Sphingosine 1-phosphate is an important endogenous cardioprotectant released by ischemic pre- and postconditioning

Exogenous sphingosine 1-phosphate (S1P) is an effective cardioprotectant against ischemic injury. We have investigated the hypothesis that S1P is also an important endogenous cardioprotectant released during both ischemic preconditioning (IPC) and ischemic postconditioning (IPOST). IPC of ex vivo rat hearts was instituted by two cycles of 3 min ischemia-5 min reperfusion prior to 40 min of index ischemia and then 40 min of reperfusion. IPC resulted in 70% recovery of left ventricular developed pressure (LVDP) upon reperfusion and a small infarct size (10%). VPC23019 (VPC), a specific antagonist of S1P(1 and 3) G protein-coupled receptors (GPCRs), when present during preconditioning blocked protection afforded by two cycles of IPC. VPC also blocked preconditioning of isolated rat cardiac myocytes subjected to hypoxia-reoxygenation injury. Increased release of S1P from myocytes in response to IPC was also demonstrated. These data indicate that S1P is released from myocytes in response to IPC and protects by binding to S1P GPCRs. In the ex vivo heart, if a third cycle of IPC was added to increase release of endogenous mediators, then the need for any individual mediator (e.g., S1P) was diminished and VPC had little effect. The adenosine antagonist 8-(p-sulfophenyl)-theophylline (8-SPT) likewise inhibited protection by two cycles but not three cycles of IPC, but VPC plus 8-SPT inhibited protection by three cycles of IPC. Similar to IPC, IPOST induced by four postindex ischemia cycles of 15 s reperfusion-15 s ischemia resulted in 66% recovery of LVDP and a 7% infarct size. When VPC was present during postconditioning and reperfusion, LVDP only recovered by 26% and the infarct size increased to 27%. Adding an additional cycle of IPOST reduced the inhibitory effect of VPC and 8-SPT individually, but not their combined effect. These studies reveal that S1P is an important mediator of both IPC and IPOST that is released along with adenosine during each cycle of IPC or IPOST.