Differential localization and regulation of two aquaporin-1 homologs in the intestinal epithelia of the marine teleost Sparus aurata

Aquaporin (AQP)-mediated intestinal water absorption may play a major osmoregulatory role in euryhaline teleosts, although the molecular identity and anatomical distribution of AQPs in the fish gastrointestinal tract is poorly known. Here, we have investigated the functional properties and cellular localization in the intestine of two gilthead seabream (Sparus aurata) homologs of mammalian aquaporin-1 (AQP1), named SaAqp1a and SaAqp1b. Heterologous expression in Xenopus laevis oocytes showed that SaAqp1a and SaAqp1b were water-selective channels. Real-time quantitative RT-PCR and Western blot using specific antisera indicated that abundance of SaAqp1a mRNA and protein was higher in duodenum and hindgut than in the rectum, whereas abundance of SaAqp1b was higher in rectum. In duodenum and hindgut, SaAqp1a localized at the apical brush border and lateral membrane of columnar enterocytes, whereas SaAqp1b was detected occasionally and at very low levels at the apical membrane. In the rectum, however, SaAqp1a was mainly accumulated in the cytoplasm of a subpopulation of enterocytes spread in groups over the surface of the epithelia, including the intervillus pockets, whereas SaAqp1b was detected exclusively at the apical brush border of all rectal enterocytes. Freshwater acclimation reduced the synthesis of SaAqp1a protein in all intestinal segments, but it only reduced SaAqp1b abundance in the rectum. These results show for the first time in teleosts a differential distribution and regulation of two functional AQP1 homologs in the intestinal epithelium, which suggest that they may play specialized functions during water movement across the intestine.     

Sodium and chloride transport in soft water and hard water acclimated zebrafish (Danio rerio)

While the zebrafish is commonly used for studies of developmental biology and toxicology, very little is known about their osmoregulatory physiology. The present investigation of Na(+) and Cl(-) transport revealed that the zebrafish is able to tolerate extremely low ambient ion concentrations and that this is achieved at least in part by a greatly enhanced apparent uptake capacity and affinity for both ions. Zebrafish maintain plasma and whole body electrolyte concentrations similar to most other freshwater teleosts even in deionized water containing only 35 microM NaCl, i.e soft water. We recorded an extremely low transport affinity constant (K(m)) of 8+/-1 microM for the active uptake of Cl(-) in soft water acclimated fish, while other transport kinetic parameters were in agreement with reports for other freshwater organisms. While both Na(+) and Cl(-) uptake in soft water clearly depends on apical proton pump activity, changes in abundance and possibly localization of this protein did not appear to contribute to soft water acclimation. Active Cl(-) uptake was strongly dependent on branchial carbonic anhydrase (CA) activity regardless of water type, while the response of Na(+) transport to a CA inhibitor was more variable. Differential response of Na(+) uptake to amiloride depending on acclimation medium suggests that different Na(+) transport mechanisms are employed by zebrafish acclimated to soft and hard water.     

两种有胃鱼消化道6种内分泌细胞的免疫组织化学比较研究

目的 应用6种胃肠激素抗血清对胡子鲶(Clarias fuscas)和瓦氏黄颡鱼(Pelteobagrus vachelli Richardson)消化道内分泌细胞进行了免疫组织化学比较.方法 应用链霉亲和素-生物素-过氧化物酶复合物技术(streptavidin biotin-peroxidase complex method,SABC法)免疫组织化学方法.结果 除胡子鲶后肠外,五羟色胺细胞在两种鱼的消化道各段均有分布,其中贲门部或幽门部密度最高,前中肠次之,食道最少;胡子鲶消化道中生长抑素细胞主要分布于胃贲门部和胃部,食管、胃幽门及肠道内未见分布,而瓦氏黄颡主要分布于食管和胃部,肠道内未见分布;胃泌素细胞在胡子鲶消化道内只在肠内检测到,瓦氏黄颡只在幽门胃和前肠两个部位检测到;胰多肽细胞只在瓦氏黄颡的幽门胃和前肠两个部位有分布;胰高血糖素和P-物质两种细胞在两种鱼的消化道各段均未见其分布.结论 两种有胃鱼消化道中六种内分泌细胞的分布既有一定的共性,体现了两者消化生理的共同点;同时又存在较大的种间差异,与各自食性及生活环境相适应.     

