Enterocyte Cell Research Articles

Metabolism of oxygenated derivatives of arachidonic acid by Caco-2 cells.

Monolayers of Caco-2 cells, a human enterocyte cell line, were incubated separately with 3H8-labeled preparations of three different lipid mediators of inflammation: 5-hydroxyeicosatetraenoic acid, 12-hydroxyeicosatetraenoic acid, and leukotriene B4. Both [3H8]5-hydroxyeicosatetraenoic and [3H8]12-hydroxyeicosatetraenoic acids were taken up and metabolized by Caco-2 cells, but [3H]leukotriene B4 remained unmetabolized in the incubation medium. [3H]5-hydroxyeicosatetraenoic acid was esterified into cellular phospholipids (15%) and triglycerides (4%) but did not undergo beta-oxidation. When [3H]12-hydroxyeicosatetraenoic acid was incubated with Caco-2 cells, 14% underwent two cycles of beta-oxidation to form [3H]8-hydroxyhexadecatrienoic acid, and 3% underwent three cycles of beta-oxidation to form [3H]6-hydroxytetradecadienoic acid, both of which were released into the media. [3H]12-Hydroxyeicosatetraenoic acid was also esterified into cellular phospholipids (13%), but none was esterified into cellular triglycerides.

Effect of 3‐hydroxy‐3‐methylglutaryl coenzyme A reductase inhibition on human gut mucosa

Mevalonic acid is an important biochemical intermediate in cholesterol synthesis and other processes involved in cell replication. 3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase is the enzyme which catalyzes mevalonic acid synthesis. To determine whether a potent competitive inhibitor of this enzyme, the drug simvastatin, may have an adverse effect on enterocyte cell replication and cholesterol metabolism, small intestinal biopsies from nine hypercholesterolemic subjects were obtained before and during treatment with simvastatin as a lipid-lowering agent. Histologic review of biopsies in a blinded manner detected no change in ratio of villous length to crypt length or in mitotic index which might indicate altered cell replication. Similarly, no significant change in measured activity of HMG-CoA reductase activity was observed. In spite of the high exposure of jejunal mucosal cells to this potent competitive inhibitor of a key enzyme, no adverse effect on growth could be detected.

Expression of liver fatty acid-binding protein/human growth hormone fusion genes within the enterocyte and enteroendocrine cell populations of fetal transgenic mice.

The intestinal epithelium establishes and maintains a precise spatial organization despite its continuous and rapid renewal. We have used transgenic mice containing liver fatty acid-binding protein/human growth hormone (L-FABP/hGH) fusion genes to begin to define the molecular mechanisms which are responsible for appropriate regional and cell-specific expression of genes in the gut. Multilabel immunocytochemical methods were employed to characterize the patterns of expression of two transgenes in the enteroendocrine and enterocytic populations of late gestation fetal mice at the time of initial cytodifferentiation of the gastrointestinal epithelium (fetal days 16-19). Surveys of the enteroendocrine cell population using a panel of antibodies directed against 11 neuroendocrine products revealed that these cells are scarce prior to fetal day 17, show a progressive increase in number through day 19, and while the relative proportion of subpopulations (defined by their principal peptide product) are somewhat different than in adults, their geographic distribution along the duodenal to colonic and intervillus(crypt) to villus axes are very similar to that encountered in adult (2-5 month old) mice. Immunoreactive L-FABP is first detectable at fetal day 17 and at this time of first appearance shows an adult pattern of regional enterocytic expression: i.e. it is present in cells overlying nascent villi but not those in the intervillus zone, it is highest in proximal small bowel, declines distally, and is absent from colonocytes. Colocalization studies indicate that L-FABP is not present in enteroendocrine cells during fetal life. Mapping studies indicate that nucleotides -596 to +21 of the rat L-FABP gene are sufficient to reproduce an appropriate temporal, cellular, and regional pattern of reporter (hGH) expression in fetal transgenic mice (with the exception that a subset(s) of enteroendocrine cells, typically containing immunoreactive gastric inhibitory peptide, support transgene but not L-FABP expression). This is in marked contrast to adult transgenic mice where inappropriate hGH accumulation occurs in crypt-associated epithelial cells, in colonocytes, and in many enteroendocrine populations. These studies indicate the importance of considering developmental stage when interpreting the results of any mapping study of cis-acting elements that regulate cell-specific and regional expression of genes in the perpetually renewing intestinal epithelium. Moreover, they also raise the possibility of using transgenes to define fundamental temporal changes in the gut's epithelial cell populations.

