H+ transport from CNS in hypercapnia and regulation of CSF [HCO3-]

CSF HCO3- increases more than plasma HCO3- in hypercapnia, and there are at least two sources for the CSF HCO3- increase--one derived from the simultaneous increase in plasma HCO3-, and the other, HCO3-formed from hydration of CO2 in the choroid plexus and glia and susceptible to inhibition by acetazolamide (J. Appl. Physiol. 38: 504-512, 1975). It was proposed that the H+ formed in the CNS in CO2 hydration is actively exchanged for plasma Na+ utilizing the Na-K ATPase pump. H+ transport from the CNS was therefore studied in four groups of dogs breathing 5% CO2 at constant VA for 4 h with repeated injections of saline, acetazolamide 5 mg/ml, ouabain 0.1 mg/ml, and acetazolamide and ouabain together into lateral cerebral ventricles. Arterial HCO3-increased 2.5 meq/l at 4 h of hypercapnia in all groups. CSF HCO3-increased 5.8 meq/l in the saline-injected animals, but it increased only about 2 meq/l and equaled plasma HCO3- rise in the other three groups. Therefore CNS HCO3- formation in hypercapnia can be blocked by inhibiting the CO2 hydration reaction with acetazolamide or by blocking H+ removal by inhibiting Na-K ATPase with ouabain. The data support the thesis of active H+ removal from the CNS in exchange for plasma Na+ in hypercapnia.