Epithelial cell proliferation in the intestine of the winter flounder,Pseudopleuronectes americanus

Summary To study epithelial cell proliferation in the North American flounder (Pseudopleuronectes americanus), fed and fasted fish received intravenous injections of ³H-thymidine and were killed 11/2 to 2 h later. Radioautographs of proximal, middle, and distal intestinal segments revealed proliferating epithelial cells at all levels of intestinal folds including the crest although labelled nuclei were most abundant in the epithelial cells on the lower half of folds and between folds. Mature appearing goblet cells with labelled nuclei were observed at all levels of the folds. The mean labelling index was greater in the epithelium of fed than fasted flounder. In fed flounder the mean labelling index was greatest in the proximal segment and least in the distal segment; no substantive differences in mean labelling indices were observed in the various segments of intestine from fasted fish. Electron microscopy revealed no major structural differences among epithelial cells along the base of folds compared to cells near the crest of folds. These findings indicate that 1) epithelial cell proliferation occurs at all levels of the folds of flounder intestine and is not compartmentalized to the base of the folds and interfold epithelium as reported in other teleosts, and 2) epithelial cell proliferation in the flounder intestine varies with feeding status.Supported be research grants AM 17537 and RR 05764 from the National Institutes of Health, Bethesda, Maryland and grant DEB7826821 AO1 from the National Science Foundation, Washington, D.C.The authors are grateful to Dr. Michael Field for stimulating discussions and suggestions and for providing facilities for collecting material from fish     

Osmoregulatory bicarbonate secretion exploits H+-sensitive haemoglobins to autoregulate intestinal O2 delivery in euryhaline teleosts

Marine teleost fish secrete bicarbonate (HCO₃ ^?) into the intestine to aid osmoregulation and limit Ca²⁺ uptake by carbonate precipitation. Intestinal HCO₃ ^? secretion is associated with an equimolar transport of protons (H⁺) into the blood, both being proportional to environmental salinity. We hypothesized that the H⁺-sensitive haemoglobin (Hb) system of seawater teleosts could be exploited via the Bohr and/or Root effects (reduced Hb-O₂ affinity and/or capacity with decreasing pH) to improve O₂ delivery to intestinal cells during high metabolic demand associated with osmoregulation. To test this, we characterized H⁺ equilibria and gas exchange properties of European flounder (Platichthys flesus) haemoglobin and constructed a model incorporating these values, intestinal blood flow rates and arterial–venous acidification at three different environmental salinities (33, 60 and 90). The model suggested red blood cell pH (pH_i) during passage through intestinal capillaries could be reduced by 0.14–0.33 units (depending on external salinity) which is sufficient to activate the Bohr effect (Bohr coefficient of ?0.63), and perhaps even the Root effect, and enhance tissue O₂ delivery by up to 42 % without changing blood flow. In vivo measurements of intestinal venous blood pH were not possible in flounder but were in seawater-acclimated rainbow trout which confirmed a blood acidification of no less than 0.2 units (equivalent to ?0.12 for pH_i). When using trout-specific values for the model variables, predicted values were consistent with measured in vivo values, further supporting the model. Thus this system is an elegant example of autoregulation: as the need for costly osmoregulatory processes (including HCO₃ ^? secretion) increases at higher environmental salinity, so does the enhancement of O₂ delivery to the intestine via a localized acidosis and the Bohr (and possibly Root) effect.     

A quantitative study of the morphological development and bacterial colonisation of the gut of the tammar wallaby Macropus eugenii eugenii and brushtail possum Trichosurus vulpecula during in-pouch development

