Intestinal Microvillus Membrane Research Articles

Glycoproteins and glycolipids of rat small intestinal microvillus and basolateral membranes.

Glycoprotein and glycolipid constituents were examined in purified microvillus and basolateral membranes isolated from rat small intestinal epithelial cells. SDS-polyacrylamide gel electrophoresis showed that the molecular weights of most of the major proteins from microvillus membranes were over 100 kD, whereas the majority of those from basolateral membranes tended to have lower molecular weights. Glycoprotein profiles were also examined using three labeling methods, and in each case marked differences were observed between microvillus and basolateral membranes. In both membranes, lectins with a specificity toward N-linked sugar chains bound to the majority of the glycoproteins, in contrast to those lectins which preferentially bind to O-linked sugar chains. Glycolipids were labeled in vivo and isolated from both membrane fractions. Some differences were observed in the fucolipids and neutral glycolipids suggesting a more complex pattern in microvillus membranes. These results indicate that there are differences in the glycoprotein and glycolipid compositions of microvillus and basolateral membranes that may reflect the functional polarity of intestinal epithelial cells.

Age and cortisone alter host responsiveness to cholera toxin in the developing gut

To assess the influence of immaturity on the responsiveness of enterocytes to specific pathogens, a dose-response curve for cholera toxin (CT)-induced fluid secretion was determined in the proximal small intestine of rats at 2 and 4 wk of age. The suckling rat was approximately 50 times more sensitive to CT in triggering the secretory response than the weaned rat, when estimated by the medium-effective dose (ED50, 0.8 vs. 38.9 nM). Cortisone, known to promote enterocyte maturation, when injected into suckling rats, decreased host sensitivity approximately 1,000 times. Neither age nor cortisone decreased the receptor binding of 125I-labeled CT to intestinal microvillus membranes. In contrast, cortisone treatment caused a threefold increase in receptor density from 14.5 to 43.0 pmol/mg protein. The enzyme responsible for the sodium pump, Na+-K+-ATPase, showed a threefold increase in activity both after weaning and after a cortisone treatment. These data indicate that the immature gut exhibited an increased host sensitivity to CT stimulation that was not correlated with initial receptor binding but was related to a lowered Na+-K+-ATPase activity, suggesting that an underdeveloped sodium pump may be partially responsible for the high incidence of secretory diarrhea in neonates.

Visualization by freeze fracture, in vitro and in vivo, of the products of fat digestion.

The technique of freeze fracture was used to visualize triglyceride (TG) hydrolysis and the production of lipolytic products (LPs) in vitro and in vivo in the presence of bile salts (BS). Three systems were investigated: pure lipolytic products (oleic acid and monoolein) in the presence of a pure bile salt (taurodeoxycholate (TDC)), lipolytic products produced from TG by pancreatic lipase in the presence of a variety of bile salts, and lipolytic products produced in the intestine of the killifish, Fundulus heteroclitus, after fat feeding. In vitro, lamellae (4-5 nm thick with 0-8-nm water spacings) appeared on the surface of TG droplets in all preparations with LP/BS molar ratios of 1.5 or greater and spherical vesicles (diameter range, 20-130 nm) were produced from these lamellae. With model killifish bile (taurocholate-cholate 1:1) at LP/BS ratios between 1.5 and 4, homogeneous vesicles or particles (mean diameter, 23.8 nm) were produced by lipase at pH 6.9. In vivo, lamellar product phases also occurred after fat feeding. The smallest visible LP/BS structures by freeze fracture electron microscopy were approximately 20 nm globular particles. Large disc-shaped micelles either were not present or were below the resolution limit of the replica (approximately 10 nm). The dominant aggregated lipolytic product phase was composed of multiple layers of rough-textured lamellae. No evidence of cubic structure was seen. These results show that lamellar and vesicular lipolytic product phases can be intermediates in intestinal fat digestion. However, no evidence for the direct endocytotic absorption of these product phases by the intestinal microvillus membrane was found.

