SIMULTANEOUS MEASUREMENT OF pH AND MEMBRANE POTENTIAL IN RAT DORSAL VAGAL MOTONEURONES DURING NORMOXIA AND HYPOXIA: A COMPARISON IN BICARBONATE AND HEPES BUFFERS. Cowan, A. I. and R. L. Martin. From.
APStracts 2:0205N, 1995.
SUMMARY AND CONCLUSIONS
1. The effects of oxygenated and hypoxic bicarbonate/CO 2 , 10 and 25 mM HEPES ( N -2-hydroxyethylpiperazine- N -2-ethanesulfonic acid) buffered artificial cerebrospinal fluid (ACSF) have been studied in a rat brain slice preparation. Double-barrelled pH-selective microelectrodes were used to measure intracellular pH (pH i ) and membrane potential in dorsal vagal motoneurones (DVMs) and to measure extracellular pH (pH e ) in the dorsal vagal motonucleus. 2. In bicarbonate ACSF pH i averaged 7.24+/-0.05 (n=21) and ranged from 6.86 to 7.79 pH units. pH e averaged 7.13+/-0.08 (n=10). 3. On changing from oxygenated bicarbonate ACSF to either 10 or 25 mM HEPES ACSF, pH i decreased by 0.13-0.15 units and the membrane depolarized by 10-11 mV. pH e also decreased in 10 mM HEPES ACSF (pH e 6.89+/-0.02, n=8) but not in 25 mM HEPES ACSF (pH e 7.15+/-0.06, n=3). In most neurones changes in pH i preceded changes in membrane potential. 4. In bicarbonate ACSF and in 25 mM HEPES ACSF there was a significant linear relationship between prehypoxic pH i 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 pH i 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 pre-existing extracellular acidification in oxygenated 10 mM HEPES buffer. 7. Various hypotheses which could give rise to a relationship between changes in membrane potential and pH are discussed; arguments are presented in favour of the concept that modulation of ion channels by either pH i or pH e , or both, is responsible for the observed correlations. 

Received 27 December 1994; accepted in final form 5 July 1995.
APS Manuscript Number J809-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 July 1995.