Proximo-distal gradient of Na+-dependent D-glucose transport activity in the brush border membrane vesicles from the human fetal small intestine

Ontogeny, growth and development of the small intestine: Understanding pediatric gastroenterology

Throughout our lifetime, the intestine changes. Some alterations in its form and function may be genetically determined, and some are the result of adaptation to diet, temperature, or stress. The critical period programming of the intestine can be modified, such as from subtle differences in the types and ratios of n3:m6 fatty acids in the diet of the pregnant mother, or in the diet of the weanlings. This early forced adaptation may persist in later life, such as the unwanted increased intestinal absorption of sugars, fatty acids and cholesterol. Thus, the ontogeny, early growth and development of the intestine is important for the adult gastroenterologist to appreciate, because of the potential for these early life events to affect the responsiveness of the intestine to physiological or pathological challenges in later life.

Development of the infant intestine: implications for nutrition support

The incidence of preterm births has continued to increase over the past 25 years, and therefore the optimal feeding of these infants is an important clinical concern. This review focuses on intestinal development and physiology, with a particular emphasis on developmentally immature functions of the preterm intestine and the resulting implications for nutrition therapies used to feed the preterm infant.

Ontogenic and longitudinal activity of Na(+)-nucleoside transporters in the human intestine

The objectives of our study were to identify the types of nucleoside transporters present in the human fetal small intestine and to characterize their developmental activity, longitudinal distribution, and transport kinetics compared with those present in the adult intestine. Nucleoside uptake by intestinal brush-border membrane vesicles was measured by an inhibitor-stop rapid filtration technique. Only the purine-specific (N1; hCNT2) and the pyrimidine-specific (N2; hCNT1) Na(+)-dependent nucleoside transporters were found to be present on the brush-border membranes of the enterocytes along the entire length of the fetal and adult small intestines. The activity of these transporters was higher in the proximal than in the distal small intestine. Both the N1 and N2 transporters found in the fetal intestine shared similar kinetic properties (Michaelis-Menten constant and Na(+)-nucleoside stoichiometry) to those in the adult intestine. During the period of rapid morphogenesis (11-15 wk gestation), no temporal differences were apparent in the activity of the N1 and N2 transporters in the fetal small intestine. These findings have implications for the absorption of drugs from the amniotic fluid by the fetus after maternal drug administration of nucleoside drugs such as the antivirals zidovudine and didanosine.

Development of intestinal transport function in mammals

Considerable progress has been made over the last decade in the understanding of mechanisms responsible for the ontogenetic changes of mammalian intestine. This review presents the current knowledge about the development of intestinal transport function in the context of intestinal mucosa ontogeny. The review predominantly focuses on signals that trigger and/or modulate the developmental changes of intestinal transport. After an overview of the proliferation and differentiation of intestinal mucosa, data about the bidirectional traffic (absorption and secretion) across the developing intestinal epithelium are presented. The largest part of the review is devoted to the description of developmental patterns concerning the absorption of nutrients, ions, water, vitamins, trace elements, and milk-borne biologically active substances. Furthermore, the review examines the development of intestinal secretion that has a variety of functions including maintenance of the fluidity of the intestinal content, lubrication of mucosal surface, and mucosal protection. The age-dependent shifts of absorption and secretion are the subject of integrated regulatory mechanisms, and hence, the input of hormonal, nervous, immune, and dietary signals is reviewed. Finally, the utilization of energy for transport processes in the developing intestine is highlighted, and the interactions between various sources of energy are discussed. The review ends with suggestions concerning possible directions of future research.

