Abstract
Acute pH changes at the cerebral cortex of the cat, after intravenous NaHCO 3 or NH 4Cl, are related to changes in expiratory and blood pCO 2 rather than to changes in blood hydrogen ion, bicarbonate or ammonium concentrations. This is because the blood-brain barrier restricts exchange of polar solutes between blood and cerebrospinal fluid and because blood is a bicarbonate buffer system. An increase in cortical pH after NH 4Cl in spontaneously breathing animals is due to hyperpneic blowing off of CO 2 and not to entry of NH 3 into the cerebrospinal fluid. Cortical pH decreases after NH 4Cl in artificially respired animals. The blood-brain barrier can be broken at the cerebrovascular endothelium by n-butanol. After this, changes in cortical pH are correlated more closely with blood pH than with pCO 2. Cortical d-c potential responses to NH 4Cl and NaHCO 3 are unaltered by breaking the barrier.
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