Reperfusion of ischemic myocardium leads to a local burst of free radicals, increased [Ca 2؉ ] i , and the release of proinflammatory cytokines. The purpose of this study was to determine whether brief exposure of cardiac fibroblasts to H 2 O 2 is associated with transient changes in [Ca 2؉ ] i levels and whether this stimulus is sufficient to induce interleukin-6 (IL-6) expression. Cardiac derived fibroblasts were isolated from adult male rats and cultured under standard conditions. Individual coverslipattached fibroblasts were loaded with the calcium probe Fura-2/AM and exposed to a single 3-min pulse of 100 M H 2 O 2 . In addition, low passage cultures were exposed to a pulse of H 2 O 2 and assayed for IL-6 expression. For the past several years, it has been known that when viable myocardium is reperfused following ischemia, there is a brief and substantial spike in reactive oxygen species (ROS) 1 levels in the tissue. Through the use of electron paramagnetic resonance spectroscopy and spin-trapping techniques, the bulk of ROS production was shown to occur during the first 5 min of reperfusion, being maximal at 2 min (1). Initial studies on this event focused on the process of myocardial stunning, the reversible reduction in contractile performance that follows nonlethal durations of ischemia (2). Studies by the laboratories of Bolli et al. (3) and Gross et al. (4) show that this impairment of contractile performance could be mitigated by treatment with ROS scavengers, and studies by McDonough et al. (5) have suggested that stunning is attributed at least in part to oxidative damage targeted to troponin species (5). ROS release also causes lipid peroxidation, and 4-hydroxynonenol has been shown to impair mitochondrial function at the level of cytochrome c oxidase (6). In addition, we have previously shown that reperfusion is followed by the activation of redox-sensitive transcription factors with the generation of proinflammatory cytokines and chemokines (7-9). Thus, a broad range of biological effects results from the post-ischemic ROS transient.In nonexcitable cells, exposure to oxidative stress has been shown to cause a calcium transient. Qin et al. (10) have shown that exposure of B cells to peroxide leads to an increase of intracellular calcium with a very rapid onset. In a series of elegant experiments, these workers showed that phospholipase C␥ mediated the hydrolysis of phosphatidylinositol 4,5-bisphosphate to IP 3 , which led to the calcium mobilization. Given the fact that the ultrastructural work by Nag et al. (11) and Eghbali et al. (12) has shown that 65-70% of the cells in the myocardium are non-myocytes with fibroblasts widely scattered throughout the tissue, these findings suggest that these cells may manifest a calcium transient following ischemia reperfusion. Given the known role of calcium as a regulator of gene transcription, it is possible that this could be a regulatory mechanism in the post-ischemic myocardium. Fibroblasts are a rich source of cytokines, chemokines, and growth...