Immunoreactive prolactin in subcellular fractions from bovine mammary tissue

Bovine corpus luteum is an extrapituitary site of prolactin production

Prolactin (PRL) is known to be synthesized not only in the anterior pituitary, but also in other organs including the ovary. Among its various functions, PRL is regarded as the most important constituent of the luteotropic complex in rodents and pigs. The purpose of the present study was to determine whether PRL is produced locally in bovine corpus luteum (CL) and to determine its possible roles in CL. In the present study, we examined changes during the luteal phase in (1) the expressions of PRL and PRL receptors (long form: l-PRLR, short form: s-PRLR) in CL and (2) the localization of PRL in CL. We also measured the levels of PRL mRNA in cultured luteal cells and luteal endothelial cells. Furthermore, the effect of PRL on progesterone (P4) and prostaglandin (PG) F2alpha production by cultured bovine luteal cells was examined. Semiquantitative RT-PCR analysis revealed that the mRNAs for PRL and its two receptors, l- and s-PRLR, were expressed in all luteal stages examined. PRL mRNA expression was less in the regressed stage (days 19-21 after ovulation) than in the other stages. Both l-PRLR and s-PRLR mRNA expressions were higher in the late luteal stage (days 15-17) than in the other stages, while the ratio of l-PRLR to s-PRLR was less in the regressed stage than in the other stages. PRL mRNA was also detected in cultured luteal cells and luteal endothelial cells. PRL protein was immunohistochemically detected only in CL of the mid- and regressed stages. It was detected in smooth muscle cells of the intraluteal arterioles and endothelial cells but not in luteal cells and other cell types of CL. Exposure of cultured luteal cells obtained from mid-stage CL (days 8-12) to bovine PRL (100, 200 ng/ml) for 24 hr did not affect P4 and PGF2alpha production by the cells. The present study demonstrates for the first time the expressions of PRL and PRLR mRNA in bovine CL throughout the luteal phase. The overall results strongly suggest that the bovine CL is an extrapituitary site of PRL production.

Transferrin and prolactin transcytosis in the lactating mammary epithelial cell

The mammary epithelial cell ferries constituents originating from blood and from stromal cells, into milk, by transcytosis. Morphological analysis of a membrane marker of transcytosis in the lactating mammary epithelial cell showed that very rapid endocytosis of surface membrane occurs from both the basolateral and the apical side of the cell. In both cases, membrane trafficking between endosomes and the Golgi complex allows communication between the endocytic and the biosynthetic pathways. Transferrin and prolactin are internalized in mammary cells and transported through multivesicular bodies and Golgi stacks. They are released into milk via different types of secretory vesicles, prolactin being carried in secretory vesicles containing casein micelles. Consequences of the intracellular transport of these proteins and physiological benefits for cell function are discussed.

Hormones in milk

Protein hormones (especially prolactin) and steroid hormones (gestagens, estrogens, corticoids, and androgens) can be detected by bioassay and radioimmunoassay in milk in a variety of species. In addition, milk contains vitamin D and beta-casomorphins (opiate-like peptides). It has been assumed that most of the hormones are transferred into milk by diffusion. However, evidence is available for active mechanisms like those for progesterone in goats and prolactin in cows. Most of the hormone profiles in milk are similar to the ones in blood plasma. Hormone concentrations in milk seem to be a good estimate of the average hormone content in plasma, especially for the measurement of longer-lasting secretory activities like progesterone and estrogen release during the estrous cycle or seasonal changes of prolactin in ruminants. Determination of progesterone and estrone sulfate in milk serves as a diagnostic tool in fertility control, especially in cows. Enzyme immunoassay kits are available for this monitoring purpose. Exogenously administered hormones are also transferred into milk. Residue studies have shown that the dilution is so great that it may be assumed that there is no potential risk for the consumer.

Target cell prolactin, II

Is the entry hypothesis compatible with all the existing data about "the" second messenger for prolactin listed in Section II? All of these messengers, in some way either participate in, or modify, prolactin's actions or, in an end point-dependent manner, may actually mimic prolactin. There remains considerable uncertainty as to whether these findings reflect phenomena, some independent of and others quite dependent upon entry, on the one hand, or merely portions of a relatively large number of molecular cascades, some (but not necessarily all) begun initially at the plasmalemma and many (if not all) orchestrated toward completion by intracellular prolactin or agonist-receptor complex.