Asexual development of Cryptosporidium parvum within a differentiated human enterocyte cell line

Unremitting diarrhea with malabsorption is associated with Cryptosporidium parvum infection of the small intestine in patients with AIDS. The lack of a well-defined in vitro model of C. parvum infection has severely hampered research into the biology of cryptosporidial invasion of the host epithelial cell and development of new pharmacologic and immunologic therapies. The adherent human intestinal epithelial cell line HT29 when grown in glucose-free medium develops morphologic and functional characteristics of the small intestine enterocyte and was used to develop an in vitro model of infection. Cryptosporidium oocysts obtained from AIDS patients were applied to a monolayer of cloned, differentiated HT29.74 cells. Cells were fixed and stained to estimate the degree of parasite infection. Schizonts were easily distinguished from the host cell by light microscopy. Twenty-four hours after 10(5) oocysts were added to approximately 10(6) HT29.74 cells, Cryptosporidium infection rates varied from 50 to 120 schizonts per 1,000 cells. Among 14 different experiments, the mean infection rate was 91 (+/- 18) schizonts per 1,000 cells. Electron microscopy at 6 and 24 h confirmed intracellular localization and development of schizonts. The morphologic features of the cryptosporidial schizonts within HT29.74 cells, which included the presence of a dense band and feeder layer, were identical to those described during cryptosporidial infection of human enterocytes in patients with AIDS. Fewer schizonts were observed at 5 days and beyond. Infection of differentiated HT29.74 cells (62 and 65 schizonts per 1,000 cells at 24 and 72 h, respectively) was over five times more efficient than infection of undifferentiated HT29.74 cells (9 and 5 schizonts per 1,000 cells at 24 and 72 h, respectively). In vitro infection of differentiated HT29.74 cells will allow a better understanding of the mechanisms by which C. parvum infects the small intestinal epithelium and will allow a systematic evaluation of new therapeutic agents.

The Process of Microbial Translocation

The process of microbial translocation was studied using Candida albicans, Escherichia coli, or endotoxin instilled into Thiry-Vella loops of thermally injured guinea pigs and rats. Translocation of C. albicans occurred by direct penetration of enterocytes by a unique process different from classical phagocytosis. Translocation between enterocytes was not observed. Internalization was associated with a disturbance of the plasma membrane and brush border, but most internalized organisms were not surrounded by a plasma membrane. Passage of the candida into the lamina propria appeared to be associated with disruption of the basal membrane with extrusion of cytoplasm of the cell and candida. Organisms in the lamina propria were commonly phagocytized by macrophages but also were found free in lymphatics and blood vessels. Translocation of E. coli and endotoxin also occurred directly through enterocytes rather than between them, but translocated endotoxin diffused through the lamina propria and muscular wall of the bowel wall by passing between rather than through the myocytes. These descriptive phenomena provide new insight into the role of the enterocyte and intestinal immune cells in the translocation process.

Chloride conductance and intracellular chloride accumulation in mouse Peyer's patch enterocytes.

