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ABSTRACT: Mammary stem cells (MaSC) provide for net growth, renewal and turnover of mammary epithelial cells, and are therefore potential targets for strategies to increase production efficiency. Appropriate regulation of MaSC can potentially benefit milk yield, persistency, dry period management and tissue repair. Accordingly, we and others have attempted to characterize and alter the function of bovine MaSC. In this review, we provide an overview of current knowledge of MaSC gained from studies using mouse and human model systems and present research on bovine MaSC within that context. Recent data indicate that MaSC retain labeled DNA for extended periods because of their selective segregation of template DNA strands during mitosis. Relying on this long-term retention of bromodeoxyuridine-labeled DNA, we identified putative bovine MaSC. These label-retaining epithelial cells (LREC) are in low abundance within mammary epithelium (<1%). They are predominantly estrogen receptor (ER)-negative and localized in a basal or suprabasal layer of the epithelium throughout the gland. Thus, the response of MaSC to estrogen, the major mitogen in mammary gland, is likely mediated by paracrine factors released by cells that are ER-positive. This is consistent with considerable evidence for cross-talk within and between epithelial cells and surrounding stromal cells. Excision of classes of cells by laser microdissection and subsequent microarray analysis will hopefully provide markers for MaSC and insights into their regulation. Preliminary analyses of gene expression in laser-microdissected LREC and non-LREC are consistent with the concept that LREC represent populations of stem cells and progenitor cells that differ with regard to their properties and location within the epithelial layer. We have attempted to modulate the MaSC number by infusing a solution of xanthosine through the teat canal and into the ductal network of the mammary glands of prepubertal heifers. This treatment increased the number of putative stem cells, as evidenced by an increase in the percentage of LREC and increased telomerase activity within the tissue. The exciting possibility that stem cell expansion can influence milk production is currently under investigation.
animal 03/2012; 6(3):382-93. · 1.74 Impact Factor
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ABSTRACT: A rapid method of 5-bromo-2'-deoxyuridine (BrdU) immunostaining was developed in cryosections of bovine mammary tissue while preserving RNA quality of the stained section. A thymidine analog that is incorporated into DNA of proliferating cells, BrdU serves as a proliferation marker. Immunostaining of BrdU-labeled cells within a histological section requires heat, enzymatic or chemical-mediated antigen retrieval to open double-stranded DNA, and exposure to the BrdU antigen. Although these established treatments permit staining, they preclude use of cells within the tissue section for further gene expression experiments. Additionally, long antibody incubations and washing steps lead to extensive RNA degradation and elution. A protocol was developed for immunolocalization of BrdU-labeled cells in cryosections of bovine mammary tissue, which does not require harsh DNA denaturation and preserved RNA integrity and quantity. This protocol used an initial acetone:polyethylene glycol 300 [9:1 (vol/vol)] fixation (2 min) followed by staining with methyl green (0.5% aqueous; 2 min) to stabilize macromolecules, antigen retrieval with deionized formamide (70% in nuclease-free phosphate buffered saline; 4 min incubation), antibody incubation in the presence of RNase inhibitors (5 min), and minimal washing to facilitate recovery of RNA from cells from the stained sections. Applicability of this protocol to other nuclear antigens was evaluated by testing its suitability for staining estrogen receptor alpha and Ki-67 antigen. In both cases, use of the protocol provided good immunostaining and tissue morphology. The RNA quality of estrogen receptor alpha- and Ki-67-stained sections was not evaluated. Quality of the isolated RNA from BrdU-stained sections was evaluated by micro-fluidic electrophoresis and its utility was confirmed using quantitative reverse transcription-PCR. Staining intensity obtained with this labeling protocol was similar to that obtained using conventional immunohistochemistry protocols. When coupled with laser microdissection and RNA or cDNA amplification, this immunostaining protocol provided a means for future transcriptome analysis of BrdU-labeled cells within a complex tissue.
