L-Arginine Improves the Diminished Cerebral Blood Flow in Rats With Acute Liver Failure: Uncertain Role of Asymmetric Dimethylarginine

Abstract

Background and Aims: Dysregulated cerebral blood flow (CBF) contributes to neurological dysfunctions accompanying hepatic encephalopathy (HE). The well-documented increase in asymmetric dimethylarginine (ADMA) in CSF of patients and brains of animals with liver failure may impair CBF by inhibition of endothelial nitric oxide synthesis (eNOS). Therefore, the correction of l-arginine (l-arg)/ADMA ratio and thereby elevation of the availability of l-arg for NO production by l-arg supplementation may improve CBF. The aim of this study was to assess whether: (i) HE in the thioacetamide (TAA) model affects CBF in the prefrontal cortex (PFC) and cerebellum (CRBL), (ii) exogenous l-arg is capable of modulating CBF and (iii) CBF changes induced by l-arg and/or TAA are correlated with extracellular cortical ADMA, l-arg, cGMP, nitrite and citrulline levels. Methods: Male Sprague Dawley rats were subjected to three intraperitoneal (i.p.) injections of TAA (300 mg/kg) at 24 h intervals. Arterial spin labeling MRI measurement was performed 24 h after the last TAA injection using a 9.4 T scanner. High resolution anatomic images were acquired with spin echo RARE sequence. For T1 relaxation time and perfusion measurement FAIR-RARE sequence was used. l-arg (200 mg/kg) was administered once by intraperitoneal cannula. The regional changes in CBF were observed before, and 30 min after l-arg injection, at first in the PFC, then in the CRBL. In the second group of anesthetized rats, bilateral microdialysis of the PFC was carried out 24 h after the last TAA dose. Six fractions were collected every 40 min. l-arg (200 mg/kg) was injected i.p. after the beginning of the 3rd fraction. Extracellular levels of l-arg, ADMA, cGMP, nitrite, citrulline were measured using positive mode electrospray LC–DMS–MS/MS, cGMP enzyme immunoassay, NO Analyzer and HPLC-FD, respectively. A repeated-measures ANOVA was applied. Results and Discussion: TAA rats exhibited a decreased basal CBF both in the PFC and CRBL (70% and 65% of the control value, respectively). l-arg increased cortical CBF both in the control (by 20%) and TAA (by 15%) rats, and cerebellar CBF in TAA rats only (by 15%). Administration of l-arg evoked durable increase of the extracellular l-arg level in the PFC of TAA rats (by 85%), while in the control group the effect was much less pronounced (by 25%) and shorter-lasting. The diminished l-arg/ADMA ratio in the TAA brain was increased after l-arg administration, whereas no effect on ADMA was observed. The increase of CBF in TAA rats was not accompanied by changes in extracellular levels of nitrite, cGMP and citrulline. It can be assumed that beneficial effect of l-arg on CBF is, as a rule, associated with eNOS activity, whereas metabolic changes noted in the extracellular fluid reflect a combination of the variable effects of acute liver failure on each of the three NOSs isoforms (Figure 1). Disclosures: This study was supported by Grant N 2013/09/B/NZ4/00536 from the National Science Centre. The equipment used for dimethylarginine determination was sponsored in part by the Centre for Preclinical Research and Technology (CePT), a project cosponsored by Eu. The authors have none to declare.