A possible role for calcium in the regulation of cell division in synchronized Ṯe̲ṯṟa̲ẖy̲m̲e̲ṉa̲

Tetrahymena was selected for this study because it displays a rapid growth rate, can be grown in axenic culture and can also be synchronized to a high degree by a series of hot and cold temperature shifts. The procedures used in this study involved basic physiological and biochemical methods. These included the use of radioisotopes to determine the patterns of calcium flux in asynchronous and synchronized Tetrahymena, the application of selected drugs known to alter calcium movements in cells and the assay of adenylate cyclase.

Calcium-45 studies showed that in asynchronous cultures there was a decrease in the rate of uptake in Tetrahymena during the log phase, while during the synchronization procedure, the rate of calcium-45 uptake per cell volume did not change. After cells were synchronized, there was a transient increase in cell associated calcium prior to cell division followed by an efflux of calcium as synchronized cell division occurred. The calcium chelating agent EGTA delayed the onset of cell division by interfering with the calcium influx while the ionophore A23187 decreased synchronized cell division by altering calcium flux across the cell membrane. Verapamil, an inhibitor of the inward flux of calcium, completely inhibited cell division.

Physiological levels of calcium stimulated the activity of adenylate cyclase, whereas, non-physiological levels (greater than 10^-6M), iii IV inhibited enzyme activity. In synchronized cells, the activity of adenylate cyclase did not respond to calcium, however, enzyme activity decreased during cell division.

These results indicate that calcium fluxes, especially the efflux from cells, is a necessary prerequisite for cell division in Tetrahymena in that cells probably must remove calcium to make available phosphate stores for use in energy requiring reactions. The calcium efflux also functions to modulate the enzymes involved in cyclic nucleotide metabolism.