title: Regulation of ornithine decarboxylase in cultured mouse mammary gland by the osmolarity in the cellular environment.
abstract: 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.
mesh_terms: D000818:Animals; D002262:Carboxy-Lyases / Q000378:metabolism*; D003470:Culture Media / Q000494:pharmacology; D003513:Cycloheximide / Q000494:pharmacology; D005260:Female; D007038:Hypotonic Solutions; D007328:Insulin / Q000494:pharmacology; D007552:Isotonic Solutions; D008321:Mammary Glands, Animal / Q000201:enzymology*; D051379:Mice; D009924:Organ Culture Techniques; D009955:Ornithine Decarboxylase / Q000378:metabolism*; D009994:Osmolar Concentration; D011247:Pregnancy; D011388:Prolactin / Q000494:pharmacology; D011700:Putrescine / Q000378:metabolism*
chemical_list: D003470:Culture Media; D007038:Hypotonic Solutions; D007328:Insulin; D007552:Isotonic Solutions; D011388:Prolactin; D003513:Cycloheximide; D002262:Carboxy-Lyases; D009955:Ornithine Decarboxylase; D011700:Putrescine