We compared the rates of change of various morphological parameters of the stomach, small intestine, caecum and colon of tammar wallabies and brushtail possums with body mass during in-pouch development. These were correlated with changes in the numbers of bacterial species in the various gut segments. In the pouch-young of both species, the wet tissue masses of all gut segments increased with body mass in a positively allometric manner (i.e. with a body mass exponent > 1), suggesting that the mass of each component was disproportionately low at birth, but increased disproportionately rapidly postnatally. However, the lengths of the wallaby stomach and small intestine scaled isometrically with respect to body mass (i.e. with a body mass exponent around 0.33), which may indicate that the shape of these components changes to the adult form during early neonatal development. Conversely, the length of the caecum and colon of both wallabies and possums scaled in a positively allometric manner with respect to body mass, showing area to volume compensation. This may indicate a more general pattern of disproportionately rapid postnatal enlargement in areas that are distal to the principal sites of neonatal digestion (i.e. the stomach). The numbers of bacterial species present in the various gastrointestinal segments of both species were low in animals aged 100 days or less but there was a significant increase in microbial diversity in the caecum of brushtail possums aged over 100 days. The possum caecum also showed the greatest rate of increase in wet tissue mass relative to body mass. It is postulated that caecal development may act as a nidus for establishment of communities of commensal microflora in the developing marsupial.     

Body fluid volume regulation in elasmobranch fish

This review addresses an often overlooked aspect of elasmobranch osmoregulation, i.e., control of body fluid volume. More specifically the review addresses the impact of changes in blood volume in elasmobranchs exposed to different environmental salinities. Measurement of blood volume in the European lesser-spotted dogfish, Scyliorhinus canicula, following acute and chronic exposure to 80% and 120% seawater (SW) is reported. In 80%, 100% and 120% SW-adapted S. canicula, blood volume was 6.3+/-0.2, 5.6+/-0.2 and 4.6+/-0.2 mL 100 g(-1) body mass, respectively. Blood volume was significantly higher and lower in 80% and 120% SW-acclimated animals compared to 100% SW controls. Comparisons are made between these results and previously published data. The role of drinking and volume regulation in elasmobranchs is discussed. For the first time measured water reabsorption rates and solute flux rates across the elasmobranch intestinal epithelia are presented. Water reabsorption rates did not differ between 100% SW-adapted bamboo shark, Chiloscyllium plagiosum, and fish acutely transferred to 140% SW. For the most part net solute flux rates and direction for both the 100% and 140% SW groups were the same with the exception of a net efflux of chloride and potassium in the 140% group and influx of these ions in the 100% adapted group. The significance of the intestine as part of the overall elasmobranch osmoregulatory strategy is discussed as is the role of the kidneys, rectal gland and gills in the regulation of body fluid volume in this class of vertebrates.     

Oocyte development in summer flounder: seasonal changes and steroid correlates

Seven ovarian stages are described in summer flounder Paralichthys dentatus. In the prespawning season plasma oestradiol levels increased in maturing fish with lipidogenic oocytes and gonadosomatic index increased in fish undergoing vitellogenesis. Atretic oocytes present in the postspawning season indicated which individuals may have spawned. The pattern of oocyte development is similar to that of other flatfishes and some teleosts. The summer flounder was unusual in having a long lipid uptake phase (oocyte diameter up to 301 μm) prior to any indication of vitellogenin (yolk protein) uptake. This information will be useful in the construction of an updated maturity schedule for the wild population.     

In vitro characterization of calcium transport along the gastrointestinal tract of freshwater rainbow trout Oncorhynchus mykiss

Using an in vitro gut-sac technique, this study examined the mechanisms of calcium (Ca) uptake along the gastrointestinal tract (GIT) of rainbow trout Oncorhynchus mykiss. Ca uptake into three different compartments (mucous-bound, mucosal epithelium and blood space) of four distinct GIT segments (stomach, anterior intestine, mid intestine and posterior intestine) was monitored after luminal exposure to 10 mM Ca saline (radiolabelled with (45) Ca). Ca transport was determined to be both time-dependent and concentration-dependent. The concentration-dependent kinetics of Ca uptake was investigated using varying luminal concentrations of Ca (1, 10, 30, 60 and 100 mM). In the blood-space compartment, Ca uptake was saturable at high Ca concentrations in the mid intestine (suggesting mediated transport), while linear uptake was found in the other gut segments. In the mucous-bound and mucosal epithelium compartments, however, saturation kinetics were found for most GIT segments, also suggesting mediated transport. Manipulation of serosal saline osmotic pressure with mannitol demonstrated that Ca uptake was not greatly affected by solvent drag. Elevated mucosal cadmium (Cd) did not appear to inhibit Ca uptake into the blood space in any of the GIT sections, and Ca uptake did not appear to be sodium dependent. Maximum transport capacities for Ca and Cd were found to be comparable between the gills and gut, but affinities were much higher at the gills (up to 3000 times).     

