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Articles in PresS, published online ahead of print January 30, 2002
Am J Physiol Cell Physiol, 10.1152/ajpcell.00378.2001
Submitted on August 6, 2001
Accepted on January 25, 2002
1 Departments of Anatomy, Histology, and Forensic Medicine, University of Florence, Florence, Italy
2 Department of Physiological Sciences, University of Florence, Florence, Italy
3 Department of Biochemical Sciences, University of Florence, Florence, Italy
4 Biophotonics Lab, National Institute of Applied Optics, Florence, Italy
* To whom correspondence should be addressed. E-mail: zecchi{at}unifi.it.
In a wide variety of cell systems, sphingosine 1-phosphate (SPP) increases cytosolic Ca2+ concentration by acting both as an intracellular mediator and extracellular ligand. We have recently demonstrated the involvement of endothelial differentiation gene (Edg) receptors specific for SPP, in agonist-mediated Ca2+ response of mouse skeletal (C2C12) myoblastic cell line (31). However, the signaling pathways and physiological significance of the SPP-mediated Ca2+ mobilization remain, so far, to be fully delineated. To this purpose, in the present study, we investigated the Ca2+ sources of SPP-mediated Ca2+ transients in C2C12 cells and the possible correlation of the ion response to changes in the cytoskeletal organization. Confocal fluorescence imaging of C2C12 cells, pre-loaded with Ca2+ dye Fluo-3, revealed that SPP elicited a transient Ca2+ increase which propagated as a wave throughout the cells. This response required extracellular and intracellular Ca2+ pool mobilization. Indeed, it was significantly reduced by both removal of external Ca2+ and pre-treatment with nifedipine, a blocker of L-type plasmamembrane Ca2+ channels, as well as by pre-treatment with inhibitors of inositol-1,4,5-trisphosphate (InsP3)-mediated Ca2+ pathway such as heparin, caffeine, 2-aminoethyldiphenylborate (2-APB) and U-73122. The involvement of Edg receptors in SPP-induced Ca2+ transients was tested by employing suramin, a specific inhibitor of Edg-3. Fluorescence associated with InsP3Rs and L-type Ca2+ channels confirmed the presence of these channels in C2C12 cells. SPP also induced C2C12 cell contraction. This event, however, was unrelated to [Ca2+]i increase, since the two phenomena were temporally shifted, contraction occurring earlier than Ca2+ changes. Therefore, the possibility that SPP may promote C2C12 myoblastic cell contraction through Ca2+-independent mechanisms has been proposed.
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