Inhibition of attachment of Escherichia coli RDEC-1 to intestinal microvillus membranes by rabbit ileal mucus and mucin in vitro.

Intestinal mucus is postulated to play a role in preventing colonization of the gastrointestinal tract by microbial pathogens. To evaluate the ability of both crude mucus and purified mucin, a glycoprotein of goblet cell origin, to inhibit mucosal adherence of enteric pathogens, we examined whether mucus and mucin derived from rabbit ileum interact with the rabbit enteropathogen Escherichia coli RDEC-1. We examined the manner in which mucus and mucin inhibited adherence of bacteria to rabbit ileal microvillus membranes (MVMs) in vitro. The purity of the mucin preparation was demonstrated by polyacrylamide gel electrophoresis before and after reduction and by showing that an antiserum raised to the mucin localized to goblet cells in rabbit intestine. Using radioactive labeling of bacteria, we quantitated attachment of RDEC-1 to MVMs, mucus, and mucin that had been immobilized on polystyrene microtiter wells. Binding of RDEC-1 to MVMs was also determined after preincubation of organisms with crude ileal mucus and purified mucin. RDEC-1 bound to both crude mucus and purified mucin when they expressed lectinlike adhesions, previously designated attachment factor rabbit 1 pili. Adherence of piliated RDEC-1 to MVMs, mucus, and mucin was significantly greater than when the bacteria were nonpiliated. Binding of piliated RDEC-1 to MVMs was decreased by preincubation of bacteria with both crude mucus (45.6 +/- 4.2% of control) and purified mucin (50.2 +/- 5.8%). These data indicate that the E. coli enteropathogen RDEC-1 can bind to purified glycoproteins of goblet cell origin and that adherence of these bacteria to mucin is mediated by expression of pili. The findings also support a role for intestinal mucus and its principal organic constituent, mucin, in preventing adherence of a known E. coli enteric pathogen to apical MVMs of enterocytes.

Lipid permeability of rat jejunum and ileum: correlation with physical properties of the microvillus membrane

It is now generally accepted that dietary lipids permeate through the cholesterol-phospholipid bilayer of the intestinal microvillus membrane during the process of intestinal absorption. Therefore, it has been assumed that rates of lipid permeation depend upon the physical properties of the microvillus membrane. In this study the lipid permeability properties of the microvilus membrane were compared in two regions of the intestine, jejunum and ileum. Since the jejunum is exposed to the majority of dietary lipid it would be reasonable to suppose that it would be more efficient at lipid absorption. The ileum was found to be less permeable to all fatty acids examined and this could be correlated with increased rigidity of ileal microvillus membrane vesicles measured with multiple fluorescent probes. Differences in membrane fluidity were found in both the outer third and central regions of the bilayer. When measurements of membrane fluidity were performed either in the presence or the absence of fatty acids, it could be demonstrated that these acids perturb the physical properties of the outer region or the membrane. Therefore, this suggests that the rate-limiting step in fatty acid permeation may be localized to the outer third of the bilayer. Pharmacologic or dietary manipulations attempting to alter rates of lipid permeability should, therefore, be directed towards altering the physical properties of this region of the microvillus membrane.

Investigation of the physical properties of dog intestinal microvillar membrane proteins by polyacrylamide gel electrophoresis: a comparison between normal dogs and dogs with exocrine pancreatic insufficiency

