Depth profiles of pH and PO2 in the isolated brain stem-spinal cord of the neonatal rat.

We have measured depth profiles of extracellular pH (pHECR) and PO2 (PtO2) as well as the kinetics of changes of pHECR in the isolated brain stem-spinal cord preparation of the neonatal rat using pH and PO2 microelectrodes that entered from the ventral surface. When the preparation was superfused with control mock cerebrospinal fluid (Control mock CSF; pH = 7.5, PO2 = 630 Torr, PCO2 = 28 Torr, at 27 degrees C), the pH in the medulla diminished with a nearly constant gradient from the surface to a depth of about 1000 microns, the slope being about 0.1 pH unit per 100 microns. A similar gradient in the 200 to 300 microns of the CSF above the surface suggested existence of unstirred layers despite continuously flowing superfusate. The pH gradient in the spinal cord was somewhat smaller than that in the medulla. The PO2 gradients in both medulla and spinal cord were about 100 Torr per 100 microns from 200 microns above to 100 to 200 microns below the surface; PO2 reached zero at about 450 (medulla) to 600 microns (spinal cord). Although the preparation was anoxic and acidic except for a small layer below the surface, respiratory activity was recorded for several hours in C4 phrenic roots. The kinetics of changes in pHECF were recorded at 100 and 200 microns depth while rapidly replacing the control mock CSF by more acidic CSF, either with increased PCO2 ("Respiratory acidosis") or by adding fixed acid ("Metabolic acidosis"). The changes in pHECF were smaller than those in pHCSF, particularly during respiratory acidosis, as a result of the buffering of the brain tissue. Our results show the importance of superficial layers of the ventral medulla in producing respiratory rhythmicity; they further suggest that somewhat alkaline CSF (pH about 7.8) should be used in this preparation to ensure physiologic surface pH values despite unstirred surface layers.