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

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