Journal of Dairy Science 06/2010; 93(6):2574-9. · 2.56 Impact Factor
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ABSTRACT: Overfeeding prepubertal heifers may impair mammary parenchymal growth and reduce milk production, but evidence suggests that increased intake of a high-protein milk replacer before weaning may be beneficial. This study was designed to evaluate effects of milk replacer (MR) composition on mass and composition of mammary parenchyma and fat pad, growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis gene expression, and putative mammary epithelial stem cells. Specifically, we hypothesized that positive effects of faster rates of gain during the preweaning period alter the development, persistence, or activity of populations of putative mammary epithelial stem cells, possibly through involvement of GH/IGF-I axis molecules. Twenty-four newborn heifers were fed 1 of 4 MR diets (n = 6/diet): control [20% crude protein (CP), 21% fat MR fed at 441 g of dry matter (DM)/d], high protein, low fat (28% CP, 20% fat MR fed at 951 g of DM/d), high protein, high fat (27% CP, 28% fat MR fed at 951 g of DM/d), and high protein, high fat+ (27% CP, 28% fat MR fed at 1,431 g of DM/d). Water and starter (20% CP, 1.43% fat) were offered ad libitum. Animals were killed on d 65 and mammary tissue was subjected to biochemical, molecular, and histological examination. No differences in mammary parenchymal mass or composition, with or without adjusting for empty body weight, were detected. Mass was increased and composition of the mammary fat pad was altered by nutrient intake. No diet differences in putative mammary epithelial stem cell abundance or abundance of transcripts for genes of the GH/IGF-I axis were detected. In this study, growth of the mammary epithelium, size of the mammary epithelial stem cell population, and components of the GH/IGF-I axis did not depend on diet. However, an underlying positive correlation between telomerase, a marker of mammary stem cells, and growth of the mammary parenchyma was detected. Implications of diet-induced effects on mammary fat pad and possible effects on subsequent development and function remain to be determined.
Journal of Dairy Science 12/2009; 92(12):5937-50. · 2.56 Impact Factor
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ABSTRACT: Our objective was to determine the effects of rate of gain and body weight (BW) on development of the mammary parenchyma. Mammary tissue samples were collected from heifers (n = 72) reared on 1 of 2 dietary treatments (restricted, 650 g/d of daily gain; or elevated, 950 g/d of daily gain) and slaughtered at 100, 150, 200, 250, 300, or 350 kg of BW. Mammary samples were excised, preserved, prepared for histology, and stained with hematoxylin and eosin. Digital images of tissue sections were captured for analysis. Tissue areas occupied by the interlobular and intralobular stroma, epithelium, and lumen were measured (mum(2)). The numbers of epithelial and luminal structures per image were tabulated to measure the complexity of ductal development. Mean percentages of mammary parenchyma occupied by the interlobular stroma, epithelium, lumen, and intralobular stroma were 29, 20, 7, and 43%, respectively. Percentage of area occupied by the intralobular stroma was affected by BW and was lower for 100-kg heifers compared with heifers 200 kg and heavier (33 +/- 4 vs. 46 +/- 4), but the percentage of area occupied by other tissue elements did not differ by BW or treatment, nor was there an interaction. However, the numbers of both epithelial (8.3 +/- 4 vs. 47 +/- 4) and luminal-containing (6 +/- 4 vs. 38 +/- 4) structures per image increased markedly between 100 and 350 kg of BW, irrespective of diet. For heifers slaughtered between 100 and 350 kg of BW, alterations in the rate of gain between 650 and 950 g/d, accomplished by feeding varying amounts of the same diet, had no significant effect on tissue characteristics or the pattern of mammary parenchymal development. These data emphasize the importance of BW and age in determining developmental characteristics of the heifer mammary parenchyma and suggest that the rate of gain per se has a minimal impact on histological development, and thus do not support the hypothesis that rate of gain has a direct negative impact on ductal development.
