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1 The Dorothy M. Davis Heart & Lung Research Institute, The Ohio State University, Columbus, OH, USA; Department of Internal Medicine (Division of Pulmonary, Critical Care and Sleep Medicine), The Ohio State University, Columbus, OH, USA
2 The Dorothy M. Davis Heart & Lung Research Institute, The Ohio State University, Columbus, OH, USA; Emergency Medicine, The Ohio State University, Columbus, OH, USA
3 The Dorothy M. Davis Heart & Lung Research Institute, The Ohio State University, Columbus, OH, USA; Molecular and Cellular Biochemistry, The Ohio State University, Columbus, OH, USA
* To whom correspondence should be addressed. E-mail: clanton.1{at}osu.edu.
Acute exposure to severe hypoxia depresses contractile function and induces adaptations in skeletal muscle that are only partially understood. Previous studies have demonstrated that antioxidants (AOXs) given during hypoxia partially protect contractile function, but this has not been a universal finding. This study confirms that specific AOXs, known to act primarily as superoxide scavengers, protect contractile function in severe hypoxia. Furthermore, the hypothesis is tested that the mechanism of protection involves preservation of high-energy phosphates (ATP, CrP) and reductions of Pi. Rat diaphragm muscle strips were treated with AOXs and subjected to 30 min of hypoxia. Contractile function was examined using twitch and tetanic stimulations and the degree of elevation in passive force occurring during hypoxia (contracture). High-energy phosphates were measured at the end of 30 min hypoxia exposure. Treatment with the superoxide scavengers, 4,5-dihydroxy-1,3-benzenedisulfonic acid (Tiron, 10 mM) or Mn(III)tetrakis(1-Methyl-4-pyridyl) porphyrin pentachloride (MnTMPyP, 50 µM) suppressed contracture during hypoxia and protected maximum tetanic force. Although N-acetylcysteine (NAC, 10 mM or 18 mM) reduced contracture during hypoxia, it had no influence on tetanic force production. Contracture during hypoxia without AOXs was also shown to be dependent on the extracellular Ca+2 concentration. Though hypoxia resulted in only small reductions in [ATP], [CrP] was decreased to approximately 10% of control. There were no consistent influences of the AOX treatments on high-energy phosphates during hypoxia. The results demonstrate that superoxide scavengers can protect contractile function and reduce contracture in hypoxia through a mechanism that does not involve preservation of high-energy phosphates.
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