The effect of inhibitors of ornithine decarboxylase on DNA synthesis in mouse mammary gland in culture. The importance of osmolarity of the medium and of the initial intracellular level of putrescine.

In cultured mammary explants from both virgin and midpregnant mice which contained a low and a high initial level of putrescine, respectively, addition of an ornithine decarboxylase inhibitor, DL-a-hydrazino-Saminovaleric acid at the onset of culture prevented an early, “osmolarity-sensitive” rise in the putrescine level that occurred prior to induction of DNA synthesis by insulin. In the presence of the drug, hormonal stimulation of DNA synthesis occurred normally in virgin mouse mammary explants cultured in isotonic medium and in midpregnant mouse mammary explants cultured in both isotonic and hypotonic media. However, in virgin mouse mammary explants cultured in hypotonic medium, the drug caused varj.ing degrees of inhibition of DNA synthesis which were inversely related to the osmolarity of the medium. The inhibitory effect of the drug on DNA synthesis was also dependent on the time of its addition, being most effective within the first 2 h of culture, but virtually ineffective at 12 h. Inhibition of DNA synthesis by the drug was reversible by addition of putrescine (0.5 to 1 p ~ ) , which was most effective within 1 h of culture and thereafter became less effective. The polyamines, spermidine, spermine, and cadaverine were ineffective in this respect. Similar results were obtained with another ornithine decarboxylase inhibitor, a-methylornithine. In contrast to the effect on DNA synthesis, DL-a-hydrazino-6-aminovaleric acid inhibited neither protein synthesis nor the uptake of amino acids and glucose and caused a much smaller degree of inhibition of RNA synthesis. These results suggest that an osmolarity-sensitive increase in putrescine is important for hormonal induction of DNA synthesis in mammary cells containing a low, initial level of putrescine which are cultured in hypotonic medium.