Journal of Dairy Science 03/2009; 92(2):499-505. · 2.56 Impact Factor
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ABSTRACT: Components of the somatotropic axis and nutrition regulate intestinal development and maturation of enterocytes. We measured gene expression in the mucosal layer of small intestine of preweaned dairy heifers to test the hypothesis that feeding increased amounts of protein and fat alters expression of somatotropic axis genes. Twenty-four newborn Holstein heifers were randomly assigned to 1 of 4 milk replacer (MR) diets: (1) 20% CP, 20% fat MR (DM basis) fed at 450 g/d (CON); (2) 28% CP, 20% fat MR fed at 970 g/d (HPLF); (3) 28% CP, 28% fat MR fed at 970 g/d (HPHF); and (4) 28% CP, 28% fat MR fed at 1,460 g/d (HPHF+). Dry calf starter (20% CP, 1.43% fat) was offered free choice. At 64 +/- 3 d of age heifers were killed and intestinal tissues were harvested for RNA isolation and histological examination. We measured the mRNA expression of growth hormone receptor (GHR), insulin-like growth factor-I (IGF-I), IGF-I receptor (IGF-IR), and IGF binding proteins (IGFBP)-1 to -6 in duodenum, jejunum, and ileum by quantitative real-time reverse transcription PCR. Expression of IGFBP-3 mRNA was lowest in the duodenum of HPHF+ and greatest in the ileum of the CON group, whereas expression of IGFBP-4 mRNA was greatest in the jejunum of the HPHF+ group. Expression of IGFBP-5 mRNA was greatest in the CON and lowest in the HPHF+. However, overall diet did not affect expression of GHR, IGF-I, IGF-IR, or IGFBP-1, -2, and -6. Expression of somatotropic axis genes differed among small intestinal locations. The GHR, IGF-IR, IGFBP-1, and IGFBP-5 mRNA were greatest in the ileum. Duodenum produced less IGF-IR, IGF-I, and IGFBP-5 mRNA. Villi were shortest in the ileum, but there was no difference in villus height between the duodenum and jejunum. There was no difference in crypt depth or villus circumference between locations. In conclusion, some components of the somatotropic axis in preweaned dairy heifers are differentially expressed in regions of the small intestine, and the gene expression tended to be affected by dietary protein and fat.
Journal of Dairy Science 10/2008; 91(9):3343-52. · 2.56 Impact Factor
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ABSTRACT: We investigated the effects of increasing dietary protein and energy on concentrations of selected blood metabolites and hormones in Holstein heifers. Twenty-four heifers were fed 1 of 4 milk replacer (MR) diets for 9 wk (n = 6/diet): control [20% crude protein (CP), 21% fat MR fed at 441 g of dry matter (DM)/d], HPLF (28% CP, 20% fat MR fed at 951 g of DM/d), HPHF (27% CP, 28% fat MR fed at 951 g of DM/d), and HPHF+ (27% CP, 28% fat MR fed at 1,431 g of DM/d). Heifers were fed twice daily; water and starter (20% CP, 1.43% fat) were offered free choice and starter orts recorded daily. Serum and plasma aliquots from blood samples collected twice weekly after a 12-h fast were analyzed for insulin-like growth factor (IGF)-I, IGF-binding proteins (IGFBP), growth hormone (GH), insulin, glucose, nonesterified fatty acids, triglyceride, and plasma urea nitrogen concentrations. Only plasma glucose, IGFBP-2, and IGFBP-3 were affected by diet. Dietary treatment differences were only noted when the control was compared with the average of the other 3 diets. The addition of fat to the MR (HPLF vs. HPHF) and increased volume of MR (HPHF vs. HPHF+) had no effect on plasma glucose concentration or relative abundance of IGFBP-2 or IGFBP-3. Heifers fed the control diet had less glucose, greater IGFBP-2, and less IGFBP-3 than the average of the other 3 diets. There was a diet by week interaction for IGF-I. Serum IGF-I concentration in control heifers varied in a quadratic manner with a nadir (20 +/- 4 ng/mL) at wk 4, whereas IGF-I increased linearly in heifers on other diets. Both insulin and triglyceride changed over time in a complex pattern (significant linear and quadratic contrast effects). The greatest concentrations were measured at wk 0.5 with nadirs at wk 6 for both insulin and triglyceride. Serum GH concentration decreased in a linear manner from wk 0.5 to wk 9 in all heifers. Relative abundance of IGFBP-2 was quadratic over time with the greatest amount of IGFBP-2 observed at wk 5. With the exception of glucose, IGF-I, IGFBP-2, and IGFBP-3, the blood variables measured were not influenced by treatment. The IGF-I -GH-IGFBP axis requires further study in heifers to deduce effects of nutrition on hypothalamic regulation of metabolism. We expected to see more treatment differences in concentrations of metabolites involved with protein and fat metabolism. It is likely that the diets used in this study were not diverse enough in composition to elicit such changes or that the efficiency of use of absorbed protein and fat was not different in these animals.
