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The Journal of Nutrition Vol. 128 No. 8 August 1998,
pp. 1302-1310
, and
Department of Biological Sciences, Mississippi State University, Mississippi State, MS 39762-5759; * Membrane Transport Research Group, Department of Physiology, Université de Montréal, Succursale Centre-Ville, Montréal, QC Canada, H3C 3J7; and College of Veterinary Medicine, Mississippi State University, Mississippi State, MS 39762-9825
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ABSTRACT |
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Abstract
Introduction
Methods
Results
Discussion
References
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Structural and functional responses of the intestine to colostrum, milk replacer, oral electrolyte solution and food deprivation were examined during the first 6 h after birth in pigs. Total intestinal weight, surface area and mucosal mass were highest (P < 0.05) in pigs fed colostrum. The other diet groups did not differ, except that food-deprived pigs had lower surface area than the other groups. Feeding colostrum was associated with higher mucosal protein content (P < 0.05). Total intestinal brush border membrane protein content of pigs fed milk replacer, oral electrolyte solution and food-deprived pigs were 61, 44 and 56%, respectively, of those fed colostrum (P < 0.05). Pigs fed colostrum had higher total mucosal maltase activities than those that were food deprived, and total brush border membrane activities were higher than in those fed oral electrolyte solution. Total intestinal brush border membrane aminooligopeptidase activity was higher in pigs fed colostrum than in those given oral electrolyte solution or deprived of food, but total intestinal homogenate activities did not differ among groups. Diet influenced lactase activity only in the mid-region, and sucrase was not responsive to diet. Intestinal glucose transport capacity by intact intestinal tissues did not differ among diet groups. The ability of brush border membrane vesicles to actively accumulate glucose was lost when pigs were fed colostrum and milk replacer, but not when fed oral electrolyte solution or deprived of food. Our findings reveal how diet during the first 6 h after birth influences the structure and functional characteristics of the intestine. The responses vary between brush border membrane proteins and intestinal regions, and appear to differ from those described for older animals.
KEY WORDS: diets · neonatal · pigs · nutrient transport · brush border membrane hydrolases
The influences of lumenal contents on intestinal characteristics begin before birth when fetuses start to swallow amniotic fluid; these influences extend into adulthood. The most dramatic examples occur during two relatively brief periods that include shifts in the qualitative and quantitative characteristics of dietary inputs and changes in intestinal structure and functions. The first is at birth when neonates start to process nutrient-rich milk and is of particular interest because of the common problems associated with neonatal nutrition. The second and best-known period is at weaning.
In the pig, which is considered a good model for human infants (Moughan et al. 1992 Animals and feeding protocol.
A total of 28 crossbred standard farm pigs of both sexes were obtained immediately after birth and before suckling from a nearby commercial producer (Prestage Farms, Deerbrook, MS). They were randomly assigned to the four diet treatments: C (n = 8), MR (n = 8), OES (n = 8) and FD (n = 4). We originally obtained one set of 16 pigs (four sets of four siblings) and these were randomly assigned to the four diets (n = 4 per diet). Analysis of the data from the first set of 16 pigs revealed several trends that were suggestive of dietary effects, but that were not significant (0.05 < P < 0.10). We therefore obtained another set of 12 pigs (four sets of three siblings), and these were randomly distributed to C, MR, and OES (n = 4 each). The FD group was not repeated. The second set of pigs was studied 6 mo after the first.
Sampling and collection of tissues.
We followed a protocol previously used to study pig intestinal development (Zhang et al. 1997 Measurements of intestinal dimensions.
The 10-cm segments from each region were first slit along their lengths. After removing digesta and adherent water, the wet weight was recorded. The circumference was measured for calculation of nominal surface area (without accounting for area amplification by villi and microvilli). The amount of mucosa was determined on a dry weight basis by gently scraping each segment with a glass slide and drying the mucosa and underlying tissues to a constant weight (48 h, 45-50°C). The amount of mucosa was expressed as a percentage of tissue dry weight. The values for weight and surface area per centimeter, and mucosal percentages were used to calculate regional weights, surface areas and mucosal mass. These were summed to calculate values for the entire intestine.
Preparation of BBMV.
We used two methods to prepare BBMV from the frozen mucosa. BBMV for assaying hydrolase activity were prepared by a Ca-based approach (Schmitz et al. 1973 Assays of protein content and hydrolase activities of homogenates and BBMV.
