CHARACTERIZATION OF A CHLORIDE CHANNEL RECONSTITUTED FROM CARDIAC SARCOPLASMIC-RETICULUM

We have characterized a voltage-sensitive chloride channel from cardiac sarcoplasmic reticulum (SR) following reconstitution of porcine heart SR into planar lipid bilayers. In 250 mM KCl, the channel had a main conductance level of 130 pS and exhibited two substrates of 61 and 154 pS. The channel was very selective for Cl- over K+ or Na+ (P-K+/P-Cl- = 0.012 and P-Na+/P-Cl- similar to 0.040). It was permeable to several anions and displayed the following sequence of anion permeability: SCN- > I- > NO3- similar to Br- > Cl- > F- HCOO-. Single-channel conductance saturated with increasing Cl- concentrations (K-m = 900 mM and gamma(max) = 488 pS). Channel activity was voltage dependent, with an open probability ranging from similar to 1.0 around 0 mV to similar to 0.5 at +80 mV. From -20 to +80 mV, channel gating was time-independent. However, at voltages below -40 mV the channel entered a long-lasting closed state. Mean open times varied with voltage, from similar to 340 msec at -20 mV to similar to 6 msec at +80 mV, whereas closed times were unaffected. The channel was not Ca2+-dependent. Channel activity was blocked by disulfonic stilbenes, arylaminobenzoates, zinc, and cadmium. Single-channel conductance was sensitive to trans pH, ranging from similar to 190 pS at pH 5.5 to similar to 60 pS at pH 9.0. These characteristics are different from those previously described for Cl- channels from skeletal or cardiac muscle SR.