Cytoplasmic free calcium in Riccia fluitans L. and Zea mays L.: Interaction of Ca(2+) and pH?

In cells of Zea mays (root hairs, coleoptiles) and Riccia fluitans (rhizoids, thalli) intracellular Ca(2+) and pH have been measured with double-barrelled microelectrodes. Free Ca(2+) activities of 109-187 nM (Riccia rhizoids), 94-160 nM (Riccia thalli), 145-231 nM (Zea root hairs), 84-143 nM (Zea coleoptiles) were found, and therefore identified as cytoplasmic. In a few cases (Riccia rhizoids), free Ca(2+) was in the lower millimolar range (2.3±0.8 mM). A change in external Ca(2+) from 0.1 to 10 mM caused an initial and short transient increase in cytoplasmic free Ca(2+) which finally levelled off at about 0.2 pCa unit below the control, whereas in the presence of cyanide the Ca(2+) activity returned to the control level. It is suggested that this behaviour is indicative of active cellular Ca(2+) regulation, and since it is energy-dependent, may involve a Ca(2+)-ATPase. Acidification of the cytoplasmic pH and alkalinization of the vacuolar pH lead to a simultaneous increase in cytoplasmic free Ca(2+), while alkalinization of pHc decreased the Ca(2+) activity. Since this is true for such remote organisms as Riccia and Zea, it may be concluded that regulation of cytoplasmic pH and free Ca(2+) are interrelated. It is further concluded that double-barrelled microelectrodes are useful tools for investigations of intracellular ion activities in plant cells.