Localization of S1P receptors in rat soleus muscle. Role of sphingosine-1-phosphate on skeletal muscle trophism.

Sphingosine 1-phosphate (S1P) mediates a number of cellular responses, including growth and proliferation. The aim of this work was to localize S1P receptors in rat skeletal muscle and to investigate whether S1P exerts a trophic action on muscle fibers. RT-PCR and Western blot analyses demonstrated the expression of S1P1 and S1P3 receptors in adult soleus muscle. Immunofluorescence revealed that S1P1 and S1P3 receptors are localized at the cell membrane of muscle fibers and in the T-tubule membranes. The receptors also decorate the nuclear membrane. and were expressed in the satellite cells. The possible trophic action of S1P was investigated by utilizing the denervation atrophy and in vivo regeneration models. Denervation of rat soleus muscle, analyzed 7 and 14 days after motor nerve cut, produced the down regulation of S1P1 and S1P3 receptors. The continuousdelivery of S1P, through a mini osmotic pumps, to the denervated muscle significantly attenuated the progress of denervation-induced muscle atrophy. During regeneration of rat soleus, induced by bupivacaine, expression of S1P1 receptor progressively increased between 3 and 7 days after degeneration, while that of S1P3 progressively decreased. The direct injection into the regenerating muscle of S1P (100 μl of 50 μM) determined an accelerated growth of regenerating fibers. In fact, in the presence of S1P, the mean cross sectional area of the 3-days regenerating fibers was significantly higher (+ 29.4 %) than in the contralateral regenerating muscle. In conclusion, the results indicate that S1P plays a significant role in muscle fiber trophism and development.