The integration of digestion and osmoregulation in the avian gut

We review digestion and osmoregulation in the avian gut, with an emphasis on the ways these different functions might interact to support or constrain each other and the ways they support the functioning of the whole animal in its natural environment. Differences between birds and other vertebrates are highlighted because these differences may make birds excellent models for study and may suggest interesting directions for future research. At a given body size birds, compared with mammals, tend to eat more food but have less small intestine and retain food in their gastrointestinal tract (GIT) for shorter periods of time, despite generally higher mass‐specific energy demands. On most foods, however, they are not less efficient at digestion, which begs the question how they compensate. Intestinal tissue‐specific rates of enzymatic breakdown of substrates and rates of active transport do not appear higher in birds than in mammals, nor is there a demonstrated difference in the extent to which those rates can be modulated during acclimation to different feeding regimes (e.g. diet, relative intake level). One compensation appears to be more extensive reliance on passive nutrient absorption by the paracellular pathway, because the avian species studied so far exceed the mammalian species by a factor of at least two‐ to threefold in this regard. Undigested residues reach the hindgut, but there is little evidence that most wild birds recover microbial metabolites of nutritional significance (essential amino acids and vitamins) by re‐ingestion of faeces, in contrast to many hindgut fermenting mammals and possibly poultry. In birds, there is some evidence for hindgut capacity to breakdown either microbial protein or protein that escapes the small intestine intact, freeing up essential amino acids, and there is considerable evidence for an amino acid absorptive capacity in the hindgut of both avian and mammalian hindgut fermenters. Birds, unlike mammals, do not excrete hyperosmotic urine (i.e. more than five times plasma osmotic concentration). Urine is mixed with digesta rather than directly eliminated, and so the avian gut plays a relatively more important role in water and salt regulation than in mammals. Responses to dehydration and high‐ and low‐salt loads are reviewed. Intestinal absorption of ingested water is modulated to help achieve water balance in one species studied (a nectar‐feeding sunbird), the first demonstration of this in any terrestrial vertebrate. In many wild avian species the size and digestive capacity of the GIT is increased or decreased by as much as 50% in response to nutritional challenges such as hyperphagia, food restriction or fasting. The coincident impacts of these changes on osmoregulatory or immune function of the gut are poorly understood.     

Comparative expression of hexose transporters (SGLT1, GLUT1, GLUT2 and GLUT5) throughout the mouse gastrointestinal tract

Hexose transporters play a pivotal role in the absorption of food-derived monosaccharides in the gastrointestinal tract. Although a basic knowledge of the hexose transporters has already been gained, their detailed distribution and comparative intensities of expression throughout the gastrointestinal tract have not been fully elucidated. In this study, we quantitatively evaluated the expression of SGLT1, GLUT1, GLUT2, and GLUT5 by in situ hybridization and real-time PCR techniques using a total of 28 segments from the gastrointestinal tract of 9-week-old mice. GLUT2 and GLUT5 mRNA expressions were detected predominantly from the proximal to middle parts of the small intestine, showing identical expression profiles, while SGLT1 mRNA was expressed not only in the small intestine but also in the large intestine. Notably, GLUT1 mRNA was expressed at a considerable level in both the stomach and large intestine but was negligible in the small intestine. Immunohistochemistry demonstrated the polarized localization of hexose transporters in the large intestine: SGLT1 on the luminal surface and GLUT1 on the basal side of epithelial cells. The present study provided more elaborate information concerning the localization of hexose transporters in the small intestine. Furthermore, this study revealed the significant expression of glucose transporters in the large intestine, suggesting the existence of the physiological uptake of glucose in that location in mice.     

