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-], [CO32-] and [CO2]. In principle, this can be accomplished using CO(2-)3 and pH-sensitive microelectrodes. Calibration requires knowledge of slope response, interference constants and corrections for [HCO3-]. complicate measurements with CO32- 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 CO32-. Measurement of CO32- slope response by this method yielded a value of 28.5 +/- 0.72 mv per decade change in [CO32-]. In Cl--containing solutions, interference coefficients 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 CO32- over HCO3- and Cl- respectively. To correct for possible changes in [HCO3-], the equilibrium relation between CO32- and HCO3- was substituted into the Nikolsky equation to yield an equation in [CO32-] and [H+]. By simultaneously measuring shifts in [H+] with a pH microelectrode, this equation is readily solved for [CO32-]. These methods were tested by measuring [HCO3-] and [CO2] in experimental solutions, and in the extracellular fluid of rat hippocampal slices.