Cytoplasmic pH Regulation in Acer pseudoplatanus Cells: II. Possible Mechanisms Involved in pH Regulation during Acid-Load.

Qualitative and quantitative aspects of the mechanisms involved in the regulation of cytoplasmic pH during an acid-load have been studied in Acer pseudoplatanus cells. Two main processes, with about the same relative importance, account for the removal of H(+) from the cytoplasm, namely a ;metabolic consumption' of protons and the excretion of protons or proton-equivalents out of the cells. The metabolic component corresponds to a change in the equilibrium between malate synthesis and degradation leading to a 30% decrease of the malate content of the cells during the period of cytoplasmic pH regulation. Various conditions which severely inhibit the activity of the plasmalemma proton pump ATPase reduce, at most by 50%, the excretion of H(+). This suggests that, besides the plasmalemma proton-pump, other systems are involved in the excretion of proton-equivalents. Indirect information on qualitative and quantitative features of these systems is described, which suggests the involvement of Na(+) and HCO(3) (-) exchanges in the regulation of cytoplasmic pH of acid-loaded cells.