Osmoregulatory physiology of pyloric ceca: regulated and adaptive changes in chinook salmon

Functions of the anatomically obvious, yet peculiar, pyloric ceca of the fish gut have been a source of conjecture for over two millennia since Aristotle hypothesized on digestive utilities. Here, we demonstrate regulated and adaptive changes in osmoregulatory physiology of ceca from chinook salmon (Onchorhynchus tshawytscha). Transfer of salmon from freshwater to seawater (both short- and long-term) significantly stimulated both fluid uptake from 5.1 to 8.3-9.3 microl/cm2/hr and also Na+/K+ -ATPase from 6.5 to 8.3-9.6 micromol/ADP/mg protein/hr. Similar changes were induced with implants of cortisol, which resulted in high physiological cortisol levels in plasma. Ceca, which can number about 200 in chinook salmon, were estimated to account for the majority of fluid uptake capacity of the intestine and, after long-term seawater adaptation, the proportion of uptake capacity was sixfold higher. Transport physiology of ceca is thus under environmental and endocrine control indicative of an important role in salt and water homeostasis.     

Osmoregulation and epithelial water transport: lessons from the intestine of marine teleost fish

For teleost fish living in seawater, drinking the surrounding medium is necessary to avoid dehydration. This is a key component of their osmoregulatory strategy presenting the challenge of excreting excess salts while achieving a net retention of water. The intestine has an established role in osmoregulation, and its ability to effectively absorb fluid is crucial to compensating for water losses to the hyperosmotic environment. Despite this, the potential for the teleost intestine to serve as a comparative model for detailed, integrative experimental studies on epithelial water transport has so far gone largely untapped. The following review aims to present an assessment of the teleost intestine as a fluid-transporting epithelium. Beginning with a brief overview of marine teleost osmoregulation, emphasis shifts to the processing of ingested seawater by the gastrointestinal tract and the characteristics of intestinal ion and fluid transport. Particular attention is given to acid–base transfers by the intestine, specifically bicarbonate secretion, which creates the distinctly alkaline gut fluids responsible for the formation of solid calcium carbonate precipitates. The respective contributions of these unique features to intestinal fluid absorption, alongside other recognised ion transport processes, are then subsequently considered within the wider context of the classic physiological problem of epithelial water transport.     

Regulation of gut function varies with life-history traits in chuckwallas (Sauromalus obesus: Iguanidae)

We examined the effects of hibernation and fasting on intestinal glucose and proline uptake rates of chuckwallas (Sauromalus obesus) and on the size of organs directly or indirectly related to digestion. These lizards show geographic variation in body size and growth rate that parallels an elevational gradient in our study area. At low elevation, food is available only for a short time during the spring; at high elevation, food may also be available during summer and autumn, depending on rainfall conditions in a given year. We hypothesized that low-elevation lizards with a short season of food availability would show more pronounced regulation of gut size and function than high-elevation lizards with prolonged or bimodal food availability. Hibernating lizards from both elevations had significantly lower uptake rates per milligram intestine for both nutrients, and lower small intestine mass, than active lizards. The combination of these two effects resulted in significantly lower total nutrient uptake in hibernating animals compared to active ones. The stomach, large intestine, and cecum showed lower masses in hibernators, but these results were not statistically significant. The heart, kidney, and liver showed no difference in mass between hibernating and nonhibernating animals. Lizards from low elevations with a short growing season also showed a greater increase in both uptake rates and small intestine mass from the hibernating to the active state, compared to those from high elevations with longer growing seasons. Thus, compared to those from long growing season areas, lizards from short growing season areas have equal uptake capacity during hibernation but much higher uptake capacity while active and feeding. This pattern of regulation of gut function may or may not be an adaptive response, but it is consistent with variation in life-history characteristics among populations. In areas with a short season, those lizards that can extract nutrients quickly and then reduce the gut will be favored; in areas where food may be available later in the year, those lizards that maintain an active gut would be favored. While other researchers have found much greater magnitudes of gut regulation when making comparisons among species, we find the different patterns of change in gut function between different populations of chuckwallas particularly intriguing because they occur within a single species.     

