Heat shock protein synthesis is affected by intracellular pH: Inhibition by monensin-induced alkalosis in C6 rat glioma cells

The effect of intracellular pH (pH(i)) on heat shock protein (HSP) synthesis was investigated in C6 rat glioma cells. pH(i) changes were analysed by means of fluorescence spectroscopy in a perfused monitoring system allowing continuous measurements before, during and after treatments. HSP induction nias determined by means of Western blots and autoradiographs. A 20 min heat shock (WS) of 44 degrees C decreased the pH(i) from 7.36 to 7.05 during exposure [17] and elicited the synthesis of heat shock proteins 2-8 h later. A pH(i) decrease, brought about by low extracellular pH (pH(e)) of 4.5 and 5.0 or 5.5, induced HSP synthesis after 1 h or 3 h, respectively. During these treatments, pH(i) decreased to values significantly lower than that caused by HS. Three h exposure to pH(e) 6.2, however, was not inductive. These results indicate that the heat shock-induced pH(i) decrease alone is not sufficient to stimulate HSP synthesis. Ln order to investigate the effect of alkaline pH(i) on the induction of HSP by heat, pH(i) was increased prior to HS treatments. Preincubation of cells at pH(e) ranging from 6.8 to 8.0 had little effect on pH(i) and on HSP synthesis. A shift of pH(i) to more alkaline values was achieved by adding the H+/Na+ exchanger monensin at alkaline pH(e), Twenty mu M monensin raised the pH(i) and inhibited the HSP induction depending on the pH(c) values: as pH(e) was increased from FH 7.2 to 8.0 HSP synthesis was increasingly inhibited. Monensin also diminished the HS-induced drop of pH(i) particularly at higher pH(e). The result showed that neither a lower pH(i) nor a drop of pH(i) during HS is a necessary prerequisite for the induction, whereas alkalosis inhibits the synthesis of HSP.