Effect of superhypercapnia on cortical pHi and cortical blood flow

In vivo panoramic imaging of regional cortical blood flow and intracellular brain pH with umbelliferone fluorescence was performed in fasted New Zealand White rabbits (n = 30) subjected to constant or stepwise increases in arterial PCO2 (PaCO2) up to 160 mmHg under 1.0% halothane anesthesia. At a PaCO2 of 40 mmHg, baseline brain pHi measured 7.03 +/- 0.04, while regional cortical blood flow was 47.0 +/- 4.3 ml.100 g-1.min-1. With an immediate but constant exposure to a PaCO2 of 60, 80, or 120 mmHg, at 15 min pHi fell to 6.94 +/- 0.03, 6.89 +/- 0.03, and 6.90 +/- 0.03 and recovered over 20, 80, and 120 min, respectively. At 160 mmHg PaCO2, pHi fell to 6.68 +/- 0.04 and remained acidotic. With stepwise increases in PaCO2 up to 80 mmHg, brain pHi decreased from 7.01 +/- 0.02 to 6.99 +/- 0.02, not significantly different despite a severe systemic acidosis of 7.022 +/- 0.034. At a PaCO2 of 120 and 160 mmHg, pHi dropped to 6.97 +/- 0.02 and 6.93 +/- 0.02, respectively. The difference in brain pHi between an immediate and stepwise exposure to a PaCO2 of 160 mmHg was significant (P < 0.005). These results demonstrate that brain pHi is tightly regulated across the cortical surface and resistant to profound changes in extracellular pH. The difference in pHi between the constant and incremental increases in PaCO2 suggests that there is upregulation of pH homeostatic mechanisms.