Determination of extracellular bicarbonate and carbon dioxide concentrations in brain slices using carbonate and pH-selective microelectrodes.
The extracellular pH of the brain is subject to shifts during neural activity. To understand these pH changes, it is necessary to measure [H+], [HCO3-], [CO3(2-)] and [CO2]. In principle, this can be accomplished using CO3(2-) and pH-sensitive microelectrodes; however, interference from HCO3- and Cl-, and physiological changes in [HCO3-], complicate measurements with CO3(2-) electrodes. Calibration requires knowledge of slope response, interference constants and corrections for [HCO3-] shifts. We show that when [HCO3-] is altered at constant [CO2] in the absence of Cl-, the HCO3- interference cancels and the Nikolsky equation reduces to the Nernst equation for CO3(2-). Measurement of CO3(2-) slope response by this method yielded a value of 28.5 +/- 0.72 mV per decade change in [CO3(2-)]. In Cl(-)-containing solutions, interference coefficient for HCO3- and Cl- were determined by altering [HCO3-] at constant [CO2], changing [CO2] at constant [HCO3-], then solving the simultaneous Nikolsky equations for each transition. The mean interference constants corresponded to selectivity ratios of 245:1 and 1150:1 for CO3(2-) over HCO3- and Cl- respectively. To correct for possible changes in [HCO3-], the equilibrium relation between CO3(2-) and HCO3- was substituted into the Nikolsky equation to yield an equation in [CO3(2-)] and [H+]. By simultaneously measuring shifts in [H+] with a pH microelectrode, this equation is readily solved for [CO3(2-)]. These methods were tested by measuring [HCO3-] and [CO2] in experimental solutions, and in the extracellular fluid of rat hippocampal slices.
Chesler
M
M
Department of Physiology, New York University Medical Center, NY 10016.
Chen
J C
JC
Kraig
R P
RP
D000818:Animals; D001639:Bicarbonates / Q000032:analysis*; D001923:Brain Chemistry / Q000502:physiology*; D002245:Carbon Dioxide / Q000032:analysis*; D002712:Chlorides / Q000737:chemistry; D005110:Extracellular Space / Q000737:chemistry*; D006624:Hippocampus / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D017736:Ion-Selective Electrodes; D008839:Microelectrodes; D051381:Rats
D001639:Bicarbonates; D002712:Chlorides; D002245:Carbon Dioxide