Enterocyte Turnover Research Articles

Crypt cell production rate, enterocyte turnover time and appearance of transport along the jejunal villus of the rat

Intestinal nutrient absorption is subject to adaptation with, for example, diabetes, diet lipid variations (isocaloric semisynthetic diets enriched with saturated (S) or polyunsaturated (P) fatty acids), ileal resection and abdominal irradiation. These models were used in rats to assess dynamic morphology and distribution of amino acid transporter along the villus. The enterocyte migration rate (EMR) was measured using [ 3H]thymidine; the vincristine metaphase arrest technique was used to determine the crypt cell production rate (CCPR); quantitative autoradiography was used to assess the time and age of enterocytes when the uptake of 1 and 20 mM [ 3H]leucine and [ 3H]lysine was initiated along the villus. The enhanced jejunal uptake of nutrients which occurs after a 50% distal enterectomy was associated with a fall in EMR and CCPR, yet the enhanced nutrient uptake which occurs in diabetes is not associated with any alteration in EMR, CCPR, enterocyte transport pool (ETP), i.e., the length of the enterocyte column along with the villus containing amino acid transporter) or expression of transporter along the villus. The reduced uptake of nutrients in rats fed P as compared with S was associated with increased rather than decreased ETP and age of the enterocytes at the tip of the villus. The reduced nutrient uptake which occurs 3 days after abdominal irradiation was associated with increased EMR and CCPR, and reduced ETP and age of enterocytes of the tip of the villus. However, 14 days after irradiation when nutrient transport remains reduced, these parameters have returned to normal. Thus, alterations in nutrient transport may be associated with changes in the dynamic morphology of the intestine, but the two processes are not necessarily interdependent. We speculate that the changes in the dynamic morphology of the intestine, and the changes of amino acid transport which occurs in these models of intestinal adaptation, are independently controlled.

Hearing impairment after bacterial meningitis: a review.

Due to the rapid turnover of the small intestinal epithelia, the rate at which enterocyte renewal occurs plays an important role in determining the level of drug metabolising enzymes in the gut-wall. Current physiologically-based pharmacokinetic (PBPK) models consider enzyme and enterocyte recovery as a lumped first-order rate. An assessment of enterocyte turnover would enable enzyme and enterocyte renewal to be modelled more mechanistically. A literature review together with statistical analysis was employed in order to establish enterocyte turnover in human and pre-clinical species. A total of 85 studies were identified reporting enterocyte turnover in 1,602 subjects in six species. In mice, the weighted combined geometric mean (WX) enterocyte turnover was 2.81±1.14 days (n=169). In rats, the weighted arithmetic mean enterocyte turnover was determined to be 2.37 days (n=501). Human exhibited a WX enterocyte turnover of 3.48±1.55 days for the gastrointestinal (GI) epithelia (n=265), displaying comparable turnover to that of Cytochrome P450 enzymes in vitro (0.96-4.33 days). Statistical analysis indicated human to display longer enterocyte turnover as compared to pre-clinical species. Extracted data was too sparse to support regional differences in small intestinal enterocyte turnover in man despite being indicated in mouse. The utilisation of enterocyte turnover data, together with in vitro enzyme turnover in PBPK modelling may improve the predictions of metabolic DDIs dependent on enzyme turnover (e.g. mechanism-based inhibition and enzyme induction) as well as absorption of nanoparticle delivery systems and intestinal metabolism in special populations exhibiting altered enterocyte turnover.

Sugar‐dependent selective induction of mouse jejunal disaccharidase activities.

1. Sugar-containing diets chosen not to affect intestinal structure or enterocyte turnover have been fed to mice previously maintained on a low carbohydrate diet in order to determine their ability to induce disaccharidase enzymes in the small intestine. 2. Glucose-, fructose- and 3-O-methyl-glucose-containing diets increased sucrase and maltase but not lactase activities in mouse jejunal homogenates. These effects were either absent or negligible in more distal regions of the small intestine. 3. Placing mice on glucose-, fructose- or 3-O-methyl-glucose-containing diets was further shown, by quantitative cytochemistry, to cause a 1.6-, 2.6- and 3.2-fold increase in the initial rate at which alpha-glucosidase activity (sucrase + maltase) appeared in the brush-border membrane of developing enterocytes. 4. The time during which alpha-glucosidase activity increased in enterocyte brush-border membranes fell from 30 h for low carbohydrate fed mice to 21, 19 and 17 h in mice fed glucose, fructose and 3-O-methyl-glucose respectively. Change of diet had no effect on the kinetics of lactase expression by developing enterocytes. 5. Maximal alpha-glucosidase activity detected in enterocyte brush-border membranes is equal to RT, where R is the initial rate of enzyme appearance and T is the time during which this rate operates. The ability of sugars to increase R selectively, but only at the expense of T, defines unexpected limits to the capacity of enterocytes to adapt to changes in luminal nutrition. 6. The above results are discussed in relation to other aspects of enterocyte differentiation recently subjected to quantitative analysis. The need to standardize other aspects of intestinal physiology and redefine the energy content of diets containing non-metabolizable substrates in this type of work is also emphasized.