Journal of Dairy Science 08/2008; 91(7):2628-40. · 2.56 Impact Factor
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ABSTRACT: Twenty-four newborn Holstein heifer calves were fed 1 of 4 milk replacers (MR): control (20% CP, 21% fat; MR fed at 441 g/d); high protein/low fat (HPLF; 28% CP, 20% fat; MR fed at 951 g/d); high protein/high fat (HPHF; 27% CP, 28% fat; MR fed at 951 g/d); and HPHF MR fed at a higher rate (HPHF+; 27% CP, 28% fat; MR fed at 1,431 g/d). Dry calf starter (20% CP, 1.43% fat) composed of ground corn (44.4%), 48% CP soybean meal (44.4%), cottonseed hulls (11.2%), and molasses (1.0%) was offered free choice. Heifers were obtained from a commercial dairy, blocked by groups of 8 in the order acquired, and randomly assigned to treatments within group. Upon arrival at the research farm, heifers were fed the control for 2 feedings. Treatments were imposed when heifers were 4 +/- 1 d of age. Heifers were on study for 61 +/- 1 d. Body weight and body size measures were taken weekly. Four-day total collection of feed refusals, feces, and urine was initiated at 57 +/- 1 d of age. Heifers were slaughtered at the end of the collection period to evaluate body composition. Preplanned contrasts were used to compare control to all, HPLF to HPHF, and HPHF to HPHF+. Heifers fed the control diet consumed more starter than those fed other treatment diets, but their total dry matter intake and apparent dry matter digestibility were lowest. Fecal output was highest in heifers fed the control diet, whereas urine output and urine N excretion were lowest. Nitrogen intake and urine N excretion were greater for heifers fed HPHF+ compared with HPHF but were not affected by MR fat content (HPLF vs. HPHF). Retention (g/d) of N and P was greater in heifers fed all nutrient-dense diets compared with those fed the control diet, but was not improved by increasing fat in the milk replacer (HPLF vs. HPHF) or by increasing the amount fed. Addition of fat to the milk replacer (HPLF vs. HPHF) increased empty body weight fat content without improving average daily gain or frame measures. Increasing the volume fed (HPHF vs. HPHF+) increased growth rate and empty body weight, but HPHF+ heifers were neither taller nor longer and their carcasses contained more fat. Clear improvements in growth and nutrient retention were observed with more nutrient-dense diets, but most of the improvements were seen with the increased protein intake relative to the control MR; adding fat to the high protein MR did not further improve lean tissue gain.
Journal of Dairy Science 08/2008; 91(8):3145-55. · 2.56 Impact Factor
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ABSTRACT: The experimental objective was to determine the effects of feeding prepubertal dairy heifers a high-energy diet for 3, 6, or 12 wk on mammary growth and composition. Holstein heifers (age = 11 wk; body weight = 107 +/- 1 kg) were assigned to 1 of 4 treatments (n = 16/ treatment). The treatment period lasted 12 wk and treatments were H0 (low-energy diet fed for 12 wk, with no weeks on the high-energy diet); H3 (low-energy diet fed for 9 wk, followed by the high-energy diet for 3 wk); H6 (low-energy diet fed for 6 wk, followed by the high-energy diet for 6 wk); and H12 (high-energy diet for all 12 wk). The low- and high-energy diets were formulated to achieve 0.6 and 1.2 kg of average daily gain, respectively. Heifers were slaughtered at 23 wk of age and mammary tissue was collected. A longer duration of feeding the high-energy diet increased total mass of the mammary gland, extraparenchymal fat, and intraparenchymal fat, but did not alter the mass of fat-free parenchymal tissue. When adjusted for carcass weight to reflect differences in physical maturity, the mass of fat-free parenchymal tissue decreased in a linear fashion with a longer duration on the high-energy diet. Total masses of mammary parenchymal DNA and RNA were not different. However, after adjustment for carcass weight, the masses of DNA and RNA decreased as heifers were fed the high-energy diet for a longer duration. The percentages of epithelium, stroma, and lumen, the number of epithelial structures, and the developmental scores of mammary parenchymal tissue were not different among treatments. However, the percentage of proliferating epithelial cells in the terminal ductal units, as indicated by Ki-67 labeling, decreased as heifers were fed the high-energy diet for a longer duration. We concluded that feeding prepubertal heifers a high-energy diet for a longer duration resulted in a linear decrease in both the percentage of mammary epithelial cells that were proliferating and in the mass of fat-free mammary parenchyma per unit of carcass. High-energy feeding hastens puberty and, in this study, decreased mammary epithelial cell proliferation in areas of active ductal expansion. These data are consistent with the idea that feeding heifers a high-energy diet will reduce mammary parenchymal mass at puberty.