1. Measurements of membrane potential, Cl- conductance and intracellular Cl-, K+, Na+ and H+ activities have been carried out in the follicle-associated epithelium of the mouse Peyer's patch to characterize further the physiological properties of antigen-transporting M cells, enterocytes and intraepithelial lymphocytes. 2. Intraepithelial lymphocytes, identified in random impalements as a second negative jump in membrane potential (Vm), were found to have higher K+ and H+ activities, lower Na+ and Cl- activities and higher negative values for Vm than either of the other two epithelial cell types. Cl- activity in these cells was higher than that predicted from the Nernst equation. 3. M cells identified as having the first negative jump in Vm in impalements involving intraepithelial lymphocytes were unable to accumulate Cl-. They also had a lower Vm than that found in apparently fully differentiated enterocytes. This Vm was partly depolarized at a low Cl- concentration. 4. Apparently fully differentiated enterocytes could be further divided into two populations depending on whether the Vm could or could not be partly depolarized by a low Cl- concentration. The mean Vm and intracellular Cl- activity of Cl(-)-sensitive cells, measured in high Cl- medium, were less than values found for Cl(-)-insensitive enterocytes. Cl- was only accumulated by enterocytes showing no depolarization at low external Cl- concentration. 5. The Vm of mature villus enterocytes was the same as that determined for Cl(-)-insensitive follicle-associated enterocytes. Villus enterocyte Vm was not depolarized at a low Cl- concentration. 6. The present ability to distinguish two apparently large populations of follicle-associated enterocytes having M cell-like or villus enterocyte-like properties is discussed in relation to current theories describing how M cells might be formed. The presence of a Cl- conductance in M cell enterocytes, the inability to accumulate Cl- and the maintenance of a low Vm could aid endocytosis of macromolecules across the apical membrane.

Manipulation of the Functional Activity of the Gut by Dietary and Other Means (Antibiotics/Probiotics) in Ruminants

The mucosa of the gut is some of the most metabolically active tissue in the body. This paper discusses the methodology used to assess enterocyte cell metabolism and nutrient uptake in the reticulorumen and small intestine of ruminant species. Metabolism of volatile fatty acids and glucose by this tissue may limit the availability of essential nutrients to peripheral tissues, and the extent to which this may vary between concentrate-based and forage-based diets is discussed. Factors that affect the development and expression of metabolite uptake by the enterocyte are considered in addition to the influence that manipulation of the microbial flora of the gut by the use of antibiotic growth promoters or probiotics may have upon this process. Data are presented to show that the use of antibiotic compounds in ruminant feeds can influence the rate of cell turnover in the small intestine and the rate of glucose uptake by isolated brush border vesicles.

Metabolism of 15-hydroxyeicosatetraenoic acid by Caco-2 cells.

Monolayers of Caco-2 cells, a human enterocyte cell line, were incubated with [1-14C]15-hydroxyeicosatetraenoic acid (15-HETE), a lipid mediator of inflammation, and [1-14C]arachidonic acid. Both fatty acids were taken up readily and metabolized by Caco-2 cells. [1-14C]Arachidonic acid was directly esterified in cellular phospholipids and, to a lesser extent, in triglycerides. When [1-14C]15-hydroxyeicosatetraenoic acid was incubated with Caco-2 cells, about 10% was directly esterified into cellular lipids but most (55%) was beta-oxidized to ketone bodies, CO2, and acetate, with very little accumulation of shorter carbon chain products of partial beta-oxidation. The radiolabeled acetate generated from beta-oxidation of [1-14C]15-hydroxyeicosatetraenoic acid was incorporated into the synthesis of new fatty acids, primarily [14C]palmitate, which in turn was esterified into cellular phospholipids, with lesser amounts in triglycerides. Caco-2 cells were also incubated with [5,6,8,9,11,12,14,15-3H]15-hydroxyeicosatetraenoic acid; most of the radiolabel was recovered either in ketone bodies or in [3H]palmitate esterified in phospholipids and triglycerides, demonstrating that most of the [3H]15-hydroxyeicosatetraenoic acid underwent several cycles of beta-oxidation. The binding of both 15-hydroxyeicosatetraenoic acid and arachidonic acid to hepatic fatty acid binding protein, the only fatty acid binding protein in Caco-2 cells, was measured. The Kd (6.0 microM) for 15-HETE was three-fold higher than that for arachidonate (2.1 microM).

