Ventilation and CSF ions during isocapnic HCl and HNO3 acidosis in conscious rabbits

In conscious rabbits with preimplanted arterial, central venous, and cisterna magna catheters, we infused HCl or HNO3 over 9 h to reduce plasma [HCO-3] by 9-14 mmol/l. We hypothesized that greater entry into cerebrospinal fluid (CSF) of a strong anion, NO-3 vs. Cl-, would reduce the strong ion difference and the [HCO-3], and we used isocapnic conditions to minimize the effects of PCO2-specific mechanisms of CSF HCO-3 regulation. Although CSF pH decreased slightly in both groups reflecting a small increase in CSF PCO2, we observed no significant change in CSF [HCO-3]. With HCl, there was no significant increase in CSF [Cl-], even though plasma [Cl-] increased 15-16 mmol/l. With HNO3, CSF [NO-3] increased 3.6-7.9 mmol/l, owing to the 18-22 mmol/l increase in plasma [NO-3], and resulted, not in a decrease in [HCO-3], as hypothesized, but in a decrease in [Cl-]. There was no difference in the ventilatory responses of the two groups. These results emphasize the importance of PCO2-dependent mechanisms in determining the CSF [HCO-3] and indicate that the strong anion mechanisms of CSF HCO-3 regulation can utilize changes in both NO-3 and Cl-.