| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Pregnancy |
Department of Animal Science and Center for Animal Biotechnology and Genomics, Texas A&M University, College Station, Texas 77843-2471
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
-amino acids. Alanine, glutamine, glycine, plus serine contributed approximately 50% of total -amino acids in fetal plasma. Fetal:maternal plasma ratios for amino acids varied greatly, being less than 1 for glutamate during late gestation, 1.5–3 for most amino acids throughout gestation, and greater than 10 for serine during late gestation. Marked changes were observed in amino acid concentrations in amniotic and allantoic fluids associated with conceptus development. Concentrations of alanine, citrulline, and glutamine in allantoic fluid increased by 20-, 34-, and 18-fold, respectively, between Days 30 and 60 of gestation and were 24.7, 9.7, and 23.5 mM, respectively, on Day 60 of gestation (compared with 0.8 mM arginine). Remarkably, alanine, citrulline, plus glutamine accounted for approximately 80% of total -amino acids in allantoic fluid during early gestation. Serine (16.5 mM) contributed approximately 60% of total -amino acids in allantoic fluid on Day 140 of gestation. These novel findings of the unusual abundance of traditionally classified nonessential amino acids in allantoic fluid raise important questions regarding their roles in ovine conceptus development.
amino acids, conceptus development, fetal fluids, sheep
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
The developing fetus, surrounded by the amniotic fluid compartment and connected with the allantoic sac via the urachus and placental vasculature, receives nutrients mainly via the umbilical vein [12]. The amniotic fluid provides a unique aqueous environment in which the fetus develops symmetrically. When it is swallowed, amniotic fluid is a significant source of nutrients for the fetus [13]. In contrast, the allantoic sac was traditionally considered to be a reservoir for fetal wastes [14]. However, recent studies with pigs have shown that the allantoic sac plays an important role in the accumulation of nutrients and metabolism of both uteroferrin (a progesterone-induced, iron-binding protein) and iron [15], suggesting a hitherto unrecognized function of the allantoic sac in fetal nutrition.
We recently reported an unusual abundance of arginine (4–6 mM) in porcine allantoic fluid during early (Day 40) gestation compared with maternal plasma arginine concentrations (0.1–0.15 mM) [16–18]. However, it is not known whether such high concentrations of arginine are unique for porcine allantoic fluid or whether they represent an important physiological phenomenon for mammals. Although there have been studies of amino acid uptake into umbilical vessels [19], placental arginine transfer [20], as well as placental and hepatic metabolism of glycine, serine, glutamate, and glutamine [21, 22] in fetal sheep during late gestation (Days 118–146), little is known regarding changes in the concentrations of amino acids in ovine fetal fluids associated with conceptus development. Such information will provide a critical database for future studies to quantify amino acid metabolism in the ovine fetus, to define fetal amino acid requirements, and to elucidate mechanisms responsible for intrauterine growth retardation and fetal origin of adult diseases.
We hypothesized that arginine was the most abundant amino acid in ovine allantoic fluid during early gestation. This hypothesis was tested by quantifying concentrations of 24 amino acids in ovine amniotic and allantoic fluids, as well as fetal and maternal plasma, between Days 30 and 140 of gestation. The measurement of amino acids other than arginine is necessary to determine its relative abundance in ovine fetal fluids.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
High-performance liquid chromatrography (HPLC)-grade water and methanol were obtained from Fisher Scientific (Fair Lawn, NJ). All other chemicals used for amino acid analysis were purchased from Sigma Chemicals (St. Louis, MO).
