Hypo-osmotic or Ca2+-rich external conditions trigger extra contractile vacuole complex generation in Paramecium multimicronucleatum.

The freshwater ciliated protozoan, Paramecium multimicronucleatum, usually possesses two contractile vacuole complexes (CVCs). The number of CVCs in a single cell, however, may vary from 1 to 7. We found that the number of cells that have more than two CVCs increased after the cells were exposed to a hypo-osmotic or a high Ca2+ condition. It is assumed that the biological significance of this increase in the number of CVCs is to enhance the cell's ability to eliminate excess water or Ca2+ from the cytosol. An extra CVC was either generated de novo in the posterior region of the cell or, when in the anterior region, by binary fission of the anterior CVC. Generation of these extra CVCs was not inhibited by aphidicolin, a potent inhibitor of DNA synthesis in the micronuclei of Paramecium, even though normal duplication of the CVC that accompanies normal cell division was completely inhibited by this inhibitor. These results suggest that generation of extra CVCs is controlled by a hypothetical regulatory mechanism that is activated either by a hypo-osmotic or by a Ca2+-rich condition and that differs from the regulatory mechanism that governs normal CVC duplication during cell division.