The immune response to dietary antigens and its influence on disease susceptibility in farm animals

Transient hypersensitivity reactions of the intestinal immune system to dietary antigens result in increases in enterocyte turnover and villous atrophy. These changes occur in the intestine of the post weaned piglet and precede the proliferation of E. coli and the development of post-weaning diarrhea. We therefore postulated that a transient cell mediated immune response to dietary antigens may increase susceptibility to disease. The interaction of dietary and microbial antigens upon the gut immune system has been investigated in mice and pigs and it has been shown that both exert powerful regulatory effects upon each other.

Effect of chronic ethanol ingestion on enterocyte turnover in rat small intestine.

Whether chronic ethanol ingestion significantly damages the small intestine remains controversial. To clarify this we have analysed the morphology of the small intestinal epithelium and quantified its renewal in chronically ethanol fed rats. Twenty adult male rats were pair fed for 28 days a nutritionally adequate liquid diet containing either ethanol as 36% of total calories or an isocaloric diet in which fat substituted for ethanol. Crypt cell production rate was determined in the jejunum and ileum by the metaphase arrest method. Weight gain and small intestinal morphology were similar in ethanol fed and control rats, but enterocyte turnover was significantly reduced in the jejunum (p less than 0.05) and ileum (p less than 0.01) of the ethanol fed rats. This effect of ethanol on the small intestine is probably systemic rather than local, because the changes in jejunum and ileum were similar, and it may contribute to the development of malnutrition in chronic alcoholics.

THE EFFECT OF CHRONIC ETHANOL CONSUMPTION ON IRON ABSORPTION IN RATS

Alcoholics often have an increased amount of iron in the liver which may contribute to the development of alcoholic liver disease, although the mechanism is unknown. It has been shown that chronic ethanol intake decreases the enterocyte turnover and enhances galactose absorption. Whether it affects iron absorption is still controversial. The aim of this study was to investigate the effect of chronic ethanol ingestion on whole body iron absorption in rats. Twenty-eight adult male Sprague-Dawley rats were pair-fed a liquid diet containing either ethanol as 36% of total calories or an isocaloric diet where fat was substituted for ethanol. On the 28th day, four-hour fasted rats were given an oral dose of 59Fe (0.5 microCi) and were immediately counted by a whole body counter. 59Fe levels were then monitored over the following nine days. Although ethanol- and control-fed rats had a similar hepatic iron content (59.5 +/- 5.8 vs 60.2 +/- 7.4 micrograms/100 mg dry liver weight) (mean +/- S.E.M.), the 59Fe total body content was greater in the ethanol group (75% +/- 3%) compared with the control group (45% +/- 4%). These results show that chronic ethanol ingestion increased iron absorption in rats. A reduction of enterocyte turnover may play a role in determining this effect.

Comparative Effects of Glucocorticoids and Prostaglandins on Small Intestine of Infant Rats

Previous studies have suggested similarities between the effects of exogenously administered glucocorticoids and prostaglandins in the developing rat small intestine. In this study the effects of exogenously administered glucocorticoids and prostaglandins were compared. In addition, the effects of prostaglandins in adrenalectomized rats were evaluated. Members of both classes of compounds stimulate small intestinal disaccharidase activities, and increase RNA to DNA ratio and brush border membrane protein synthesis. Hydrocortisone accelerates enterocyte turnover, whereas prostacyclin does not. Enteral administration of 16,16-dimethyl prostaglandin E2 stimulates disaccharidase activities in intact as well as sham-operated and adrenalectomized animals. The data suggest that certain prostaglandins may play a role in small intestinal metabolism which is similar to that of the glucocorticoids but is independent of the adrenal-intestinal axis.

PROSTACYCLIN (PCI2) AND HYDROCORTISONE (HC) MEDIATES BRUSH BORDER PROTEIN (BBMP) SYNTHESIS

Glucocorticoids are known mediators of small intestinal maturation. In this study, the effects of exogenously administered HC and PGI2 on BBMP and DNA synthesis along with other indices of maturation were evaluated. Suckling rats were administered PGI2, HC or saline between days 10 to 13 of life and sacrificed on day 14. DNA and BBMP synthesis were analyzed using in vivo 3H-thymidine and 3H-leucine incorporation into DNA and BBMP in separate groups of animals. DNA synthesis was not significantly altered by PGI2 or HC. The ratio of 3H-leucine incorporation into BBMP to total intestinal homogenate protein was elevated in both PGI2 and HC administered animals as follows:RNA and RNA to DNA ratios were elevated in animals administered PGI2 or HC. Intestinal lactase was elevated in PGI2 administered animals, whereas, sucrase, maltase and enterocyte turnover time was considerably less in PGI2 than in HC administered littermates. These data suggests a preferential stimulation of brush border to total intestinal protein synthesis. A specific effect of PGI2 on lactase and HC on sucrase is also suggested.

A comprehensive description of brush border membrane development applying to enterocytes taken from a wide variety of mammalian species.

