conductance (corrected for inactivation) was voltage dependent also, increasing sigmoidally with depolarization to a maximum of 25--30 muS at +10 to +20 mV. Half-maximal Na conductance occurred at a membrane potential of -8 mV and from -15 to -5 mV, a 5 mV change in membrane potential produced an e-fold change in steady-state Na conductance. 4. Steady-state inactivation of Na conductance (hNa(infinity)) was voltage dependent with half-inactivation occurring at a membrane potential of -32 mV. Recovery from Na inactivation followed an exponential time course with a voltage-dependent time constant. 5. During a maintained depolarization Ca currents activated then decayed (inactivated) more slowly than Na currents. The decay was exponential with time constant tauH. The decay of Ca current was not an artifact porduced by an outward current. The amplitude of calcium tail currents, produced by voltage steps back to epsilonK at different times during the decay of ICa, decayed also with a time constant close to tauH. 6. Ca conductance (after correction for inactivation) could be described approximately by the expression G'Ca(t) = G'Ca(infinity) (1 - exp [-t/tauM])p but it was necessary to vary p from 1 to 2 at different potentials. No value of p gave as good a fit to this model as that obtained for Na currents. 7. taum and tauH were voltage dependent. In the range of potentials from 0 to +60 mV, tauM varied from 9 to 5 msec and tauH from 300 to 50 msec (13.5 degrees C). Steady-state Ca conductance (corrected for inactivation) was voltage dependent also, increasing sigmoidally with depolarization to a maximum of 10--15 muS at +30 to +40 mV. Half-maximal Ca conductance occurred at a membrane potential of +12 mV, and from +10 to +20 mV a 6 mV change in membrane potential produced an e-fold change in Ca conductance. 8. Steady-state inactivation of Ca conductance (hCa(infinity)) varied with holding potential (VH). Half-inactivation occurred with depolarization to -20 mV. At potentials more negative than -40 mV, hCa(infinity) was less than at -40 mV, i.e. hyperpolarization produced Ca 'inactivation'. 9...