Glutamine-dependent inhibition of pial arteriolar dilation to acetylcholine with and without hyperammonemia in the rat

Abstract

Glutamine has been shown to influence endothelial-dependent relaxation and nitric oxide production in vitro, possibly by limiting arginine availability, but its effects in vivo have not been well studied. Hyperammonemia is a pathophysiological condition in which glutamine is elevated and contributes to depressed CO 2 reactivity of cerebral arterioles. We tested thehypothesis that acute hyperammonemia decreases pial arteriolar dilation to acetylcholine in vivo and that this decrease could be prevented by inhibiting glutamine synthetase with L-methionine-S-sulfoximine (MSO) or by intravenous infusion of L-arginine. Pial arteriolar diameter responses to topical superfusion of acetylcholine were measured in anesthetized rats before and at 6 h of infusion of either sodium or ammonium acetate. Ammonium acetate infusion increased plasma ammonia concentration from 3 0 to ~ 600 μM and increased cerebral glutamine concentration fourfold. Arteriolar dilation to acetylcholine was intact after infusion of sodium acetate in groups pretreated with vehicle or with MSO plus methionine, which was coadministered to prevent MSO-induced seizures. In contrast, dilation in response to acetylcholine was completely blocked in hyperammonemic groups pretreated with vehicle or methionine alone. However, MSO plus methionine administration before hyperammonemia, which maintained cerebral glutamine concentration at control values, preserved acetylcholine dilation. Intravenous infusion of L-arginine during the last 2 h of the ammonium acetate infusion partially restored dilation to acetylcholine without reducing cerebral glutamine accumulation. Superfusion of 1 or 2 mM L-glutamine through the cranial window for 1 h in the absence of hyperammonemia attenuated acetylcholine dilation but had no effect on endothelialindependent dilation to nitroprusside. We conclude that 1) hyperammonemia reduces acetylcholineevoked dilation in cerebral arterioles, 2) this reduction depends on increased glutamine rather than ammonium ions, and 3) increasing arginine partially overcomes the inhibitory effect of glutamine. Keywordsarginine; cerebral circulation; endothelial-dependent dilation; hepatic encephalopathy; hyperammonemia; nitric oxide Patients with hepatic failure have been reported to have a depressed cerebrovascular response to hypercapnia (9). Depressed cerebral vasodilation to hypercapnia also has been observed in experimental models of acute hyperammonemia in a variety of species, including monkey, dog, cat, and rat (2,6,15,16,33 Hyperammonemia produces substantial increases in brain glutamine concentration (7). Inhibition of glutamine synthetase preserves cerebrovascular reactivity to hypercapnia and hypocapnia during acute hyperammonemia (13,15). Thus the loss of CO 2 reactivity is related to glutamine accumulation rather than to direct vascular effects of ammonium ions. Moreover, intravenous infusion of glutamine in the absence of hyperammonemia decreases hypercapnic vasodilation, although not to the same extent as hyperammonemia (22). The...

Glutamine: A Trojan horse in ammonia neurotoxicity

“…50 Likewise, MSO restored ammonia-induced reduced CBF responsiveness to hypercapnia 51 and depressed vasodilatory response to acetylcholine. 52 Recently, a 1 H-MRS examination revealed a good correlation between cerebral glutamine level (glutamate/ glutamine signal) and the severity of neuropsychological effects induced by hyperammonemia in patients with cirrhosis. 53 Additionally, the brain concentration of glutamine, as measured by microdialysis, almost perfectly correlated with levels of intracranial pressure.…”

Role of astrocytes in cerebrovascular regulation

“…Astrocytes are enriched in glutamine synthetase which forms glutamine from ammonia and glutamate (22,85). During hyperammonemia, increases in tissue glutamine result in depressed cerebrovascular reactivity to CO 2 and acetylcholine due, in part, to glutamine's interference with NOS activity (46,47,57,86,108). Astrocyte swelling and increased extracellular K + activity related to the osmotic effect of glutamine accumulation may also contribute to impaired vascular reactivity (106,109,116).…”

Effects of l-glutamine supplementation on maternal and fetal hemodynamics in gestating ewes exposed to alcohol

“…Two diverse effects of L-glutamine on vascular reactivity have been observed. In vivo and in vitro studies have shown that L-glutamine influences endothelial-dependent relaxation by inhibiting availability of NADPH (Wu et al 2001) and by inhibiting recycling of citrulline to arginine to limit nitric oxide release (Kawaguchi et al 2005; Arnal et al 1995; Meininger and Wu 1997; Okada et al 2000), whereas other studies have postulated that a longer duration vasodilatory effect of L-glutamine is by its donation of nitrogen molecule for vasodilator synthesis (Xu and Pearl 1994). Thus, we hypothesized that repeated maternal alcohol exposure would lead to: (1) maternal and fetal hypercapnic acidemia; (2) alteration of systemic hemodynamics; (3) a decrease in uterine blood flow; and (4) subsequent changes in fetal regional brain blood flow.…”