Interstitial P-CO2 and pH, and their role as chemostimulants in the isolated respiratory network of neonatal rats

1. CO2-H+-sensitive microelectrodes were used for simultaneous measurements of the partial pressure of CO2 (P-CO2) and extracellular pH (pH(o)) in the ventral respiratory group (VRG) of the isolated brainstem-spinal cord of neonatal rats. Some of the data were analysed using diffusion equations. 2. With increasing recording depth within the boundaries of the VRG (300-600 mu m below the tissue surface), P-CO2 increased from 77 to 95 mmHg and pH(o) fell from 7.0 to 6.8 at steady state in standard saline equilibrated with 5% CO2 and 95% O-2. 3. Elevating bath CO2 from 5 to 10-12.5% produced a mean increase in P-CO2 of 18 mmHg, a fall in pH(o) of 0.13 pH units, and a 50-250% increase in the frequency of respiration-related spinal (C2) nerve bursts. Similar effects on C2 activity and PHo were observed upon lowering bath [HCO3-] from 25 to 10 mM, leading to a mean decrease in P-CO2 of 4.4 mmHg in the VRG. 4. Raising bath [HCO3-] to 50 mM produced a substantial frequency decrease, a rise in pH(o) of 0.24 pH units and an elevation in P-CO2 of 9.3 mmHg. C2 activity was not profoundly affected upon doubling the CO2-HCO3- content, leading to a mean increase in pH(o) of 0.13 pH units and elevation of P-CO2 by 30 mmHg. 5. In a CO2-HCO3--free, Hepes-suffered solution, P-CO2 decreased to 18 mmHg in the VRG and pH(o) fell by 0.15 pH units with no major effect on rhythmic activity. Subsequent anoxic exposure for more than 15 min produced a further fall in P-CO2 to below 1 mmHg, a decrease in pH(o) of 0.55 pH units, and blockade of respiration-related activity. In three out of the six preparations tested, C2 activity could be restored by reapplication of CO2-HCO3- in the absence of O-2. 6. C2 activity persisted at a reduced frequency, even up to 30 min, during anoxia in the CO2-HCO3--buffered saline, leading to an elevation in P-CO2 of 15 mmHg and a fall in pH(o) of 0.18 pH units. 7. The diffusion coefficient of CO2 in the tissue was found to be equal to that in saline. Two mean estimates for anoxic tissue of the function lambda(2)/alpha of tortuosity (lambda) and extracellular volume fraction (alpha), affecting extracellular diffusion of bicarbonate, were 4.7 and 4.1. The mean rate of acid production by anoxic tissue was 1.1 mequiv 1(-1) min(-1). 8. The results suggest that extracellular H+ is the primary stimulating factor in central chemosensitivity, which may often mask the less evident effects of CO2. A model of diffusion of acid equivalents in brain tissue is proposed.