Cerebral hyperemia and nitric oxide synthase in rats with ammonia-induced brain edema

doi:10.1016/S0168-8278(00)00069-6

Journal of Hepatology

Volume 34, Issue 4, April 2001, Pages 548-554

https://doi.org/10.1016/S0168-8278(00)00069-6 Get rights and content

Abstract

Background/Aim: Brain edema is a common fatal complication in acute liver failure. It is related to an acute change in brain osmolarity secondary to the glial accumulation of glutamine. Since high cerebral blood flow (CBF) precedes cerebral herniation in fulminant hepatic failure we first determined if an increase in brain water and glutamine are prerequisite to a rise in CBF in a model of ammonia-induced brain edema. Secondly, we determined if such a cerebral hyperperfusion is mediated by nitric oxide synthase (NOS).

Methods: Male rats received an end-to-side portacaval anastomosis (PCA). At 24 h, they were anesthetized with ketamine and infused with ammonium acetate (55 μM/kg per min). Studies were performed at 60, 90, 120, 150 and 180 min after starting the ammonia infusion and once the intracranial pressure had risen three-fold (mean 210′). Brain water (BW) was measured using the gravimetry method and CBF with the radioactive microsphere technique. Glutamine (GLN) in the CSF was sampled via a cisterna magna catheter. The neuronal NOS was specifically inhibited by 1-2-trifluoromethylphenyl imidazole (TRIM, 50 mg/kg intraperitoneally) and in separate studies nonspecifically by N-omega-nitro-l-arginine (l-NNA, 2 μg/kg per min intravenously)

Results: At 90′, brain water was significantly increased (P<0.015) as compared to the 60′ group while CBF was significantly different at 150′. A significant correlation was observed between values of CBF and brain water (r=0.88, n=36, P<0.001). Administration of either TRIM or l-NNA did not prevent the development of cerebral hyperperfusion and edema.

Conclusion: We observed that cerebral hyperemia follows an initial rise in brain water content, rather than in the cerebrospinal fluid concentration of glutamine. The rise in CBF further correlated with brain water accumulation and was of critical importance for the development of intracranial hypertension. The unique mechanism for the rise in CBF in hyperammonemia was not prevented by NOS inhibition indicating that NO is not the mediator of high CBF and intracranial hypertension.

Introduction

Cerebral edema and intracranial hypertension commonly complicate the clinical course of fulminant hepatic failure (FHF) [1]. The pathogenesis of intracranial hypertension is not settled but accumulating evidence points at the importance of disturbances in the regulation of cerebral blood flow (CBF) [2], [3]. Indeed, high CBF is correlated with cerebral herniation in patients with FHF [4], [5], [6]. The exact mechanism responsible for the increase in CBF remains poorly understood, but two possibilities arise. In the first, cerebral hyperperfusion results from changes in the peripheral circulation, i.e. systemically-induced cerebral hyperemia [3]. However, we have recently shown a lack of correlation between CBF and cardiac output in patients with FHF [7]. The second possibility is that cerebral vasodilatation arises from metabolic changes in endothelial cells, astrocytes and/or neurons [3], [8].

A high arterial ammonia plasma concentration is correlated to cerebral herniation in patients with FHF [9], a finding that supports previous conclusions from a large number of experimental studies [10]. Ammonia is detoxified in brain astrocytes to glutamine. Inhibition of glutamine synthesis with methionine-sulfoximine (MSO) prevents the development of ammonia-induced brain edema in normal rats [11] and ameliorates brain edema in rats after portacaval anastomosis (PCA) receiving an ammonia infusion [8]. An increase in brain glutamine is associated with a marked rise in CBF at the time of an increase in brain water and intracranial pressure [12]. From this laboratory, it has been demonstrated that MSO ameliorates brain edema by inhibiting both glutamine synthesis and the development of cerebral hyperemia [8]. We proposed that activation of neuronal nitric oxide synthase (nNOS) with subsequent nitric oxide-induced cerebral arterial vasodilatation could be the link between an increased brain glutamine and the change in CBF seen in FHF [[3], [8]].

Rats after PCA receiving an ammonia infusion predictably exhibit both brain edema and cerebral hyperemia when studied at 3–3.5 h of infusion. In the present study, we first determined the sequence of alterations in brain water, intracranial pressure and cerebral blood flow, hoping to establish a predictable time for the increase in CBF. Subsequently, studies were performed to determine if increased production of nitric oxide could be responsible for the increase in CBF. Both non-specific and specific inhibitors of nNOS were used to examine this hypothesis.

Section snippets

Performance of portacaval anastomosis

This study was performed in male Sprague–Dawley rats (Charles River, Wilmington, DE) weighing 300–400 g. Once received, the animals were housed in cages, fed a regular chow diet with free access to water, and kept on a 12:12 light/dark cycle. After 1 week, they were anesthetized with methoxyflurane and an end-to-side portacaval anastomosis (PCA) was constructed using a continuous suture technique (7-0 Surgilene, Davis &Geck, Danbury, CT). The anastomosis was completed within 15 min. The abdomen

Systemic variables

Body weight was similar in all groups. No significant differences in MAP, heart rate or renal blood flow were seen between the seven groups (Table 1). Arterial oxygen content was also similar between groups.

Ammonia and glutamine levels

Plasma arterial ammonia concentrations progressively increased with the duration of ammonia infusion, though levels plateau at 150 min. Ammonia levels were already significantly elevated within 60′ of infusion (P<0.01) as compared to the control group (108±15 μmol/l), (Fig. 1). Glutamine

Discussion

In this study, a rise in brain water preceded the development of cerebral hyperemia by approximately 1 h and is consistent with the notion that the signal for cerebral vasodilatation arises from the swollen brain. Once CBF began to increase, brain water progressively rose, and the contribution of cerebral hyperemia to brain edema is suggested by a correlation between CBF and brain water content (Fig. 2). Accordingly it can be concluded that cerebral accumulation of water is responsible for the

Acknowledgements

This work was supported by a grant (FSL) provided by the Danish Association for the Study of the Liver in co-operation with Schering–Plough, the Veterans Administration Research Service and the Stephen S. Tips Memorial Fund at Northwestern Memorial Hospital.

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Cerebral hyperemia and nitric oxide synthase in rats with ammonia-induced brain edema

Abstract

Introduction

Section snippets

Performance of portacaval anastomosis

Systemic variables

Ammonia and glutamine levels

Discussion

Acknowledgements

J Hepatol

Gastroenterology

Gastroenterology

Brain Res

Hepatology

Hepatic encephalopathy and cerebral edema

Semin Liver Dis

Cerebral circulation in liver failure

Semin Liver Dis

Cerebral hemodynamic and metabolic changes in fulminant hepatic failure: a retrospective study

Hepatology

Cerebral blood flow and metabolism in fulminant liver failure

Hepatology

Transcranial doppler sonography may predict brain death in patients with fulminant hepatic failure

Transplant Proc

Cerebral perfusion, arterial pressure and cardiac output in patients with acute liver failure

Crit Care Med

Cerebral blood flow and the development of ammonia-induced brain edema in rats after portacaval anastomosis

Hepatology

Cerebral herniation in patients with acute liver failure is correlated with arterial ammonia concentration

Hepatology

Brain edema and hepatic encephalopathy

Semin Liver Dis