Simultaneous measurement of pH and membrane potential in rat dorsal vagal motoneurons during normoxia and hypoxia: a comparison in bicarbonate and HEPES buffers.

1. The effects of oxygenated and hypoxic bicarbonate/CO2, 10 and 25 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)-buffered artificial cerebrospinal fluid (ACSF) have been studied in a rat brain slice preparation. Double-barreled pH-selective microelectrodes were used to measure intracellular pH (pHi) and membrane potential in dorsal vagal motoneurons (DVMs) and to measure extracellular pH (pHe) in the dorsal vagal motonucleus. 2. In bicarbonate ACSF, pHi averaged 7.24 +/- 0.05 (mean +/- SE, n = 21) and ranged from 6.86 to 7.79 pH units. pHe averaged 7.13 +/- 0.08 (n = 10). 3. On changing from oxygenated bicarbonate ACSF to either 10 or 25 mM HEPES ACSF, pHi decreased by 0.13-0.15 units, and the membrane depolarized by 10-11 mV. pHe also decreased in 10 mM HEPES ACSF (pHe 6.89 +/- 0.02, n = 8) but not in 25 mM HEPES ACSF (pHe 7.15 +/- 0.06, n = 3). In most neurons changes in pHi preceded changes in membrane potential. 4. In bicarbonate ACSF and in 25 mM HEPES ACSF, there was a significant linear relationship between prehypoxic pHi and the direction and amplitude of the hypoxia-induced membrane potential change (either an hyperpolarization or a depolarization). 5. In 10 mM HEPES ACSF, hypoxia always induced a depolarization; there was no correlation between prehypoxic pHi and the membrane potential response. 6. In bicarbonate ACSF and in 10 and 25 mM HEPES ACSF, hypoxia resulted in intracellular and extracellular acidification. However, the extracellular acidification in hypoxic 10 mM HEPES buffer was most pronounced (pH 6.40 +/- 0.11, n = 8), reflecting a preexisting extracellular acidification in oxygenated 10 mM HEPES buffer. 7. Various hypotheses that could give rise to a relationship between changes in membrane potential and pH are discussed; arguments are presented in favor of the concept that modulation of ion channels by either pHi or pHe, or both, is responsible for the observed correlations.