In vivo measurement of regional brain and tumor pH using [14C]dimethyloxazolidinedione and quantitative autoradiography. II: Characterization of the extracellular fluid compartment using pH-sensitive microelectrodes and [14C]sucrose

Abstract

We measured the extracellular (interstitial) pH (pHe) of RG-2 rat gliomas using H+-sensitive microelectrodes and estimated the volume of tumor extracellular space based on the tissue-plasma ratio of [14C]sucrose. The average RG-2 pHe was 7.63 +/- 0.15 (mean +/- SD, n = 6), whereas the average pHe of contralateral brain tissue was 7.34 +/- 0.10 (n = 3) and arterial pH was 7.36 +/- 0.02. RG-2 extracellular space water volume was estimated to be 0.3 ml water/g tissue. In separate experiments in normal, nontumored rats, intracellular pH (pHi) was calculated for nine gray and white matter regions based on measurements of tissue and plasma [14C]dimethyloxazolidinedione concentration. pHi values ranged from 6.80 to 6.94, and no consistent gray-white differences were observed. Our data suggest that tumor pHi is not more acidic than that of normal brain tissue and that the observed alkalinity of primary brain tumors is due to the presence of a large alkaline extracellular space.

FIG. 1

Typical recording of RG-2 glioma extracellular pH (pH_e) using a double-barreled H⁺ ion-selective microelectrode. Simultaneous direct-current signal is not illustrated. The microelectrode was advanced with a micromanipulator to 2,000–2,500 μm below the tumor surface to record pH_e = 7.84 (dashed line). The electrode was then raised in 500-μm increments (up arrowheads). pH_e reached a maximum of 7.88 before a rapid fall to 7.46 (after the last up arrowhead). This latter change in pH reflected movement of the microelectrode from glioma interstitial space into the superfusion Ringer’s. The electrode was again advanced into the tumor by 100-μm increments (down arrowheads). Note the standing gradient pH_e near the surface of the tumor before pH_e again reached 7.84 (dashed line) at ~400 μm in depth.

FIG. 2

Estimates of RG-2 tumor extracellular space based on tumor and arterial plasma [¹⁴G]sucrose concentrations. Percentage extracellular space (EGS) was calculated in each experiment as described in the text. Points on the graph are averages of all experiments from each time point, and the equation for the least-squares fit is y = 31.2 (1 – e^−1.058t), where the asymptote (31.2) is assumed to represent tissue ECS, 1.058 is the equilibration rate constant for sucrose, and t is time in hours. Averages of individual experimental values at each time point were as follow: for 2-h experiments, 27.2 ± 6.9% ECS (mean ± SD); for 4-h experiments, 32.2 ± 10.2%; for 6-h experiments, 28.0 ± 5.2%; and for 8-h experiments, 33.1 ± 3.8%. Mean arterial blood pressure at the time of death ranged from 85 to 120 mm Hg. Arterial pH was 7.33–7.43, Pco₂ 22.2–41.8 mm Hg, Po₂ 88.3–105.2 mm Hg, and hematocrit 38–48%.