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1 Department of Human Anatomy and Physiology, University of Padova, Padova, Italy
2 Department of Biomedical and Experimental Sciences, University of Padova, Padova, Italy
3 Institute of Neuroscience, Muscle Biology and Physiopathology, National Research Council (CNR), Padova, Italy
4 Department of Biology and Heart Institute, San Diego State University, San Diego, California, USA
* To whom correspondence should be addressed. E-mail: daniela.danieli{at}unipd.it.
Sphingomyelin derivatives exert various second messenger actions in numerous tissues. Sphingosine (SPH) and sphingosine 1-phosphate (S1P) are two major sphingomyelin derivatives present at high levels in blood. The aim of present work was to investigate whether S1P and SPH exert relevant actions in mouse skeletal muscle contractility and fatigue. The exogenous S1P and SPH supplementation caused a significant reduction of tension decline during fatigue of EDL muscle (final tension after the fatiguing protocol was 40% higher with respect to untreated muscle). Interestingly, N,N-dimethylsphingosine, an inhibitor of SPH kinase (SK), abolished the effect of supplemented SPH and not that of S1P, suggesting that SPH acts through its conversion into S1P. Moreover, SPH was not effective in Ca2+-free solutions, in conformance with the hypothesis that SPH action is dependent on its conversion to S1P by the Ca2+ requiring enzyme, SK. In contrast to SPH, S1P produced its positive effects on fatigue in Ca2+-free conditions, indicating that S1P action does not require Ca2+ entry and, most likely, is receptor mediated. The effects of S1P could be in part ascribed to its ability to prevent the reduction (-20 mV) of action potential amplitude caused by fatigue. In conclusion, these results indicate that extracellular S1P has protective effects during the development of muscle fatigue and that the extracellular conversion of SPH to S1P may represent a rheostat mechanism to protect skeletal muscle from possible cytotoxic actions of SPH.
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