Relationship between central nervous system hydrogen ion regulation and amino acid metabolism in hypercapnia, II.

Resting level of ventilation is affected by change in extracellular fluid hydrogen ion concentration [H+] in the central nervous system (CNS) and by certain amino acid neurotransmitters within or near the medulla oblongata. Hypercapnia alters both cerebrospinal fluid (CSF) [H+] and CSF ammonia metabolized to glutamine, a precursor of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Therefore, the effect of 1 to 2 h of hypercapnia on cerebral cortical and medullary contents of selected amino acids and bicarbonate (HCO-3) fixation rates was studied in anesthetized mongrel dogs using 11C-labeled HCO-3. Medullary taurine, glycine, alanine, and glutamate concentrations were not significantly altered by hypercapnia, but mean medullary glutamine and GABA concentrations both increased significantly (p less than 0.05), with a high correlation (r = 0.82, n = 8) between individual values. Medullary GABA and glutamine increased linearly with CSF [H+]. The rate of CNS HCO-3 fixation into CSF glutamine was negligibly small and decreased during hypercapnia, compared with the rate of medullary tissue HCO-3 fixation, which increased linearly with CSF [H+]. These observations show that there is a significant interrelationship between medullary metabolism of GABA, glutamine, bicarbonate, and CNS hydrogen ion regulation during hypercapnia.