Objectives: Sphingosine 1-phosphate (Sph-1-P), a bioactive lipid derived from activated platelets, may play an important role in coronary artery spasm and hence the pathogenesis of ischemic heart diseases, since we reported that a decrease in coronary blood flow was induced by this lysophospholipid in an in vivo canine heart model [Cardiovasc. Res. 46 (2000) 119]. In this study, metabolism related to and cellular responses elicited by Sph-1-P were examined in human coronary artery smooth muscle cells (CASMCs). Methods and results: [³H]Sphingosine (Sph), incorporated into CASMCs, was converted to [³H]Sph-1-P intracellularly, but its stimulation-dependent formation and extracellular release were not observed. Furthermore, the cell surface Sph-1-P receptors of S1P family (previously called EDG) were found to be expressed in CASMCs. Accordingly, Sph-1-P seems to act as an extracellular mediator in CASMCs. Consistent with Sph-1-P-elicited coronary vasoconstriction in vivo, Sph-1-P strongly induced CASMC contraction, which was inhibited by JTE-013, a newly-developed specific antagonist of S1P₂ (EDG-5). Furthermore, C3 exoenzyme or Y-27632 inhibited the CASMC contraction induced by Sph-1-P, indicating Rho involvement. Finally, exogenously-added [³H]Sph-1-P underwent a rapid degradation. Since lipid phosphate phosphatases, ectoenzymes capable of dephosphorylating Sph-1-P, were expressed in CASMCs, Sph-1-P may be dephosphorylated by the ectophosphatases. Conclusions: Sph-1-P, derived from platelets and dephosphorylated on the cell surface, may induce the contraction of coronary artery smooth muscle cells through the S1P₂/Rho signaling.