Ewes
Columbia cross-bred ewes were mated to Suffolk rams when detected as being in estrus (Day 0) and at 12 and 24 h later. Ewes were then assigned randomly to be hysterectomized (n = 4 per day) on either Day 30, 40, 60, 80, 100, 120, or 140 of gestation to allow collection of fetal fluids as well as of maternal and fetal blood. These gestational ages were selected to include early, mid-, and late gestation in sheep (term, 147 days). In preliminary studies, we determined that concentrations of amino acids in fetal plasma, amniotic fluid, or allantoic fluid were similar between twin fetal lambs from the same ewes. Thus, samples were obtained from only one fetus per ewe for amino acid analysis when twin fetal lambs were present. None of the ewes in the study had more than twin fetuses. Throughout gestation, ewes had free access to water and were fed individually 1.4 kg/day of an alfalfa-based diet (consisting of 13.6% corn, 1.9% rice bran, 4.5% cottonseed meal, 3.0% cottonseed hull, 73.31% dehydrated alfalfa, 2.5% liquid binder, 0.4% soy oil, 0.5% ground limestone, 0.065% vitamin mixture, 0.15% salt mixture, and 0.075% mineral oil) that met the recommended National Research Council (NRC) requirements [23]. The diet contained the following nutrients: 90.9% dry matter, 58.7% total digestible nutrients, 15.8% crude protein, 3.7% fat, 27.0% acid detergent fiber, 35.0% neutral detergent fiber, 1.23% calcium, 0.441% chloride, 0.285% magnesium, 0.306% phosphorus, 2.01% potassium, 0.135% sodium, 0.221% sulfur, 0.385 ppm of cobalt, 7.67 ppm of copper, 0.258 ppm of iodine, 335 ppm of iron, 38.4 ppm of manganese, 0.555 ppm of selenium, 23.7 ppm of zinc, 147 554 IU/kg of vitamin A, 316 IU/kg of vitamin D, 100 IU/kg of vitamin E, and 6.45 mg/kg of vitamin K. Ewes consumed all of the feed provided daily. This study was approved by the Texas A&M University Institutional Agricultural Animal Care and Use Committee.
Hysterectomy and Sample Collection
Hysterectomies were performed between 0800 and 0900 h at 24 h after the last feeding. All ewes were administered isofluorane (5%) to induce anesthesia, and anesthesia was maintained with isofluorane (1%–5%). A midventral laparotomy was then performed to expose the reproductive tract. Maternal uterine arterial blood (3 ml) and fetal umbilical venous blood (1 ml) were collected into heparinized tubes. Blood could not be obtained from the fetal umbilical vein on Day 30 of gestation because of the small size of the vessels. Amniotic and allantoic fluids were obtained through the amniochorion and chorioallantoic membranes, respectively. Blood was centrifuged at 3000 x g at 4°C for 10 min to obtain plasma. Plasma, amniotic, and allantoic samples (0.5 ml) were deproteinized with 0.5 ml of 1.5 M HClO4 and neutralized with 0.25 ml of 2 M K2CO3. Recovery of free amino acids from blood and fetal fluids was determined by adding known amounts of amino acid standards as previously described [17] and was found to be greater than 95% for all amino acids.
Analysis of Amino Acids
Amino acids, except for proline, were determined by fluorometric HPLC methods involving precolumn derivatization with o-phthaldialdehyde as previously described [24]. The values for total cysteine in plasma and fluids represent free cysteine plus 
cystine. Proline was measured using a fluorometric HPLC method involving precolumn derivatization with 9-fluorenylmethyl chloroformate [24]. Amino acids in samples were quantified on the basis of authentic standards (Sigma) using Millenium-32 Software (Waters, Milford, MA).
Calculations and Statistical Analysis
Amino acid concentrations in plasma, amniotic fluid, and allantoic fluid were calculated on the basis of the recovery rates of amino acids from ovine plasma, amniotic fluid, and allantoic fluid, respectively. For each gestational age, total content of amino acids in amniotic and allantoic fluids was calculated by multiplying amino acid concentrations by amniotic and allantoic fluid volumes, respectively. Concentrations of total -amino acids (all measured amino acids except for ß-alanine and taurine) were a mathematical sum of the individual -amino acids. Data were subjected to least-squares analyses of variance using the PROC GLM, PROC REG, and PROC CORR procedures of SAS [25]. Differences between means were determined by the Student-Newman-Keuls multiple-comparison test [25]. Statistical significance was set at a probability value of 0.05 or less.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Data regarding fetal weights as well as allantoic and amniotic fluid volumes are summarized in Table 1. Fetal weights increased with gestational age (P < 0.01, quadratic). The absolute growth rate of the fetal lamb was low (2.0 g/day) from Day 30 to Day 60 of gestation but increased rapidly (50.8 g/day) from Day 60 to Day 140. Weight gain from Day 120 to Day 140 of gestation was similar to that during the first 4 mo of gestation, indicating increased requirements for amino acids for protein deposition during late gestation. The volume of amniotic fluid increased progressively from Day 30 to Day 100 of gestation and then declined (P < 0.01, quadratic). The volume of allantoic fluid increased (P < 0.01) progressively from Day 40 to Day 120 of gestation and did not differ (P > 0.05) between Days 120 and 140 of gestation. Interestingly, the increase in allantoic fluid volume was closely correlated with the increase in fetal weight between Days 40 and 120 of gestation (r = 0.89, P < 0.01).