Structure and functions of the digestive tract of Alvinella pompejana, a hydrothermal vent polychaete

The different parts of the digestive tract of Alvinella pompejana are described. The buccal apparatus presents a ventral pharyngeal organ, which is thought to be protrusible, and a dorsal anterior oesophageal gland. Unusual racks are observed in this part, a fact that confirms the deposit feeding behaviour of the worm which has been hypothetized. The serous cells of the oesophageal gland may be the origin of the proteolytic activity detected in the lumen. Intracellular enzymatic activities are detected in the stomach cells. These cells are characterized by numerous iron concretions and polyphagosomes. Comparative elemental analysis of these concretions lead us to think that this digestive segment is a homeostatic organ. The intestine is composed of an absorbant epithelium including enterocytic cells, myocytes, and cells containing zymogenic granules. As much as 90% of ingested protein matter occurs in a water-soluble state (half of it free amino acids, principally glycine) immediately on passing through the anterior part of the gut lumen. In the hindgut, solid faeces are mostly composed of elemental sulfur and are glycine depleted, making them good candidates to form the bulk of the suspended particles found in the waters closely surrounding the worm colonies. The whole gut content contains a high concentration of elemental sulfur. The digestive process of the worm is discussed.

The effect of inclusion of virginiamycin in feed on glucose and alanine transport by intestinal brush border membrane vesicles of sheep

Manipulation of the bacterial population of the intestine has been shown to result in changes in the structure and function of mucosal tissues. The objective of the present experiment was to investigate the effect of inclusion of a non-therapeutic antibiotic, virginiamycin, in the diet of sheep on the activity of the Na* -dependant transport of glucose and alanine by enterocyte cells. These transport mechanisms can be studied by isolation of brush border membrane vesicles following sub-cellular fractionation of the mucosal tissues.Tissue was obtained from two separate growth trials in which the effect of inclusion of virginiamycin on lamb growth was being studied. In Trial 1 (glucose studies) the animals had been fed a concentrate diet plus hay ad libitum and virginiamycin was included at 100 ppm in the treatment group. In Trial 2 (alanine studies) the sheep were fed a concentrate diet with virginiamycin included at 50 ppm in the treatment group.

The influence of inclusion of a probiotic in the diet on the development of enzyme activity in the small intestine of the pig

The objective of the present study was to investigate the development of hydrolase enzyme activity at six sites along the small intestine of the pig before and after weaning and the influence inclusion of a probiotic preparation had upon this. The enzymes studied lactase, dipeptidase and tripeptidase are primarily associated with the brush border of the enterocyte cell and expression of hydrolase activity has been shown to be affected by manipulation of the microbial population of the gut.

Use of transgenic mice to study the routing of secretory proteins in intestinal epithelial cells: analysis of human growth hormone compartmentalization as a function of cell type and differentiation.

