Bicarbonate formation in cerebrospinal fluid: role in sodium transport and pH regulation.

MAREN, THOMAS H. Bicarbonate formation in cetebrospinal fluid: role in sodium transport and pH regulation. Am. J. Physiol. 222(4) : 885-899. 1972.-Two series of experiments were performed which clarified the role of HCOaformation and movement into cerebrospinal fluid (CSF), and its relation to sodium transport and pH regulation. In the first, the rates of access of HC03, Na+, and Clfrom plasma to CSF were measured in the dogfish Squalus acanthias by means of their isotopes. The rate constant (kin) values for the three ions were 1.9, 0.19, and 0.14 hr-l, respectively. Assuming that these represent net influx, the composition of CSF as secreted has the same sodium concentration as plasma, with HC03much higher and Cllower than plasma. The rate data and the effects of carbonic anhydrase inhibition suggest that HCOsU reaches the CSF by hydroxylation of gaseous COz at the choroid plexus, and probably at glia. The relations among the rates of the three ions show that the movement of HCOShas an important influence on the movement of sodium and of fluid. In the second series, the PCO~ of the fish was elevated fourfold, and the effect upon CSF HCOawas measured. The procedure caused a striking elevation of CSF HC03 concentration, to fourfold normal, in about 3 hr. As a result, pH regulation was maintained in the CSF during a profound respiratory acidosis in the plasma. The rate of HCOaformation in CSF was reduced to half by carbonic anhydrase inhibition. The data of the two series agree in showing that HCOaformation is a principal feature of CSF formation and that there is a clear homology to the formation of aqueous humor.