|
Amino Acids in Maternal Uterine Arterial and Fetal Umbilical Venous Plasma
Changes (P < 0.01) were observed in the concentrations of all amino acids, except for proline and tyrosine, in maternal plasma during gestation (Table 2). Glycine was the most abundant amino acid in maternal plasma at all gestational ages (P < 0.01), accounting for approximately 25% of total -amino acids. Alanine, glutamine, glycine, and serine were the most abundant amino acids in fetal umbilical venous plasma, which together contributed approximately 50% of total -amino acids (Table 3). Marked changes (P < 0.01) were observed in the concentrations of all amino acids, except for alanine, aspartate, isoleucine, leucine, phenylalanine, tryptophan, and valine, in fetal umbilical venous plasma between Days 40 and 140 of gestation.
|
|
Fetal:maternal plasma ratios for amino acids varied greatly, being lowest for glutamate (e.g., 0.3 on Day 100 of gestation), between 1.5 and 3 for most amino acids throughout gestation, and greatest for serine (e.g., 15 on Day 120 of gestation) (Table 4). Interestingly, fetal:maternal plasma ratios for asparagine, isoleucine, leucine, and valine did not differ (P > 0.05) during gestation. In contrast, changes (P < 0.01) were observed in fetal:maternal plasma ratios for all other amino acids measured between Days 40 and 140 of gestation (Table 4).
|
Amino Acids in Amniotic Fluid
Concentrations of amino acids in amniotic fluid are summarized in Table 5. As in fetal plasma, alanine, glutamine, glycine, and serine were the most abundant -amino acids at all gestational ages and contributed approximately 50% of total -amino acids. Marked changes were observed in the concentrations of all amino acids in amniotic fluid during pregnancy. Between Days 30 and 60 of gestation, concentrations of the following amino acids decreased (P < 0.01): glutamine, glycine, histidine, lysine, methionine, serine, and threonine. In contrast, concentrations of the following amino acids increased (P < 0.01) from Day 30 to Day 60 of gestation: arginine, aspartate, citrulline, cysteine, glutamate, leucine, ornithine, proline, taurine, tryptophan, and tyrosine. Except for ß-alanine, aspartate, citrulline, cysteine, ornithine, serine, and taurine, the lowest concentrations of all amino acids were observed on Days 80–100 of gestation. Between Days 120 and 140 of gestation, concentrations of proline and threonine decreased (P < 0.01), whereas concentrations of the following amino acids increased (P < 0.01): ß-alanine, arginine, citrulline, glutamine, glutamate, glycine, histidine, isoleucine, lysine, ornithine, and tryptophan.
|
Marked changes were observed in the total content of individual amino acids in amniotic fluid during gestation (Table 6). Between Days 30 and 60 of gestation, the total content of -amino acids increased (P < 0.01) by approximately 80-fold. Between Days 60 and 80 of gestation, the total content of ß-alanine, cysteine, and serine did not differ (P > 0.05); the total content of aspartate, ornithine, phenylalanine, and taurine increased (P < 0.01); and the total content of other amino acids decreased (P < 0.01). Except for ornithine and taurine, the total content of all amino acids in amniotic fluid increased (P < 0.01) from Day 80 to Day 140 of gestation. The marked decrease in concentrations of total -amino acids on Day 80 of gestation was inversely related to the increase in amniotic fluid volume (Table 1). However, the increases in concentrations of total -amino acids in amniotic fluid during late gestation could not be accounted for by changes in amniotic fluid volume (Table 1).