Developmental aspects of lipid and lipoprotein synthesis and secretion in human gut

This review article focuses on the ontogeny and the regulatory mechanisms involved in the modulation of the intracellular events governing the assembly and delivery of lipoproteins in human gut. The human fetal intestine organizes villi covered with well-differentiated enterocytes during the end of the first trimester in utero. One striking event is the formation of villi in the colonic mucosa similar to those of the small intestine. The small intestine exhibits very early (14-20 weeks) the capacity to absorb lipids, to elaborate most of the major lipoprotein classes (chylomicrons, very-low-density lipoproteins, low-density lipoproteins, high-density lipoproteins), and to efficiently export these lipoproteins from the intestinal cells. The ontogenic changes of lipid and lipoprotein synthesis are correlated with specific patterns of regulatory enzymes (HMG-CoA reductase, ACAT, MGAT) that are representative of key patterns such as the cholesterol pathway, cholesterol esterification, and neutral lipid pathway. The human fetal colon also has the capability to synthesize lipids, lipoproteins, and apolipoproteins. However, comapred with the small intestine, it is much less efficient at exporting these lipoproteins. Epidermal growth factor, insulin, and hydrocortisone, which are known modulators of the brush border digestive functions of the human gut, differentially modulate the synthesis and secretion of lipoproteins in the small intestine and colon. The use of human fetal gut represents a unique model to further our understanding of the complex biosynthetic molecular events essential for the formation and secretion of lipoproteins relevant to human intestine, both in normal or pathological conditions.

Development of the human gastrointestinal tract: twenty years of progress

A combination of approaches has begun to elucidate the mechanisms of gastrointestinal development. This review describes progress over the last 20 years in understanding human gastrointestinal development, including data from both human and experimental animal studies that address molecular mechanisms. Rapid progress is being made in the identification of genes regulating gastrointestinal development. Genes directing initial formation of the endoderm as well as organ-specific patterning are beginning to be identified. Signaling pathways regulating the overall right-left asymmetry of the gastrointestinal tract and epithelial-mesenchymal interactions are being clarified. In searching for extrinsic developmental regulators, numerous candidate trophic factors have been proposed, but compelling evidence remains elusive. A critical gene that initiates pancreas development has been identified, as well as a number of genes regulating liver, stomach, and intestinal development. Mutations in genes affecting neural crest cell migration have been shown to give rise to Hirschsprung's disease. Considerable progress has been achieved in understanding specific phenomena, such as the transcription factors regulating expression of sucrase-isomaltase and fatty acid-binding protein. The challenge for the future is to integrate these data into a more complete understanding of the physiology of gastrointestinal development.

Diet influences development of the pig (Sus scrofa) intestine during the first 6 hours after birth

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.

Intestinal brush-border membrane enzyme activities and transport functions during prenatal development of pigs

Enzyme activities and rates of leucine and glucose uptake were measured using brush-border membrane vesicles prepared from the small intestine of 7-, 8-, 10-, and 12-week fetal (43, 49, 61, and 74% of gestation) and unsuckled, neonatal pigs. Lactase was detected in 7-week fetuses, with a large increase in activity between 10 weeks of gestation and birth. gamma-Glutamyltranspeptidase activity was stable throughout gestation, whereas sucrase activity was not detected. Active L-leucine uptake was already present at 7 weeks of gestation, with an increasing distal-to-proximal gradient observed at birth. D-glucose uptake was low at 7 weeks, but by 8 weeks it exhibited a typical overshoot phenomenon and established a decreasing proximal-to-distal gradient by 12 weeks. D-glucose uptake at all ages was directly related to incubation temperature, but less so for 7- and 10-week fetuses. By 12 weeks strict Na(+)-dependency of D-glucose uptake was observed along the entire length of the small intestine. Kinetic analysis of Na(+)-D-glucose cotransport showed a shift from the presence of both high- and low-affinity systems at 8 weeks of gestation to a single high-affinity Michaelian component at birth. In light of similarities with human fetuses, the pig may be a valuable model for studying development of intestinal transport during gestation, particularly during the final trimester, when availability of human tissue is limited.

Chloride-dependent amino acid transport in the small intestine: occurrence and significance

