Characteristics of two types of chloride channel in sarcoplasmic reticulum vesicles from rabbit skeletal muscle

A comparison is made of two types of chloride-selective channel in skeletal muscle sarcoplasmic reticulum (SR) vesicles incorporated into lipid bilayers, The IN relationships of both channels, in 250/50 mM Cl- (cis/trans), were linear between -20 and +60 mV (cis potential,) reversed 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 cisternae. BCl and SCI channels responded differently to cis SO42-(30-70 mM), 4,41-diisothiocyanatostilbene 2,2'-disulfonic acid (8-80 mu M) and to bilayer potential. The BCl channel open probability was high at all potentials, whereas SCI channels exhibited time-dependent activation and inactivation at negative potentials and deactivation at positive potentials. The duration and frequency of SCI channel openings were minimal at positive potentials and maximal at -40 mV, and were stationary during periods of activity. A substate analysis was performed using the Hidden Markov Model (S. H. Chung, J. B. Moore, L, Xia, L. S, Premkumar, and P. W. Gage, 1990, Phil. Trans. R. Sec. Lend. B., 329:265-285) and the algorithm EVPROC (evaluated here), SCI channels exhibited transitions between 5 and 7 conductance levels. BCl channels had 7-13 predominant levels plus many more short-lived substates. SCI channels have not been described in previous reports of Cl- channels in skeletal muscle SR.