|
Amino Acids in Allantoic Fluid
Concentrations of amino acids in allantoic fluid are summarized in Table 7. Remarkable changes (P < 0.01) were observed in the concentrations of all amino acids in allantoic fluid during gestation. Concentrations of alanine, citrulline, and glutamine in allantoic fluid increased (P < 0.01) by 20-, 34-, and 18-fold, respectively, from Day 30 to Day 60 of gestation and were approximately 80-, 30-, and 60-fold, respectively, those in ovine fetal plasma on Day 60 of gestation. Alanine, citrulline, plus glutamine accounted for approximately 80% of total -amino acids in allantoic fluid during early pregnancy. Serine was the most abundant amino acid in allantoic fluid on Days 100 and 140 of gestation (P < 0.01), contributing 45%–62% of total -amino acids. Concentrations of serine in allantoic fluid during late gestation were approximately 30-fold those in ovine fetal plasma (Table 3). The increase in concentrations of allantoic fluid serine was closely correlated with the increase in fetal weight (r = 0.69, P < 0.01).
|
Arginine was not among the most abundant amino acids in ovine allantoic fluid during early gestation (Table 7). However, arginine and citrulline were the second and third most abundant -amino acids, respectively, in allantoic fluid on Day 140 of gestation (P < 0.01) and contributed approximately 10% of total -amino acids (Table 7). Between Days 100 and 140 of gestation, allantoic fluid was particularly rich in two ß-amino acids: ß-Alanine (1.5–4.2 mM), and taurine (2.5–4.4 mM) (Table 7).
Except for arginine, cysteine, isoleucine, leucine, lysine, methionine, ornithine, phenylalanine, proline, tryptophan, tyrosine, and valine on Day 60 of gestation and for cysteine, methionine, and phenylalanine on Day 40 of gestation, the total content of amino acids was much greater (P < 0.01) in allantoic fluid than in amniotic fluid at all gestational ages. As in amniotic fluid, marked changes were observed in the total content of individual amino acids in allantoic fluid during gestation (Table 8). Between Days 30 and 60 of gestation, the total content of amino acids, except for phenylalanine, tyrosine, and valine, increased markedly (P < 0.01). Between Days 60 and 80 of gestation, the total content of arginine, cysteine, leucine, lysine, phenylalanine, and proline increased (P < 0.01), whereas the total content of alanine, ß-alanine, asparagine, citrulline, glutamine, glutamate, histidine, taurine, and tyrosine decreased (P < 0.01). Except for leucine, the total content of all amino acids in allantoic fluid increased (P < 0.01) between Days 80 and 100 of gestation. Indeed, highest values were observed for alanine, glutamine, glycine, histidine, isoleucine, methionine, phenylalanine, threonine, tyrosine, and valine on Day 100 of gestation. Interestingly, total -amino acid content in ovine allantoic fluid was fairly constant between Days 100 and 140 of gestation.
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
To our knowledge, this is also the first report of ovine fetal:maternal plasma ratios for amino acids throughout the entirety of gestation. Fetal:maternal plasma ratios for glutamate and serine were remarkably low and high, respectively, during late gestation (Table 4). In vivo studies have demonstrated that little uterine uptake of glutamate occurs from maternal plasma but that placental uptake of glutamate (derived largely from glutamine hydrolysis in fetal liver) occurs in the ovine fetus [29]. Thus, extensive placental catabolism of glutamate [31] and a high rate of fetal utilization of glutamate (a major amino acid in tissue protein) [30] likely are the major factors responsible for its low concentrations in fetal plasma compared with maternal plasma. In contrast, uterine uptake of serine from maternal plasma occurs, but little transplacental transport of serine to the ovine fetus takes place because of its catabolism by uteroplacental tissues [31, 32]. Thus, large amounts of serine are synthesized from glycine and N5,N10-methylenetetrahydrofolate via serine hydroxymethyltransferase and from 3-phosphoglycerate (an intermediate of glycolysis) and glutamate via phosphoglycerate dehydrogenase and phosphoserine aminotransferase [29, 32]. All these enzymes are present in fetal ovine liver and kidney, with the liver being the major organ for serine synthesis in fetal sheep [34, 35].