Journal of Dairy Science 06/2008; 91(5):1926-35. · 2.56 Impact Factor
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ABSTRACT: A proteomics approach was used to characterize biochemical and cellular mechanisms governing effects of peripubertal feeding on heifer mammary development. Mammary parenchymal tissue from 24 Holstein heifers randomly assigned to treatments arranged in a 2 x 2 factorial design was used to generate 2-dimensional protein maps of mammary tissue extracts. Heifers were reared on 1 of 2 dietary treatments, restricted (650 g/ d of daily gain) or elevated (950 g/d of daily gain) and killed at 1 of 2 body weights (BW, 200 or 350 kg). Cytosolic mammary gland extracts were prepared from frozen mammary parenchyma. Proteome maps of extracts were constructed using PDQuest software. Densities of 820 protein spots were analyzed using the MIXED procedure of SAS. Protein spots were characterized by changes in profiles of expression in response to increased BW, dietary treatment, or both. Dietary treatment influenced the expression of 131 protein spots, whereas heifer BW influenced the expression of 108 spots. The 22 most highly influenced (statistically) spots were excised and submitted for mass spectrometric analyses. Returned protein names and accession numbers were used in National Center for Biotechnology Information database searches to obtain information on the identified proteins. For example, one of the proteins that differed by dietary treatment, transferrin, a binding protein of insulin-like growth factor binding protein-3, was identified via these methods. Possible roles of this and other proteins in mammary development are described. We concluded that a proteomic approach is an effective tool for identifying the proteins involved in bovine mammary development.
Journal of Dairy Science 12/2006; 89(11):4276-88. · 2.56 Impact Factor
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ABSTRACT: The objective was to determine whether increased energy and protein intake between 2 and 14 wk of age would increase growth rates of heifer calves without fattening. At 2 wk of age, Holstein heifer calves were assigned to 1 of 4 treatments in a 2 x 2 factorial arrangement with 2 levels of protein and energy intake (moderate [M]; high [H]) in period 1 (2 to 8 wk of age) by 2 levels of protein and energy intake (low [L]; high [H]) in period 2 (8 to 14 wk of age) to produce similar initial BW for all 4 treatments. Treatments were ML, MH, HL, and HH, indicating moderate or high energy and protein intake during the first period and low or high intake during the second period. The M diet consisted of a standard milk replacer (21.3% CP, 21.3% fat) fed at 1.1% of BW on a DM basis and a 16.5% CP grain mix fed at restricted intake to promote 400 g of average daily gain (ADG), whereas the L diet consisted only of the grain mix. The H diet consisted of a high-protein milk replacer (30.3% CP, 15.9% fat) fed at 2% of BW on a DM basis and a 21.3% CP grain mix available ad libitum. Calves were weaned gradually from milk replacer by 7 wk and slaughtered at 8 (n = 11) or 14 wk of age (n = 41). In periods 1 and 2, ADG and the gain:feed ratio were greater for calves fed the H diet. Calves fed the H diet were taller after both periods 1 and 2. No difference was observed in carcass composition at 8 wk, but at 14 wk calves fed MH and HH had less water and more fat than calves fed ML and HL. Plasma IGF-I concentrations were greatest for calves fed the H diet during either period. Plasma leptin concentrations were increased in calves fed the H diet during period 1 from 4 to 6 wk of age. Increasing energy and protein intake from 2 to 8 wk and 8 to 14 wk of age increased BW, withers height, and gain:feed ratio. Calves fed the H diet from 8 to 14 wk of age had more body fat than calves fed the L diet. Increased energy and protein intake can increase the rate of body growth of heifer calves and potentially reduce rearing costs.