The intestinal epithelium is a heterogeneous cell monolayer that undergoes continuous renewal and differentiation along the crypt-villus axis. We have used transgenic mice to examine the compartmentalization of a regulated endocrine secretory protein, human growth hormone (hGH), in the four exocrine cells of the mouse intestinal epithelium (Paneth cells, intermediate cells, typical goblet cells, and granular goblet cells), as well as in its enteroendocrine and absorptive (enterocyte) cell populations. Nucleotides -596 to +21 of the rat liver fatty acid binding protein gene, when linked to the hGH gene (beginning at nucleotide +3) direct efficient synthesis of hGH in the gastrointestinal epithelium of transgenic animals (Sweetser, D. A., D. W. McKeel, E. F. Birkenmeier, P. C. Hoppe, and J. I. Gordon. 1988. Genes & Dev. 2:1318-1332). This provides a powerful in vivo model for analyzing protein sorting in diverse, differentiating, and polarized epithelial cells. Using EM immunocytochemical techniques, we demonstrated that this foreign polypeptide hormone entered the regulated basal granules of enteroendocrine cells as well as the apical secretory granules of exocrine Paneth cells, intermediate cells, and granular goblet cells. This suggests that common signals are recognized by the "sorting mechanisms" in regulated endocrine and exocrine cells. hGH was targeted to the electron-dense cores of secretory granules in granular goblet and intermediate cells, along with endogenous cell products. Thus, this polypeptide hormone contains domains that promote its segregation within certain exocrine granules. No expression of hGH was noted in typical goblet cells, suggesting that differences exist in the regulatory environments of granular and typical goblet cells. In enterocytes, hGH accumulated in dense-core granules located near apical and lateral cell surfaces, raising the possibility that these cells, which are known to conduct constitutive vesicular transport toward both apical and basolateral surfaces, also contain a previously unrecognized regulated pathway. Together our studies indicate that transgenic mice represent a valuable system for analyzing trafficking pathways and sorting mechanisms of secretory proteins in vivo.

Receptors for and bioresponses to 1,25-dihydroxyvitamin D in a human colon carcinoma cell line (HT-29)

Human colon carcinoma (HT-29) cells were examined for their capacity to bind and respond to 1,25-dihydroxycholecalciferol [1,25-(OH) 2D 3]. These cells are known to differentiate and increase their population doubling time when galactose is substituted for glucose in their media. High-affinity and specific binding of 1,25-(OH) 2[ 3H]D 3 was observed in extracts of these cells grown in glucose. The binder sedimented in sucrose gradients and eluted from DEAE-cellulose columns in a manner indistinguishable from rabbit intestinal 1,25-(OH) 2D 3-receptor. Smaller amounts of this binder were seen in HT-29 cells grown in galactose. Both glucose-fed and galactose-fed cells exhibited a dose-dependent decrease in growth rate on exposure to 10 −12 to 10 −6 mol/L 1,25-(OH) 2D 3. Ultrastructural examination of galactose-fed and glucose + 1,25-(OH) 2D 3-treated cells showed enterocytic differentiation and features that were not distinguishable between these groups. Sucrase activity was higher in galactose-fed cells and did not change with 1,25-(OH) 2D 3 treatment. However, the lower sucrase activity in glucose-fed cells increased after exposure to 10 −8 mol/L 1,25-(OH) 2D 3. These results indicate receptor content and bioresponsivity to 1.25-(OH) 2D 3 in a human enterocytic cell line, suggesting that it will be a useful model for the study of the mechanisms of action of this sterol.

Low‐Dose Methotrexate‐Induced Changes in Intestinal Permeability Determined by Polyethylene Glycol Polymers

The impact of chemotherapeutic agents on small bowel mucosa has not been investigated adequately in children. To determine the effect of low‐dose methotrexate (MTX) on the permeability and mucosal integrity of the small bowel, we administered low molecular weight polyethylene glycol (PEG) to 19 children who were in the maintenance phase of treatment of acute lymphocytic leukemia. PEG was administered before and after the children received their weekly oral or intramuscular doses of MTX. PEG recoveries from a 6‐h urine collection were calculated by a mathematical formula to determine the polymer with maximal recovery (PCD0) and the polymer whose recovery was 50% of PCDo (N1/2). Compared with pre‐MTX levels, a significant increase was observed in the polymer with maximal recovery (PCDo) in 15/19 children after MTX administration (p < 0.015) which indicated increased transmucosal passage of PEG. A decrease in the PEG parameter (N1/2) was observed in 11/38 studies, and may have indicated diminished enterocyte cell mass. These changes indicate that treatment with MTX results in an increase in intestinal permeability by the paracellular pathway, which may affect the absorption of other molecules.