Amniotic fluid is composed of water and electrolytes from both the fetus (kidneys, lungs, epidermis, and fetal blood vessels in the placenta and umbilical cord) and the mother (decidual blood vessels via amniotic membranes) [13]. This fluid is removed by both the fetus and the mother through the same channels, along with the participation of the fetal intestine after swallowing [13]. Thus, with the development of intestinal amino acid transport systems during gestation [36], the drinking of amniotic fluid provides a source of amino acids for fetal utilization. The nutritional significance of amniotic fluid is graphically illustrated by the finding that esophageal ligation, which prevents entry of this fluid into the small intestine, results in intrauterine growth retardation in fetal sheep [37]. Glutamine, a major fuel for enterocytes [1] and an abundant amino acid in amniotic fluid (Table 5), may be an important nutrient in this fluid that stimulates intestinal growth and development.
Allantoic fluid is derived from fetal and maternal secretions but primarily from placental transport mechanisms [15]. Although early anatomical studies suggested that the allantoic sac served as a reservoir for fetal wastes, it is now clear that allantoic fluid nutrients may be absorbed by the allantoic epithelium into the fetal-placental circulation and utilized by fetal-placental tissues [15]. The increases in concentrations of total amino acids in allantoic fluids during early pregnancy could not be accounted for by changes in allantoic fluid volume (Table 1) or by changes in concentrations of amino acids in maternal and fetal plasma (Tables 2 and 3). Interestingly, between Days 30 and 60 of gestation, Na+ and Cl- concentrations in ovine allantoic fluid decrease by 40 and 39 mEq/L, respectively, with a combined decrease of 79 mEq/L [15]. Such a decrease in electrolyte concentrations is closely matched by an increase of 73 mmol/L amino acids in allantoic fluid (Table 7), suggesting an important role for amino acids as regulators of osmolality in allantoic fluid.
Ovine allantoic fluid was particularly rich in four of the traditionally classified nonessential amino acids: alanine, citrulline, glutamine, and serine (Table 7). In contrast to alanine, glutamine, and serine, citrulline (a nonprotein amino acid) is not a building block for tissue protein synthesis and is virtually absent from the diet. Citrulline was the third most abundant amino acid in ovine allantoic fluid on Day 60 of gestation (Table 7). Although glutamine concentrations may be as high as 20 mM in human skeletal muscle [33], the unusual abundance of alanine and citrulline in ovine allantoic fluid on Day 60 of gestation and of serine during late gestation has not been reported for any other biological fluid in animals (e.g., [1, 2, 11, 16, 19, 29, 33]). For comparison, concentrations of alanine, glutamine, citrulline, and serine in porcine allantoic fluid were 0.28, 3.4, 0.07, and 0.90 mM, respectively, on Day 40 of gestation [16] and 0.67, 0.70, 0.03, and 1.2 mM, respectively, on Day 110 of gestation [18].
Our results raise important questions regarding the origin and function of alanine, glutamine, citrulline, and serine in ovine fetal-placental nutrition and metabolism. Both glutamine and alanine may be synthesized from branched-chain amino acids in fetal ovine skeletal muscle [38]. On the basis of maternal arterial-fetal umbilical venous differences in amino acid concentrations (Tables 2 and 3), we suggest that the ovine placenta synthesizes citrulline. Indeed, our in vitro studies demonstrated the formation of citrulline from glutamine in incubated ovine placenta (data not shown) as previously reported for the porcine small intestine [39]. As noted above, serine is synthesized from glycine and from 3-phosphoglycerate (an intermediate of glycolysis) and glutamate (a metabolite of glutamine) in the liver and kidneys of fetal sheep [34, 35]. Alanine, glutamine, and serine are major glucogenic precursors in humans [1] and ewes [40]. Serine also plays an important role in one-carbon unit metabolism essential for 2'-deoxythymidylate synthesis and methylation [11]. Glutamine is a major fuel for the fetus [6] and is essential for the synthesis of nucleotides, NAD(P)+, and aminosugars (glucosamine-6-phosphate, UDP-N-acetylgalactosamine, and UDP-N-acetylglucosamine, a precursor for the formation of all macromolecules containing amino sugars) [1, 4]. In addition, serine participates in the synthesis of phosphatidylserine and ceramide (signaling molecules) [1]. All of these events are critical for DNA synthesis and, thus, cell proliferation.