Journal of Dairy Science 03/2005; 88(2):585-94. · 2.56 Impact Factor
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ABSTRACT: The objective of this study was to determine if increased energy and protein intake from 2 to 14 wk of age would affect mammary development in heifer calves. At 2 wk of age, Holstein heifer calves were assigned to 1 of 4 treatments in a 2 x 2 factorial arrangement with 2 levels of protein and energy intake (moderate, M; high, H) in period 1 (2 to 8 wk of age) and 2 levels of protein and energy intake (low, L; high, H) in period 2 (8 to 14 wk of age), so that mean initial body weights were approximately equal for all 4 treatments (ML, MH, HL, and HH). The M diet in period 1 consisted of a standard milk replacer (21.3% CP, 21.3% fat) fed at 1.1% of BW on a DM basis and a 16.5% CP grain mix fed at restricted intake to promote 400 g of daily gain, whereas the L diet in period 2 consisted only of the grain mix. The H diet in period 1 consisted of a high-protein milk replacer (30.3% CP, 15.9% fat) fed at 2.0% of body weight on a DM basis and a 21.3% CP grain mix available ad libitum. In period 2, the H diet consisted of just the 21.3% grain mix. Calves were gradually weaned from milk replacer by 7 wk and slaughtered at 8 (n = 11) or 14 wk of age (n = 41). Parenchyma from the distal region, midgland, and proximal region relative to the teat from one half of the udder was collected, fixed, and embedded in paraffin. The other half of the gland was used to determine parenchymal mass, protein, fat, DNA, RNA, and extraparenchymal mass. Total parenchymal tissue, parenchymal DNA, parenchymal RNA, and concentrations of DNA and RNA were higher for calves on the H diet during period 1, but were not affected by diet during period 2. Parenchymal fat percentage was increased by the H diet during period 2. The H diet increased extraparenchymal fat during both periods. The area of parenchyma occupied by epithelium was not affected by treatment, but at the end of period 2, the percentage of proliferating epithelial cells as indicated by Ki67, an marker of cell proliferation, expression was greater for calves on the M diet in period 1 compared with calves on the H diet in period 1. Diets did not influence parenchymal protein percentage or the ratio of RNA to DNA. Higher energy and protein intake from 2 to 8 wk of age increased parenchymal mass and parenchymal DNA and RNA in mammary glands of heifer calves without increasing deposition of parenchymal fat. Diet also influenced histological development of mammary parenchyma and subsequent proliferation of ductal epithelial cells. Implications of these effects for future milk production potential are unknown.
Journal of Dairy Science 03/2005; 88(2):595-603. · 2.56 Impact Factor
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ABSTRACT: We investigated the effects of increasing dietary protein and energy on concentrations of selected blood metabolites and hormones in Holstein heifers. Twenty-four heifers were fed 1 of 4 milk replacer (MR) diets for 9 wk (n = 6/diet): control [20% crude protein (CP), 21% fat MR fed at 441 g of dry matter (DM)/d], HPLF (28% CP, 20% fat MR fed at 951 g of DM/d), HPHF (27% CP, 28% fat MR fed at 951 g of DM/d), and HPHF+ (27% CP, 28% fat MR fed at 1,431 g of DM/d). Heifers were fed twice daily; water and starter (20% CP, 1.43% fat) were offered free choice and starter orts recorded daily. Serum and plasma aliquots from blood samples collected twice weekly after a 12-h fast were analyzed for insulin-like growth factor (IGF)-I, IGF-binding proteins (IGFBP), growth hormone (GH), insulin, glucose, nonesterified fatty acids, triglyceride, and plasma urea nitrogen concentrations. Only plasma glucose, IGFBP-2, and IGFBP-3 were affected by diet. Dietary treatment differences were only noted when the control was compared with the average of the other 3 diets. The addition of fat to the MR (HPLF vs. HPHF) and increased volume of MR (HPHF vs. HPHF+) had no effect on plasma glucose concentration or relative abundance of IGFBP-2 or IGFBP-3. Heifers fed the control diet had less glucose, greater IGFBP-2, and less IGFBP-3 than the average of the other 3 diets. There was a diet by week interaction for IGF-I. Serum IGF-I concentration in control heifers varied in a quadratic manner with a nadir (20 ± 4 ng/mL) at wk 4, whereas IGF-I increased linearly in heifers on other diets. Both insulin and triglyceride changed over time in a complex pattern (significant linear and quadratic contrast effects). The greatest concentrations were measured at wk 0.5 with nadirs at wk 6 for both insulin and triglyceride. Serum GH concentration decreased in a linear manner from wk 0.5 to wk 9 in all heifers. Relative abundance of IGFBP-2 was quadratic over time with the greatest amount of IGFBP-2 observed at wk 5. With the exception of glucose, IGF-I, IGFBP-2, and IGFBP-3, the blood variables measured were not influenced by treatment. The IGF-I –GH–IGFBP axis requires further study in heifers to deduce effects of nutrition on hypothalamic regulation of metabolism. We expected to see more treatment differences in concentrations of metabolites involved with protein and fat metabolism. It is likely that the diets used in this study were not diverse enough in composition to elicit such changes or that the efficiency of use of absorbed protein and fat was not different in these animals.
Journal of Dairy Science.
