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J. Biol. Chem., Vol. 282, Issue 45, 32780-32791, November 9, 2007

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Inhibition of Skeletal Muscle ClC-1 Chloride Channels by Low Intracellular pH and ATP^*

From the St. Vincent's Institute, Fitzroy, Victoria 3065, Australia

Skeletal muscle acidosis during exercise has long been thought to be a cause of fatigue, but recent studies have shown that acidosis maintains muscle excitability and opposes fatigue by decreasing the sarcolemmal chloride conductance. ClC-1 is the primary sarcolemmal chloride channel and has a clear role in controlling muscle excitability, but recombinant ClC-1 has been reported to be activated by acidosis. Following our recent finding that intracellular ATP inhibits ClC-1, we investigated here the interaction between pH and ATP regulation of ClC-1. We found that, in the absence of ATP, intracellular acidosis from pH 7.2 to 6.2 inhibited ClC-1 slightly by shifting the voltage dependence of common gating to more positive potentials, similar to the effect of ATP. Importantly, the effects of ATP and acidosis were cooperative, such that ATP greatly potentiated the effect of acidosis. Adenosine had a similar effect to ATP at pH 7.2, but acidosis did not potentiate this effect, indicating that the phosphates of ATP are important for this cooperativity, possibly due to electrostatic interactions with protonatable residues of ClC-1. A protonatable residue identified by molecular modeling, His-847, was found to be critical for both pH and ATP modulation and may be involved in such electrostatic interactions. These findings are now consistent with, and provide a molecular explanation for, acidosis opposing fatigue by decreasing the chloride conductance of skeletal muscle via inhibition of ClC-1. The modulation of ClC-1 by ATP is a key component of this molecular mechanism.

Received for publication, April 18, 2007 , and in revised form, August 2, 2007.

^* This work was supported in part by a grant from the National Health and Medical Research Council of Australia (to M. W. P. and B. A. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ An Australian Research Council Federation fellow.

² To whom correspondence may be addressed: Present address: Howard Florey Institute and Dept. of Pharmacology, University of Melbourne, Victoria 3010, Australia. Tel.: 61-3-8344-1849; Fax: 61-3-9347-0466; E-mail: brett.cromer{at}florey.edu.au.

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