Single-channel recordings of chloride currents in cultured human skeletal muscle.

The Cl- channels in human myoballs were investigated with several recording techniques. Three types of channels were found and dubbed "small", "intermediate", and "large", according to their different conductance. The intermediate Cl- channel was observed most frequently. It was active at the resting potential immediately after seal formation in cell-attached as well as in excised patches. Its Cl- selectivity was rather high (PCl/PNa = 9.46; PCl/PMeSO4 = 7.85 where P denotes permeability) and the slope conductance at the reversal potential with [Cl-]o/[Cl-]i equal to 160 mM/42 mM was 31 pS. The channel showed an open-channel substructure with two subconductance levels having equal amplitudes. It can conduct two kinetically different currents that correspond to the activating and the inactivating Cl- current components described by Zachar et al. (1992). The small Cl- channel had a conductance of 10 pS at the reversal potential, a PCl/PNa of 2.7, and a PCl/PMeSO4 of 22.6. Its open probability was biggest negative to -85 mV, resulting in an inactivating whole-cell Cl- current component. Because of the small channel density and conductance the contribution of this channel type to the whole-cell current seems to be small. Patches with only one small channel were never observed which suggests that this channel type occurs in clusters. A third type of channel with very large conductance (250 pS) was seen only four times.