Brain extracellular fluid pH and blood flow during isocapnic and hypocapnic hypoxia

Cerebrovascular responses to 30 min of isocapnic hypoxia [arterial O2 partial pressure (PaO2) = 33 +/- 1 Torr; means +/- SE] were examined in eight chloralose-urethan-anesthetized, paralyzed, and artificially ventilated New Zealand White rabbits. Cerebral blood flow (Q) was measured using the radioactive microsphere technique. Vascular resistance (R) was calculated from arterial pressure and Q. Brain extracellular fluid (ECF) pH was measured continuously in the same animals using pH microelectrodes (1- to 2-micrometers tip diameter) placed stereotaxically in the diencephalon. Diencephalon Q increased from 40 +/- 2 to 69 +/- 4 ml . 100 g-1 . min-1 (P less than 0.05) as R decreased (P less than 0.05) after 4-6 min of isocapnic hypoxia. Total brain Q and R changes resembled those of the diencephalon. The ECF pH of the diencephalon increased by 0.016 +/- 0.006 (P less than 0.05) after 1 min of isocapnic hypoxia and remained significantly elevated through the first 20 min of hypoxia. Ten minutes after the return of normoxia Q and R were at control levels, whereas diencephalon ECF pH was 0.043 +/- 0.006 below control (P less than 0.05). Five additional rabbits were prepared as described above then made hypocapnic [arterial CO2 partial pressure (PaCO2) = 21 +/- 0.3 Torr] for 18 min. Diencephalon and total brain Q and R remained at control levels through 12-14 min of hyperventilation, whereas diencephalon ECF pH was elevated by 0.03 +/- 0.006 (P less than 0.05). Hyperventilation was then continued with hypoxic gas to lower PaO2 to 35 +/- 4 Torr for 30 min. Both diencephalon and total brain R decreased (P less than 0.05), with no change in Q after 4-6 min of hypocapnic hypoxia. Diencephalon ECF pH was not significantly different from control throughout the hypocapnic-hypoxic period. We conclude that the early cerebral vasodilation during hypoxia is not mediated by increased brain ECF acidity.