Regulation of cell shape in Euglena gracilis. II. The effects of altered extra- and intracellular Ca2+ concentrations and the effect of calmodulin antagonists.

When cultures of Euglena gracilis Z., normally grown in medium containing 180 microM-Ca2+, are resuspended in Ca2+-free medium cells assume round shapes within 10 min, from which they recover slowly when Ca2+ is returned to the cultures. Cultures grown in 10 microM-Ca2+ do not display the typical circadian rhythm in cell shape even though the photosynthesis and cell division circadian rhythms are unaffected. Elevating intracellular Ca2+ levels by the addition of the Ca2+ ionophore A23187 prevents cells from undergoing the two daily shape changes characteristic of growth-synchronized cultures, but does not alter the ability to maintain the cell shapes found at the time of ionophore addition. When the calmodulin inhibitors trifluoperazine or chlorpromazine are added to cultures, the cells always respond by rounding. Cells are not able to maintain any cell shape other than spherical in the presence of these inhibitors and therefore cannot change shape throughout the daily cycle as is found in the control populations.