Kinetic and voltage-dependent characteristics of the inactivating action of incoming calcium currents were investigated in the somatic membrane of rat spinal ganglia neurons using an intracellular dialysis technique. It was shown that the "tail" of low-threshold calcium current could be reliably described by one exponent with a time constant of τℓ=1.2-1.8 msec at a repolarization potential of -90 mV. The "tail" of the high-threshold calcium current represented the sum of several exponents. The time constant of the main component which expressed inactivation of the high threshold calcium current was τh=250-350 μsec. It was also shown that τℓ and τh remained virtually unchanged for repolarization potentials in the subthreshold region; they increase, however, if the repolarizing potential is close to those potentials at which the corresponding component of calcium current is initially activated. A dependence was observed between the levels τℓ and τh and duration of the depolarizing shift. Findings are discussed in the context of a three-tier model of calcium channels.
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