Procedures have been validated for the investigation of the physical properties of canine microvillar membrane proteins by SDS-polyacrylamide gel electrophoresis. These have been used to examine mucosal samples from eight control dogs and from five dogs with naturally occuring exocrine pancreatic insufficiency (EPI) in order to evaluate the potential role of the pancreas in the normal turnover of microvillar membrane proteins in the dog. Gel scanning showed that the proportion of total membrane protein in bands corresponding to a molecular mass greater than 200 kDa was up to 20-times higher in dogs with EPI than in control dogs. In particular, a band of apparent molecular mass 218 kDA represented between 8 and 28% of membrane protein in all affected dogs, compared with only 0.5 to 1.8% in controls, and is most likely to contain single chains of both pro-maltase-glucoamylase and pro-sucrase-isomaltase. Incubation of microvillar membranes in vitro with either trypsin or canine pancreatic juice resulted in degradation of this high molecular mass band and a corresponding increase in the amount of protein in three bands representing molecular masses of 150, 133 and 106 kDa. In samples from control dogs aminopeptidase N was identified in the 133 kDa band by Western blotting and incubation with monospecific antiserum. These findings suggest that pancreatic enzymes play a major role in the normal post-translational processing of intestinal microvillar membrane proteins in the dog.

Lipid peroxidation of rabbit small intestinal microvillus membrane vesicles by iron complexes

Fe(II)- and Fe(III)-induced lipid peroxidation of rabbit small intestinal microvillus membrane vesicles was studied. Ferrous ammonium sulphate, ferrous ascorbate at a molar ratio of 10:1, and ferric citrate, at molar ratios of 1:1 and 1:20, did not stimulate lipid peroxidation. Ferrous ascorbate, 1:1, induced low stimulation, while ferrous ascorbate, 1:20 gave higher stimulation of lipid peroxidation. These results show that in our experimental system, ascorbate is a promotor rather than an inhibitor of lipid peroxidation. Ferric nitrilotriacetate (at molar ratios of 1:2 and 1:10), at an iron concentration of 200 μM, was by far the most effective in inducing lipid peroxidation. Superoxide dismutase, mannitol and glutathione had no effect, while catalase, thiourea and vitamin E markedly decreased ferrous ascorbate l:20-induced lipid peroxidation. Ferric nitrilotriacetate-induced lipid peroxidation was slightly reduced by catalase and mannitol, significantly reduced by superoxide dismutase, and completely inhibited by thiourea. Glutathione caused a 100% increase in the ferric nitrilotriacetate-induced lipid peroxidation. These results suggest that Fe(II) in the presence of trace amounts of Fe(III), or an oxidizing agent and Fe(III) in the presence of Fe(II) or a reducing agent, are potent stimulators of lipid peroxidation of microvillus membrane vesicles. Addition of deferoxamine completely inhibited both ferrous ascorbate, 1:20 and ferric nitrilotriacetate-induced lipid peroxidation, demonstrating the requirement for iron for its stimulation. Iron-induced peroxidation of microvillus membrane may have physiological significance because it could already be demonstrated at 2 μM iron concentration.

Cholesterol modulates alkaline phosphatase activity of rat intestinal microvillus membranes.

Experiments were conducted, using a nonspecific lipid transfer protein, to vary the cholesterol/phospholipid molar ratio of rat proximal small intestinal microvillus membranes in order to assess the possible role of cholesterol in modulating enzymatic activities of this plasma membrane. Cholesterol/phospholipid molar ratios from 0.71 to 1.30 were produced from a normal value of 1.05 by incubation with the transfer protein and an excess of either phosphatidylcholine or cholesterol/phosphatidylcholine liposomes for 60 min at 37 degrees C. Cholesterol loading or depletion of the membranes was accompanied by a decrease or increase, respectively, in their lipid fluidity, as assessed by steady-state fluorescence polarization techniques using the lipid-soluble fluorophore 1,6-diphenyl-1,3,5-hexatriene. Increasing the cholesterol/phospholipid molar ratio also decreased alkaline phosphatase specific activity by approximately 20-30%, whereas decreasing this ratio increased this enzymatic activity by 20-30%. Sucrase, maltase, and lactase specific activities were not affected in these same preparations. Since the changes in alkaline phosphatase activity could be secondary to alterations in fluidity, cholesterol, or both, additional experiments were performed using benzyl alcohol, a known fluidizer. Benzyl alcohol (25 mM) restored the fluidity of cholesterol-enriched preparations to control levels, did not change the cholesterol/phospholipid molar ratio, and failed to alter alkaline phosphatase activity. These findings, therefore, indicate that alterations in the cholesterol content and cholesterol/phospholipid molar ratio of microvillus membranes can modulate alkaline phosphatase but not sucrase, maltase, or lactase activities. Moreover, membrane fluidity does not appear to be an important physiological regulator of these enzymatic activities.

