Original articleEffect of Allopurinol Supplementation on Nitric Oxide Levels in Asphyxiated Newborns
Introduction
Hypoxic-ischemic encephalopathy of the newborn has been studied extensively in various animal models and in humans to investigate the pathogenesis and treatment options. Treatment protocols of hypoxic-ischemic encephalopathy in newborn infants range from no therapy at all to extensive use of a variety of medications. New management strategies include the use of inhibitors of xanthine oxidase, excitatory amino acid antagonist, calcium channel blockers, and antioxidant enzymes [1], [2]. Most of the animal research has pointed to the role of excitatory amino acids, cytokines, and nitric oxide (NO) determining the extent of the pathology [3], [4], [5]. Ergenekon et al. reported that cerebrospinal fluid NO levels elevated with increasing severity of hypoxic-ischemic encephalopathy during the first 24-72 hours of the hypoxic insult in asphyxiated newborns [6]. Recently the production of NO, a free radical, has been demonstrated to be increasing during cerebral hypoxia-ischemia [7]. NO is formed directly from the guanidino nitrogen of l-arginine by NO synthase. Nitric oxide synthase is an unusual oxidative enzyme in that most other enzymes consume one or two electrons for similar functions. Once it was recognized that NO synthase is a calmodulin-dependent enzyme, NO synthase could be purified to homogeneity from cerebellum [8]. Soon thereafter, purified NO synthase could be isolated from a variety of other brain tissues in a variety of species [9]. It was demonstrated that there were varying effects of cerebral NO in asphyxiated knockout mice for different isoforms of NO synthase [3]. Neuronal NO synthase is activated shortly after the asphyxial event and is known to cause an increase in the neuronal NO which produces several neurotoxic effects. After the first 12-24 hours of asphyxia, inducible NO synthase in the glial cells becomes activated, causing large increases in cerebral NO, which is now considered to have a major role in the ongoing neurotoxicity following asphyxia [3]. Despite a great deal of evidence about the double-edged effects and the role of NO in animals in hypoxic-ischemic encephalopathy, few studies have been conducted in asphyxiated human newborns with regard to cerebral NO production [6], [10].
Xanthine oxidase–derived superoxide exerts its actions in the overall context of various endogenous oxidant and antioxidant systems. For example, NO can act as an endogenous suppressor of xanthine oxidase activity [11]. Because in many pathophysiologic conditions there is an impairment of endogenous NO production, a reduced level of the tonic, NO-mediated suppression of xanthine oxidase may actually lead to increased superoxide generation and pathophysiologic positive feed-forward cycles. At the same time, there may also be a xanthine oxidase–derived superoxide-dependent tonic suppression of NO synthase activity, which may lead to an enhancement of NO production in response to xanthine oxidase inhibition in vivo [12]. There is some evidence that xanthine oxidase can catalyze the reduction of nitrite and nitrate (normally the decomposition products of NO) back to NO [13]. These activities are more prominent under acidic conditions and may contribute to a pathophysiologic enhancement of NO generation in ischemic or hypoxic tissues. Xanthine oxidase–derived superoxide can rapidly react with NO or nitrosothiols in its vicinity to form the cytotoxic oxidant species peroxynitrite, which can lead to a variety of oxidative and nitrosative injury to proteins, lipids, and deoxyribonucleic acid in the vicinity of xanthine oxidase [14]. It is conceivable that after binding of circulating xanthine oxidase to the endothelium, xanthine oxidase–derived superoxide can combine with endothelially derived NO (produced by the endothelial NO synthase), and the subsequent formation of peroxynitrite and the activation of downstream pathways of cell injury can lead to endothelial and tissue injury in various pathophysiologic conditions [15], [16]. Therefore administration of xanthine oxidase–inhibiting compounds such as allopurinol might have the potential to decrease cerebral injury of the asphyxiated newborns. The aim of the present study was to investigate the effect of allopurinol on cerebrospinal fluid and serum NO concentrations in cerebral injury following birth asphyxia as well as to observe if there is a therapeutic window after the hypoxic insult with regard to NO production.
Section snippets
Patients and Methods
From January 1, 2003, to February 1, 2005, a total of 3658 neonates were admitted to the neonatal intensive care unit of Gevher Nesibe Hospital. Patients with gestation of 37 weeks or more and with history of perinatal asphyxia documented by three or more of the following were considered eligible for the study: abnormal level of consciousness; abnormal muscle tone; abnormal reflexes; seizures within the first 72 hours of life; and three or more of the following: low Apgar scores (5-minute Apgar
Results
The clinical characteristics of infants are summarized in Table 1. The asphyxiated and control infants had statistically similar birth weights, sex, delivery mode, and gestation (P > 0.05). The clinical characteristics of Groups I and II are detailed in Table 2. The groups were also similar according to birth weight, sex, delivery mode, Apgar score at 5 minutes, arterial blood base deficit, and gestation (P > 0.05). Cerebrospinal fluid and blood cultures drawn at the time of sampling revealed
Discussion
In this study, serum NO levels in the moderately and severely asphyxiated newborns were significantly higher than in control subjects, and cerebrospinal fluid levels of NO were correlated with the degree of hypoxic-ischemic encephalopathy, supporting the role of NO in the pathophysiology following asphyxia. In order to assess the role of NO in ischemic damage, it is necessary to know where and when NO is produced in the brain during and after ischemia. It appears that the production of NO
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