Voltage-dependent kinetics of an anionic channel of large unit conductance in macrophages and myotube membranes

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Abstract
Using the patch-clamp technique single-channel parameters and kinetic properties of an anionic channel are studied in cell-attached and exised membrane patches from peritonal macrophages of mouse and cultured chicken myotubes. The channel has a unit conductance of about 340 pS with a Q10 of 1.3. In addition a subconductance state of about 210 pS in frequently adopted. The selectivity ratio of PCl/PNa is about 5. In excised membrane patches the activation of the channel appears to be independent of Ca either in the cytoplasmic or the extracellular medium. The channel induced current fluctuations appear in a burst like pattern. At least three non-conducting channel states could be distinguished kinetically. The mean lifetime of one of these states exhibits a strikingly steep voltage dependence which could be correlated to the mean shut interval between consecutive bursts. A similar steep voltage dependence was found for the mean liefetimes of bursts. The burst kinetic shows an about bell-shaped dependence on voltage. The results suggest that the burst kinetic and the kinetic within bursts are regulated by independent voltage sensitive mechanisms. The burst kinetic was analyzed by ensemble averages of voltage-jump current relaxations performed on the single channel level. A model of two voltage-sensitive gates is proposed for a description of the burst kinetic.