The unidirectional influx of amino acids, D-glucose and ions across the brush-border membrane of the small intestine of different species has been measured in vitro with emphasis on characterization of topographic and species differences and on chloride dependence. The regional differences in transport along the small intestine are outlined and shown to be caused by variation in transport capacity, while the apparent affinity constants are unchanged. Rabbit small intestine is unique by exhibiting maximal rates of transport in the distal ileum and a very steep decline in the oral direction from where tissues are normally harvested for preparation of brush-border membrane vesicles. Transport in the guinea pig and rat is much more constant throughout the small intestine. Since the capacity of nutrient carriers is regulated by their substrates it is possible that bacterial breakdown of peptides and proteins in rabbit distal ileum increases the concentration of amino acids leading to an upregulation of the carriers. Chloride dependence is a characteristics of the carrier rather than the transported amino acid, and is used to improve the classification of amino acid carriers in rabbit small intestine. In this species the imino acid carrier, the beta-amino acid carrier, and the beta-alanine carrier, which should be renamed the B0,+ carrier, are chloride-dependent. The steady-state mucosal uptake of classical substrates for these carriers in biopsies from the human duodenum is also chloride-dependent. The carrier of beta-amino acids emerges as ubiquitous and chloride-dependent, and evidence of cotransport with both sodium and chloride is reviewed. A sodium:chloride:2-methyl-aminoisobutyric acid coupling stoichiometry of approx. 2:1:1 is suggested by ion activation studies. Direct measurements of coupled ion fluxes in rabbit distal ileum confirm that sodium, chloride and 2-methyl-aminoisobutyric acid are cotransported on the imino acid carrier with an identical influx stoichiometry. Control experiments and reference to the literature on the electrophysiology of the small intestine exclude alterations of the membrane potential as a feasible explanation of the chloride dependence. Thus, it is concluded that chloride is cotransported with both sodium and 2-methyl-aminoisobutyric acid across the brush-border membrane of rabbit distal ileum.

Ontogeny of Na+/D-glucose cotransport in guinea-pig jejunal vesicles: only one system is involved at both 20 degrees C and 35 degrees C

The kinetic parameters of Na+/D-glucose cotransport were examined in fetal, newborn and adult guinea-pig jejunal brush-border membrane vesicles using a displacement curve and non-linear regression procedure. Our data indicated the presence of a single system with a Km of 0.34 +/- 0.04 mM at both 20 degrees C and 35 degrees C. Vmax was increased by about 4-fold when the kinetic experiments were performed at 35 degrees C. Since our results were not in agreement with the findings of Brot-Laroche et al. (J. Biol. Chem. (1986) 261, 6168-6176) which indicated the existence of a distinct D-glucose transport system in the adult guinea-pig jejunum at 35 degrees C, we verified the influence of their experimental conditions on initial rate uptake measurements. In the presence of D-sorbitol instead of D-mannitol in the transport media, 70% inhibition of D-glucose uptake was observed, an effect which was attributable to contamination of sorbitol preparations by D-glucose. After removal of glucose contamination D-sorbitol did not significantly reduce the initial rate of D-glucose transport. These results led us to conclude the existence of a single D-glucose transport system in the guinea-pig small intestine and to stress the choice of experimental conditions as being crucial for an accurate estimation of kinetic parameters.

Characterization of human placental activity for transport of L-alanine, using brush border (microvillous) membrane vesicles

To characterize the placental activity for the transport of amino acids, the uptake of L-alanine was investigated by rapid membrane filtration using brush border membrane vesicles separated from the human placenta of early pregnant (12-13 gestational weeks) and late pregnant (37-38 gestational weeks) women. The uptake of L-alanine into the brush border membrane vesicles at early and late stage of gestation showed a pattern of transport dependent on the intra- and extravesicular Na+ concentration gradient (extravesicular Na+ greater than intravesicular Na+). The Na+ concentration gradient-dependent uptake of L-alanine into the brush border membrane vesicles at early and late stage of gestation also showed a dependency on the potential difference of the internal and external membrane. The transport of L-alanine into the brush border membrane vesicles was markedly augmented at late stage of gestation. On the basis of the double reciprocal plotting of the L-alanine concentration and the Na(+)-dependent uptake of L-alanine into the vesicles, Km and Vmax were calculated as parameters of the Na(+)-dependent uptake of L-alanine into the vesicles. In the early stage of gestation Km was 0.78 mM, and in the late stage of gestation was 0.80 mM. In the early stage of gestation Vmax (nmol/mg protein/20 sec) was 0.62, and in the late stage of gestation was 3.53. From the results, it was considered that the placental active transport mechanism of L-alanine is the same in the early and late stages of gestation, and it was shown that its transport activity increases greatly in late stages compared to the early stages of gestation.

