Intra- and extraerythrocyte pH at 37°C and during long term storage at 4°C: ³¹P NMR measurements and an electrochemical model of the system

A ³¹P NMR method based on pH dependent variation of the chemical-shift-difference between the resonances of orthophosphate and methylphosphonate was used to measure simultaneously intracellular pH (pH_i) and extracellular pH (pHc) during long term storage of erythrocytes; pH was determined at both 4°C and 37°C. An equation describing the equilibrium distribution of membrane-permeant ions was derived by consideration of the electrochemical and osmotic constraints in the RBC suspension. Calculations using the model-equation and the measured pH_i yielded the Donnan ratio and therefore pH_o; the relationship between experimentally determined pH_i and pH_o values was accurately predicted by the model. Sensitivity analysis of the model-equation revealed that the observed increase in transmembrane pH gradient during storage is principally due to the alteration of the total net charge of intracellular (poly)-anions.