Difference between actions of high $$P_{{\text{CO}}_2 } $$ and low [HCO 3 ? ] on neurons in the rat medullary chemosensitive areas in vitro

To evaluate the contribution of extracellular fluid (ECF) pH in stimulating the ventral medullary chemosensors, effects on neuronal activities of changing ECF \(P_{{\text{CO}}_2 } \) and/or [HCO ₃ ⁻ ] were studied in tissue slices taken from the medulla oblongata of the rat. In many cases changes in neuronal discharges produced by a high \(P_{{\text{CO}}_2 } \) -normal [HCO ₃ ⁻ ] solution differed from those produced by a low [HCO ₃ ⁻ ]-normal \(P_{{\text{CO}}_2 } \) solution although the ECF pH was reduced to the same degree (from 7.40 to about 7.15). Only 9 of a total of 76 neurons showed an increase in discharge in response to both acid solutions. Neuronal activation due to high \(P_{{\text{CO}}_2 } \) was augmented when the ECF pH was returned to a normal value (7.40) by simultaneous increase in [HCO ₃ ⁻ ]. High \(P_{{\text{CO}}_2 } \) -high [HCO ₃ ⁻ ] solution (pH 7.40) increased the activity of many neurons which were either inhibited or uninfluenced by high \(P_{{\text{CO}}_2 } \) -normal [HCO ₃ ⁻ ] (pH 7.15). Neuronal activation due to low [HCO ₃ ⁻ ] was partially suppressed by compensating for the pH reduction with a concomitant decrease in \(P_{{\text{CO}}_2 } \) . The results suggest that CO₂ and HCO ₃ ⁻ independently influence the activity of neurons. Possible roles of ECF H⁺, CO₂ and HCO ₃ ⁻ in activating the ventral medullary chemosensitive structures are discussed.