Regulation of cell shape in Euglena gracilis. V. Time-dependent responses to Ca2+ agonists and antagonists

The daily changes in cellular shape observed in growth-synchronized cultures of Euglena gracilis Klebs strain Z, were altered by exposure to Ca2+ channel agonists and antagonists. The response of the cells to these pharmacological agents depended, in part, on the time in the growth cycle that the cells were exposed. The Ca2+ channel blockers verapamil and nifedipine and the intracellular Ca2+ antagonist TMB-8 all caused cell rounding when elongated cells from the middle of the light cycle were treated. These results were the same as with other methods used to deprive cells of extracellular Ca2+, such as exposure to EGTA or resuspension in Ca2+-free medium. The cell response in mid light cycle to the channel blockers was reversible by simultaneous exposure to CaCl2, and the nifedipine response was also reversed by simultaneous exposure to the structurally related Ca2+ agonist BAY-K 8644. Exposure of cells in the first hour of the light cycle to verapamil, nifedipine or TMB-8 caused an unexpected result. Instead of preventing the round cells from elongating in the first portion of the light cycle, as do LaCl3, EGTA or resuspension in Ca2+-free medium, a greater than expected percentage of elongated cells was found in the treated population. This represents the first instance in which the biological clock control over the rate and extent of cell elongation was accelerated. The calcium agonist CGP-28392 did not have an effect on cell elongation in the early portion of the light cycle but caused cell rounding in the middle of the light cycle. The calcium agonist BAY-K 8644 did not cause any shape changes alone, but was capable of reversing the effects of nifedipine in the middle of the light cycle.