In contrast to high concentrations of arginine (4–6 mM) in porcine allantoic fluid during early gestation [16–18], arginine was not a major amino acid in ovine allantoic fluid during early gestation (Table 7). Concentrations of arginine in ovine allantoic fluid on Days 60, 100, and 140 of gestation were only 16%, 11%, and 8.5%, respectively, of those of serine (Table 7). Thus, our findings do not support the hypothesis that arginine is the most abundant amino acid in ovine allantoic fluid. Why is citrulline, but not arginine, particularly abundant in ovine allantoic fluid during early gestation? Several answers to this intriguing question are possible. First, arginase activity is present in ovine allantoic fluid on Days 30 and 60 of gestation but is not detectable in porcine allantoic fluid on Days 30 to 60 of gestation (unpublished data). The presence of arginase in ovine allantoic fluid would hydrolyze arginine and reduce its availability to the fetus. Because citrulline is an effective precursor for arginine synthesis in all animals because of the widespread presence of argininosuccinate synthase and lyase in animal tissues [2], high concentrations of citrulline in ovine allantoic fluid would serve as an efficient reservoir of precursor for arginine in the fetus. Second, citrulline is a neutral amino acid. Thus, unlike arginine (a basic amino acid), citrulline does not disturb the acid-base balance in ovine allantoic fluid, even at high concentrations. Third, citrulline is an efficient antioxidant, protecting DNA, lipids, and proteins from hydroxyl radical-induced oxidative damage [41]. This effect of citrulline may contribute to a protective environment for fetal growth and development in sheep.
Collectively, our findings indicate that different strategies are used by different animal species to conserve arginine, the most abundant nitrogen carrier in tissue proteins [30, 42]. Whatever the differences among species, the unusual abundance of either citrulline (an effective precursor of arginine) in ovine allantoic fluid (Table 7) or arginine in porcine allantoic fluid [16–18] during early gestation raises intriguing and important questions regarding the physiological significance of arginine-dependent pathways in fetal-placental nutrition and development. In this regard, it is noteworthy that maternal undernutrition decreases arginine concentrations in porcine fetal plasma and allantoic fluid [17], impairs fetal growth [17, 27], and may also program permanent structural, metabolic, and functional alterations [43, 44]. We recently determined that maternal undernutrition in sheep (50% of NRC nutrient requirements) from Day 28 to Day 78 of gestation decreased concentrations of citrulline, serine, glutamine, and alanine in fetal plasma and allantoic fluid by 35%–45% and retarded fetal growth by 32% on Day 78 of gestation (unpublished data). Because recent epidemiological studies in humans suggest links between intrauterine growth retardation and development of chronic disease (e.g., diabetes, hypertension, and coronary heart disease) later in life [43, 44], our novel results may have important implications for both intrauterine growth retardation and fetal origins of diseases in adults.
In conclusion, remarkable changes occur in the concentrations of amino acids in ovine fetal allantoic fluid between Days 30 and 140 of gestation. In this fluid, alanine, citrulline, plus glutamine contributed approximately 80% of total -amino acids during early gestation, and serine accounted for approximately 60% of total -amino acids during late gestation. These novel findings raise important questions regarding placental and fetal metabolism of the traditionally classified nonessential amino acids, and provide a new database for further studies to define their roles in ovine conceptus development.
|
ACKNOWLEDGMENTS |
|---|
|
FOOTNOTES |
|---|
2 Correspondence: Guoyao Wu, Department of Animal Science, Texas A&M University, 2471 TAMU, College Station, TX 77843-2471. FAX: 979 845-6057; g-wu{at}tamu.edu
Received: 7 November 2002.
First decision: 27 November 2002.