Lactase (EC 3.2.1.23), sucrase (EC 3.2.1.48) and maltase (EC 3.2.1.20) activities were determined by the method of Dahlqvist (1964) Measurements of DNA content.
A microfluorometric method using the 33258 Hoechst fluorochrome was used to measure DNA content of mucosal homogenates (Cesarone et al. 1979 Measurements of BBMV aldohexose uptake.
Initial rates of uptake were measured at 25°C by using a fast sampling, rapid filtration apparatus with nine samples collected over 4.5 s. Our previous studies with tissues from fetal and neonatal pigs showed that this range of time is appropriate for defining initial rates of hexose uptake by BBMV (Buddington and Malo 1996 Na+ cotransport of glucose and galactose by intact tissues.
BBMV are generally more appropriate for studying kinetics of nutrient uptake because of reduced unstirred layer influences. However, in our previous study (Zhang et al. 1997 Chemicals.
All salts and chemicals used to prepare solutions were of the highest purity available. Radioisotopes for BBMV studies were purchased from New England Nuclear (Mississauga, Canada), and those for measuring intact tissue uptakes were obtained from American Radiolabelled Chemicals (St. Louis, MO) and New England Nuclear (Boston, MA).
Statistics.
The experimental design for the first set of pigs was a randomized complete block design with litter membership as the blocking variable. The experimental design for the second set of pigs was a completely randomized design. An ANOVA combining the two designs was constructed as discussed by Cochran and Cox (1957) Results are presented as averages for the two sets, unless the variances for the two sets were not similar. When this occurred, the sets were analyzed and presented separately.
Body weights and intestinal dimensions.
Initial and final body weights did not differ among groups (Table 1). Pigs fed milk replacer gained weight (P < 0.05), whereas the other groups did not have a significant change in weight.
Homogenate and BBM protein content.
Pigs fed colostrum had more mucosal protein compared with the other groups, with significantly higher values in the proximal and mid-, but not the distal regions of the small intestine (Table 2). In the distal intestine, MR pigs had more protein than the FD pigs but were not different from C and OES pigs. However, the distal intestine of C pigs had more mucosal protein than those of OES and FD pigs. Corresponding with this, when total mucosal protein was normalized to g mucosa, values for C pigs exceeded those of all other groups (P < 0.001), and MR pigs had more protein per g mucosa than OES and FD pigs (P < 0.005; from Tables 1 and 2).
Mucosal DNA content.
There was no effect of diet on total mucosal DNA content (Table 2), and no significant diet effect was detected for the three regions of the small intestine.
Hydrolase activities.
On the basis of homogenates, diet did not have a significant effect on total intestinal activity for lactase (Fig. 1), AOP (see Fig. 3) sucrase (data not presented). A significant diet effect was seen for maltase with C pigs having higher total homogenate maltase activity than FD pigs (Fig. 2) and tending to have higher maltase activity than MR (P = 0.0501) and OES (P = 0.070) pigs. Total intestinal BBMV lactase activity was not responsive to diet, but a significant diet effect was detected for maltase and AOP. For both enzymes, total BBMV activities for C pigs were higher than those for OES pigs, but were not different than MR pigs. C pigs had higher total BBMV activity than FD for AOP, but not for maltase.
Aldohexose uptake by intact tissues and BBMV.
Rates of glucose uptake per milligram intact tissue at the saturating concentration of 50 mmol/L did not differ among groups in any region (Fig. 4). There were also no differences among groups for maximal rates of glucose and galactose uptake (Vmax; Table 3) and associated affinity constants (Km). Total intestinal transport capacities for glucose were not significantly affected by diet, although the difference between C and MR pigs approached significance (P = 0.0517). Rates of uptake and regional uptake capacities declined from proximal to distal (P = 0.0001).
Even though the intestines of neonatal pigs have sufficient capacities to meet requirements for energy and nutrients, the onset of feeding initiates rapid changes that lead to even higher capacities for some functions (Buddington 1992 Influence of diet on body mass and intestinal growth
Responses of intestinal brush border membrane functions to diet
Brush border membrane hydrolases.
Enzyme activities normalized to protein content (specific activity) are often used to describe age- and diet-related influences. However, data from this and previous studies (Zhang et al. 1997 Glucose cotransport.
