Single anion-selective channel and its ion selectivity in the vascular smooth muscle cell
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The properties of voltage-gated Cl channels of cultured smooth muscle cell prepared from embryonic rat aorta were studied. In the excised patch (inside-out configuration), we observed the activity of channels, opening and closing spontaneously, when the membrane potential was held at around 0 mV. The channels were active at a potential range between +10 and −10 mV. A step change of the membrane potential from the active potential range in either a positive or a negative direction closed the channel to an apparently inactivated state. The time course of this inactivation process became faster as the amplitude of the step change was increased. Returning the membrane potential to 0 mV allowed the channel to recover from the inactivated state. The channel had at least two open conductance states. Ca ions at the cytoplasmic face were not required for the activation of the channel. Adenosine nucleotides at the same side of the membrane had no effect on channel activity. The channels were selective to anions rather than cations, and they had a large single channel conductance of 340.5±20.4 pS in symmetrical 150 mM TEA-Cl. The reversal potential of the channel was shifted by −15.2±2.6 and 17.0±1.7 mV, when the Cl concentration at the intracellular side was changed to 75 mM or 300 mM, respectively. The permeability sequence of halides was I−>Br−>Cl−>F− (1.4∶1.3∶1.0∶0.7), whereas the conductance sequence was Cl−>Br−>F−>I− (1.00∶0.89∶0.86∶0.83). The internal dimension of the channel was estimated by measuring the permeability of various anions with different molecular cross section. We suggest that the smallest cross section of the channel pore is about 32Å2.
Key wordsAnion-selective Cl channel Cultured smooth muscle cell Ion selectivity Voltage-dependent
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