Uptake of putrescine by 3T3 and SV3T3 cells and its effect on ornithine decarboxylase activity

The addition of 50 μM exogenous putrescine to Swiss 3T3 cells during serum stimulation resulted in a sixfold increase in intracellular putrescine content and prevented the rise in ornithine decarboxylase activity normally seen in response to serum. SV3T3 cells had putrescine levels ten times higher than those in the 3T3 cells. When SV3T3 cells were exposed to 50 μM putrescine, the intracellular content increased less than twofold. Despite the high levels of intracellular putrescine, ornithine decarboxylase was stimulated by addition of serum to SV3T3 cells. These results indicate that the system by which ornithine decarboxylase activity is regulated by its product is much less sensitive in the transformed SV3T3 cells. The ability of the 3T3 cells to take up putrescine changed with changes in the growth state, increasing when growth was stimulated by serum. These changes were manifested by changes in the maximum rate of transport (Vmax) rather than the Km. The increased transport rate was prevented when protein synthesis was inhibited by cycloheximide. The uptake of putrescine by SV3T3 cells did not change significantly under any of the conditions tested and the activity of this system in the SV3T3 cells was less than that of the 3T3 cells during exponential growth. This distinguishes the putrescine uptake system from those for amino acid uptake and suggests that the activity of the system may be regulated coordinately with the need for polyamines for cell growth. Finally, cadaverine (1,5‐diaminopentane), a diamine not usually present in fibroblasts, was taken up much more slowly than putrescine and uptake was not saturable under the conditions tested. The slower uptake may account for the lesser ability of cadaverine to reduce ornithine decarboxylase activity in the 3T3 cells.