Figure 4.

Changes in Adenosine and Excitability Correlate with Changes in Buffer pH and Are pH Dependent

(A) fEPSPs are plotted versus calculated pH_i (R² = 0.758). Inset: CA1 pyramidal cell loaded with BCECF-AM dye (black circle = 20% CO₂, n = 6 for pH_i measurement, n = 23 for fEPSP measurement; red circle = 10% CO₂, n = 6 for pH_i measurement, n = 13 for fEPSP measurement; yellow triangle = 20 mM propionic acid, n = 4 for pH_i measurement, n = 5 for fEPSP measurement; green triangle = isohydric hypercapnia, n = 7 for pH_i measurement, n = 7 for fEPSP measurement; blue square = 2% CO₂, n = 8 for pH_i measurement, n = 8 for fEPSP measurement). (B) Changes in extracellular adenosine are plotted versus calculated pH_i (R² = 0.848) (black circle = 20% CO₂, n = 23 for adenosine measurement; red circle = 10% CO₂, n = 13 for adenosine measurement; yellow triangle = 20 mM propionic acid, n = 5 for adenosine measurement; green triangle = isohydric hypercapnia, n = 7 for adenosine measurement; blue square = 2% CO₂, n = 8 for adenosine measurement). (C) fEPSPs are plotted versus buffer pH (R² = 0.992). (D) Changes in extra-cellular adenosine are plotted versus buffer pH (R² = 0.996). (E) Inhibition due to 10% CO₂ requires changes in buffer pH. Isohydric hypercapnia significantly attenuates the inhibition caused by 10% CO₂ (n = 7; **p < 0.01, ANOVA, Fisher PLSD). Intracellular acidification alone (propionic acid exposure) causes significantly less inhibition than 10% CO₂ (n = 5; ***p < 0.001, ANOVA, Fisher PLSD). (F) Increased extracellular adenosine caused by 10% CO₂ requires changes in pH_e. Isohydric hypercapnia completely blocks the adenosine release caused by 10% CO₂ (n = 7; **p < 0.01, ANOVA, Fisher PLSD). Intracellular acidification via propionic acid exposure was not sufficient to cause adenosine release (n = 5; ***p < 0.001 compared to 10% CO₂, ANOVA, Fisher PLSD). Error bars indicate standard error.