Multiple pathways for amino acid transport in brush border membrane vesicles isolated from the human fetal small intestine

The present study was undertaken to identify the different amino acid transport pathways present in the human small intestine during the early gestational period. The uptake time courses of neutral (L-leucine, L-alanine, L-methionine), acidic (L-glutamic and D-aspartic acids), basic (L-lysine), and imino (L-proline) acids have been studied in brush border membrane vesicles isolated from both proximal and distal parts of the human fetal small intestine. Both Na(+)-dependent and Na(+)-independent uptake pathways have been identified all along the small intestine. The Na(+)-dependent systems are as follows: (a) the NBB system for neutral amino acids such as L-leucine and L-alanine; (b) the PHE system for L-methionine; (c) the x-ag system for L-glutamic and D-aspartic acids; and (d) the IMINO system for L-proline. The Na(+)-independent pathways are represented by the L system for most of the neutral amino acids and maybe L-proline and by the basic amino system y+ for L-lysine uptake. These results demonstrate that the different uptake pathways for transport of amino acids are present in the human fetal intestine and that their characteristics in terms of Na+ requirement and proximodistal activity gradient are already established in the early stages of the human development.

Analysis of kinetic data in transport studies: new insights from kinetic studies of Na(+)-D-glucose cotransport in human intestinal brush-border membrane vesicles using a fast sampling, rapid filtration apparatus

Using the fast sampling, rapid filtration apparatus (FSRFA) recently developed in our laboratory (Berteloot et al., 1991, J. Membrane Biol. 122:111-125), we have studied the kinetic characteristics of Na(+)-D-glucose cotransport in brush-border membrane vesicles isolated from normal adult human jejunum. True initial rates of transport have been determined at both 20 and 35 degrees C using a dynamic approach which involves linear-regression analysis over nine time points equally spaced over 4.5 or 2.7 sec, respectively. When the tracer rate of transport was studied as a function of unlabeled substrate concentrations added to the incubation medium, a displacement curve was generated which can be analyzed by nonlinear regression using equations which take into account the competitive inhibition of tracer flux by unlabeled substrate. This approach was made imperative since at 20 degrees C, in the presence of high substrate concentrations or 1 mM phlorizin, no measurable diffusion was found and the resultant zero slope values cannot be expressed into a classical v versus S plot. All together, our results support the existence of a single Na(+)-D-glucose cotransport system in these membranes for which Na+ is mandatory for uptake. This conclusion is at variance with that of a recent report using the same preparation (Harig et al., 1989. Am J. Physiol. 256:8618-8623). Since the discrepancy seems difficult to resolve on the consideration of experimental conditions alone, we have determined the kinetic parameters of D-glucose transport using one time point measurements and linear transformations of the Michaelis-Menten equation, in order to investigate the potential problems of such a widely used procedure. Comparing these approaches, we conclude that: (i) the dynamic uptake measurements give a better understanding of the different uptake components involved: (ii) it does not matter whether a dynamic or a one time point approach is chosen to generate the uptake data provided that a nonlinear-regression analysis with proper weighting of the data points is performed; (iii) analytical procedures which rely on linearization of Michaelian process(es) are endowed with a number of difficulties which make them unsuitable to resolve multicomponent systems in transport studies. A more general procedure which uses a nonlinear-regression analysis and a displacement curve is proposed since we demonstrate that it is far superior in terms of rapidity, data interpretation, and visual information.

