Bimodal kinetics of a chloride channel from human fibroblasts

Excised patches were used to study the kinetics of a Cl channel newly ident ified in cultured human fibroblasts (L132). The conductance of ca. 70 pS in 150 mM symmetrical Cl, and the marked outward rectification ascribe this c hannel to the ICOR family. Long single-channel recordings (>30 min) reveale d that the channel spontaneously switches from a kinetic mode characterized by high voltage dependence (with activity increasing with depolarization; mode 1), into a second mode (mode 2) insensitive to voltage, and characteri zed by a high activity in the voltage range +/-120 mV. On patch excision th e channel always appeared in mode 1, which was maintained for a variable ti me (5-20 min). In most instances the channels then switched into mode 2, an d never were seen to switch back, in spite of the eight patches that cumula tively dwelled in this mode 2.33-fold as compared to mode 1. Stability plot s of long recordings showed that the channel was kinetically stable in both modes, allowing standard analysis of steady-state kinetics to be performed . Open and closed time distributions of mode 1 and mode 2 revealed that the apparent number of kinetic states of the channel was the same in the two m odes. The transition from mode 1 into mode 2 was not instantaneous, but req uired a variable time in the range 5-60 sec. During the transition the chan nel mean open time was intermediate between mode 1 and mode 2. The intermed iate duration in the stability plot however is not to be interpreted as if the channel, during the transition, rapidly switches between mode 1 and mod e 2, but represents a distinct kinetic feature of the transitional channel.