Antecedent hydrogen sulfide elicits an anti-inflammatory phenotype in postischemic murine small intestine

Ischemia followed by reperfusion (I/R) is now well-recognized as one form of acute inflammation in which leukocytes play a key role. Recognition of the importance of the inflammatory process to the pathogenesis of I/R injury has led to an intensive research effort directed at identifying strategies to prevent leukocyte infiltration into post-ischemic tissues. Indeed, work conducted over the past 15 years has led to the development of the concept that oxidant-induced leukocyte/endothelial cell interactions are largely responsible for the microvascular dysfunction induced by reperfusion. Preconditioning is a phenomenon through which antecedent exposure to a particular stimulus confers protection against a subsequent prolonged ischemic event. The development of a protected phenotype occurs in response to a diverse array of preconditioning stimuli, including short periods of ischemia, heat shock, ethanol, lipopolysacharide, calcitonin gene-related peptide, adenosine and bradykinin. Each of these preconditioning stimuli appears to promote the production of the gaseous monoxide, nitric oxide (NO), as an initial triggering event in the acquisition of tolerance to I/R. Recent work has shown that NO acts as an endogenous regulator of a second gaseous signaling molecule with vasorelaxant properties, hydrogen sulfide (H2S). While it had been assumed that H2S acts solely as a toxic, environmental pollutant with minimal physiological and pathophysiological significance, it is now apparent that H2S is also synthesized endogenously in mammalian tissue. Furthermore, the vasorelaxation induced by NO is synergistically affected 13-fold by H2S. Additionally, the production of H2S is up-regulated by NO. These observations suggest that, like NO, H2S has the ability to fulfill a physiologic role in regulating cardiovascular function, distinct from its toxicologic effect. In light of these observations, we postulated that H2S inhibits inflammation after I/R injury, through four separate, yet not necessarily distinct, mechanisms. Although the temporal relationships of these mechanisms are important, this project has focused on the involvement of each mechanism, without specific elucidation of sequential order. The aims of this dissertation addressed the hypothesis that H2S elicits a preconditioning stimulus and protects against I/R injury through an eNOS-/p38 MAPK-/K channel-/HO-1 dependent mechanism. REVIEW OF THE LITERATURE Introduction Ischemia, or inadequate supply of blood to a part of the body, is caused by partial or total occlusion of an artery. Early restoration of blood flow to ischemic tissues is an absolute prerequisite to halt the progression of cellular injury associated with decreased oxygen and nutrient delivery following ischemia. Although minimizing ischemic time is an important intervention for diminishing the extent of ischemic injury, reperfusion itself may lead to accelerated and additional tissue injury beyond that generated by ischemia alone (11, 56). It is now clear that reperfusion of ischemic tissues initiates a complex series of pathologic events that produce the same end result as prolonged hypoxia, i.e. cellular dysfunction and necrosis, collectively referred to as ‘reperfusion injury’. (26, 30, 31, 67, 71). Recognition of the fact that reperfusion can initiate a cascade of deleterious processes that exacerbate the tissue injury induced by ischemia has resulted in an intensive research effort directed at defining the cellular and molecular events that underlie reperfusion injury. One proposed mechanism of reperfusion injury involves the generation, accumulation and release of various reactive species along with simultaneous consumption of endogenous antioxidants. Reinfusion of previously ischemic tissue with hypoxic blood prevents reperfusion injury, while gradually increasing oxygen concentration in the reperfusate progresses injury, as with normoxic blood (73). These studies provide direct evidence that postischemic cell damage occurs by an oxygen-dependent mechanism. Indirect evidence supporting the role of ROS in I/R