Cardiovascular regulation by endogenous nitric oxide is essential for survival after acute haemorrhage

Our previous studies have indicated that endogenous nitric oxide serves as a physiologically important inhibitor of vascular tone during acute haemorrhage. This vasodilator action attenuates the concomitant reflex adrenergic constriction and thereby prevents critical reduction of tissue blood flow. The present study aimed to evaluate the overall importance of this nitric oxide regulation for survival after acute haemorrhage. This was done by comparative observations of survival time and circulatory, metabolic and histopathological changes after an acute standardized lethal blood loss (45%) in cats exposed to nitric oxide synthase (NOS) inhibition and in matched control animals with intact nitric oxide regulation. NOS inhibition was instituted by intravenously administered N omega-nitro-L-arginine methyl ester. The survival time averaged 2 h 49 min in the NOS-blocked animals and 10 h 14 min in the control animals (P < 0.001). NOS inhibition thus reduced the posthaemorrhagic survival time to < 30% of that in the control cats. Haemorrhage in the NOS-blocked animals led to rapidly developing arterial hypotension, increased anaerobic metabolism, metabolic lactacidosis, hyperkalaemia, and morphological tissue damage especially in heart and liver, in spite of maintained arterial normoxia, which signifies tissue hypoxia caused by seriously impaired nutritional blood supply. At the time of death of the NOS-blocked cats, the control animals still exhibited a virtually normal circulatory/metabolic state. A much later, and more slowly developing circulatory/metabolic deterioration was observed in the control animals. These differences between the two groups of animals indicate that nitric oxide release, by its vasodilator action, to a significant extent helps to maintain an adequate nutritional blood supply to the tissues in acute haemorrhage.