Endothelial progenitor cells are rapidly recruited to myocardium and mediate protective effect of ischemic preconditioning via "imported" nitric oxide synthase activity.

The function of bone marrow-derived endothelial progenitor cells (EPCs) in repair of ischemic tissue has been the subject of intense scrutiny, and the capacity of these cells to contribute significantly to new blood vessels remains controversial. The possibility that EPCs could act as reservoirs of cytokines has been implied by several observations; however, a specific role for cytokine delivery has not been identified. We performed a series of experiments that revealed the rapid recruitment of EPCs to the myocardium by very short periods of ischemia, so-called ischemic preconditioning. The recruited EPCs express an array of potentially cardioprotective cytokines including nitric oxide synthase isoforms. Bone marrow transplantation studies, using donor marrow null for nitric oxide synthase isoforms, revealed that both endothelial and inducible nitric oxide synthase derived from bone marrow cells play essential roles in the cardioprotective effect that normally occurs after ischemic preconditioning. These findings provide novel insights about the role of bone marrow-derived cells in ischemic preconditioning and also reveal that distinct mechanisms regulate recovery after ischemia-reperfusion and chronic ischemic injury.