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The impacts of a teat being suckled or not in first parity on its development, gene expression, and milk yield in the next parity were studied. Forty-seven first-parity sows (Sus scrofa) were divided into 2 groups: i) the same teats suckled in 2 subsequent lactations (controls, CTL; n = 22); and ii) different teats suckled in 2 subsequent lactations (treated, TRT; n = 25). In the first lactation, over half of the teats (Teats 1, 2, 5, 6, and 7 from 1 side of the udder, and Teats 3, 4, and 7 from the other side) were sealed with tape so that they were nonfunctional. During the next lactation, the CTL group had the same teats sealed as in the first lactation, whereas the opposite teats were sealed for the TRT group. In both parities, litters were standardized to 7 piglets around birth and to 6 piglets (1 piglet per available teat) at 48 h postpartum. During the second lactation, piglets were weighed at birth and on d 2, 4, 7, 14, 21, 35, and 56 postpartum. Weaning was at 17 ± 1 d of age. Behavioral measures were obtained (using 24-h video recording) on d 3 and d 10 of lactation on 15 sows per treatment to evaluate satiety of piglets, using aggressiveness and nursing behavior as indicators. At weaning in the second lactation, 16 sows per treatment were slaughtered and 4 functional mammary glands were collected for compositional analyses and parenchyma from 2 nonfunctional glands was collected to measure mRNA abundance for selected genes. Piglets from CTL sows weighed 1.12 kg more than piglets from TRT sows (P < 0.05) on d 56, and functional mammary glands from CTL sows contained more parenchymal tissue, more DNA, and more RNA (P < 0.01) than those from TRT sows. The relative mRNA abundance of prolactin in parenchymal tissue tended to be greater in CTL than TRT sows (P < 0.10). Behavioral measures indicated a greater hunger level for piglets using teats that were not previously suckled. Current findings clearly show that teats that were suckled in first lactation produce more milk and have a greater development in the second lactation than nonsuckled teats.

The ATP-binding cassette (ABC) transporters ABCA1 and ABCG1 play an important role in cellular cholesterol homeostasis, but their function in mammary gland (MG) tissue remains elusive. A bovine MG model that allows repeated MG sampling in identical animals at different functional stages was used to test whether 1) ABCA1 and ABCG1 protein expression and subcellular localization in mammary epithelial cells (MEC) change during the pregnancy-lactation cycle, and 2) these 2 proteins were present in milk fat globules (MFG). Expression and localization in MEC were investigated in bovine MG tissues at the end of lactation, during the dry period (DP), and early lactation using immunohistochemical and immunofluorescence approaches. The presence of ABCA1 and ABCG1 in MFG isolated from fresh milk was determined by immunofluorescence. The ABCA1 protein expression in MEC, expressed as arbitrary units, was higher during the end of lactation (12.2±0.24) and the DP (12.5±0.22) as compared with during early lactation (10.2±0.65). In contrast, no significant change in ABCG1 expression existed between the stages. Throughout the cycle, ABCA1 and ABCG1 were detected in the apical (41.9±24.8 and 49.0±4.96% of cows, respectively), basal (56.2±28.1 and 54.6±7.78% of cows, respectively), or entire cytoplasm (56.8±13.4 and 61.6±14.4% of cows, respectively) of MEC, or showed combined localization. Unlike ABCG1, ABCA1 was absent at the apical aspect of MEC during early lactation. Immunolabeling experiments revealed the presence of ABCA1 and ABCG1 in MFG membranes. Findings suggest a differential, functional stage-dependent role of ABCA1 and ABCG1 in cholesterol homeostasis of the MG epithelium. The presence of ABCA1 and ABCG1 in MFG membranes suggests that these proteins are involved in cholesterol exchange between MEC and alveolar milk., (Copyright © 2011 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.)

The transport of lipids across mammary gland epithelial cells (MEC) determines milk lipid content and composition. We investigated the expression of lipid transporters and their regulators in comparison to blood metabolites during lactation and dry period (DP) in dairy cows. Repeated mammary gland biopsies and blood samples were taken from 10 animals at 7 stages of the pregnancy-lactation cycle. Expression levels of the specific mRNAs were determined by quantitative reverse transcription-PCR, whereas ABCA1 was localized by immunohistochemistry. Blood serum metabolites were determined by common enzymatic chemistries. Elevated mRNA profiles of ABCA1 and ABCA7 were found during DP as compared with lactation and were inversely associated with blood cholesterol levels. Elevated levels of ABCG2, NPC1, SREBP1, SREBP2, LXR alpha, and PPAR gamma were found postpartum, whereas ABCG1 did not differ between the functional stages of the mammary gland. The ABCA1 protein was localized in MEC and showed differential activity between DP and lactation suggesting a role of ABCA1 in the removal of excess cellular cholesterol from MEC during the DP. The expression profiles of ABCA7 and NPC1 may reflect a role of these transporters in the clearance of apoptotic cells and the intracellular redistribution of cholesterol, respectively. Regulation of lipid transporters in the mammary gland is partially associated with transcription factors that control lipid homeostasis.