Development of the Gastrointestinal Mucosal Barrier: Changes in Phospholipid Head Groups and Fatty Add Composition of Intestinal Microvillus Membranes from Newborn and Adult Rats

Phospholipids in microvillus membrane (MVM) prepared from the proximal small intestine of newborn (less than 24 h of age) and adult rats were characterized to determine if differences in composition existed. Our hypothesis is that developmental differences in composition of the MVM may account for mucosal barrier differences to microorganisms and antigens during the perinatal period. Lipid analysis showed that both cholesterol and total phospholipids, per mg of membrane protein, were much higher in the newborn MVM than the adult, but the molar ratio of these two lipid classes remained unchanged. Increased phosphatidylcholine and decreased phosphatidylinositol levels were found in the newborn MVM compared to the adult. In addition, the major difference in fatty acid composition of total lipid extracts and three major membrane phospholipids, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol appeared to be an increased ratio of palmitate (16:0) to stearate (18:0) and of oleate (18:1) to linoleate (18:2) in the newborn. These data suggest that changes in the proportion of phospholipid polar head groups and fatty acyl groups as well as changes in the amount of total phospholipids and cholesterol exist in the newborn MVM. These observations are consistent with differences in MVM fluidity previously noted and may help explain the increased uptake of antigens and other mucosal barrier defects in the newborn intestine.

Specific receptors for epidermal growth factor in rat intestinal microvillus membranes.

Epidermal growth factor (EGF) is present in high concentrations in milk, salivary, and pancreaticobiliary secretions. EGF, delivered to the intestinal lumen by these fluids, appears to influence intestinal proliferation. Because EGF exerts its mitogenic effect through binding to specific membrane-bound receptors, binding studies of 125I-labeled EGF to purified microvillus membrane (MVM) preparations from fetal, newborn, and adult rat small intestine were performed. Using the membrane filter technique, binding of 125I-EGF to adult MVM was specific, saturable, and reversible. Adult and fetal MVM binding was rapid and reached a plateau after 30 min at both 20 and 37 degrees C. No binding was detected at 4 degrees C. Specific binding increased linearly from 0 to 75 micrograms MVM protein. Scatchard analysis revealed a single class of receptors in fetal and adult MVM with an association constant of 1.0 +/- 0.35 X 10(9) and 2.3 +/- 1.6 X 10(9) M-1, respectively. Binding capacity was 435.0 +/- 89 and 97.7 +/- 41.3 fmol 125I-EGF bound/mg MVM protein for fetal and adult MVM, respectively. Newborn MVM binding was negligible. After binding, cross-linking utilizing disuccinimidyl suberate, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, autoradiography revealed a 170-kDa receptor. These data demonstrate specific receptors for EGF on MVM of rat small intestine and, thus, suggest a mechanism for the intraluminal regulation of enterocyte proliferation by EGF.

Food proteins and maturation of small intestinal microvillus membranes (MVM). I. Binding characteristics of cow's milk proteins and concanavalin A to MVM from newborn and adult rats.

