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The Journal of Nutrition Vol. 128 No. 8 August 1998, pp. 1302-1310

Diet Influences Development of the Pig (Sus scrofa) Intestine during the First 6 Hours after Birth1,2

Hongzheng Zhang, Christiane Malo*, Carolyn R. Boyledagger , and Randal K. Buddington3

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 dagger  College of Veterinary Medicine, Mississippi State University, Mississippi State, MS 39762-9825

    ABSTRACT
Abstract
Introduction
Methods
Results
Discussion
References

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

    INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References

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), 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

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.

Colostrum was collected from sows that were farrowing or had completed farrowing within 2 h. This avoided the changes in milk composition that occur during the first 24 h of lactation (Klobasa et al. 1987). 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).

Beginning immediately after birth and every hour for 6 h, the pigs were given 15 g/h of their respective diets using a feeding tube. This rate of feeding was based on preliminary observations of hourly weight increases by newborn pigs that were suckling and accounted for urinary and fecal losses. During the 6-h feeding period, the pigs were kept warm in a box placed on top of a heating pad.

All phases of the study involving the experimental pigs were approved by the Mississippi State University Institutional Animal Care and Use Committee.

Sampling and collection of tissues.  We followed a protocol previously used to study pig intestinal development (Zhang et al. 1997). 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.

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), 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).

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). Aminooligopeptidase (AOP, EC 3.4.11.2) activity was assayed by the method of Wojnarowska and Gray (1975) using 0.17 mmol/L leucyl-beta -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).

Total intestinal homogenate and BBM activities (µmol hydrolyzed/min) were calculated by summing the products of specific activity in each region times the regional protein content.

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). 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.

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). 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).

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), 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.

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) 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.

For some variables, diets were compared within intestinal region and intestinal regions were compared within a diet. Each of these analyses was carried out independently by using either a combined or separate analysis as appropriate.

Analyses were performed by using the SAS (Statistical Analysis System, Version 6.11, Cary, NC) procedure MIXED (Littell et al. 1996). 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.

Kinetics of D-glucose transport by intact tissues were determined by nonlinear regression analysis (Michaelis-Menten) using the Enzfitter software package (Robin G. Leatherbarrow, copyright 1987, Biosoft Elsevier). A model equation for one transport system was used because equations for multiple transporter sites did not improve the fit. Initial rates of hexose accumulation by BBMV were calculated by linear regression analysis over the linear part of the uptake-time curves. If uptake-time curves deviated from linearity, a second degree polynomial analysis was used with the initial rate represented by the first degree coefficient of the polynomial (Chenu and Berteloot 1993).

    RESULTS
Abstract
Introduction
Methods
Results
Discussion
References

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.

 
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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

The longest small intestines were measured in C pigs and the shortest intestines in OES and FD pigs (Table 1). For set one pigs, the intestinal lengths of MR pigs were not different from the C pigs, but were longer than OES and FD pigs. In set two, MR pigs did not differ from either C or OES pigs. Total intestinal surface area was responsive to diet, with greater area for C pigs and the least area measured in FD pigs. These differences can be attributed largely to the effect of diet on length because circumference did not differ among groups in any region (data not presented).

Total intestinal weight was greatest in C pigs and comparable among the other groups (Table 2). This diet-related pattern was detected in each of the three regions of small intestine (P < 0.05 for proximal, mid- and distal intestine for comparisons of C pigs with each of the other three groups).

 
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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

Total mucosal mass and values for the proximal and mid-regions were greatest in C pigs compared with each of the other groups (P < 0.05), which did not differ. In the distal intestine, the MR pigs were not different from the C or the OES and FD pigs. The percentages of the different intestinal regions represented by mucosa were not affected by diet (data not presented).

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).

Total intestinal BBMV protein content was highest for C pigs (P < 0.05). Values for MR, OES and FD pigs were 61, 44 and 56% of values for C pigs, with no significant differences among the other three groups. The same pattern held for the proximal region; in the distal region, however, values for FD pigs were lower than those for OES pigs. There were no significant differences among diets for the mid-region.

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.

Mucosal protein content normalized to DNA was higher for C pigs (P < 0.05; from Tables 1 and 2), averaging 133, 138 and 156% higher than MR, OES and FD pigs, respectively, indicating more protein per cell.

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.


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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.


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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.


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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.

Diet effects were not detected for BBMV specific activities of lactase (Fig. 1), maltase (Fig. 2), sucrase (data not shown) or AOP (Fig. 3) in the three regions of the small intestine. Significant diet effects were seen for regional homogenate specific activities of all hydrolases, with the exception of sucrase. For lactase, the effect of diet was restricted to the mid-intestine with C and MR pigs having lower activity than OES and FD pigs. Maltase activity was higher in homogenates from the mid- and distal regions of OES pigs compared with the other groups, which did not differ; values for the proximal region did not differ among groups. There were significant effects of diet on homogenate AOP activity in all three regions. Activities of C and MR pigs were lower than those of FD pigs in all three regions (Fig. 3; P < 0.05). Activities of OES pigs were not different from those of FD pigs, but were higher than C pigs in the mid- and distal regions and MR pigs in the mid-region only.

Enrichments (specific activities of BBMV relative to homogenates) for the three disaccharidases ranged from a high of 3.4 for lactase to a low of 0.5 for sucrase, and 2.0 for maltase (averages from all three regions). These values are markedly lower than those for fetal and unsuckled newborn pigs (Buddington and Malo 1996) 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.