Accepted: 16 December 2002.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  A. Lassala, F. W. Bazer, T. A. Cudd, P. Li, X. Li, M. C. Satterfield, T. E. Spencer, and G. Wu Intravenous Administration of L-Citrulline to Pregnant Ewes Is More Effective Than L-Arginine for Increasing Arginine Availability in the Fetus J. Nutr., April 1, 2009; 139(4): 660 - 665. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. Gao, G. Wu, T. E. Spencer, G. A. Johnson, and F. W. Bazer Select Nutrients in the Ovine Uterine Lumen. III. Cationic Amino Acid Transporters in the Ovine Uterus and Peri-Implantation Conceptuses Biol Reprod, March 1, 2009; 80(3): 602 - 609. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. Gao, G. Wu, T. E. Spencer, G. A. Johnson, X. Li, and F. W. Bazer Select Nutrients in the Ovine Uterine Lumen. I. Amino Acids, Glucose, and Ions in Uterine Lumenal Flushings of Cyclic and Pregnant Ewes Biol Reprod, January 1, 2009; 80(1): 86 - 93. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
X. Zeng, F. Wang, X. Fan, W. Yang, B. Zhou, P. Li, Y. Yin, G. Wu, and J. Wang Dietary Arginine Supplementation during Early Pregnancy Enhances Embryonic Survival in Rats J. Nutr., August 1, 2008; 138(8): 1421 - 1425. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 W. S. Jobgen, S. P. Ford, S. C. Jobgen, C. P. Feng, B. W. Hess, P. W. Nathanielsz, P. Li, and G. Wu Baggs ewes adapt to maternal undernutrition and maintain conceptus growth by maintaining fetal plasma concentrations of amino acids J Anim Sci, April 1, 2008; 86(4): 820 - 826. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. E Spencer, O. Sandra, and E. Wolf Genes involved in conceptus-endometrial interactions in ruminants: insights from reductionism and thoughts on holistic approaches Reproduction, February 1, 2008; 135(2): 165 - 179. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. Wu, F. W. Bazer, J. M. Wallace, and T. E. Spencer BOARD-INVITED REVIEW: Intrauterine growth retardation: Implications for the animal sciences J Anim Sci, September 1, 2006; 84(9): 2316 - 2337. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. Song, F. W. Bazer, G. F. Wagner, and T. E. Spencer Stanniocalcin (STC) in the Endometrial Glands of the Ovine Uterus: Regulation by Progesterone and Placental Hormones Biol Reprod, May 1, 2006; 74(5): 913 - 922. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. R. Miles, C. E. Farin, K. F. Rodriguez, J. E. Alexander, and P. W. Farin Effects of Embryo Culture on Angiogenesis and Morphometry of Bovine Placentas During Early Gestation Biol Reprod, October 1, 2005; 73(4): 663 - 671. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. Wu, F. W. Bazer, T. A. Cudd, C. J. Meininger, and T. E. Spencer Maternal Nutrition and Fetal Development J. Nutr., September 1, 2004; 134(9): 2169 - 2172. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. Kwon, S. P. Ford, F. W. Bazer, T. E. Spencer, P. W. Nathanielsz, M. J. Nijland, B. W. Hess, and G. Wu Maternal Nutrient Restriction Reduces Concentrations of Amino Acids and Polyamines in Ovine Maternal and Fetal Plasma and Fetal Fluids Biol Reprod, September 1, 2004; 71(3): 901 - 908. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. T. Self, T. E. Spencer, G. A. Johnson, J. Hu, F. W. Bazer, and G. Wu Glutamine Synthesis in the Developing Porcine Placenta Biol Reprod, May 1, 2004; 70(5): 1444 - 1451. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. Kwon, G. Wu, C. J. Meininger, F. W. Bazer, and T. E. Spencer Developmental Changes in Nitric Oxide Synthesis in the Ovine Placenta Biol Reprod, March 1, 2004; 70(3): 679 - 686. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. Kwon, G. Wu, F. W. Bazer, and T. E. Spencer Developmental Changes in Polyamine Levels and Synthesis in the Ovine Conceptus Biol Reprod, November 1, 2003; 69(5): 1626 - 1634. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
