, Volume 398, Issue 4, pp 324-330

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

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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.