Intracellular pH in Dictyostelium: a 31P nuclear magnetic resonance study of its regulation and possible role in controlling cell differentiation.

Intracellular pH (pHi) has been measured in Dictyostelium discoideum cells by 31P nuclear magnetic resonance. Ax2 cells, newly harvested from growth medium, maintained a pHi of 7.33 +/- 0.04 (17) at an extracellular pH ranging from 3.5 to 6.5. Below pH 3.5 the cells tend to lyse, whereas at pH values above 6.5 their pHi rises though they remain viable. pHi regulation in acid medium is not dependent on external Na+ or any other inorganic ion and so most probably involves the electrogenic plasma membrane proton pump. No significant change in pHi was detected during development through to the slug stage. Mature stalk cells gave a very acidic phosphate signal (pH less than or equal to 5.5) which was probably vacuolar in origin. Indirect experiments had suggested that pHi might regulate the development of Dictyostelium cells, with low pHi favouring stalk cell and high pHi favouring spore cell differentiation. In particular, two inhibitors of the plasma membrane proton pump, diethylstilbestrol and zearalenone, had been shown to be stalk cell inducers. In the present studies measurements of pHi of cells exposed to these inducers failed to detect the expected drop in pHi. In addition, DIF-1 (a low Mr factor), the natural inducer of stalk cell formation, caused, if anything, a slight alkalinization of the cells. Thus the original theory linking pHi and cell differentiation is not supported by these results and therefore appears to require some modification. Finally, extract experiments revealed the existence of two unidentified abundant phospho-compounds with resonant frequencies close to inorganic phosphate. The existence of these compounds can complicate the interpretation of spectra gained from living Dictyostelium cells.