Transient and steady state responses of pulmonary ventilation to the medullary extracellular pH after approximately rectangular changes in alveolar PCO2

Abstract

The extracellular pH (pHe) either on the ventral surface of the medulla oblongata or the parietal cortex, the tidal volume, the expiratory PCO2 and the arterial blood pressure were continuously recorded in anaesthetized or unanaesthetized decerebrate cats. The concentration of the inspired CO2 was manipulated in order to obtain a nearly rectangular increase in the end-tidal PCO2. The responses of VT.f, VE and pH to such a change in PCO2 were observed. The observations from such a preparation were: 1. pHe responded with a delay of 5-7 s to a rectangular variation in end-tidal PCO2. 2. The time constant of the change in the medullary extracellular pH was in the range of 50 s and a similar value was found for VT and VE. 3. The response of VT or VE to a change in the medullary pHe was approximately linear in anaesthetized as well as in unanaesthetized decerebrate cats. The slope of the respiratory response of VT to pHe in decerebrate cats was about 3 times greater than that in anaesthetized cats. There were only slight differences in the relation of VT to pHe between the transient and steady state responses. This means that the "upstroke" of the on-transient of VT was approximately the same as the 'downstroke' of the off-transient. On the other hand, a slight delay was observed when VE was plotted against pHe. Pronounced delay occurred when respiratory frequency was plotted against pHe for on- and off-CO2 transients. 4. A marked hyteresis was observed when VT or VE was plotted against the cortical pHe for on- and off-CO2 inhalation. 5. Such a precise time correlation of the medullary surface pH and VT changes could only be possible if the pH on the ventral medullary surface is representative for the pH at the sensor.