Early mammary gland responses to hormones

Historical perspectives of prolactin and growth hormone as mammogens, lactogens and galactagogues--agog for the future!

Around 80 years ago researchers first established that the pituitary gland regulates mammary gland function as demonstrated by the ability of its extracts to promote both mammogenesis and lactogenesis in animal models. Little did they realize that in fact two hormones, prolactin (PRL) and growth hormone (GH), were contributing to these effects. By the mid 1930s PRL had been purified as a distinct lactogen, while the galactopoietic effect of GH was confirmed after its purification in the 1940s. Interest in these hormones initially centered about their potential for increasing milk production, while in the latter half of the twentieth century it became obvious that these hormones also had the potential to influence mammary cancer development. During the past 50 years large strides have been made into understanding how these hormones signal to, and within, cells of the mammary gland, paralleling rapid developments in the fields of cellular and molecular biology. In compiling this review we have summarized the progress that has been made to date regarding roles for these hormones in the mammary gland, with a goal of ensuring that some of the seminal literature is not diluted or forgotten. In doing so it is clear that there are lessons to be learned from past experiences, where new methods and technologies will continue to present exciting new opportunities to revisit lingering questions regarding these fascinating hormones and this fascinating organ.

Growth and biochemical changes in mammary glands of ewes from 1 to 18 months of age

Growth and development of the mammary gland of ewe lambs was characterized by changes of various biochemical constituents. Mammary glands were removed from 49 ewe lambs ranging between 1 and 18 mo of age, weighed before and after trimming, and samples of the trimmed glands were fat extracted and dried. The dried fat-free tissues were weighed, ground, and analyzed for nucleic acids, nitrogen, sodium, potassium, and chloride contents. Fresh samples were used for glycogen determination. Trimmed weight of mammary glands increased rapidly after the 9th mo of age. The pattern was similar for nucleic acid content of mammary glands. Dry matter and fat contents of mammary glands increased up to the 9th mo, fat decreased thereafter, and dry matter remained constant. Protein content was low up to the 9th mo and increased thereafter. Sodium and glycogen content in mammary glands increased, but potassium decreased linearly from 1 to 18 mo of age. Allometric growth of mammary tissue started at about 3 mo of age, before onset of puberty at 8 mo of age, but intense mammary metabolic activity started only after the ewe attained puberty.

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.

Effect of amino acid concentration on orotic acid production by bovine mammary tissue

Investigations were to determine if bovine mammary tissue could synthesize orotate and to assess the influence of amino acids on production of orotate. Mammary tissue from 6 Holstein cows was obtained immediately after slaughter. Tissue slices were incubated in either a modified Krebs-Ringer bicarbonate buffer, the same medium containing a physiological concentration of amino acids, or a third medium containing a fourfold concentration of the amino acid mixture. In all experiments, tissue slices were preincubated in the physiological amino acid medium for 5, 10, and 15 min (30 total min) with fresh medium changes after each incubation with one of the three treatment media for three consecutive 20-min periods. Tissue slices incubated in the physiological amino acid medium for a total of 90 min synthesized 28.5 microgram orotate/g wet tissue after correction for the initial tissue contribution. Orotate secreted by the tissue slices increased as concentration of amino acids increased. Lactose released into the medium was not affected by amino acid concentration. Bovine mammary tissue can synthesize substantial amounts of orotate, and tissue synthesis of orotate but not lactose increased as amino acid concentration increased.

Glucocorticoid binding in mammary tissue slices of cattle in various reproductive states

Unlabeled cortisol and dexamethasone reduced tritiated cortisol and tritiated dexamethasone binding to 700 X g supernatant and precipitate fractions of mammary tissue slices from virgin heifers and from multiparous cows that were 1-mo prepartum (nonlactating), lactating (non-pregnant), or dry (nonpregnant, nonlactating). Unlabeled progesterone, testosterone, and 17beta-estradiol had no effect on tritiated glucocorticoid binding in 700 X g supernatant and precipitate fractions from these mammary tissue slices. The 700 X g fractions in mammary tissue slices from all cattle bound cortisol and dexamethasone with high affinity (Kd 10-10M). There were 1263 and 1955 molecules of cortisol and dexamethasone bound per mammary cell, respectively, in mammary tissue slices from lactating non-pregnant cows; in comparison virgin heifers bound 413 and 651 molecules of cortisol and dexamethasone; dry, non-pregnant cows bound 336 and 536 molecules of cortisol and dexamethasone; and 1-mo prepartum nonlactating cows bound 532 molecules of cortisol. Mammary tissue slices from cattle in reproductive states examined contained a major non-specific component which bound cortisol in both 700 X g tissue fractions. Since mammary tissue slices from lactating cattle bound more molecules of glucocorticoids than mammary slices from cattle in other reproductive states, we speculate specific glucorticoid binding may be associated with lactation.

Optimum amino acid complement for protein synthesis by rat mammary cells in tissue culture

The amino acid requirement of rat mammary cells for milk protein synthesis was investigated in dispersed cell culture. A three-dimensional central composite design utilizing three variables (X1 = lysine; X2= methionine, valine, and arginine; X3 = isoleucine, tryptophan, threonine, phenylalanine, and histidine) at five concentrations each, was duplicated twice with mammary cells from lactating Sprague-Dawley rats. The optimum combination of amino acids for maximum milk protein synthesis from multiple regression models was X1 15.0-, X2 4.5-, and X3 1.5-fold their quantities in Eagle's minimal essential medium with leucine, tyrosine, cystine, and glutamine at the base 1-fold in the medium.

Cellular studies of mammary tissue from cows hormonally induced into lactation: histology and ultrastructure

Cytological changes were investigated in mammary parenchyma of cows hormonally induced to lactate. Four cows were induced with injections of 17 beta-estradiol plus progesterone (days 1 to 7) and dexamethasone (days 17 to 19). Tissue biopsies were obtained on days 0, 8, 16, and 26; and milking commenced on day 18 or 21 of the treatment period. Mammary tissue of cow A, the most successful animal, showed marked cellular proliferation and differentiation by day 8. By day 16, the parenchyma of cow A had undergone lactogenesis and exhibited histological and ultrastructural characteristics associated with actively secreting mammary tissue. Mammary epithelia of cow D, the least successful animal, showed minimal development by day 8 and ultrastructurally resembled nondifferentiated mammary cells. Mammary parenchyma of cow D had not undergone differentiation and was involuting by day 16. Mammary tissues of cow B and C showed intermediate responses with only slight development by day 16 and only limited areas of secretory activity by day 26. Results suggest that animals successfully induced into lactation undergo critical periods of cellular proliferation and differentiation which unsuccessful animals do not experience. Cytological differences were first evident by day 8, further magnified by day 16 of the treatment period, and subsequently expressed in terms of milk production (peak milk yield equaled 30.5, 10.0, 9.2, and 2.4 kg/day for cows A, B, C, and D).