Regulation of intracellular pH in neuronal and glial tumour cells, studied by multinuclear NMR spectroscopy

Abstract

The effect of extracellular pH (pH_e) on intracellular pH (pH_i) and cellular metabolism was examined by multinuclear NMR spectroscopy of cells in vivo and in vitro. A decrease in pH_e from 7.4 to 6.4 led to a significant drop in pH_i, in both neuronal and glial tumour cells, as detected by in vivo ³¹P NMR of cells embedded in basement membrane gel threads. A more than 50% decrease in both the phosphocreatine (PCr) level and derivatives of glycolysis (i.e., glycerol 3-phosphate) was observed, concomitantly to the fall in pH_i. A 50% decrease in intracellular lactate levels was seen in in vivo ¹H NMR spectra under these conditions. Reperfusion with fresh medium (pH_e 7.4) resulted in the full recovery of pH_i, simultaneously with an increase in both PCr and intracellular lactate back to their control levels. Perchloric acid and lipid extract measurements confirmed the observations made by in vivo ³¹P and ¹H NMR spectroscopy and further showed a decrease both in tricarboxylic acid cycle activity and phospholipid synthesis. The data revealed no significant differences between the neuronal and glial tumour cells investigated. pH_i measurements in the presence of inhibitors of the various pH regulatory mechanisms showed that the Na⁺/H⁺ exchanger, the carbonic anhydrase and at least one of the bicarbonate-transport systems are involved in pH regulation of both cell types. The results suggest that Na⁺/H⁺ exchange is the preferred mechanism by which both neuronal and glial cells regulate their pH_i after extracellular acidification.