Low-dose methotrexate-induced changes in intestinal permeability determined by polyethylene glycol polymers.

The impact of chemotherapeutic agents on small bowel mucosa has not been investigated adequately in children. To determine the effect of low-dose methotrexate (MTX) on the permeability and mucosal integrity of the small bowel, we administered low molecular weight polyethylene glycol (PEG) to 19 children who were in the maintenance phase of treatment of acute lymphocytic leukemia. PEG was administered before and after the children received their weekly oral or intramuscular doses of MTX. PEG recoveries from a 6-h urine collection were calculated by a mathematical formula to determine the polymer with maximal recovery (PCD0) and the polymer whose recovery was 50% of PCD0 (N1/2). Compared with pre-MTX levels, a significant increase was observed in the polymer with maximal recovery (PCD0) in 15/19 children after MTX administration (p less than 0.015) which indicated increased transmucosal passage of PEG. A decrease in the PEG parameter (N1/2) was observed in 11/38 studies, and may have indicated diminished enterocyte cell mass. These changes indicate that treatment with MTX results in an increase in intestinal permeability by the paracellular pathway, which may affect the absorption of other molecules.

A novel marker glycoprotein for the microvillus membrane of surface colonocytes of rat large intestine and its presence in small-intestinal crypt cells.

Murine mAbs were produced against purified microvillus membranes of rat colonocytes in order to establish a marker protein for this membrane. The majority of antibodies binding to the colonic microvillus membrane recognized a single protein with a mean apparent Mr of 120 kD in both proximal and distal colon samples. The antigen is membrane bound as probed by phase-partitioning studies using Triton X-114 and by the sodium carbonate extraction procedure and is extensively glycosylated as assessed by endoglycosidase F digestion. Localization studies in adult rats by light and electron microscopy revealed the microvillus membrane of surface colonocytes as the principal site of the immunoreaction. The antigen was not detectable in kidney or liver by immunoprecipitation but was present in the small intestine, where it was predominantly confined to the apical membrane of crypt cells and much less to the microvillus membrane of differentiated enterocytes. During fetal development, the antigen appears first in the colon at day 15 and 1-2 d later in the small intestine. In both segments, it initially covers the whole luminal surface but an adult-like localization pattern develops soon after birth. The antibodies were also used to develop a radiometric assay for the quantification of the antigen in subcellular fractions of colonocytes in order to assess the validity of a previously developed method for the purification of colonic brush-border membranes (Stieger, B., A. Marxer, and H.P. Hauri. 1986. J. Membr. Biol. 91:19-31.). The results suggest that we have identified a valuable marker glycoprotein for the colonic microvillus membrane, which in adult rats may also serve as a marker for early differentiation of enterocyte progenitor cells in small-intestinal crypt cells.

