Intracellular acidosis in murine fibrosarcomas coincides with ATP depletion, hypoxia, and high levels of lactate and total Pi.

Bioenergetic and metabolic status of murine FSaII tumours were evaluated using 31P MRS, acid extracts ('global' techniques) and quantitative bioluminescence ('microregional' assay). Data obtained from s.c. tumours of varying sizes (44-600 mm3) have been correlated with the oxygenation status evaluated using O2-sensitive needle electrodes. beta-NTP/Pi and phosphocreatine (PCr)/Pi ratios derived from 31P MRS were positively correlated to the median tissue pO2 values. pH declined during growth with intracellular acidosis being evident in tumours > 350 mm3. Whereas lactic acid formation greatly contributed to this decline in small- and medium-sized tumours, ATP hydrolysis and slowing down of the activities of pumps involved in pHi regulation seem to be major factors responsible for intracellular acidification in bulky tumours. PCr levels decreased at an early growth stage, whilst ATP concentrations dropped in bulky malignancies only, coinciding with a decrease in adenylate energy charge and a substantial rise in the levels of total Pi. MRS observable (mobile) Pi was consistently lower than [Pi] measured in acid extracts. On average, median pO2 values of ca 10 mmHg represent a critical threshold for energy metabolism. At higher median O2 tensions, levels of ATP, phosphomonoester and total Pi were relatively constant. This coincided with intracellular alkalosis or neutrality and stable adenylate ratios. On average, median pO2 values < 10 mmHg coincided with intracellular acidosis, ATP depletion, a drop in energy charge and rising Pi levels.