Milk yield of the dairy cow follows a pattern termed the lactation curve. We have investigated the cellular background for this pattern. Seven mammary biopsies were obtained from each of 10 cows: at the end of lactation (d 347, equal to d 77 before next parturition); during the dry period at d 48 (4 d after dry off); 16 d before parturition; and during lactation at d 14, 42, 88, and 172. The fraction of proliferating (staining positive for Ki-67) alveolar cells was higher during the dry period (8.6%) than during lactation (0.5%). The fraction of apoptotic (staining positive by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling) alveolar cells was higher immediately after dry off (0.37%) and in early lactation (0.76%) than during other periods (0.15%). The enzyme activities of fatty acid synthetase, acetyl CoA-carboxylase, and galactosyl transferase were approximately 12-, 11-, and 4-fold higher, respectively, during lactation than during the dry period. In conclusion, mammary cell proliferation is substantial in a period near parturition but otherwise low, and apoptosis is elevated at dry off and in early lactation. The increase in apoptosis in early lactation may be due to discarding nonfunctional or senescent cells or to removal of a surplus of newly synthesized cells. The activity of selected enzymes central for milk synthesis is probably not limiting for milk production.

The possible roles of dietary protein level and feed restriction in regulating mammary development of prepubertal gilts were investigated. Cross-bred gilts were fed a commercial diet until 90 d of age and then divided into four nutritional regimens based on two pelleted diets (as-fed basis): a high-protein diet (HP = 13.8 MJ of ME, 1.0% total lysine, 18.7% CP) and a low-protein diet (LP = 13.8 MJ of ME, 0.7% total lysine, 14.4% CP). Nutritional regimens were as follows: 1) HP ad libitum until slaughter (n = 22, T1); 2) HP ad libitum until 150 d of age followed by LP until slaughter (n = 20, T2); 3) LP ad libitum until slaughter (n = 21, T3); and 4) HP with a 20% feed restriction until slaughter (n = 19, T4). Gilts were weighed, their backfat thickness was measured, and jugular blood samples were obtained on d 90, 150, and at slaughter to determine concentrations of prolactin, IGF-I, leptin, and glucose. Gilts were slaughtered 8+/-1 d after their first or second estrus (202.7+/-14.5 d of age). Mammary glands were excised, parenchymal and extraparenchymal tissues were dissected, and composition of parenchymal tissue (protein, fat, DM, DNA, protein/DNA) was determined. The T4 gilts weighed less (P < 0.01) and had less backfat (P < 0.01) than did gilts on other treatments on d 150 and at slaughter. Treatments had no significant effects on prolactin, IGF-I, or glucose concentrations, but there was a treatment x day interaction (P < 0.01) for leptin, with concentrations being lower at slaughter in restricted-fed (T4) vs. LP (T3) gilts (P < 0.05). There was less extraparenchymal mammary tissue (P < 0.01) in T4 gilts than in gilts from the other groups and a tendency (P = 0.13) for the amount of parenchymal tissue to be lower in T4 gilts. In conclusion, a lower lysine intake during prepuberty did not hinder mammary development of gilts, but a 20% feed restriction decreased mass of parenchymal and extraparenchymal tissues. The effect of feed restriction on extraparenchymal tissue is most likely associated with the lower fat deposition.

Prolactin is required for mammary development in various species but its possible role for mammogenesis in pigs is not known. The goal of the present study was therefore to determine the effect of prolactin inhibition by bromocriptine during the last third of gestation on mammary gland development in gilts. Twenty-eight primigravid gilts were assigned as controls (n = 15) or received 10 mg of bromocriptine orally thrice daily (n = 13) from d 70 to 110 of gestation. Jugular blood samples were collected on d 70 of gestation and every 8 d thereafter and were assayed for prolactin, IGF-I, estradiol, and progesterone. Gilts were slaughtered on d 110 of gestation and fetuses were counted and weighed. One row of mammary glands was used for dissection of parenchymal and extraparenchymal tissues and for determination of DNA, RNA, dry matter, protein, and fat contents. Tissue from the other row was used for measures of prolactin receptor number and affinity. Concentrations of prolactin were drastically reduced throughout the bromocriptine treatment period (P < .001), whereas there was no overall treatment effect on progesterone and IGF-I levels (P > .10). Total weight and extraparenchymal tissue weight of the mammary glands were unaffected by treatment (P > or = .1), but weight of parenchymal tissue, total DNA, and total RNA decreased (P < .01) with bromocriptine treatment. Percentages of fat and dry matter in parenchymal tissue increased with bromocriptine treatment (P < .01) and the percentage of protein decreased (P < .01). Number of prolactin receptors in parenchymal tissue decreased with bromocriptine treatment (P < .001) and receptor affinity increased (P < .001). Average fetal weight was lower in gilts receiving bromocriptine than in control gilts (P = .05), but fetal number did not differ (P > .1). These results clearly demonstrate that prolactin is essential for normal mammary gland development and can affect fetal growth during the last third of gestation in gilts.

