Nitric oxide and extravasation in endotoxaemia : an experimental study

Abstract: Background: Despite advances in intensive care the mortality of patients in septic shock is still high. In early stages the condition is characterised by circulatory failure that is a consequence of myocardial depression, vasodilatation with decreased sensitivity to catecholamine stimulation and increased loss of protein and fluid from the circulation. For each of these components nitric oxide (NO) has been proposed as a mediator. In the thesis focus is on extravasation and the influence of NO on this.Methods: As a model of sepsis chloralose anaesthetised rats were subjected to endotoxaernia induced by E. coli lipopolysaccharide (LPS) 3 mg kg-1 i.v. Mean arterial pressure (MAP), heart rate (HR) and haematocrit (Hct) were followed. NO synthesis was evaluated by the detection of methaemoglobin (metHb), nitrite and nitrate in blood. Extravasation was investigated by a double isotope method of labelled albumin (albumin clearance) and by the determination of extravascular water. The effects of endotoxaemia were studied for 3 or, in the majority of cases, 5 h. To evaluate, the involvement of inducible nitric oxide synthase (iNOS) aminoguanidine (AG), an inhibitor of iNOS was employed. Finally, the presence of iNOS in several tissues was investigated by immunohistochemistry.Main Results: LPS increased formation of NO metabolites. Initially after LPS MAP was decreased and HR elevated. Het was decreased over the 5 h observation period. After LPS the leakage of albumin, as evidenced by albumin clearance, was decreased in several vascular beds, mostly in the gastrointestinal tract. Extravascular water was not increased in any tissue. AG had no influence on circulatory changes to LPS or on albumin clearance despite the inhibition of NO synthesis as indicated by metHb and nitrate. iNOS was present in several tissues of LPS treated as well as control rats but not in the heart or in any vessels; only in the lung was there a significant increase of iNOS positive inflammatory cells after LPS as compared to controls.Conclusion: In this rat model of endotoxaemia LPS increased the formation of NO and induced circulatory changes commonly seen in rat endotoxaernia. Yet there were no signs of increased extravasation, on the contrary results are compatible with decreased or unchanged losses of fluid and albumin. It is proposed that this result is depending on changed microvascular haemodynamics. It also shows that increased NO formation does not necessarily lead to increased extravasation and indicates that NO could  protect against extravasation in endotoxaemia or at least be neutral for the process. It also points to the importance of carefully evaluating the experimental models used in the study of endotoxaemia.