Ca2+ currents in cerebral artery smooth muscle cells of rat at physiological Ca2+ concentrations.
Single Ca2+ channel and whole cell currents were measured in smooth muscle cells dissociated from resistance-sized (100-microns 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 x 10(-4) and 2 x 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 approximately 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.
Rubart
M
M
Department of Pharmacology, University of Vermont, Burlington 05405, USA.
Patlak
J B
JB
Nelson
M T
MT
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002536:Cerebral Arteries / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D005260:Female; D009130:Muscle, Smooth / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley
D015220:Calcium Channels; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D002118:Calcium