ATP INHIBITION AND RECTIFICATION OF A CA2-ACTIVATED ANION CHANNEL IN SARCOPLASMIC-RETICULUM OF SKELETAL-MUSCLE()

We describe ATP-dependent inhibition of the 75-105-pS tin 250 mM Cl-) anion channel (SCI) from the sarcoplasmic reticulum (SR) of rabbit ske letal muscle. In addition to activation by Ca2+ and voltage, inhibitio n by ATP provides a further mechanism for regulating SCI channel activ ity in vivo. Inhibition by the nonhydrolyzable ATP analog 5'-adenylyli midodiphosphate (AMP-PNP) ruled out a phosphorylation mechanism. Cytop lasmic ATP (similar to 1 mM) inhibited only when Cl- flowed from cytop lasm to lumen, regardless of membrane voltage. Flux in the opposite di rection was not inhibited by 9 mM ATP. Thus ATP causes true, current r ectification in SCI channels. Inhibition by cytoplasmic ATP was also v oltage dependent, having a K-1 of 0.4-1 mM at -40 mV (Hill coefficient similar to 2), which increased at more negative potentials. Luminal A TP inhibited with a K-1 of similar to 2 mM at +40 mV, and showed no bl ock at negative voltages. Hidden Markov model analysis revealed that A TP inhibition 1) reduced mean open times without altering the maximum channel amplitude, 2) was mediated by a novel, single, voltage-indepen dent closed state (similar to 1 ms), and 3) was much less potent on lo wer conductance substates than the higher conductance states. Therefor e, the SCI channel is unlikely to pass Cl- from cytoplasm to SR lumen in vivo, and balance electrogenic Ca2+ uptake as previously suggested. Possible roles for the SCI channel in the transport of other anions a re discussed.