To study maturational changes of food protein and lectin binding to rat small intestinal microvillus membranes (MVM), MVM were prepared from newborn and adult animals by a modified CaCl2 precipitation technique. Radiolabeled cow's milk proteins [alpha-lactalbumin, alpha-casein, beta-lactoglobulin, bovine serum albumin (BSA)] and the lectin concanavalin A (Con A) were used for incubations. Binding assays were done using miniature ultracentrifugation for separation of unbound material. Binding of Con A to MVM from newborn and adult rats was strong, specific, and saturable. Binding of Con A was inhibited by cold Con A and by the sugar ligand polymer mannan. Adult MVM bound more Con A than newborn preparations. Unlike Con A, binding of cow's milk proteins by MVM was weak, nonspecific, and noninhibitable. Newborn MVM bound more cow's milk proteins than adult controls. This was true for all the proteins tested (p less than 0.001). Binding rose with decreased molecular weight of cow's milk proteins, but molecular weight was not the only determining factor for binding. Trypsin treatment of MVM caused a marked increase of BSA binding in adult but not in newborn preparations. This finding indicated the importance of protein components of MVM for cow's milk protein binding. Maturational changes in protein-lipid interactions and membrane fluidity possibly influence nonspecific cow's milk protein binding to MVM. Differences in binding between newborns and adults were not directly related to maturational shifts in membrane glycosylation that are indicated by differential Con A binding. Increased cow's milk protein binding in newborn individuals might increase the potential risk to develop an adverse reaction to food proteins.

Food Proteins and Maturation of Small Intestinal Microvillus Membranes (MVM). I. Binding Characteristics of Cow's Milk Proteins and Concanavalin A to MVM from Newborn and Adult Rats

SummaryTo study maturational changes of food protein and lectin binding to rat small intestinal microvillus membranes (MVM), MVM were prepared from newborn and adult animals by a modified CaCl2 precipitation technique. Radiolabeled cow's milk proteins [α‐lactalbumin, α‐casein, β‐lactoglobulin, bovine serum albumin (BSA)] and the lectin concanavalin A (Con A) were used for incubations. Binding assays were done using miniature ultracentrifugation for separation of unbound material. Binding of Con A to MVM from newborn and adult rats was strong, specific, and saturable. Binding of Con A was inhibited by cold Con A and by the sugar ligand polymer mannan. Adult MVM bound more Con A than newborn preparations. Unlike Con A, binding of cow's milk proteins by MVM was weak, nonspecific, and noninhibitable. Newborn MVM bound more cow's milk proteins than adult controls. This was true for all the proteins tested (p < 0.001). Binding rose with decreased molecular weight of cow's milk proteins, but molecular weight was not the only determining factor for binding. Trypsin treatment of MVM caused a marked increase of BSA binding in adult but not in newborn preparations. This finding indicated the importance of protein components of MVM for cow's milk protein binding. Maturational changes in protein‐lipid interactions and membrane fluidity possibly influence nonspecific cow's milk protein binding to MVM. Differences in binding between newborns and adults were not directly related to maturational shifts in membrane glycosylation that are indicated by differential Con A binding. Increased cow's milk protein binding in newborn individuals might increase the potential risk to develop an adverse reaction to food proteins.

Food proteins and maturation of small intestinal microvillus membranes (MVM). II. Binding of gliadin hydrolysate fractions and of the gliadin peptide B3142.

To investigate in vitro interactions between gliadin peptide fractions that have been shown to be toxic to celiac small intestinal mucosa in humans and small intestinal microvillus membranes (MVM) from rats during postnatal maturation, MVM were prepared from newborn, 18-day-old preweanling, and adult rats. Partially hydrolyzed gliadin peptide fractions B1-B4, and the pure gliadin peptide B3142 were radioiodinated and used for binding assays. Miniature ultracentrifugation was used for separation of unbound material. Binding of gliadin fractions to MVM was weak and nonspecific in terms of lacking saturation and inhibition. There was no inhibition of binding by mannan. Enzyme pretreatment of MVM (trypsin, neuraminidase, phospholipase C) did not result in any significant change of binding. Compared with peptides prepared from bovine serum albumin as a control, there was no significant difference in binding of gliadin peptide fractions to MVM. Thus, a lectin-like effect of gliadin peptides toward MVM, or the existence of a specific intestinal surface receptor for gliadin peptides appeared improbable. There were, however, consistent maturational changes in MVM binding in that newborn MVM bound more B1-B4 and B3142 compared with adult controls (p less than 0.001). Nonspecific binding of gliadin fractions to MVM might be related to the initiation of nonspecific in vitro effects of gliadin, particularly toward the immature small intestine. The MVM binding model in the rat clearly does not provide a system for studying celiac disease pathogenesis, but it might help clarify basic processes in the interaction between food-derived substances and elements of the gastrointestinal mucosal barrier.

