In vivo Measurement of Regional Brain and Tumor pH Using [¹⁴C]Dimethyloxazolidinedione and Quantitative Autoradiography

Using [¹⁴C]dimethyloxazolidinedione ([¹⁴C]-DMO) and quantitative autoradiography, we estimated tissue pH (pH_t) and intracellular pH (pH_i) in nine regions of the normal rat brain and in intracerebrally implanted RG-2 gliomas. Calculations of regional pH_t, based on equilibrium tissue and arterial plasma [¹⁴C]DMO concentration, ranged from 6.83 to 6.94; pH_i, calculated assuming an extracellular water volume of 0.15 ml/g for gray matter and 0.11 ml/g for white matter, ranged from 6.61 to 6.78. No consistent difference was found in pH, or pH_i between white and gray matter regions. Tumor tissue water content was determined by drying to constant weight, and extracellular space water volume ( V_e) was estimated with [¹⁴C]sucrose in nephrectomized rats using quantitative autoradiography. Tumor pH_t ranged from 7.08 to 7.18. For V_e = 0.17 (measured), pH_i was 6.94–7.06; for V_e = 0.30 (assumed), the corresponding range for pH_i was 6.63–6.90. Thus, the RG-2 glioma is not more “acidic” than adjacent brain tissue and its “alkaline” pH_t probably reflects a large extracellular water content and plasma-like extracellular pH.