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 +], [HCO 3 −], [CO 3 2−] and [CO 2]. In principle, this can be accomplished using CO 3 2− and pH-sensitive microelectrodes; however, interference from HCO 3 − and Cl −, and physiological changes in [HCO 3 −], complicate measurements with CO 3 2− electrodes. Calibration requires knowledge of slope response, interference constants and corrections for [HCO 3 −] shifts. We show that when [HCO 3 −] is altered at constant [CO 2] in the absence of Cl −, the HCO 3 interference cancels and the Nikolsky equation reduces to the Nernst equation for CO 3 −. Measurement of CO 3 − slope response by this method yielded a value of 28.5 ± 0.72 mV per decade change in [CO 3 2−]. In Cl −-containing solutions, interference coefficients for HCO 3 − and Cl − were determined by altering [HCO 3] at constant [CO 2], changing [CO 2] at constant [HCO 3], then solving the simultaneous Nikolsky equations for each transition. The mean interference constants corresponded to selectivity ratios of 245:1 and 1150:1 for CO 3 2− over HCO 3 − and Cl − respectively. To correct for possible changes in [HCO 3 2−], the equilibrium relation between CO 3 2 and HCO 3 − was substituted into the Nikolsky equation to yield an equation in [CO 3 2−] and [H +]. By simultaneously measuring shifts in [H +] with a pH microelectrode, this equation is readily solved for [CO 3 2−]. These methods were tested by measuring [HCO 3 −] and [CO 2] in experimental solutions, and in the extracellular fluid of rat hippocampal slices.