Remodeling of hepatic microvascular responsiveness after ischemia/reperfusion.

Although there is substantial evidence suggesting that the integrity of the microcirculation is an important determinant of tissue viability during reperfusion after ischemia in the liver, as well as other tissues, the mechanisms responsible for microvascular failure are not fully understood. It is now recognized that the microvascular response to reperfusion, similar to the whole organism response to shock, can consist of either a rapid exacerbation of injury after a severe ischemic episode or, alternatively, a more slowly developing alteration in responsiveness that occurs after a less severe insult. In the more slowly developing response, the alterations in vascular status are the result of up-regulation of stress-induced vascular mediators such as endothelin, nitric oxide synthase (NOS), and heme oxygenase, as well as changes in the reactivity of the effector cells to the mediators. The mechanisms for change in reactivity of vascular cells range from changes in receptor expression to overt phenotypic transformation, as can occur in the hepatic stellate cells in response to repeated injury. When maintained in balance, these counteracting constrictor and dilator influences can be protective; however, local imbalance can result in focal ischemia, thus propagating the injury. Thus, the remodeling of the hepatic microvascular responsiveness during reperfusion after ischemia may serve as a useful paradigm for consideration of the overall response of the organism to shock.