Sphingosine 1-phosphate (S1P) influences heart rate, coronary artery caliber, endothelial integrity, and lymphocyte recirculation through five related high affinity G-protein-coupled receptors. Inhibition of lymphocyte recirculation by non-selective S1P receptor agonists produces clinical immunosuppression preventing transplant rejection but is associated with transient bradycardia. Understanding the contribution of individual receptors has been limited by the embryonic lethality of the S1P 1 knock-out and the unavailability of selective agonists or antagonists. A potent, S1P 1 -receptor selective agonist structurally unrelated to S1P was found to activate multiple signals triggered by S1P, including guanosine 5 -3-O-(thio)triphosphate binding, calcium flux, Akt and ERK1/2 phosphorylation, and stimulation of migration of S1P 1 -but not S1P 3 -expressing cells in vitro. The agonist also alters lymphocyte trafficking in vivo. Use of selective agonism together with deletant mice lacking S1P 3 receptor reveals that agonism of S1P 1 receptor alone is sufficient to control lymphocyte recirculation. Moreover, S1P 1 receptor agonist plasma levels are causally associated with induction and maintenance of lymphopenia. S1P 3 , and not S1P 1 , is directly implicated in sinus bradycardia. The sustained bradycardia induced by S1P receptor nonselective immunosuppressive agonists in wild-type mice is abolished in S1P 3 ؊/؊ mice, whereas S1P 1 -selective agonist does not produce bradycardia. Separation of receptor subtype usage for control of lymphocyte recirculation and heart rate may allow the identification of selective immunosuppressive S1P 1 receptor agonists with an enhanced therapeutic window. S1P 1 -selective agonists will be of broad utility in understanding cell functions in vitro, and vascular physiology in vivo, and the success of the chemical approach for S1P 1 suggests that selective tools for the resolution of function across this broad lipid receptor family are now possible.Sphingosine 1-phosphate (S1P), 1 through its high affinity G-protein-coupled receptors, is a physiological mediator with widespread effects upon multiple physiological systems (1). It regulates heart rate (2), coronary artery blood flow (3), blood pressure (4), endothelial integrity in lung (5, 6) and most recently has been shown to regulate the recirculation of lymphocytes (7-11). Many of the physiologically relevant functions occur in the low nanomolar range, including activation of endothelial nitric oxide synthase (12, 13), vasorelaxation (14), and inhibition of thymic egress and lymphocyte recirculation (11). Free plasma levels of S1P are tightly regulated by protein binding to albumin and high density lipoprotein to avoid the deleterious effects of systemic S1P receptor subtype activation at high concentrations of ligand, such as bradycardia and coronary artery vasospasm (3, 15). The choice of S1P, through its receptors, as an acute regulator of the number of blood lymphocytes may represent an interesting evolutionary choice by the immun...