Genetic differences between Upton-Meishan (UM, n = 13) and Large White (LW, n = 14) gilts were studied with regard to mammary gland development and concentrations of hormones. Gilts were weighed and their backfat measured at mating, and at d 70 and 109 of gestation. Jugular blood samples were also collected at these times and assayed for prolactin, cortisol, IGF-I, insulin, glucose, progesterone, and estradiol. Gilts were slaughtered on d 110 of gestation. One row of mammary glands was used for dissection and biochemical analyses. The other row was used for determination of prolactin receptor number and affinity. UM gilts weighed less (P<0.05) and had more backfat (P<0.01) than LW gilts at all times. Parenchymal tissue weight was less (P<0.05) in UM gilts. Percent fat (P<0.001) and dry matter (P<0.001) in parenchymal tissue were greater in UM gilts while that of protein (P<0.001) was lower. Total protein weight in parenchyma was also lower in parenchyma was also lower in UM gilts (P = 0.01). Both DNA (P<0.001) and RNA (P<0.001) contents were lower in UM gilts while RNA/DNA remained similar (P>0.1). Number of prolactin receptors were lower (P = 0.06) and affinity greater (P<0.05) in UM gilts. Cortisol levels were greater (P<0.01) in UM gilts while other hormones were not affected (P> 0.1). Results clearly demonstrate genetic differences with regard to mammogenesis in gilts and suggest that the less mammary gland development in Upton-Meishan compared with Large White breed of gilts may be related to lower number of prolactin receptors.

Third-parity sows received s.c. injections of sterile water (CTL, n = 12) or 15 mg of recombinant porcine prolactin (pPRL, n = 12) at 0730, 1530, and 2330 from d 2 to 23 of lactation. Litters were standardized to 11 or 12 pigs and were weighed weekly until weaning (d 24). On d 22 of lactation, milk production was estimated, and a milk sample was obtained the next day. Jugular blood samples were collected from sows on d 2, 7, 14, and 21 of lactation. Sows were slaughtered and mammary glands collected at d 24. Injections of pPRL doubled the serum concentrations of prolactin (P < .001) on d 7, 14, and 21 of lactation and decreased IGF-I concentrations on d 14 (P = .07) and 21 (P < .01). Weight, backfat, and milk yield of sows and mean pig weights were not affected by pPRL (P > . 1), yet the mean duration of intervals between nursings was reduced by 4.2 min (P = .06) in pPRL litters (45.9 vs 41.7 min). Dry matter and fat percentages in milk were lower in pPRL sows (P < .01). Weights of parenchymal and extraparenchymal tissues were not altered by pPRL treatment (P > .1). Number of prolactin receptors in parenchymal tissue as well as receptor affinity were similar in both groups (P > .1). Results indicate that virtually all prolactin receptors were saturated in CTL sows. This is probably the reason why additional exogenous prolactin had negligible effects.

Forty-eight heifers (3 mo old) were subjected for 35 wk to twice daily s.c. injections of saline or human somatotropin-releasing factor (5 microgram.kg-1 BW) and photoperiod of 8 h (short day) or 16 h (long day) of light daily according to a 2 X 2 factorial design. There was no treatment effect on slaughter weight, withers height, and carcass length. Somatotropin-releasing factor increased thymus weight, metacarpal length, and protein content of the carcass but decreased carcass fat content. Long day photoperiod increased chest girth, pituitary weight, and carcass yield. Effects of somatotropin-releasing factor and photoperiod were not additive on volume of mammary parenchymal tissue; there was also a treatment interaction on liver and lung weights. However, extraparenchymal mammary tissue was decreased by human somatotropin-releasing factor and short day photoperiod. In conclusion, use of somatotropin-releasing factor and photoperiod treatments affected carcass parameters and composition and mammary growth.

Nine pairs of identical twins were used to examine the effect of exogenous bovine somatotropin on mammary growth and development in dairy heifers. One twin received a daily subcutaneous injection of somatotropin (20 IU) and the other received excipient. Treatments commenced at 8.0 mo of age (179 kg live weight) and continued for 15.6 wk, at which time heifers were slaughtered and mammary development evaluated. Treatment with somatotropin resulted in an increase in mammary parenchyma and a decrease in extraparenchymal tissue and weight of mammary glands. Increases in parenchyma were 46% as determined by computer assisted x-ray tomography and 18% as determined by dissection. Differences between the two techniques related to the ability of computer assisted x-ray tomography technique to exclude fat deposits and connective tissue from parenchymal estimates. Chemical composition (39% water, 7% protein, and 54% fat) as well as histological and cytological appearance of the mammary parenchyma were not affected by the treatment. Therefore, treatment with exogenous somatotropin around puberty enhances the growth rate of mammary parenchymal tissue. Additional studies are required to examine whether this enhanced mammary development will increase milk yield during subsequent lactations.