The higher total glucose transport capacities of C pigs indicate that colostrum stimulates absorptive processes, with more pronounced responses in the proximal small intestine. Because our uptake values are corrected for passive influx, the higher values for C pigs can be explained by the insertion of more transporters, an increase in the turnover rate of existing carriers or a shift in the relative proportions of different transporters. The last-mentioned does not appear likely because kinetic characteristics for proximal intestine did not differ among groups. Although transporter site density was not quantified in this study, an increase would be consistent with previous reports of a relationship between rates of uptake and carrier densities (Ferraris et al. 1993 Perspectives
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
), the onset of suckling causes marked increases of intestinal dimensions (Widdowson et al. 1976) and tissue architecture (Xu et al. 1992). These structural changes are accompanied by increased synthesis of some brush border membrane (BBM)4 functional proteins (e.g., lactase; Burrin et al. 1994). However, the relationships between intestinal growth and functional capacities are not understood. Therefore, this study uses a multidisciplinary approach to examine the effect of dietary inputs on the structural and functional development of the neonatal pig intestine. We examined intestinal structure and functions 6 h after birth and onset of feeding because colostrum induces significant changes within this period of time (Burrin et al. 1992, Zhang et al. 1997). We used four diet treatments, which included colostrum (C), a sow's milk replacer (MR), an oral electrolyte solution (OES) and food deprivation (FD). These diet groups were selected to distinguish between the respective effects of the following: 1) nutrients, 2) biologically active components present in sow's milk and colostrum, 3) physical presence of exogenous fluid in the gut and 4) responses that are independent of ingestion and might be related to birth itself. The last-mentioned include the release of glucocorticoids induced by the stress of birth and which are potent mediators of intestinal maturation in pigs (Sangild et al. 1993) and other species (rats and mice, Henning 1987). By determining responses in three different regions of the intestine we were able to calculate hydrolytic and transport capacities of the entire length of small intestine and to search for regional influences of diet. Finally, we examined whether synthesis and/or insertion of constituent membrane enzymes is influenced by dietary inputs by assaying hydrolase activities in mucosal homogenates and brush border membrane vesicles (BBMV).
MATERIALS AND METHODS
Abstract
Introduction
Methods
Results
Discussion
References
). Pooled colostrum from several sows was stored at 
70°C until fed, at which time it was thawed, shaken and warmed. The milk replacer (SPF Lac, Pet Ag, Hampshire, IL) has a reported composition of (g/L) crude protein (40), crude fat (50), crude fiber (0) and ash (10); carbohydrate, not reported. The oral electrolyte solution (Ricelyte, Mead Johnson; Evansville, IN) contains 30 g/L rice syrup solids with (mEq/L) sodium (50), potassium (25), chloride (45) and citrate (34).
). Briefly, after 6 h of feeding or food deprivation pigs were killed (Beuthanasia; 1 mL/kg body weight; intravenously); the small intestine from the pyloric sphincter to the ileocolonic junction was rapidly removed and placed in cold (2-4°C), aerated (95% O2-5% CO2) mammalian Ringers. The associated mesenteries were severed, allowing the intestine to be straightened along a table top and its length measured in a relaxed state. The intestine was then divided into three segments of equal length, which were designated as proximal, mid- and distal regions. From the middle of each region, a 10-cm segment was removed for measurements of intestinal dimensions. An adjacent proximal segment of ~10-15 cm was used for measuring rates of Na+/D-glucose cotransport by intact tissues. Mucosa was removed from the remainder of each segment and stored at 70°C.
), which is reported to retain enzyme activity better than Mg-based methods (Ibrahim and Balasubramanian 1995). For transport studies, BBMV were prepared by MgCl2 precipitation (Hauser et al. 1980), because this reduces the activity of Ca-activated phospholipases, and suspended in 50 mmol/L Tris-HEPES buffer (pH 7.5) with 0.1 mmol/L MgSO4, 200 mmol/L KCl and 125 mmol/L mannitol. Aliquots (25 µL, with 10-40 mg protein/mL) were stored in liquid nitrogen until used for transport measurements (within 48 h of final preparation).