Food Proteins and Maturation of Small Intestinal Microvillus Membranes (MVM). II. Binding of Gliadin Hydrolysate Fractions and of the Gliadin Peptide B3142

SummaryTo investigate in vitro interactions between gliadin peptide fractions that have been shown to be toxic to celiac small intestinal mucosa in humans and small intestinal microvillus membranes (MVM) from rats during postnatal maturation, MVM were prepared from newborn, 18‐day‐old preweanling, and adult rats. Partially hydrolyzed gliadin peptide fractions B1‐B4, and the pure gliadin peptide B3142 were radioiodinated and used for binding assays. Miniature ultracentrifugation was used for separation of unbound material. Binding of gliadin fractions to MVM was weak and nonspecific in terms of lacking saturation and inhibition. There was no inhibition of binding by mannan. Enzyme pretreatment of MVM (trypsin, neuraminidase, phospholipase C) did not result in any significant change of binding. Compared with peptides prepared from bovine serum albumin as a control, there was no significant difference in binding of gliadin peptide fractions to MVM. Thus, a lectin‐like effect of gliadin peptides toward MVM, or the existence of a specific intestinal surface receptor for gliadin peptides appeared improbable. There were, however, consistent maturational changes in MVM binding in that newborn MVM bound more B1‐B4 and B3142 compared with adult controls (p < 0.001). Nonspecific binding of gliadin fractions to MVM might be related to the initiation of nonspecific in vitro effects of gliadin, particularly toward the immature small intestine. The MVM binding model in the rat clearly does not provide a system for studying celiac disease pathogenesis, but it might help clarify basic processes in the interaction between food‐derived substances and elements of the gastrointestinal mucosal barrier.

Differential binding kinetics of cholera toxin to intestinal microvillus membrane during development.

A complete randomized block design was used to compare the binding kinetics of cholera toxin to developing rat enterocyte microvillus membranes prepared from newborn, 2-week-old, 4-week-old, and adult animals. Saturation-binding isotherms were generated on 16 independent samples (four blocks) under steady-state and reversible conditions. Scatchard analyses suggested positive cooperative binding to a single class of receptors, and the isotherms were analyzed by both the Hill-Waud and Michaelis-Menten functions. Receptor density varied significantly with age (P = 0.013). An abrupt rise in receptor density occurred after the neonatal period and normalized in the adult animal. The half-dissociation constant also varied significantly with age (P = 0.019). Microvillus membranes from suckling animals had a slightly higher apparent affinity than those from weaned animals. Neither receptor concentration nor membrane purity confounded these observations. Whereas age-related changes in apparent affinity correlated with cellular responses, changes in receptor density did not. This study suggests that developmental changes in membrane structure which influence binding affinity but not receptor density may, in part, contribute to the increased sensitivity of suckling rats to cholera toxin exposure.

Pilus-mediated interactions of the Escherichia coli strain RDEC-1 with mucosal glycoproteins in the small intestine of rabbits

