Current-Voltage Curves of Single CI⁻Channels which Coexist with Two Types of K⁺Channel in the Tonoplast ofChara corallina

A Cl⁻ channel and two types of K⁺ channel have been observed, by the use of the patch-clamp technique, in the membrane surrounding cytoplasmic droplets from Chara corallina. Measurements on cell-attached patches showed that the channel selective for Cl⁻ had a chord conductance of 21 pS at the resting membrane p.d. (mean = 11 mV, n = 19) and when open, passed an outward current of 1.4 pA (n = 24 patches) at the resting p.d., with reversal of the direction of current at −54 mV (130 mol m⁻³ Cl⁻ in the external solution). The Cl⁻ concentration in the cytoplasmic droplet calculated from the reversal p.d. was 15 mol m⁻³. The channel strongly rectified outward current flow, but this rectification disappeared with symmetrical Cl⁻ concentrations across detached patches of membrane. It is concluded that rectification observed in cell-attached patches is primarily due to asymmetric ^Cl− concentrations rather than an asymmetry in energy barriers to Cl⁻ permeation in the channel or any voltage-dependent kinetics of the channel. The channel was rarely observed in detached patches despite being commonly observed in cell-attached patches. However, the absence of Ca²⁺ at the cytoplasmic face of the membrane allowed observation of the channel in detached patches for brief periods, during which ion substitution experiments revealed a permeability sequence of $NO3->Cl->$ aspartate (76:33:1). A 100 pS K⁺ channel previously described by Luhring (1986) was frequently observed, in some instances simultaneously, with a channel having a conductance of 60 pS and displaying outward rectification. This rectification was due to the channel remaining open almost continuously for positive membrane potential differences (p.d.) and remaining shut almost continuously for negative p.d.'s. The 60 pS channel, like the 100 pS K⁺ channel, reversed current flow at the resting p.d., suggesting that it was also permeable to K⁺.