Single Ca2+ channel and whole cell currents were measured in smooth muscle cells dissociated from resistance-sized (100-μm diameter) rat cerebral arteries. We sought to quantify the magnitude of Ca2+ channel currents and activity under the putative physiological conditions of these cells: 2 mM [Ca2+]o, steady depolarizations to potentials between -50 and -20 mV, and (where possible) without extrinsic channel agonists. Single Ca2+ channel conductance was measured over a broad range of Ca2+ concentrations (0.5-80 mM). The saturating conductance ranged from 1.5 pS at 0.5 mM to 7.8 pS at 80 mM, with a value of 3.5 pS at 2 mM Ca (unitary currents of 0.18 pA at -40 mV). Both single channel and whole cell Ca2+ currents were measured during pulses and at steady holding potentials. Ca2+ channel open probability and the lower limit for the total number of channels per cell were estimated by dividing the whole-cell Ca2+ currents by the single channel current. We estimate that an average cell has at least 5,000 functional channels with open probabilities of 3.4 × 10-4 and 2 × 10-3 at -40 and -20 mV, respectively. An average of 1-10 (-40 mV and -20 mV, respectively) Ca2+ channels are thus open at physiological potentials, carrying ∼0.5 pA steady Ca2+ current at -30 mV. We also observed a very slow reduction in open probability during steady test potentials when compared with peak pulse responses. This 4-10-fold reduction in activity could not be accounted for by the channel's normal inactivation at our recording potentials between -50 and -20 mV, implying that an additional slow inactivation process may be important in regulating Ca2+ channel activity during steady depolarization.
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