Cyclic nucleotide responses and radiation-induced mitotic delay in Physarum polycephalum

The response of the plasmodial levels of cyclic AMP and cyclic GMP in Physarum polycephalum to several putative phosphodiesterase inhibitors and to ionizing radiation has been measured. Isobutylmethylxanthine (2 mM) induces a rapid transient threefold elevation of cyclic AMP alone, with maximum response in about 10 min and return to the base line in about 30 min. Theophylline (2 mM) induces a rapid, sustained twofold elevation of cyclic GMP only. Caffeine (2 mM) and Ro-20-1724 (18 microM) both elicit a rapid transient rise in cyclic AMP, resembling the isobutylmethylxanthine response, and a slow transient elevation of the cyclic GMP level. Of particular interest is the rapid threefold transient elevation of the cyclic AMP, but not of the cyclic GMP, level by gamma radiation. These results (1) emphasize the organism and agent specificity of cyclic nucleotide responses, (2) indicate that the kinetics and apparent refractory response are compatible with the function of previously identified membrane receptor systems, i.e., a blue light receptor and a choleragen receptor mediating cyclic AMP responses, and (3) suggest a relationship between the agent-specific cyclic nucleotide responses and the modification of gamma-radiation-induced delay of mitosis by the same agents, i.e., a decrease in the delay by those which elevate cyclic GMP levels and an increase in the delay by those which elevate only cyclic AMP levels.