DEPTH PROFILES OF PH AND P(O2) IN THE ISOLATED BRAIN STEM-SPINAL CORDOF THE NEONATAL RAT

We have measured depth profiles of extracellular pH (pHECR) and P(O2) (Pt(O2)) as well as the kinetics of changes of pHECR in the isolated b rain stem-spinal cord preparation of the neonatal rat using pH and P(O 2) microelectrodes that entered from the ventral surface. When the pre paration was superfused with control mock cerebrospinal fluid (Control mock CSF; pH = 7.5, P(O2) = 630 Torr, P(CO2) = 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 mum, the slope being about 0 .1 pH unit per 100 mum. A similar gradient in the 200 to 300 mum of th e CSF above the surface suggested existence of unstirred layers despit e continuously flowing superfusate. The pH gradient in the spinal cord was somewhat smaller than that in the medulla. The P(O2) gradients in both medulla and spinal cord were about 100 Torr per 100 mum from 200 mum above to 100 to 200 mum below the surface; P(O2) reached zero at about 450 (medulla) to 600 mum (spinal cord). Although the preparation was anoxic and acidic except for a small layer below the surface, res piratory activity was recorded for several hours in C4 phrenic roots. The kinetics of changes in pHECF were recorded at 100 and 200 mum dept h while rapidly replacing the control mock CSF by more acidic CSF, eit her with increased P(CO2) (''Respiratory acidosis'') or by adding fixe d acid (''Metabolic acidosis''). The changes in pHECF were smaller tha n those in pHCSF, particularly during respiratory acidosis, as a resul t of the buffering of the brain tissue. Our results show the importanc e of superficial layers of the ventral medulla in producing respirator y rhythmicity; they further suggest that somewhat alkaline CSF (pH abo ut 7.8) should be used in this preparation to ensure physiologic surfa ce pH values despite unstirred surface layers.