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Neurones from the dorsal root ganglia of 1-day-old rat pups were grown in dissociated culture and voltage clamped using patch electrodes for whole-cell recording. The pipettes were filled with either 140 mM-KCl or CsCl. Depolarizing voltage jumps activated net inward calcium currents in all neurones, which in a subpopulation of 46% were followed by slowly decaying inward tail currents accompanied by large increases in membrane conductance. During voltage jumps to membrane potentials more positive than 0 mV the inward calcium current was contaminated by a slow outward relaxation only in those neurones with slow inward tail currents. The availability curve for the slow inward tail current was U shaped, with a peak at approximately +5 mV in medium containing 2.5 mM-Ca2+; further depolarization reduced the amplitude of the tail current. During perfusion with calcium-free solution, or in the presence of the calcium-channel blockers cadmium or cobalt, or on substitution of barium for calcium, both the slow inward tail currents and outward relaxations were reversibly blocked. The reversal potential of the slow inward tail current, measured using a twin-pulse protocol, was approximately -10 mV. Replacement of sodium by tetraethylammonium (TEA) did not reduce the slow inward tail current, nor change its reversal potential. Reduction of the extracellular chloride activity produced a large increase in the amplitude of the slow inward tail current suggesting an increase in permeability to anions. This conductance, which behaves as though activated by prior or concurrent calcium entry triggered by membrane potential depolarization, is referred to as ICl(Ca). The activation and deactivation kinetics of ICl(Ca) are complex: envelope experiments measuring peak tail current amplitude revealed activation to be described by a single exponential function, of time constant approximately 100 ms at -10 to +8 mV. The integral of the tail currents increased with the duration of depolarizing pre-pulses suggesting accumulation of intracellular calcium. The decay of tail currents activated by short depolarizing voltage jumps was described by a single exponential function of time constant approximately 200 ms at -60 mV; more complex decay kinetics were recorded following activation by voltage jumps of duration greater than 60 ms. Tail current decay was voltage sensitive, becoming faster with hyperpolarization and increasing e-fold per 120 mV change in membrane potential.
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