. Aminooligopeptidase (AOP, EC 3.4.11.2) activity was assayed by the method of Wojnarowska and Gray (1975) using 0.17 mmol/L leucyl-
-naphthylamide. Activities [µmol substrate hydrolyzed/min (IU)] were normalized to protein content (specific activity) determined by the Coomassie Blue method (Bio-Rad Laboratories, Hercules, CA) and a bovine serum albumin standard. When necessary because of high activity, BBMV were diluted in 20 mmol/L Tris-HEPES/100 mmol/L mannitol buffer (pH 7.5).
). Fluorescent readings were made at excitation and emission wavelengths of 360 and 450 nm, respectively, and compared with a standard prepared with calf thymus DNA.
). Final concentrations in the incubation medium were 50 mmol/L Tris-HEPES buffer (pH 7.5) with 0.1 mmol/L MgSO4, 192 mmol/L NaCl, 8 mmol/L KCl, 125 mmol/L D-mannitol with 4 µmol/L 3H-D-aldohexose (glucose or galactose). Accumulation of labeled aldohexose was quantified using liquid scintillation counting and expressed as pmol/s·mg protein).
), glucose accumulation by BBMV from pigs suckled for 6 h was only 24% of that at birth, with the decline continuing to 24 h. In contrast, rates of uptake by intact tissues did not decline appreciably; the only significant difference was lower uptake by the mid-intestine 24 h after birth. Therefore, we used intact tissues to understand the influences of diet on maximum rates of glucose transport in the three regions of the small intestine and to estimate the kinetics of aldohexose (glucose and galactose) uptake by the proximal region. The procedures used for preparing and incubating the tissues, and measuring uptake followed our previous protocols (Zhang et al. 1997). Maximum rates of glucose uptake in the three regions were measured using 50 mmol/L glucose, which is sufficiently high to saturate the carriers. Glucose transport capacities of the entire length of small intestine (µmol/min) were then calculated by multiplying the average for rates of uptake at 50 mmol/L by the three regions by total intestinal weight; these values represent maximum capacities. The relationships between galactose and glucose concentrations and rates of uptake were defined by exposing tissues from the proximal intestine to Ringers with 0.1, 1, 5, 10, 25 and 50 mmol of each aldohexose/L. All tissues were incubated for 2 min. Rates of uptake were normalized to tissue weight.
whenever the variation was sufficiently similar for the two sets of pigs. This condition was tested using the F statistic at the 0.30 level of significance (Sokal and Rohlf, 1981). When the homogeneity of variances assumption was rejected, data were analyzed separately for each set of pigs.
). The Least Significant Difference was used to separate significant main effect means. Unless otherwise noted, all analyses were performed at the 0.05 level of significance.
).
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
View this table:
Table 1.
Body weights and small intestinal lengths and surface areas of pigs fed colostrum (C), milk replacer (MR) or
oral electrolyte solution (OES) or food-deprived (FD) for the first 6 h after birth1
View this table:
Table 2.
Total and regional weight, mucosal and brush border membrane protein, and mucosal DNA content of the small intestines of pigs fed colostrum (C), milk replacer (MR) or oral electrolyte solution (OES) or food-deprived (FD) for the first 6 h after birth1
View larger version (27K):
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Fig 1.
Total intestinal lactase activity (panel A) and specific activity (panel B) for homogenates and brush border membrane vesicles from the proximal (P), mid- (M) and distal (D) small intestines of pigs fed colostrum (C), milk replacer (MR), oral electrolyte solution (OES) or food-deprived (FD) for the first 6 h after birth. Values represent means ± SEM, n = 8 for C, MR and OES, n = 4 for FD. Means within a region that do not share a letter are significantly different (P < 0.05). Total intestinal activity was calculated by summing the products of specific activity times protein content in each region.
View larger version (30K):
[in a new window]
Fig 3.
Total intestinal aminooligopeptidase activity (panel A) and specific activity (panel B) for homogenates and brush border membrane vesicles from the proximal (P), mid- (M) and distal (D) small intestines of pigs fed colostrum (C), milk replacer (MR), oral electrolyte solution (OES) or food-deprived (FD) for the first 6 h after birth. Values represent means ± SEM, n = 8 for C, MR and OES, n = 4 for FD. Means within a region that do not share a letter are significantly different (P < 0.05). Total intestinal activity was calculated by summing the products of specific activity times protein content in each region. P-value for total BBVM AOP activity for MR pigs compared with C pigs (superscript 1) is 0.06.