Ontogeny of osmoregulation in postembryonic fish: a review

Salinity and its variations are among the key factors that affect survival, metabolism and distribution during the fish development. The successful establishment of a fish species in a given habitat depends on the ability of each developmental stage to cope with salinity through osmoregulation. It is well established that adult teleosts maintain their blood osmolality close to 300 mosM kg(-1) due to ion and water regulation effected at several sites: tegument, gut, branchial chambers, urinary organs. But fewer data are available in developing fish. We propose a review on the ontogeny of osmoregulation based on studies conducted in different species. Most teleost prelarvae are able to osmoregulate at hatch, and their ability increases in later stages. Before the occurrence of gills, the prelarval tegument where a high density of ionocytes (displaying high contents of Na+/K+-ATPase) is located appears temporarily as the main osmoregulatory site. Gills develop gradually during the prelarval stage along with the numerous ionocytes they support. The tegument and gill Na+/K+-ATPase activity varies ontogenetically. During the larval phase, the osmoregulatory function shifts from the skin to the gills, which become the main osmoregulatory site. The drinking rate normalized to body weight tends to decrease throughout development. The kidney and urinary bladder develop progressively during ontogeny and the capacity to produce hypotonic urine at low salinity increases accordingly. The development of the osmoregulatory functions is hormonally controlled. These events are inter-related and are correlated with changes in salinity tolerance, which often increases markedly at the metamorphic transition from larva to juvenile. In summary, the ability of ontogenetical stages of fish to tolerate salinity through osmoregulation relies on integumental ionocytes, then digestive tract development and drinking rate, developing branchial chambers and urinary organs. The physiological changes leading to variations in salinity tolerance are one of the main basis of the ontogenetical migrations or movements between habitats of different salinity regimes.     

Ionoregulatory physiology of two species of African lungfishes Protopterus dolloi and Protopterus annectens

Basic ionoregulatory physiology was characterized in two species of African lungfish, slender African lungfish Protopterus dolloi and West African lungfish Protopterus annectens , largely under aquatic conditions. There were no substantive differences between the two species. Plasma [Na], [Cl] and [Ca] were only 60–80% of those typical of freshwater teleosts, and plasma Ca activity was particularly low. Unidirectional Na and Cl influx rates from water were also very low, only c . 10% of teleost values, whereas unidirectional Ca influx rates were comparable with teleost rates. Protopterus spp. were fed a 3% ration of bloodworms every 48 h. The bloodworm diet provided similar amounts of Na and Ca as uptake from water, but almost no Cl. Efflux rates of Na and Cl through the urine were greater than via the faeces, whereas the opposite was true for Ca. Net ion flux measurements and ionic balance sheet calculations indicated that (1) both water and dietary uptake routes are important for Na and Ca acquisition; (2) the waterborne route predominates for Cl uptake; (3) unidirectional ion effluxes across the body surface (gills and skin) rather than urine and faeces are the major routes of loss for Na, Cl and Ca. Tissues (muscle, liver, lung, kidney, intestine and heart) and plasma ions were also examined in P. dolloi 'terrestrialized' in air for up to 5 months, during which plasma ion concentrations (Na, Cl, Ca and Mg) did not change and there were only a few alterations in tissue ions, that is, increased [Na] in intestine, decreased [Cl] in kidney and increased [Ca] in liver and kidney.     

Luminal and systemic signals trigger intestinal adaptation in the juvenile python

Juvenile pythons undergo large rapid upregulation of intestinal mass and intestinal transporter activities upon feeding. Because it is also easy to do surgery on pythons and to maintain them in the laboratory, we used a python model to examine signals and agents for intestinal adaptation. We surgically isolated the middle third of the small intestine from enteric continuity, leaving its mesenteric nerve and vascular supply intact. Intestinal continuity was restored by an end-to-end anastomosis between the proximal and distal thirds. Within 24 h of the snake's feeding, the reanastomosed proximal and distal segments (receiving luminal nutrients) had upregulated amino acid and glucose uptakes by up to 15-fold, had doubled intestinal mass, and thereby soon achieved total nutrient uptake capacities equal to those of the normal fed full-length intestine. At this time, however, the isolated middle segment, receiving no luminal nutrients, experienced no changes from the fasted state in either nutrient uptakes or in morphology. By 3 days postfeeding, the isolated middle segment had upregulated nutrient uptakes to the same levels as the reanastomosed proximal and distal segments, but it still lacked any appreciable morphological response. These contrasting results for the reanastomosed intestine and for the isolated middle segment suggest that luminal nutrients and/or pancreatic biliary secretions are the agents triggering rapid upregulation of transporters and of intestinal mass and that systemic nerve or hormonal signals later trigger transporter regulation but no trophic response.     

