CSF bicarbonate regulation in metabolic acidosis: role of HCO3- formation in CNS.

In metabolic acidosis, cerebrospinal fluid bicarbonate content (CSF [HCO3-]) falls in parallel with reductions in CSF CO2 tension (PCO2), and the fall is minimal with isocapnia. Regulation of CSF HCO3- was therefore investigated during 6 h of isocapnic metabolic acidosis in dogs. One group received intraventricular injections of acetazolamide to inhibit the centrally located carbonic anhydrase, essential in central nervous system (CNS) HCO3- formation, while the control group received intraventricular saline. Plasma [HCO3-] was reduced by 10 meq/l with iv infusion of 0.2 N HCl. CSF [HCO3-] fell in the control group from 22.8 to 17.7 meq/l at 6 h, whereas in the acetazolamide group it fell from 22.9 to 13.0 meq/l. Brain ammonia content was 1,286 +/- 153 microgram/100 g in the controls and 666.2 +/- 103 microgram/100 g in the acetazolamide-treated group at 6 h. Therefore, some reduction in CSF [HCO3-] occurred during 6 h of isocapnic metabolic acidosis along the chemical concentration gradient between CSF and blood, but further falls, in CSF [HCO3-] were minimized by de novo, carbonic anhydrase-dependent HCO3- formation within the CNS. Some of H+ formed were buffered by the increase in brain ammonia. These central mechanisms contribute to local CNS H+ homeostasis in metabolic acidosis.