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).


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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.

 
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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

Rates of aldohexose accumulation by BBMV showed diet effects. BBMV prepared from C and MR pigs exhibited minimal abilities to transport glucose (Table 4). For each region of C and MR pigs, only one of the preparations exhibited active accumulation of isotope. In a previous study (Zhang et al. 1997), 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.

 
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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

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), 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.

Influence of diet on body mass and intestinal growth

The positive weight gains of C and MR pigs, but not those fed OES, show that nutrients stimulate postnatal growth in body mass. However, colostrum elicits faster intestinal growth; this has been attributed to the presence of nonnutritive components such as immunoglobulins and biologically active substances (Sheard and Walker 1988). 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).

Postnatal increases in mucosal protein content (Xu et al. 1992, 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.

Intact protein absorption by C pigs would be further enhanced by the presence of protease inhibitors in colostrum (Lindberg 1982) 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 beta -lactoglobulin, have a shorter gastric retention time than other proteins and are poorly digested during the neonatal period (Kiriyama 1992, Yvon et al. 1993).

Colostrum stimulates synthesis of mRNA, protein (Burrin et al. 1992, 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).

The reason for the higher total BBM protein in C pigs is uncertain. Binding of colostral proteins to BBM receptors could contribute. However, the lack of differences among groups when BBM protein was normalized to mucosal weight and DNA content (data not presented) suggests that it is not caused by an increase in the average amount of BBM protein per cell. The higher values for C pigs may be related to the presence of more enterocytes (discussed above), which would increase the total amount of BBM and associated proteins.

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) 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).

Total homogenate and BBM activities provide some of the first insights about the relative importance of synthesis and subsequent insertion of active enzyme into the apical membrane during the neonatal period. They also reveal three different responses to diet during the first 6 h after birth. The higher homogenate activities for maltase in C pigs are indicative of diet-induced increases in de novo synthesis and/or the maturation of precursors existing in the cells at the time of birth. The presence of intergroup differences for total BBM maltase indicates that insertion of the active form is also responsive to diet during the first 6 h after birth. Lactase synthesis is stimulated by colostrum (Burrin et al. 1994). 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.

Total homogenate and BBMV sucrase activities provided a second pattern in nonresponsiveness to diet. Pigs cannot modulate lactase and sucrase activity in response to dietary inputs for the first 20 d after birth (unpublished data).

AOP provided a third contrasting pattern of response. A previous studied showed that total intestinal homogenate and BBMV AOP activities increase during the first 24 h after birth and the onset of suckling (Zhang et al. 1997). 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.

The different patterns of postnatal development for the hydrolases and the respective responses to diet are also apparent from recovery values (total BBM activity divided by total homogenate activity, expressed as a percentage). The highest values were calculated for lactase (45%; no differences among groups), with the lowest for sucrase (11%). Values for maltase were intermediate for C, MR and OES pigs (23%; no differences). Interestingly, maltase recovery values for FD pigs were comparable to those for lactase (44%) and higher than those for the other groups (P < 0.005). The increased insertion of AOP into the BBM when colostrum was fed was corroborated by higher recovery values for C pigs (31%) compared with the other groups (MR, 22%; OES, 23%; and FD, 19%; P < 0.002).

The cellular mechanisms responsible for the changes in BBM enzyme activity during the neonatal period have not been clarified. The lower proportion of the mature 160-kD form of lactase relative to the several precursor isoforms, particularly the 180-kD isoform, in homogenates from neonatal pigs fed colostrum (Burrin et al. 1994) 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.

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). 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).

We are unable to explain why intact tissues from C and MR pigs retain the ability to transport aldohexoses, but not BBMV isolated from the same animals. Furthermore, the loss is specific to aldohexoses because BBMV prepared from suckled pigs are intact and retain the ability to actively accumulate amino acids (Zhang et al. 1997). 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).

Perspectives

These findings highlight how genetic determinants and dietary inputs interact to influence structural and functional development of the small intestine. The differences between the two groups of pigs, though limited to only some of the structural and functional characteristics, provide evidence that there are differences in the genetic determinants of even relatively inbred groups of production animals. This may complicate comparisons of data sets, particularly those based on different strains of pigs. Despite this limitation, the differences between strains can provide insights about population variation that may be clinically relevant.

The role of biologically active substances (BAS) in accelerating intestinal development has been of interest because of clinical implications and has recently been reviewed (Odle et al. 1996). 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.

Most of what is known about regulation of BBM functions is based on the adult intestine. The lower enrichments for the BBM hydrolases at the end of gestation (Buddington and Malo 1996) 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.

    FOOTNOTES
1   Supported by funds provided by the University of Montreal and Mississippi State University with travel costs paid by a North Atlantic Treaty Organization Collaborative Research Grant to R.K.B. and C.M.
2   The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.
3   To whom correspondence and reprint requests should be addressed.
4   Abbreviations used: AOP, aminooligopeptidase; BAS, biologically active substances; BBM, brush border membrane; BBMV, brush border membrane vesicles; C, colostrum-fed pigs; FD, food-deprived pigs; MR, milk replacer-fed pigs; OES, pigs fed an oral electrolyte solution.

Manuscript received 16 April 1997. Initial reviews completed 30 May 1997. Revision accepted 17 April 1998.

    ACKNOWLEDGMENTS

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.

    LITERATURE CITED
Abstract
Introduction
Methods
Results
Discussion
References

0022-3166/98 $3.00 ©1998 American Society for Nutritional Sciences



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