Glutamate and glutamine in the brain: influence of acute pCo 2 changes in normal rats and in rats under sustained hypercapnia or hypocapnia

Free amino acid concentrations in vitreous humor and cerebrospinal fluid in relation to the cause of death and postmortem interval

We studied free amino acids in vitreous humor and cerebrospinal fluid from 58 cadavers in the course of routine medicolegal autopsies in the city of Granada. The main objective was to establish whether free amino acids contents in these fluids were related with the cause of death, postmortem interval, and severity of the classic signs of asphyxia. The amino acids (aspartic acid, glutamic acid, serine, glutamine, glycine/threonine/histidine, citruline, arginine, alanine, taurine, GABA, tirosine, valine, methionine, isoleucine, phenylalanine/tryptophan, leucine, and lysine) were quantified by high performance liquid chromatography. There were no statistically significant differences in amino acids concentrations in vitreous humor when the different causes of death were considered. Our results did not show any statistically significant relationship when asphyxial score was plotted against the vitreous content of each amino acid. A statistically significant increase with postmortem interval was observed in vitreous taurine (r = 0.3191, p = 0.01461), glutamate (r = 0.4323, p = 0.0007) and particularly in aspartate (r = 0.4508, p = 0.0003).

Glutamine protects neuronal function against cerebral hypoxia: a study using the in vitro hippocampal slice preparation

Pretreatment of hippocampal slices with glutamine doubled the recovery rate of the synaptic function (electrically evoked population spike) from a standardized hypoxic insult in CA1 pyramidal neurons. This protective effect of glutamine was dose-dependent and biphasic; recovery of synaptic function was observed in 44% of the control slices and in 55%, 90%, 92% and 0% of slices pretreated with 0.1, 1.0, 5.0 and 10.0 mM glutamine, respectively. The synaptic function did not recover after hypoxia in slices pretreated with glutamic acid (1.0 mM).

Acid-base balance and plasma glutamine concentration in man

Plasma glutamine concentrations were measured in chronic metabolic acidosis and alkalosis in healthy male volunteers. Metabolic acidosis resulted in a significant drop in glutamine concentration while metabolic alkalosis significantly elevated glutamine levels. These changes in glutamine concentration correlated with both the bicarbonate and PCO2 levels. To determine whether bicarbonate or PCO2 levels influence the glutamine concentrations, respectively acidosis was induced by respiring 5% CO2. This resulted in a significant elevation in both PCO2 and glutamine while bicarbonate levels remained unchanged. The results demonstrate an effect of acid-base alterations upon plasma glutamine concentration mediated by PCO2.

CSF HCO3- regulation in isosmotic conditions: the role of brain PCO2 and plasma HCO3-

We have studied in anesthetized cats the response of CSF HCO3- to (1) an increase in CSF PCO2, plasma HCO3- controlled at the normal value; (2) an increase in plasma HCO3-, CSF PCO2 controlled at the normal value; and (3) an increase in both CSF PCO2 and plasma HCO3-. Plasma HCO3- was controlled via HCO3-/Cl- exchange without altering plasma Na concentration or osmolarity using the technique of peritoneal dialysis. The results suggest that CSF HCO3- regulation in these conditions is dependent on (1) a PCO2-dependent process in which HCO3- ions are formed in brain cells or choroid plexus and (2) an ionic movement of HCO3- from plasma to CSF dependent on an increase in plasma HCO3-. The first process is largely completed by 3 hours; the second takes 6 hours or more. An analysis of plasma and CSF HCO3-, Cl- and unmeasured anions (UA), (Na+ + K+ -CL- -HCO3-), suggests that CSF HCO3- regulation may be complex involving ionic exchanges of three or more anions between brain extracellular fluid, plasma and brain cells.

Brain amino acids in conscious rats in chronic normocapnic and hypocapnic hypoxemia

The influence of hypoxemia on the brain content of several organic acids and NH+4, AND ITS RELATIONship to the accompanying hypocapnia was studied in unanesthetized rats subjected to hypoxemia for periods ranging between 2 hours and 7 days. Under acute conditions, 'mild' hypoxemia (FO2 = 6--7%), these increases were greater and accompanied by increased gamma-aminobutyric acid (GABA) and decreased glutamic and aspartic acid levels; glutamine and NH+4 remained normal. When hypocapnia was prevented, 'severe' hypoxemia induced only a rise in GABA and slight elevations in lactic and alpha-ketoglutaric acid. During prolonged severe hypoxemia, the effects on the brain amino acids were maintained throughout, indicating that they are independent from the intracerebral pH which should progressively normalize. The effect on lactic acid gradually disappeared. The results show that during hypocapnic hypoxemia the rise in brain GABA is hypoxemia dependent, the decrease in glutamic and aspartic acid is hypocapnia dependent and the increase in lactic acid is in a large way alkalosis dependent.

Glutamate and glutamine in the brain of the neonatal rat during hypercapnia

In order to study the influence of hypercapnia on the content of glutamate and glutamine in the developing brain, pregnant rats and their offspring were kept in CO2 rich (6-10%) atmosphere and the litters were killed at different ages between 4 and 28 days. In the hypercapnic rats the content of both amino acids in the brain increases with age with almost the same time course as in normocapnic rats. At any age the glutamate content is lower in the hypercapnic animals than in control rats, whereas the glutamine content, beyond the first 8 days of life is increased. Both effects are rapidly reversible on return to air breathing. Although the glutamate-glutamine system is in full development, the influence of hypercapnia can be compared to that observed in adult rats. Hypercapnia did not change the glutaminase and the glutamine synthetase activity of the brain.

The CSF HCO3 increase in hypercapnia relationshp to HCO3, glutamate, glutamine and NH3 in brain

To study the role of carbonic anhydrase in the CSF [HCO3] increase in respiratory acidosis and its effect on brain ammonia, anesthetized rats were subjected to hypercapnia (7% CO2) for 2 hours. The animals received periodic intraventricular injections of either 'mock' CSF or 'mock' CSF and acetazolamide for 45 minutes prior and during hypercapnia when: (a) plasma [HCO3-] was allowed to increase normally and (2) plasma [HCO3] increase was prevented by i.v. HC1 infusion, CSF [HCO3] increased 8.5 mM/L after 2 hours of hypercapnia (delta PCO2 40) in the rats with intraventricular 'mock' CSF injections, and only 6 mM/L in the animals with acetazolamide injections. CSF [HCO3-] increased 7 mM/L during hypercapnia and HCl infusion with intraventricular 'mock' CSF injections, but only 2 mM/L with acetazolamide injections. Changes in total brain CO2 (increase) and brain glutamic acid (decrease) in hypercapnia were not affected by intraventricular acetazolamide and i.v. HCl. The increase of brain NH4+ and glutamine in hypercapnia was reduced in these conditions. It is concluded that there are at least two sources for the CSF [HCO3-] increase in hypercapnia; one formed in the CNS and dependent on carbonic anhydrase, and the other derived from plasma [HCO3-] increase.