Activation of casein synthesis by prostaglandins plus spermidine in mammary gland explants of mice

Prolactin stimulation of iodide uptake and incorporation into protein is polyamine-dependent in mouse mammary gland explants

Previous studies have demonstrated that the prolactin stimulation of most lactational processes (casein, lactose, and triglyceride synthesis) requires an earlier stimulating effect of prolactin on the synthesis of the polyamines. Spermidine appears to be the specific polyamine required for prolactin to enhance milk product synthesis. Inorganic iodide is present in milk at more than an order of magnitude higher concentration than that of the maternal plasma. Since prolactin stimulates iodide accumulation in milk, the goal of these studies was to determine the role of the polyamines in this hormone response. Two drugs were employed in these studies: DFMO (difluoromethylornithine), which inhibits ornithine decarboxylase, and MGBG [methylglyoxal bis(guanyl-hydrazone)], which inhibits S-adenosyl methionine decarboxylase. In mammary gland explants from midpregnant (10-14 days of pregnancy) mice, MGBG at 100 microM abolished the prolactin stimulation of iodide uptake and incorporation into milk proteins, whereas DFMO caused a concentration-dependent inhibition of the PRL response. Selected sensitivity of the MGBG and DFMO inhibitions was validated by a reversal of the drug inhibitions with the addition of 1 mM spermidine to the culture medium. These data suggest that the polyamine signaling pathway is involved in the prolactin stimulation of iodide uptake into milk.

Effects of cyclic nucleotides on ornithine decarboxylase activity in mammary gland explants from mid-pregnant mice

Dibutyryl cAMP and prolactin stimulated ornithine decarboxylase activity in mouse mammary gland explants which had been preincubated with insulin and cortisol for 1 day; maximally stimulatory concentrations of dibutyryl cAMP and prolactin produced a response which was greater than the sum of the responses of prolactin and dibutyryl cAMP when tested alone. 8-Bromo-cGMP inhibited ornithine decarboxylase activity whereas other derivatives of cyclic nucleotides were without effect. Cortisol concentrations were found to be important for optimizing the dibutyryl cAMP and prolactin responses. Optimal prolactin responses were obtained with cortisol concentrations greater than 10(-7) M, whereas optimal dibutyryl cAMP responses were observed with cortisol concentrations less than 10(-7) M. Despite the differing optimal cortisol concentrations for the prolactin and dibutyryl cAMP responses, it is concluded that prolactin and dibutyryl cAMP probably stimulate ornithine decarboxylase activity in the mammary gland via the same mechanism.

Amino acid utilization by lactating mammary gland

Amino acids utilization comprises two phases, cellular uptake and intracellular metabolism. The paper reviews reports on amino acid uptake in different species and at different stages of lactation and presents new data on changes at lactogenesis. The relative importance of changes in factors which determine uptake, mammary blood flow, arterial concentration, and mammary extraction, are discussed. In addition to direct incorporation into milk protein, some absorbed amino acids are an energy source and carbon precursors of other amino acids synthesized in mammary cells. Branched-chain amino acids and ornithine act as nitrogen precursors in these syntheses. Ornithine, which is derived partly from absorbed arginine, is also a precursor of spermidine, a postulated intracellular mediator of hormone action. The question of whether milk protein yield is limited by supply to the mammary gland of certain amino acids is discussed. Much evidence indicates the importance of certain essential amino acids that are transferred stoichiometrically from blood to milk. Results of experiments involving administration of exogenous growth hormone to goats are presented. Increases in milk yield and protein yield are associated with increased rates of mammary blood flow, emphasizing the role of the latter in determining uptake.

Prostaglandin I2 modifies both prolactin binding capacity and fluidity of mouse liver membranes

The objective of this study was to determine if prostaglandins alter the fluidity of hepatic membranes and if this, in turn, modifies their ability to bind prolactin. Liver homogenates of adult C3H female mice incubated with 1-1000 nM prostaglandin A1, A2, B2, F1 alpha, 6-keto-F1 alpha, E1, E2, I2, or thromboxane B2 provided the 100,000 X g membrane pellets for subsequent ovine prolactin binding and membrane fluidity studies. Only membrane preparations treated with prostaglandin I2 showed an increase in specific binding of ovine prolactin. The effect (40-50%) was maximal at 100 nM prostaglandin I2 after 30 min of incubation and was due to an increase in the number of receptor sites. Under the same conditions, prostaglandin I2 induced a 17% decrease in membrane microviscosity. These data suggest that specific prostaglandins may modulate the number of prolactin receptors in vivo by modifying the fluidity of the lipid bilayer and the subsequent ease with which receptors can assume active configurations within the matrix.

[Effect of indomethacin on the lactogenic action of prolactin]

The action of indomethacin on the lactogenic activity of prolactin has been evaluated usind the technique of rabbit mammary gland organ culture. Indomethacin is totally unable to inhibit prolactin action as estimated by lactose synthetase activity and casein synthesis. These data suggest, as opposed to previous works, that prostaglandins are not involved in the mechanism of prolactin action on lactogenesis.

[Human prolactin (author's transl)]

Human prolactin (hPRL) is the most recent anterior pituitary hormone in human endocrinology, whose structure has been elucidated in 1977. The possibility to measure hPRL in serum has led to a rapid increase of our knowledge of prolactin-physiology and -pathophysiology in men. hPRL is the only anterior pituitary hormone which is under predominantly inhibitory hypothalamic control. The effects of prolactin in the various species differ considerably, whereas in men it acts mainly upon the mammary gland and the gonadal system. Hyperprolactinemia leads typically to hypogonadism, amenorrhea and frequently galactorrhea. The hyperprolactinemia-hypogonadism-syndrome has been identified as a separate entity in recent years. Because of the relative frequency of this disease prolactin measurements have become of great importance in the diagnosis of sterility. Depending on the cause of hyperprolactinemia a neurosurgical, radiotherapeutical or medical treatment is indicated.