Effects of respiratory and (isocapnic) metabolic arterial acid-base disturbances on medullary extracellular fluid pH and ventilation in cats.

Ventilation is influenced by the brain extracellular fluid (ecf) pH which is sensed by the central chemoreceptors. In the present experiments we have investigated to what extent ventilatory effects of brain ecf pH changes depend on the origin of these pH changes. With this aim we have compared the effects of 'respiratory' (via changes in PaCO2) and 'metabolic' (via isocapnic pHa changes) ecf pH changes on steady state ventilatory activity. Experiments were performed in anaesthetized (both artificially ventilated and spontaneously breathing) cats with cut sinus nerves; medullary surface ecf pH was measured with a glass electrode with a flat pH-sensitive surface. We found that ecf pH changes caused by changes in PaCO2 give rise to greater ventilatory responses than the same ecf pH changes caused by (isocapnic) changes in pHa. Moreover, within the pH ranges measured, isocapnic pHecf-ventilatory response lines at higher PaCO2 are shifted upwards compared with those at lower PaCO2 levels. It was concluded that with the present technique it is impossible to show a unique relation between ecf pH and ventilation.