A CALCIUM-ACTIVATED CHLORIDE CHANNEL IN SARCOPLASMIC-RETICULUM VESICLES FROM RABBIT SKELETAL-MUSCLE

A Ca2+-activated Cl- channel is described in sarcoplasmic reticulum (S R) enriched vesicles of skeletal muscle incorporated into lipid bilaye rs. Small chloride (SCl) channels (n = 20) were rapidly and reversibly activated when cis- (cytoplasmic) [Ca2+] was increased above 10(-7) M , with trans- (luminal) [Ca2+] at either 10(-3) or 10(-7) M. The open probability of single channels increased from zero when cis-[Ca2+] was 10(-7) M to 0.61 +/- 0.12 when [Ca2+] was 10(-4) M. High- and low- co nductance levels in single-channel activity were activated at differen t cis-[Ca2+]. Channel openings to the maximum conductance, 65-75 pS (2 50/50 mM Cl-, cis/trans), were active when cis-[Ca2+] was increased ab ove 5 x 10(-6) IM. In contrast to the maximum conductance, channel ope nings to submaximal levels between 5 and 40 pS were activated at a low er cis-[Ca2+] and dominated channel activity between 5 x 10(-7) and 5 x 10(-6) M. Activation of SCl channels was Ca2+ specific and not repro duced by cytoplasmic Mg2+ concentrations of 10(-3) M. We suggest that the SCl channel arises in the SR membrane. The Ca2+ dependence of this channel implies that it is active at [Ca2+] achieved during muscle co ntraction.