Studies on the mechanism of intestinal ischemic preconditioning – the role of neuronal nitric oxide synthase

Abstract

Small bowel transplantation (SBTX) is a potentially life-saving solution for patients suffering from intestinal failure. Novel immunosuppressive strategies have greatly improved the clinical outcome of SBTX,butthe survival of grafts is still poor due to non-immunological reactions caused byhypoxic damage or ischemia/reperfusion (IR) injury. Both warmischemiaand cold ischemic periods(i.e. SBTX)arecharacterized by severebiochemical and microcirculatory consequences which affect theintegrity of the highly vulnerable mucosal barrier. These alterationsmay range from increased bowel permeability totranslocation of gut-derived endotoxin and intraluminal bacteria,causing sepsis,multiple organ failureor death. The long-term consequences of SBTX also include intestinal motility changes caused by denervation during the surgical procedure.Ischemic preconditioning (IPC) involvestransient, brief periods of ischemia,followed by short intervals of reperfusion prior to the final ischemic challenge.Some of the detrimental consequences of non-immune-mediated bowel injury can be ameliorated by IPC, but the details of the underlying, protective mechanisms are still largely unknown.Since microcirculatory reactions can affecttheblood supply as well asmicrovascular inflammatory reactionsand cancritically influence the integrity of the mucosal barrier during IR injury, major emphasiswas put on the effect of IPC on these changes in relation to SBTXin the present studies. Firstly,the consequences of local intestinal IPC wereexamined in a detailed fashion in a large animal model, where bowel innervation is more similar to that inhumansthan in smaller species. In this system,we demonstrated that IPC improvesmacrohemodynamics and intestinal microcirculationand reduces leukocyte-mediated tissue injury during reperfusion. Nitric oxide (NO) has previously been implicated as a mediator in the mechanism of IPC, andneuronal NOsynthase (nNOS) is a dominant NOS isoform in the gastrointestinal tract. Therefore, the aim of ourfurtherstudieswas to investigate the role of nNOS in IPC-induced protection after mesenteric IR using the selective enzyme inhibitor 7-nitroindazole. Among the positive effects of IPC, NO availability, mucosal pH recovery, amelioration of leukocyte accumulation and mast cell degranulation were detected,andmorphological injury was reducedby thetreatment. Therefore, it was concluded that IPC represents an effective tool to ameliorate reperfusion injury during SBTX, and the data indirectly provedthat NO generated by intestinal nNOS plays a pivotal role in IPC-linked tissue protection during IRinjury.