Modifications of cardiovascular response to ischemia and trauma

Abstract

Ischemia and trauma are two clinical problems characterized by a common response: inflammation and potentially deleterious effects on the organism. Surgery is a form of trauma, and the most advanced surgical operations, including open heart surgery, aortic surgery, and organ transplantation, cause trauma and ischemia of the cardiovascular system. It is a big challenge to develop pharmacological and surgical methods of organ protection in this situation. Organ protection can be achieved by preconditioning, an intrinsic defence mechanism triggered by brief episodes of ischemia and reperfusion. Unravelling the mechanisms underlying preconditioning may provide new therapeutic options in the treatment of inflammatory disease.

The aims of this thesis were to investigate a possible therapeutic effect of lazaroid pretreatment and its mechanism of action in a model of hemorrhagic shock and trauma, with focus on heat shock protein (HSP) 72 induction; to study the role of intraperitoneal injection and intermittent limb ischemia on heart function and the role of mitogen activated protein kinases, inducible nitric oxide synthase and nuclear factor kappa-B (NFkappaB) in the signaling of protection; to explore the role of tumor necrosis factor alpha (TNFalpha) and NFkappaB in preconditioning by hyperoxia and in bum injuryinduced myocardial intolerance to ischemia.

Rats were subjected to controlled hemorrhagic shock and trauma. The outcome of hemorrhagic shock was improved in rats pretreated with the second-generation corticosteroid analogue drug U-83836E, evident as improved maintenance of blood pressure and acid-base balance. This improvement concurred with an increased cardiac production of HSP72.

Mice were subjected to intraperitoneal injections of fluid, and 24 hours later their hearts were isolated and Langendorff-perfused with induced global ischemia and reperfusion. Intraperitoneal injection provided a preconditioning-like protection of the heart with improved postischemic function and reduced infarct size. The myocardial protection did not involve signalling with activation of NFkappaB or phosphorylation of mitogen activated protein kinases. Intermittent episodes of hind limb ischemia and reperfusion protected the heart 24 hours later, activating NFkappaB in hearts and skeletal muscle, and increasing cardiac expression of inducible nitric oxide synthase. When hind limb preconditioning was performed in mice deficient of the NFkappaB p105 subunit or of inducible nitric oxide synthase, the protective effect was abolished.

When mice were exposed to hyperoxia (>95% oxygen) before heart isolation, postischemic function was improved and infarct size reduced. Pulmonary expression of TNFalpha and its synthetizing enzyme TACE was increased after hyperoxic exposure. Hyperoxic pretreament did not afford myocardial protection of mice genetically mutated to lack TNFalpha or its receptor 1. Hyperoxic pretreatment was also efficient in protecting vascular function from the detrimental effects of exposure to TNFalpha.

TNFalpha, but not NFkappaB, was increased in hearts of mice subjected to minor cutaneous bums, which suffered an increased susceptibility to ischemia/reperfusion. Pharmacological blockade of TNFalpha by etanercept abolished this effect.

In conclusion, cardiovascular protection can be obtained by different stimuli that induce a state of self protection, resulting in tolerance to trauma and ischemia. Lazaroids induce expression of the beneficial HSP72. The main findings of this thesis are that some components of innate immunity, such as NFkappaB and the proinflammatory cytokine TNFalpha, are necessary to evoke organ protection by preconditioning. However, TNFalpha is also important for developing decreased tolerance to ischemia following bum injury. Finding the balance between the protective and detrimental effects for pharmacological treatment of patients undergoing major surgery is a challenge for the future.