View larger version (26K):
[in a new window]
Fig 2.
Total intestinal maltase activity (panel A) and specific activity (panel B) for homogenates and brush border membrane vesicles from the proximal (P), mid- (M) and distal (D) small intestines of pigs fed colostrum (C), milk replacer (MR), oral electrolyte solution (OES) or food-deprived (FD) for the first 6 h after birth. Values represent means ± SEM, n = 8 for C, MR and OES, n = 4 for FD. Means within a region that do not share a letter are significantly different (P < 0.05). Total intestinal activity was calculated by summing the products of specific activity times protein content in each region. P-values for comparison of MR and OES pigs with C pigs (superscripts 1 and 2, respectively) are 0.0501 and 0.07.
) and humans (Malo and Berteloot 1987), and in older life stages of pigs (unpublished data) and other species (Galand and Forstner 1974). Similarly, enrichments for AOP were low, ranging from 2.8 for C pigs to 0.9 for FD pigs.
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[in a new window]
Fig 4.
Total intestinal glucose cotransport capacity (panel A) and regional rates (panel B) measured at 50 mmol/L glucose using intact tissues from the proximal, mid- and distal small intestines of pigs fed colostrum (C), milk replacer (MR), oral electrolyte solution (OES) or food-deprived (FD) for the first 6 h after birth. Values represent means ± SEM, n = 8 for C, MR and OES, n = 4 for FD. Total transport capacities were calculated by summing the products of rates of uptake times regional weight.
View this table:
Table 3.
Maximum rates of glucose and galactose transport (Vmax) and affinity constants (Km) for intact tissues from the proximal intestines of pigs fed colostrum, milk replacer or oral electrolyte solution or food-deprived for the first 6 h after birth
), we reported a similar inability of BBMV prepared from pigs suckled for 6 h to accumulate glucose. The same BBMV, however, retained the ability to transport amino acids. In contrast to C and MR pigs, glucose accumulation was evident for all BBMV preparations from OES and FD pigs, with strong proximal to distal gradients for both groups and higher uptakes in the proximal region for FD compared with OES pigs (P < 0.05). Galactose uptake was minimal by BBMV from C and MR pigs. Although BBMV prepared from OES and FD pigs retained the ability to transport galactose, rates of accumulation were lower than those for glucose and a proximal to distal gradient was not evident.
View this table:
Table 4.
Rates of D-glucose and galactose uptake by brush border membrane vesicles from the small intestines of pigs fed colostrum (C), milk replacer (MR) or oral electrolyte solution (OES) or food-deprived (FD) for the first 6 h after birth1
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
), whereas others decline, notably receptor-mediated endocytosis (Westrom et al. 1989). What has remained uncertain is how diet might influence the relations between structural changes and functional abilities. Our previous study (Zhang et al. 1997) revealed that 6 h of suckling is sufficient to cause significant changes in intestinal structure and functions. The present results show that some of the changes during this period are responsive to the composition of dietary inputs.
). The greater nominal surface area of C pigs is actually an underestimate of the actual difference due to the area amplification caused by the expansion of villi and microvilli associated with ingestion of colostrum (Xu et al. 1992). Although rapid postnatal intestinal growth occurs in dogs (Schwartz and Heird 1994) and several other mammals (Widdowson 1985), this response is not universal. For example, intestinal dimensions of cats do not change during the first week after birth (Buddington and Diamond 1992, Buddington and Lepine 1997).
, Zhang et al. 1997) are affected by diet composition (this study). The higher values for the distal intestine of MR pigs relative to pigs that were food deprived correspond with the ability of enterocytes in the distal intestine of neonatal pigs to nonselectively absorb proteins and other macromolecules (Ekstrom and Westrom 1991). The even higher values for C pigs may be the result of receptor-mediated endocytosis of the immunoglobulins present in colostrum, but not in the milk replacer.
) and higher concentrations of protein relative to mature milk and milk replacer (Widdowson 1985). The protein composition of colostrum and milk replacer could also play an important role in the observed differences between C and MR pigs. The most abundant colostral proteins, immunoglobulin G and -lactoglobulin, have a shorter gastric retention time than other proteins and are poorly digested during the neonatal period (Kiriyama 1992, Yvon et al. 1993).
