Intracellular pH and intracellular buffering power of the cat brain

Abstract

In 49 cats the intracellular pH (pHi) of the brain was estimated from the distribution of 5,5-dimethyloxazolidine-2,4-dione-2-C14 (DMO) between brain, plasma, and CSF, at arterial pCO2 of 14.5–83.2 mm Hg. From six cats cortical biopsies were taken; in the remainder, brain DMO distribution was determined on postmortem samples from various brain regions, both white and gray. pHi of the samples was calculated, assuming extracellular space to be equal to either 3 or 12% of brain weight, and extracellular composition to be equal to composition of either arterial, cerebral venous, or mean capillary plasma, or CSF. Bicarbonate concentration of cell water and total CO2 concentration of brain were derived from the data. Results: 1) there were no significant regional differences in pHi; 2) when extracellular space was taken as 3% of brain weight, and its composition as equal to that of mean capillary plasma, the following relations were found: a) pHi = .494 pH arterial + 3.471, and thus pHi = 7.13 at arterial pH = 7.40 (and arterial pCO2 = 40); b) pHi = 7.83 – .42 log pCO2 tissue, and thus Δ log pCO2 tissue/Δ pHi = 2.38; c) Δ [HCO3]/ Δ pHi = 36.7 mEq/kg cell water; d) Δ [total CO2]/Δ pCO2 tissue = .33 ml/100 g cortical tissue in the pCO2 range of 30–50 mm Hg. The basic assumptions of the DMO method were critically examined. A general equation was developed which expresses the transmembrane steady-state distributions of the two members of the conjugate pair of a weak acid in terms of their relative permeabilities, intracellular and extracellular H+ ion concentrations, and membrane voltage. The equation was applied to the present experiments.