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

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Abstract
The time course and voltage dependence of Na and Ca conductance changes produced by depolarization of the soma of the neuron R15 in the abdominal ganglion of A. juliana were examined at temperatures of 10-14.degree. C. During a maintained depolarization, Na currents turned on then decayed (inactivated). Inactivation was exponential with time constant .tau.h. Activation (after correction of inactivation) was reasonably well-described by an expression over a wide range of potentials. .tau.m and .tau.h were voltage dependent. In the range -20 to +40 mV, .tau.m varied from 5-0.5 ms and .tau.h from 25-8 ms (13.5.degree. C). Steady-state Na conductance (corrected for inactivation) was voltage dependent also, increasing sigmoidally with depolarization to a maximum of 25-30 .mu.S 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. Steady-state inactivation of Na conductance (hNa (.infin.)) 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. During a maintained depolarization Ca currents activated, then decayed, more slowly than Na currents. The decay was exponential with time constant .tau.H. The decay of Ca current was not an artifact produced by an outward current. The amplitude of Ca tail currents, produced by voltage steps back to .epsilon.K at different times during the decay of Ca current amplitude decayed also with a time constant close to .tau.H. Ca conductance (after correction for inactivation) could be described approximately by an expression, but it was necessary to vary constant p from 1-2 at different potentials. No value of p gave as good a fit to this model as that obtained for Na currents. .tau.M and .tau.H were voltage dependent. In the range of potentials from 0 to +60 mV, .tau.M varied from 9-5 ms and .tau.H from 300-50 ms (13.5.degree. C). Steady-state Ca conductance (corrected for inactivation) was voltage dependent also, increasing sigmoidally with depolarization to a maximum of 10-15 .mu.S 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. Steady-state inactivation of Ca conductance (hCa(.infin.)) varied with holding potential (VH). Half-inactivation occurred with depolarization to -20 mV. At potentials more negative than -40 mV, hCa(.infin.) was less than at -40 mV, i.e., hyperpolarization produced Ca inactivation. Recovery from Ca inactivation did not follow an exponential time course with a single time constant but appeared to consist of 2 phases, the 1st with a time constant in the order of ms and the second with a time constant of seconds.