Cyclic Nucleotide Changes in X-Irradiated Synchronized Tetrahymena

Abstract

Intracellular levels of cyclic AMP and cyclic GMP both fluctuate in relation to the division cycle in synchronized Tetrahymena pyriformis (a ciliate protozoan). The amounts of cAMP per cell are highest before the onset of cell division, begin to decrease during the peak of cell division, and reach a minimum level (a twofold decrease) at the end of the division cycle. On the other hand, cGMP levels are lowest in cells prior to division, begin increasing at the onset of cell division, and reach a maximum (100-fold increase) during the peak of cell division. X irradiation (5000 R), when given at the end of the synchronizing treatment, delays cell division approximately 35 min with some decrease in synchrony. Corresponding changes in intracellular levels of cyclic nucleotides reflect somewhat different patterns in irradiated cells than observed for controls. Cyclic GMP levels in irradiated cells are lowest just before the onset of cell division, rising rapidly during the peak of cell division, and reach a maximum (about 175-fold increase) at the end of the division cycle. Cyclic AMP phosphodiesterase activity increases after EHS, decreases during cell division and is about one-half of peak levels at the end of the division cycle formore » both control and x-irradiated cells. Cyclic GMP phosphodiesterase activity decreases in control cells about 50% during the peak of cell division as compared to the predivision state after EHS. In irradiated cells, cGMP-associated phosphodiesterase activity increases to a maximum prior to the onset of cell division and drops by about two-thirds of peak values at the end of cell division. Radiation studies indicate that phosphodiesterase activity is not altered significantly by x rays. The results on changing levels of cyclic nucleotides indicates, and is consistent with the hypothesis, that marked increases (100-fold) in cGMP signal the initiation of cell division.« less