Escherichia coli strain RDEC-1 (serotype 015:NM) is an effacing adherent enteropathogen that binds to the intestine of rabbits in a manner morphologically identical to the binding of human enteropathogenic E. coli strains to human intestine. The rabbit enteropathogen adheres to mucosal enterocytes in vivo and to microvillus membranes in vitro. Binding of RDEC-1 to ileal brush borders and to M cells overlying Peyer's patches is mediated by pili (fimbriae) expressed on the cell surface of bacteria. To examine whether similar binding occurs to glycoproteins present in the intestinal lumen, RDEC-1 was fed to rabbits and the intestinal luminal contents were examined for in vivo colonization by RDEC-1. In addition, preparations of rabbit luminal glycoproteins were tested for their ability to agglutinate RDEC-1 in vitro. After the infection of rabbits, RDEC-1 organisms were found in the intestinal lumen associated with glycoproteins, as defined by positive histochemical staining of luminal material by periodic acid-Schiff and Alcian blue. In vitro aggregation of RDEC-1 by luminal glycoproteins of rabbit intestine, a luminal glycoprotein fraction purified on column chromatography and rabbit ileal microvillus membranes, was determined using a microtiter plate assay and an aggregometer. Nonpiliated RDEC-1 did not interact with either of the intestinal glycoprotein preparations or microvillus membranes. RDEC-1 expressing mannose-resistant AF/R1 pili or mannose-sensitive type 1 pili coaggregated with both rabbit luminal glycoprotein preparations and rabbit ileal microvillus membranes at equivalent protein concentrations. These studies demonstrate that RDEC-1 are associated with luminal glycoproteins during in vivo infection of rabbits, and that piliated RDEC-1, but not nonpiliated, coaggregate with rabbit glycoprotein preparations in vitro. Luminal glycoproteins contained within the mucus layer may serve as a site for replication and colonization of bacteria before bacterial enteroadherence.

High molecular mass forms of intestinal microvillar membrane proteins in dogs with exocrine pancreatic insufficiency

High molecular mass forms of intestinal microvillar membrane proteins in dogs with exocrine pancreatic insufficiency

Analysis of canine intestinal microvillar membranes by two-dimensional electrophoresis

Analysis of canine intestinal microvillar membranes by two-dimensional electrophoresis

Alterations in piglet small intestine after cholesterol deprivation.

Mammalian cells require cholesterol for normal cell function. This requirement can be fulfilled by endogenous biosynthesis or by extracellular supplementation. Infants fed with human milk receive greater quantities of cholesterol than those fed commercial formulas. Whether this lack of cholesterol in commercial formulas poses a threat to normal neonatal cell function is not known. We compared small intestinal microvillus membrane fluidity, hydrolase activities, protein concentration, permeability to nonabsorbable markers, and weight gain in neonatal piglets receiving restricted intake of isocaloric formulas containing either normal amounts of cholesterol (145 mg/dl) or very low levels of cholesterol (less than 2 mg/dl). Using the fluorescent probe, diphenylhexatriene, and fluorescence polarization, microvillus membranes from cholesterol deprived piglets demonstrated higher fluidities than did microvillus membranes from animals fed normal concentrations of cholesterol. Cholesterol-deprived animals, even though their caloric intake was similar to cholesterol-fed animals, demonstrated a net weight loss per animal whereas the cholesterol-fed animals demonstrated a weight gain. These results demonstrate that in a pig model on a restricted intake, cholesterol deprivation alters the biophysical properties of the microvillus membrane.

Structural and functional maturation of rat gastrointestinal barrier with thyroxine.

It has been noted that the closure of the intestinal barrier to immunoglobulins is a normal maturational process in the rat. It has also been noted that the microvillus membrane (MVM) of newborn animals differs from adult MVM. The purpose of this study is to document whether thyroid hormone can induce closure in vivo in the rat and to relate this effect of thyroxine to the structural and functional maturation of the intestinal MVM. To assess closure, 2-wk-old rats were fed a rat immunoglobulin G (IgG), and serum antibody binding activity was measured 4 h later. The antibody binding activity of treated animals (T) was 1.5-2 times less than that of controls (C) (P less than 0.001), indicating that thyroxine stimulates closure. The MVM similarly showed signs of maturation. Structural maturation was demonstrated by the lower fluidity of the thyroid-treated animals' membranes. Under the influence of thyroxine, the number of receptors on the MVM for IgG had decreased [2.8 X 10(-7) M (C) vs. 1.7 X 10(-7) M (T)], while the Ka remained the same, demonstrating the functional maturation of the MVM. In conclusion, thyroid hormone can induce both structural and functional maturation of the intestinal MVM and can enhance the intestinal mucosal barrier by decreasing the penetration of antibodies.