Factors modulating the activity of ornithine decarboxylase in rat HTC cells.

Dibutyryl cyclic AMP (dibu-cAMP), N1-acetylputrescine and both N1-acetylspermidine and N8-acetylspermidine regularly induce ornithine decarboxylase activity in both a time- and a concentration-dependent manner when added to HTC cells in suspension culture. N,N'-Diacetyl-1,6-diaminohexane and N-acetyl-1,3-diaminopropane, aliphatic analogues of acetylated putrescine containing the carboxamide bond but not known to occur naturally in animal tissues, are also effective inducing agents. The addition of 10(-2)M putrescine to the cell culture at the time of maximal induction of ODC in HTC cells by dibu-cAMP causes a marked decrease in the apparent half-life of ODC in less than 10 minutes. Ten-fold concentration of HTC cells at the time of their maximal induction by dibu-cAMP also results in an immediate decrease both in the specific activity of ODC and in its apparent half-life. When the population density of induced HTC cells is increased the relative effect of exposure of cells to increasing levels of putrescine is lessened. Some of the implications of these observations are discussed.