Pieces of small intestine taken from rabbit, rat, mouse, guinea-pig, hamster and pig have been used to determine microvillus length in enterocytes located at different points along the crypt-villus axis. Thymidine labelling has also been used to convert measurements of enterocyte position into age of enterocyte. Microvillus lengths showed lower and upper plateaux (a and a + c respectively) with fairly rapid transition from one to the other defined by an exponential coefficient b. The mid-point of elongation (m) usually occurred within 60 micron of the crypt-villus junction. Correlations were found to exist between a and m and between a, m and the depth of the intestinal crypt. Values of b and bc/4, the maximal rate of microvillus elongation, were also found to be correlated with the size of the crypt. None of these parameters were related in any way to the villus height or enterocyte turnover time. The possibility that some factor associated with the physical size of the crypt might be exerting positional and temporal control over the subsequent structural differentiation of enterocytes is discussed.

Intestinal adaptation to protein deficiency

Feeding weanling rats a protein-deficient but energetically adequate diet leads to a marked change in intestinal architecture with no obvious change in the overall capacity to absorb amino acids ( Syme, 1982). Using autoradiographic localization of tritiated amino acids within enterocytes ( King et al., 1981), one can show that valine transport across the microvillar membrane of enterocytes is confined to the upper half of villi, whether or not these villi come from rats fed a high or low protein diet. Adaptation of the rat intestine to a protein-deficient diet occurs by a combination of reduction of villus height and mitotic rate. The net result of these changes is to preserve absorptive function at a reduced energy cost to the animal while at the same time maintaining a constant rate for enterocyte turnover.

Enterocyte turnover and content in fat-fed hamsters.

Cryptal lecithin-synthesizing enzyme specific activities have been previously demonstrated to be increased by feeding a fat-supplemented diet to hamsters. To determine if a physiological basis exists for such changes [3H]thymidine incorporation, cellular migration rate, and mucosal concentration of DNA, protein, and lecithin were measured. Radioautographic studies showed that the labeling index and cell migration rate throughout the intestine in the fat-fed hamsters and in the proximal three-quarters of the intestine of the control group were the same. Both parameters were reduced in the distal quarter of the control intestine. The protein/DNA ratio was significantly increased in the proximal three-quarters and modestly in the distal quarter of the intestine of the fat-fed group as compared to controls, suggesting cellular hypertrophy. The lecithin content of the proximal three-quarters of intestine was the same in both groups but reduced in the distal quarter of the gut of the fat-fed group. These studies suggest that lipid feeding in the hamster can have profound effects on intestinal cellular content and turnover.

Rotavirus infection in lambs: Pathogenesis and pathology

Experimental lamb rotavirus infections were studied by immunofluorescence, histopathology and electron microscopy of tissues from infected gnotobiotic lambs killed at intervals from the incubation period to recovery. The rotavirus was demonstrated by immunofluorescence only in epithelial cells of villi in the small and large intestine, and virus antigen was most abundant during the incubation period. An increased enterocyte turnover rate was suggested by the rapid movement of virus-infected cells to the villus tip, and this increase may be one of the basic pathogenic mechanisms of rotavirus infection. Principal histopathological changes were shortening of villi and sloughing of epithelial cells. These were greatest in the middle and posterior small intestine at the onset of diarrhoea, but regeneration occurred within a few hours. Virus morphology in tissues was similar to that reported in other species, and virus presence correlated well with histopathological change.

Effects of local delayed hypersensitivity on the small intestine.

There are many T and B cells in the small intestinal mucosa and local T cell immunity could have a role both in protective immunity and as a cause of disease (i.e. hypersensitivity). This latter aspect has been investigated by using several animal models to assess the effects of local delayed hypersensitivity on the structure and function of the small intestine. Heterotopically transplanted grafts of fetal small intestine in mice (isografts and allografts) have been examined by conventional histology, scanning and transmission electron microscopy, by making direct measurements of villi, crypts, and lymphoid cell infiltrate, and by counting the number of mitoses per crypt. This cell-mediated immune reaction causes lymphocyte infiltration which is most marked in the lamina propria, hyperplasia of the crypts of Lieberkühn, increased cell loss with villous atrophy and a flat surface, but the individual enterocytes appear fairly normal. Graft-versus-host disease cause exactly the same changes in structure and in cell kinetics as does rejection. However, crypt hyperplasia has been found to precede villous atrophy by several days. Preliminary experiments on local contact hypersensitivity suggest that intraluminal injection of oxazolone in the gut of sensitized mice also produces villous atrophy and crypt hyperplasia. It is postulated that these effects are likely to be produced via lymphokines: by an 'enteropathic' factor which damages the lamina propria and basement membrane, and a factor which is mitogenic for crypt stem cells. In mice infected with Giardia lamblia, crypt hyperplasia and lymphocyte infiltration of the epithelium are present and there is accelerated epithelial cell turnover. In rats infected with Nippostrongylus brasiliensis, the flat mucosa has been shown to be due to the thymus-dependent immune response and not directly to the damage produced by the parasite itself. A common factor in the variety of conditions associated with villous atrophy and crypt hyperplasia may well be a local cell-mediated immune reaction to food, microbial, parasite or other antigens which causes changes in enterocyte turnover rate and malabsorption.