Zinc binding in intestinal brush-border membrane isolated from pig

Zinc binding to brush-border membrane vesicles isolated from pig jejunum was investigated by a rapid filtration method, for long incubation periods (up to 180 min). Zn2+ influx revealed a large accumulation of the metal, reaching an apparent intravesicular volume of 160 microliters/mg protein at equilibrium, a volume 45-times that of an osmotically reactive sugar, sorbitol (3.6 microliters/mg protein). Changes in medium osmolarity had no effect on zinc accumulation. These results suggested a large degree of zinc binding to vesicular components (membrane or core). 65Zn efflux measurements led to the conclusion that two vesicular pools of zinc existed: a small external pool, accessible to different chelators (EGTA) or competitive cations, and a large intravesicular pool. Accumulated 65Zn was quickly removed from its internal sites only after the membrane had been permeabilized by the cation ionophore A23187 in association with an exchange molecule or a chelator. Scatchard plot analyses revealed, on one hand a first class of high-affinity extravesicular zinc binding sites (Ka = 8.6.10(3) M-1, n = 0.455 nmol Zn2+/mg protein) and a second class of extravesicular sites having a very low affinity (Ka = 22 M-1, n = 25.35 nmol Zn2+/mg protein) and, on the other hand one type of intravesicular sites (Ka = 3.3.10(4) M-1, n = 550 nmol Zn2+/mg protein). The intravesicular sites have a high affinity for zinc and are specific, since only nonlabelled zinc (or cadmium) but not calcium present in the bathing medium is exchanged with the internally accumulated labelled cation.

Separation of two distinct Na+/D-glucose cotransport systems in the human fetal jejunum by means of their differential specificity for 3-O-methylglucose

Based on kinetic arguments, we have recently proposed the existence of two distinct Na+/D-glucose cotransporters in brush-border membrane vesicles isolated from the human fetal jejunum (Biochim. Biophys. Acta 938 (1988) 181-188). In order to further test this hypothesis, inhibition studies of the zero-trans influx of substrate have been performed under Na(+)-gradient and voltage-clamped conditions. Initial rates of D-glucose uptake were totally abolished by D-glucose, D-galactose, alpha-methylglucose and phlorizin while 3-O-methylglucose and phloretin induced only a 65% inhibition even at the highest concentrations used. The residual activity of D-glucose uptake is thus compatible with substrate flux through a low-affinity transport system which is insensitive to phloretin and does not accept 3-O-methylglucose as substrate. This substrate specificity has been used to separate kinetically the two putative pathways for glucose transport. The data obtained are compatible with the existence of the following two systems: (i) a low-affinity, high-capacity system with a Km of 4.7 mM and a Vmax of 22 nmol/min per mg of protein, and; (ii) a high-affinity, low-capacity system with a Km of 0.57 mM and a Vmax of 10.7 nmol/min per mg of protein. These data thus demonstrate clearly the existence of two distinct Na(+)-dependent D-glucose carriers in the human jejunum during the early gestation period since these systems can be differentiated not only by their kinetic properties but also by their differences in both substrate and inhibitor specificities.

Preparation of microvillus membrane vesicles from the intestine of embryonic and young chicks

1. A procedure is described for the isolation of microvillus membranes from 19-day old embryonic, newly hatched, and 2-day old chicken intestine. 2. The magnesium concentration of epithelial cell homogenates is shown to be a crucial factor in obtaining membranes of equal purity from the three age groups. 3. Microvillus membranes are purified 20-25 fold over the original homogenate and form vesicles which are tightly sealed based on the Na+-dependent accumulation of glucose to levels four to five times equilibrium values. 4. These membrane preparations should prove useful in future studies concerning the embryonic and neonatal development of microvillus enzymes and nutrient transport systems in the chicken.

Kinetic evidence for heterogeneity in Na+-D-glucose cotransport systems in the normal human fetal small intestine

Zero-trans kinetic studies of Na+-D-glucose cotransport have been performed under voltage-clamped conditions in brush-border membrane vesicles isolated from both jejunum and ileum of 17-20-week-old normal human fetuses. Varying glucose concentrations in the incubation medium led to curvilinear Eadie-Hofstee plots in the jejunum only, thus suggesting the presence of both high-affinity, low-capacity (Km 0.37 mM; Vmax 8.3 nmol/min per mg protein) and low-affinity, high-capacity (Km 4.2 mM; Vmax 30.9 nmol/min per mg protein) systems in the proximal small intestine, and of a single carrier (Km 1.2 mM; Vmax 4.9 nmol/min per mg protein) in the distal small intestine. Sodium activation curves provide further evidence for heterogeneity in glucose transport systems in the fetal small intestine: Hill coefficients of 2 and 1 were found for the jejunal high-affinity and ileal systems, and for the jejunal low-affinity system, respectively. It is concluded that there is early differentiation of a functional heterogeneity in glucose transport capacity along the human fetal small intestine.