Transient Cytoplasmic pH Changes in Correlation with Opening of Potassium Channels inEremosphaera

Steigner, W. Kāhler, K., Simonis, W. and Urbach, W. 1988. Transient cytoplasmic pH changes in correlation with opening of potassium channels in Eremosphaera.—J. exp. Bot. 39: 23–36. The role of the cytoplasmic pH (pHc) of Eremosphaera viridis in the signal transduction chain after light-off from the chloroplasts to the K+ channels in the plasmalemma of this unicellular alga was investigated. The temporary opening of K+ channels is indicated by a transient hypcrpolarization (TP). To record rapid changes of pHc, continuous measurements with pH sensitive micro-electrodes were carried out. (i) Under normal conditions pHc in the light (7·56 ±0·2) did not differ from pHc in the dark (7·62 ±0·2). (ii) The vacuole pH ranged between 4·8 and 5·2. (iii) After light-off a rapid transient acidification of δpHc O19±0·07 occurred and a TP was released, (iv) In every case, the start of the transient acidification after light-off preceded the hyperpolarization by about 3s. (v) Light-on caused a rapid transient alkalinization but never a TP. (vi) Change to acid external medium (3.2) transiently acidified the cytoplasm and was able to release a TP. (vii) After addition of NH4Cl, pHc again showed a rapid transient acidification and the release of a TP. The origin of the protons appearing in the cytoplasm after light-off is discussed critically with respect to the buffer capacity. Either direct or indirect translocation is a possible mechanism for the movement of H+ from the chloroplasts into the cytoplasm. The intracellular acidification and its relation to the opening of potassium channels in the plasmalemma leads us to suggest that a sudden change of pHc is a potent internal signal factor in Eremosphaera viridis.