CHARACTERISTICS OF 2 TYPES OF CHLORIDE CHANNEL IN SARCOPLASMIC-RETICULUM VESICLES FROM RABBIT SKELETAL-MUSCLE

A comparison is made of two types of chloride-selective channel in ske letal muscle sarcoplasmic reticulum (SR) vesicles incorporated into li pid bilayers, The IN relationships of both channels, in 250/50 mM Cl- (cis/trans), were linear between -20 and +60 mV (cis potential,) rever sed near E(Cl) and had slope conductances of similar to 250 pS for the big chloride (BCl) channel and similar to 70 pS for the novel, small chloride (SCI) channel. The protein composition of vesicles indicated that both channels originated from longitudinal SR and terminal cister nae. BCl and SCI channels responded differently to cis SO42-(30-70 mM) , 4,41-diisothiocyanatostilbene 2,2'-disulfonic acid (8-80 mu M) and t o bilayer potential. The BCl channel open probability was high at all potentials, whereas SCI channels exhibited time-dependent activation a nd inactivation at negative potentials and deactivation at positive po tentials. The duration and frequency of SCI channel openings were mini mal at positive potentials and maximal at -40 mV, and were stationary during periods of activity. A substate analysis was performed using th e Hidden Markov Model (S. H. Chung, J. B. Moore, L, Xia, L. S, Premkum ar, and P. W. Gage, 1990, Phil. Trans. R. Sec. Lend. B., 329:265-285) and the algorithm EVPROC (evaluated here), SCI channels exhibited tran sitions between 5 and 7 conductance levels. BCl channels had 7-13 pred ominant levels plus many more short-lived substates. SCI channels have not been described in previous reports of Cl- channels in skeletal mu scle SR.