[Role of membrane colchicine binding proteins in the transmission of prolactin message to casein genes in the rabbit mammary gland]

Lactogenic hormones: binding sites, mammary growth, secretory cell differentiation, and milk biosynthesis in ruminants

Roles of the lactogenic hormones prolactin and placental lactogen in mammary development in ruminants were reviewed. In contrast with other ruminants, failure to detect lactogenic activity in the serum of pregnant cows (in excess of that attributed to prolactin) suggests that placental lactogen may have little direct effect on mammary growth or lactogenesis. However, replacement and ablation experiments using ergocryptine provide definitive evidence that increased periparturient secretion of prolactin is necessary for maximal milk production in cattle. Quantitative microscopy indicates a relative failure of mammary cells in cows with inhibited secretion of prolactin to differentiate structurally. Prolactin induces synthesis and secretion of alpha-lactalbumin in prepartum bovine mammary tissue. Temporary disruption of mammary microtubules immediately prepartum in pregnant heifers reduced subsequent milk production, biosynthetic capacity, and cellular differentiation. For maximal milk production, mammary secretory cells apparently must respond to lactogenic hormone stimulation during the immediate periparturient period. Colchicine may desensitize the mammary epithelium to prolactin action. Membrane binding of radiolabeled human growth hormone to ruminant mammary gland provides a measure of lactogenic hormone binding sites. Specific binding to 600 micrograms of mammary membrane protein was 296% greater in lactating, compared with nonlactating, pregnant (65 days of gestation) ewes. Binding capacity (fmol/mg membrane protein) averaged 275 +/- 57 in mammary membranes from nonlactating, pregnant ewes (100 days gestation, n = 2) and 2,325 +/- 521 in mammary membranes from lactating ewes (n = 6, 14 to 21 days postpartum). Greater understanding of hormonal regulation of the ruminant mammary gland likely will result in development of techniques to produce milk more efficiently and perhaps capability to evaluate production potential of young animals.

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.

Intramammary infusion technique for genetic engineering of the mammary gland

The mammary gland is an appropriate substrate for genetic engineering because of its capacity to synthesize and secrete molecules of biological importance. An approach to mass production of such molecules involves transfer of genes into the lactating cell by infusion via the teat and duct system. We describe an infusion technique with the rat, a useful animal in which to develop such technology. By dye maker, trypan blue, and the ultrastructural marker, ferritin-concanavalin A, infusions by this route can permeate the entire gland and deliver molecules to apical membranes of lactating cells.

In vivo effects of colchicine on milk fat globule membrane

Milk secretion in lactating goats was suppressed reversibly by infusing colchicine (2.5 to 5 mg) into one half of the udder via the teat canal. Fat globules were isolated from milks before, during and after (96 h post-infusion) this suppression. Protein, phospholipid, cholesterol (free and esterified), 5'-nucleotidase activity and peptide patterns by gel electrophoresis of these globule samples were determined. Association of [14C]colchicine with milk fat globules in vivo and in vitro also was investigated. Amounts of protein, phospholipid and free cholesterol per g of globule and 5'-nucleotidase per mg of globule protein fall following colchicine infusion. The nature of these changes suggests that the supply of membrane for milk secretion is restricted as a result of the drug treatment. Patterns of globule peptides by gel electrophoresis were qualitatively similar during the experimental period. However, a major globule glycoprotein, Mr = 52 000, showed a significant (3-fold) increase relative to the other principal peptide bands during the period of reduced milk flow. Analysis of milks for radioactivity following infusion of [14C]colchicine revealed that a portion of activity returning in milk is associated with fat globules. This activity peaked at 72 h post-infusion. Evaluation of [14C]colchicine binding to milk fat globules in vitro yielded evidence that the drug binds to the cytoplasmic, but not the exterior surface of the globule membrane. Colchicine's inhibition of milk synthesis and secretion is discussed.