ECF pH dynamics within the ventrolateral medulla: a microelectrode study.

Using pH-sensitive microelectrodes, we evaluated pH dynamics of extracellular fluid (ECF) within the ventrolateral medulla (VLM) beneath the central chemoceptive areas in anesthetized, spontaneously breathing cats. Static ECF pH was acid in the superficial layers (less than 1 mm), compared with the overlying cerebrospinal fluid pH that became alkaline gradually during the experiments. In the deeper VLM areas (1-3 mm), no systematic gradients of ECF pH were observed. We found various, isolated regions where intravertebral artery injections of CO2-saturated saline evoked acidic shift of ECF pH in the time course analogous to ventilatory augmentation. Those responsive regions were found to be scattered not only in the superficial layers but also in the deeper VLM areas, although many nonresponsive regions were also intermingled among them. Occlusions of the principal vessels supplying the tested VLM regions diminished but failed to abolish the ECF pH responses to the CO2 loadings, suggesting a collateral blood flow by fine pial vessels. The present study suggests a possibility that the pH-dependent central chemoreceptors, if any, would be scattered in the deeper VLM areas as well as the superficial layers.