CA2-ARTERY SMOOTH-MUSCLE CELLS OF RAT AT PHYSIOLOGICAL CA2+ CONCENTRATIONS( CURRENTS IN CEREBRAL)

Single Ca2+ channel and whole cell currents were measured in smooth mu scle cells dissociated from resistance-sized (100-mu m diameter) rat c erebral arteries. We sought to quantify the magnitude of Ca2+ channel currents and activity under the putative physiological conditions of t hese cells: 2 mM [Ca2+](0) steady depolarizations to potentials betwee n -50 and -20 mV, and (where possible) without extrinsic channel agoni sts. Single Ca2+ channel conductance was measured over a broad range o f Ca2+ concentrations (0.5-80 mM). The saturating conductance ranged f rom 1.5 pS at 0.5 mM to 7.8 pS at 80 mM, with a value of 3.5 pS at 2 m M 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 hol ding 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 th at an average cell has at least 5,000 functional channels with open pr obabilities of 3.4 x 10(-4) and 2 x 3 10(-3) at -40 and -20 mV, respec tively. An average of 1-10 (-40 mV and -20 mV, respectively) Ca2+ chan nels are thus open at physiological potentials, carrying similar to 0. 5 pA steady Ca2+ current at -30 mV. We also observed a very slow reduc tion in open probability during steady test potentials when compared w ith peak pulse responses. This 4-10-fold reduction in activity could n ot be accounted for by the channel's normal inactivation at our record ing potentials bet between -50 and -20 mV, implying that an additional slow inactivation process may be important in regulating Ca2+ channel activity during steady depolarization.