ATP inhibition and rectification of a Ca2+-activated anion channel in sarcoplasmic reticulum of skeletal muscle

Abstract

We describe ATP-dependent inhibition of the 75-105-pS (in 250 mM Cl-) anion channel (SCI) from the sarcoplasmic reticulum (SR) of rabbit skeletal muscle. In addition to activation by Ca2+ and voltage, inhibition by ATP provides a further mechanism for regulating SCI channel activity in vivo. Inhibition by the nonhydrolyzable ATP analog 5'-adenylylimidodiphosphate (AMP-PNP) ruled out a phosphorylation mechanism. Cytoplasmic ATP (~1 mM) inhibited only when Cl- flowed from cytoplasm to lumen, regardless of membrane voltage. Flux in the opposite direction was not inhibited by 9 mM ATP. Thus ATP causes true, current rectification in SCI channels. Inhibition by cytoplasmic ATP was also voltage dependent, having a K1 of 0.4-1 mM at - 40 mV (Hill coefficient ~2), which increased at more negative potentials. Luminal ATP inhibited with a K1 of ~2 mM at +40 mV, and showed no block at negative voltages. Hidden Markov model analysis revealed that ATP inhibition 1) reduced mean open times without altering the maximum channel amplitude, 2) was mediated by a novel, single, voltage-independent closed state (~1 ms), and 3) was much less potent on lower conductance substates than the higher conductance states. Therefore, 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 are discussed.