Regulation of ornithine decarboxylase in cultured mouse mammary gland by the osmolarity in the cellular environment.

The biphasic increase of ornithine decarboxylase activity in mouse mammary gland in organ culture occurs with a hormone-independent first peak and a hormone-dependent second peak. The data presented indicate that a change in the osmolarity of the cellular environment is the major contributing factor for the emergence of the hormone-independent ornithine decarboxylase activity in mammary explants. Thus, incubation of mammary explants for 3 h in a medium diluted 53% with distilled water results in approx. 1000-fold stimulation of enzyme activity over the initial level, whereas a similar dilution of the medium with 0.18 M NaCl or 0.3 M sucrose blocks the increase. The increase in enzyme activity is similarly affected by a reduction of the concentration of NaCl in the culture medium. The hypoosmotic stimulation of ornithine decarboxylase activity appears to be affected at a posttranscriptional level, and is enhanced further by the actions of insulin and prolactin. The hypoosmotic enhancement of ornithine decarboxylase activity produces a large increase in the intracellular concentration of putrescine in mammary explants. However, neither the concentration of spermidine and spermine nor the activity of S-adenosyl-L-methionine decarboxylase is affected. In addition, studies of putrescine transport in mammary explants show that hypotonicity causes an increase in the rate of influx and a decrease in efflux of putrescine with enhancement of intracellular putrescine accumulation. On the other hand, the uptake of spermidine, spermine, amino acids, sugar, and a lipophilic cation, triphenylmethylphosphonium is unaffected. These data suggest a possibility that osmotic alteration in cellular environment causes an incresed need for putrescine in mammary cells, resulting in stimulation of ornithine decarboxylase activity, which may represent a cellular mechanism for maintaining the homeostasis of the intracellular cationic environment.