Intestinal cadmium permeability in mature and immature rats

To compare the intrinsic permeability properties of the small intestine in adult (average body wt 300 g) and 25- to 27-day-old (average body wt 50 g) male rats, the uptake rates of cycloleucine and of cadmium were measured in intestinal segments isolated in situ with their blood supply intact. Uptake rates were expressed on the basis of that of ethanol, a solute whose absorption depends primarily on the size, rather than the composition, of the available surface area and on the presence of unstirred layers. These layers may be concluded to affect movement of cycloleucine, cadmium, and ethanol to the same extent. The ratio of uptake rates, therefore, provides in arbitrary units a measure of the intrinsic permeability of the luminal surface area to cadmium and to cycloleucine. On this basis, no developmental change in cycloleucine permeability could be detected. In contrast, the rate of cadmium uptake relative to that for ethanol decreased with age by about 50%. Possible mechanisms are discussed for this significant change in the intrinsic cadmium permeability of the jejunum in post-"closure" animals.     

Intestinal anion exchange in teleost water balance

Simultaneous measurements of all major electrolytes including HCO3(-) and H+ as well as water demonstrated that fluids absorbed by the anterior intestine of the marine gulf toadfish under in vivo-like conditions on an overall net basis are hypertonic at 380 mOsm and acidic ([H+] = 27 mM). This unusual composition of fluids absorbed across the intestinal epithelium is due to the unusual intestinal fluid chemistry resulting from seawater ingestion and selective ion and water absorption along the gastro-intestinal tract. Measurement under near symmetrical conditions with high NaCl concentrations and low MgSO4 concentrations revealed absorption of iso-osmotic and much less acidic fluids by the intestinal epithelium, a situation resembling that of other water absorbing leaky vertebrate epithelia. Reduced luminal NaCl concentrations seen in vivo results in lower absolute water absorption rates but higher Cl-/HCO3(-) exchange rates which are associated with higher net H+ absorption rates. It appears that apical anion exchange is important for net Cl- uptake by the marine teleost intestine especially when luminal NaCl concentrations are low and/or when MgSO4 concentrations are high. Observations indicate that fluid absorption from solutions of low NaCl but high MgSO4 concentrations is energetically more demanding than absorption from NaCl rich solutions at the level of the intestinal epithelium. Furthermore, the high luminal MgSO4 concentration which is an unavoidable consequence of seawater ingestion projects a demand for renal and branchial compensation for intestinal MgSO4 uptake and absorption of hypertonic and acidic fluid by the intestine.     

Luminal hydrolysis of recombinant human epidermal growth factor in the rat gastrointestinal tract: segmental and developmental differences

Epidermal growth factor (EGF), present in high concentrations in the milk of various species, is biologically active following oral administration to young animals. Although in vivo studies show gastrointestinal processing of dietary EGF during early postnatal development, the relative importance of luminal and mucosal digestion in such processing is undefined. To characterize the luminal metabolism of dietary EGF in the developing gastrointestinal tract, we incubated human recombinant 125I-EGF in vitro at 37 degrees with luminal fluid from the stomach and various segments of the small intestine of 12 day old suckling and 31 day old weanling rats and analyzed the resulting reaction products. The rate of EGF hydrolysis as determined by generation of acid soluble material was greater in weanling small intestine than in suckling, with maximal hydrolytic capacity observed in the mid-jejunum and ileum. Minimal hydrolysis was observed with stomach fluid from both age groups, and EGF retained its ability to elute as a single species on Sephadex G-25 columns and to bind to monospecific affinity columns and placental membrane receptors. Incubation with suckling small intestinal fluid produced little change in the chromatographic profile on Sephadex G-25, but a reduction in antibody and receptor binding was observed. In contrast, incubation with weanling small intestinal fluid yielded both a more pronounced loss of EGF-like material on G-25 columns and a greater reduction in receptor and antibody binding. We conclude that little luminal EGF degradation occurs in the rat stomach during the suckling and weanling periods, but that in the lumen of the small intestine breakdown increases during postnatal development.