Regulation of cholesterol esterification by micellar cholesterol in CaCo-2 cells

The regulation of acylcoenzyme A:cholesterol acyltransferase (ACAT) activity by cholesterol was studied in an established enterocyte cell line. CaCo-2 cells were grown in culture to confluency and dome formation. They were characterized morphologically by light and transmission electron microscopy. During the culture period, ACAT activity remained stable while the activities of the brush border enzymes sucrase and alkaline phosphatase progressively increased with time and plateaued 12 days after plating. As determined by the rate of incorporation of oleic acid into the individual lipid classes, the rate of triglyceride synthesis was twice that of phospholipid and 15 times that of cholesteryl ester synthesis in these cells. Incubating CaCo-2 cells with cholesterol solubilized in taurocholate micelles resulted in a significant increase in ACAT activity (149 +/- 5 pmol/dish per 2 hr vs. 366 +/- 5, (P less than 0.001) without changing the rates of triglyceride or phospholipid synthesis. The stimulation of ACAT activity by micellar cholesterol was rapid, occurring within 5 min and reaching a maximal effect by 2 hr. The regulation of ACAT activity by cholesterol was directly dependent upon the concentration of cholesterol solubilized in the micelle and was independent of protein synthesis. Incubating CaCo-2 cells with micellar cholesterol did not increase the esterification of, nor did the cholesterol enter the pool of, newly synthesized or performed cholesterol within 2 hr. The micellar cholesterol that was taken up by the cells was esterified within 5 min after starting the incubation. Progesterone, a known ACAT inhibitor, significantly decreased the rate of esterification of intracellular micellar cholesterol proving that the cholesterol taken up by CaCo-2 cells was indeed entering the ACAT pool. Despite increasing amounts of unesterified cholesterol entering the cells via micelles, the percent of cholesterol that was esterified at any one time remained constant at 1%. The results suggest that ACAT activity in CaCo-2 cells is stimulated by cholesterol delivered to the cells by way of taurocholate micelles. The rapid entry of this sterol into the ACAT substrate pool suggests that ACAT activity in CaCo-2 cells is regulated by the expansion of the cholesterol substrate pool that is being utilized by an unsaturated ACAT enzyme.

Pathological changes in the midgut epithelium of grayling, Thymallus thymallus L., larvae reared on different kinds of food, and their relation to mortality and growth

Abstract. The histological differentiation of the digestive tract in grayling, Thymallus thymallus L., larvae was studied during the first 20 days after hatching. The normal development was assessed in larvae fed with nauplii of two Anemia strains. These results were compared with a starved control. Grayling larvae reared on lake zoo‐plankton suffered mortality which was ten times higher than that in the Artemia‐fed groups. The histopathology of the zooplankton‐fed larvae was characterized by excessive fat accumulation in the apices of columnar absorptive cells, complete absence of fat transport into the villus lamina propria, and disintegration of enterocyte cell membranes. This pathology is compared with the clinical findings in human abetalipoproteinaemia. It is concluded that the histopathological manifestations and the resulting elevated mortality in larval grayling were provoked by a disturbance of the fat absorption process in the enterocytes. This disturbance is likely to be due to the inability to synthesize chylomicrons. It is further hypothesized that lake plankton either contains a toxic agent which interferes with the build‐up of chylomicrons or is deficient in some lipoprotein moiety which is essential for chylomicron synthesis.

An integrated model for intestinal development in the fetal sheep.

The interrelationships between 22 parameters relating to small intestinal morphology and enterocyte cell kinetics were investigated. It was found that all significant correlations could be related to changes in intestinal diameter. The area of accessory and mucosal coats in cross section was computed and a constancy of relative composition, independent of experimental perturbation, was noted. It appears likely that despite seemingly trophic responses in specific tissue components, an overall balance of these components is maintained in the developing intestine. The need to develop integrated models of gut structure and function to allow adequate interpretation of results is discussed.

Initial cholesterol uptake by everted sacs of rat small intestine: kinetic and thermodynamic aspects.

The kinetics of initial cholesterol uptake by everted rat proximal and distal small intestinal sacs were evaluated in vitro. The mucosal incubation solution consisted of 0.05 mM cholesterol solubilized in 4.8 mM sodium taurocholate micellar solution at pH 7.4 Experiments were performed at temperatures from 26 to 38 C. The rate of cholesterol uptake followed a linear relationship when plotted against time indicating an apparent zero-order kinetics mechanism for initial uptake. An Arrhenius plot of the results of uptake versus temperature remained linear over the entire range of temperatures studied. The large free energy of activation (20 kcal/mole) suggests that an energy barrier for cholesterol uptake exists at the enterocyte luminal cell membrane and may be an important limiting step in cholesterol uptake. It is proposed that a transient association between cholesterol and a component of the enterocyte luminal cell membrane is formed during initial uptake of cholesterol. The transient association may be an activated complex formed with proteins present at or within the luminal enterocyte cell membrane.