, Simmen et al. 1990) and DNA (Xu et al. 1992, Zhang et al. 1997). Although unknown proportions of values measured at 6 h in all groups would be from the bacteria that rapidly colonize the intestine after birth (Swords et al. 1993), the higher values for C pigs indicate proliferation of enterocytes is enhanced by colostrum. However, higher ratios of mucosal protein to DNA after 6 h of colostrum (Zhang et al. 1997, this study) indicate that accumulation of protein exceeds the rate of DNA synthesis, resulting in the hypertrophy of the enterocytes that line the intestine at birth (Xu et al. 1992).
) indicate that this approach can be misleading during the neonatal period. The dramatic increases in the mucosal protein content of C and MR pigs coincided with lower specific activities of all enzymes relative to OES and FD pigs. However, calculations of total mucosal activities from homogenates revealed higher or comparable values for the C and MR groups compared with OES and FD pigs. Thus, the higher homogenate specific activities of OES and FD pigs were not caused by the presence of more enzymes, as happens after longer periods of food deprivation (Núñez et al. 1996), but simply by less protein per unit of activity. This is corroborated by a lack of differences among groups when homogenate activities for all four hydrolases were normalized to DNA content (data not shown).
). In this study, the lack of significantly lower specific activity in the proximal and distal intestine of C pigs compared with OES and FD pigs is nevertheless suggestive of a diet effect. Specifically, the significantly higher protein content in the homogenates and BBMV of C pigs would have resulted in significantly lower specific activities of lactase if the colostrum diet had not stimulated an increase in lactase activity.
). This study reveals that the magnitude of increase for homogenates is not affected by diet, but that insertion into the apical membrane is increased when neonatal pigs are fed colostrum.
) suggests that synthesis is stimulated by colostrum. It is possible that the 6-h feeding period was not sufficiently long for the pigs used in this study to synthesize and insert enough catalytically active lactase to cause a significant increase in activity. Tivey et al. (1994) reported that the postnatal increase in lactase is due to higher activity in enterocytes already present on the villus at birth, not to the production of enzyme by new enterocytes. The low enrichment factors for the BBM hydrolases indicate that the cellular distribution of active enzyme is different during early suckling compared with other life stages.
). Alternatively, changes in the physical and chemical characteristics of the BBM after onset of suckling (Omodeo-Sale et al. 1991) may enhance the activities of existing transporters. Notable is the apical aldohexose carrier, SGLT-1, which is sensitive to the BBM environment (Meddings et al. 1990).
). The loss of glucose accumulation by BBMV from suckled pigs is not unique and has been seen in studies of developing mice and rats (unpublished data).
). However, the identities of specific BAS in colostrum that influence intestinal development remain uncertain. The rapid internalization of receptors after the onset of suckling (Kelly et al. 1992) suggests that the responses to BAS may be short-lived. This should be considered when examining the influences of BAS during the perinatal period. However, our findings also show that nutrients and even just the physical presence of material in the intestine can alter some intestinal characteristics.
) and particularly during early suckling (Burrin et al. 1992, Galand and Forstner 1974, Zhang et al. 1997, this study), combined with the loss of aldohexose accumulation by BBMV, highlight how the fetal enterocytes that line the pig intestine at birth differ structurally and functionally from those produced postnatally (Buddington 1997). The different patterns of change for total homogenate and BBM activities indicate how the levels of regulation (e.g., transcription, synthesis, processing or insertion) differ among the functional proteins. It is also possible that these processes in suckling animals differ from those known for older animals. The increasing availability of molecular and genetic probes will allow us to address these questions and improve our understanding of neonatal intestinal development and the role of diet.
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FOOTNOTES |
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Manuscript received 16 April 1997. Initial reviews completed 30 May 1997. Revision accepted 17 April 1998.
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ACKNOWLEDGMENTS |
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We thank Prestage Farms of Mississippi for allowing us to have access to farrowing sows for collection of colostrum and for providing the neonatal pigs. The milk replacer was donated by Milk Specialties (Dundee, IL) and the oral electrolyte solution was from Mead Johnson (Evansville, IN). Phil Oldham (Chemistry, MSU) assisted with the fluorescent measurements of DNA content.
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LITERATURE CITED |
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Abstract
Introduction
Methods
Results
Discussion
References
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