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 (pHt) and intracellular pH (pHi) in nine regions of the normal rat brain and in intracerebrally implanted RG-2 gliomas. Calculations of regional pHt, based on equilibrium tissue and arterial plasma [14C]DMO concentration, ranged from 6.83 to 6.94; pHi, 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 pHt or pHi between white and gray matter regions. Tumor tissue water content was determined by drying to constant weight, and extracellular space water volume (Ve) was estimated with [14C]sucrose in nephrectomized rats using quantitative autoradiography. Tumor pHt ranged from 7.08 to 7.18. For Ve = 0.17 (measured), pHi was 6.94-7.06; for Ve = 0.30 (assumed), the corresponding range for pHi was 6.63-6.90. Thus, the RG-2 glioma is not more "acidic" than adjacent brain tissue and its "alkaline" pHt probably reflects a large extracellular water content and plasma-like extracellular pH.