Intracellular cyclic AMP was found to be significantly elevated in V-79 cells by addition to the culture medium of Prostaglandin E1, or of compounds inhibiting cyclic AMP phosphodiesterase. Cells in which elevated cyclic AMP had been induced by these means showed modified survival after X-irradiation. For high cyclic AMP cells the extent of the shoulder on the survival curve was increased while the exponential slope of the curve was decreased. Thus, more high cyclic AMP cells survive low doses of radiation than do control cells, but with increasing dose the high cyclic AMP cells become progressively more radiosensitive compared with controls. The increased size of the shoulder on the survival curve for high cyclic AMP cells is not correlated with an increased capacity to repair sublethal damage in these cells.

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