Dynamics of brain extracellular fluid pH and phrenic nerve activity in cats after end-tidal CO2 forcing.

Ventilation is influenced by the interstitial [H+] of the brain. The pHecf, which in turn is determined largely by ventilation (via PaCO2) is sensed by the central chemoreceptors. In order to investigate the dynamics of both pHecf and neural tidal volume, we measured in cats with cut vagi and sinus nerves the dynamic medullary pHecf changes and the associated changes in integrated phrenic nerve activity after end-tidal CO2 forcing. The medullary surface ecf pH was measured with a glass electrode with a flat pH-sensitive surface. After CO2 up-steps, the pHecf changed with a time constant of about 43 sec, after down-steps 30 sec was found. The central time constant of the neural tidal volume response was 50 sec (mode) in both cases, whereas the overall response had a (modal) time constant of 80 sec. The results indicate that pHecf dynamics and the dynamic characteristics of the central neural respiratory organization are about equally important in determining the dynamic neural tidal volume response. It is argued that when PaCO2 changes, the dynamic pHecf change is perfusion limited and macroscopically homogeneous within the brainstem. Therefore, in our view it seems that the location of the central chemoreceptors within the brainstem is of minor importance in determining the dynamic neural tidal volume response to PaCO2 changes.