Chloride dependent amino acid transport in the human small intestine

Effects of glucocorticoids on the gene expression of nutrient transporters in different rabbit intestinal segments

Glucocorticoids (GCs) are counterregulatory hormones with broad effects on the digestion and absorption of dietary carbohydrates, lipids and proteins, but the underlying molecular mechanisms of these effects remain unclear. The present experiment was conducted to investigate the main expression sites of nutrient transporters and the effects of GCs on the gene expression of these transporters in the rabbit small intestine. The results showed that peptide transporter 1 (PepT1), facultative amino acid transporter (rBAT), neutral amino acid transporter (B0AT), excitatory amino acid transporter 3 (EAAT3), sodium-glucose transporter 1 (SGLT1) and glucose transporter 5 (GLUT5) were mainly expressed in the distal segment, glucose transporter 2 (GLUT2) and fatty-acid-binding protein 4 (FATP4) were mainly expressed in the proximal segment and cationic amino acid transporter 1 (CAT1) was mainly expressed in the middle segment of the rabbit small intestine. In addition, we analysed the effects of 3 h (short-term) or 7 days (long-term) dexamethasone (DEX) treatment on the gene expression of most nutrient transporters. The results showed that short-term DEX treatment significantly decreased PepT1, B0AT, EAAT3, rBAT and SGLT1 expressions in all small intestinal segments, while it significantly decreased GLUT2 in the duodenum and FATP4 in the duodenum and ileum (P < 0.05). Long-term DEX treatment also significantly decreased PepT1, CAT1, B0AT, EAAT3, rBAT and SGLT1 in all small intestinal segments and significantly decreased GLUT2 in the jejunum and FATP4 in the ileum (P < 0.05). In conclusion, DEX could decrease the gene expression of most nutrient transporters (except GLUT5) and affect the transport of intestinal amino acids, monosaccharides and fatty acids.

Association of clinical severity of cystic fibrosis with variants in the SLC gene family (SLC6A14, SLC26A9, SLC11A1 and SLC9A3)

INTRODUCTION

Cystic fibrosis (CF) manifests with clinical and histopathological variability depending on environmental and genetic factors. Moreover, the genes encoding ion channels[rs3788766(SLC6A14), rs7512462(SLC26A9), rs17235416(SLC11A1) and rs17563161(SLC9A3)] have been insufficiently studied as modifier genes. Then, our objective was associate the variants in the genes of SLC family with 43 CF severity markers.

METHODS

The variants were identified by real-time-PCR in 188 CF patients considering the CFTR genotype. Statistical analyses were performed by parametric and nonparametric tests. The correction by multiple testing was performed by the False Rate Discovery test, alpha=0.05.

RESULTS

Depending on the CFTR mutations, we found association of: (i) rs3788766*CC with mucoid Pseudomonas aeruginosa (OR=0.171; 95%CI=0.029-0.696), non-mucoid P. aeruginosa (OR=0.283; 95%CI=0.094-0.853) and Staphyloccocus aureus (OR=4.443; 95%CI=1.019-40.64), largest FEF_max(p=0.041) and best response to bronchodilator for FEF_50%(p=0.033) and FEV₁/FVC(p=0.044); (ii) rs3788766*CT with early start of pulmonary symptom (OR=3.524; 95%CI=1.229-10.1) and osteoporosis (OR=0.203; 95%CI=0.022-0.883); (iii) rs3788766*TT with lowest body mass index (OR=4.242; 95%CI=1.505-11.95), presence of mucoid P. aeruginosa (OR=3.176; 95%CI=1.29-7.819) and S. aureus (OR=0.116; 95%CI=0.004-0.881), highest Bhalla score (p=0.047) and lowest FEF_max(p=0.028) and FEF_25%(p=0.031) values; (iv) rs7512462*CC with highest Shwachman-Kulczycki score (p=0.019), FVC(p=0.043), FEV₁(p=0.047), FEV₁/FVC(p=0.022), FEF_50%(p=0.038) and FEF_25-75%(p=0.016); (v) rs7512462*CT with lowest values of FVC(p=0.034), FEV₁(p=0.047), FEV₁/FVC(p=0.022), FEF_25%(p=0.012), FEF_50%(p=0.038), FEF_75%(p=0.008), FEF_25-75%(p=0.016) and ERV(p=0.023); (vi) rs7512462*TT with best response to the inhaled bronchodilator for FEV₁(p=0.011), FEF_50%(p=0.019), FEF_75%(p=0.036) and FEF_25-75%(p=0.008); (vii) rs17234516*Normal allele with lowest value of SaO₂ (p=0.010) and S. aureus (OR=3.333; 95%CI=1.085-10.24); (viii) rs17563161*GG with lowest age for onset of digestive symptoms (OR=2.564; 95%CI=1.234-5.33).

CONCLUSIONS

The clinical and laboratory variability of CF were associated with the variants in the genes of SLC family in our sample.

NaCl potentiates human fibrocyte differentiation

Excessive NaCl intake is associated with a variety of fibrosing diseases such as renal and cardiac fibrosis. This association has been attributed to increased blood pressure as the result of high NaCl intake. However, studies in patients with high NaCl intake and fibrosis reveal a connection between NaCl intake and fibrosis that is independent of blood pressure. We find that increasing the extracellular concentration of NaCl to levels that may occur in human blood after high-salt intake can potentiate, in serum-free culture conditions, the differentiation of freshly-isolated human monocytes into fibroblast-like cells called fibrocytes. NaCl affects the monocytes directly during their adhesion. Potassium chloride and sodium nitrate also potentiate fibrocyte differentiation. The plasma protein Serum Amyloid P (SAP) inhibits fibrocyte differentiation. High levels of extracellular NaCl change the SAP Hill coefficient from 1.7 to 0.8, and cause a four-fold increase in the concentration of SAP needed to inhibit fibrocyte differentiation by 95%. Together, our data suggest that NaCl potentiates fibrocyte differentiation. NaCl-increased fibrocyte differentiation may thus contribute to NaCl-increased renal and cardiac fibrosis.

Deciphering the mechanisms of intestinal imino (and amino) acid transport: The redemption of SLC36A1

The absorption of zwitterionic imino and amino acids, and related drugs, is an essential function of the small intestinal epithelium. This review focuses on the physiological roles of transporters recently identified at the molecular level, in particular SLC36A1, by identifying how they relate to the classical epithelial imino and amino acid transporters characterised in mammalian small intestine in the 1960s-1990s. SLC36A1 transports a number of D- and L-imino and amino acids, beta- and gamma-amino acids and orally-active neuromodulatory and antibacterial agents. SLC36A1 (or PAT1) functions as a proton-coupled imino and amino acid symporter in cooperation with the Na+/H+ exchanger NHE3 (SLC9A3) to produce the imino acid carrier identified in rat small intestine in the 1960s but subsequently ignored because of confusion with the IMINO transporter. However, it is the sodium/imino and amino acid cotransporter SLC6A20 which corresponds to the betaine carrier (identified in hamster, 1960s) and IMINO transporter (identified in rabbit and guinea pig, 1980s). This review summarises evidence for expression of SLC36A1 and SLC6A20 in human small intestine, highlights the differences in functional characteristics of the imino acid carrier and IMINO transporter, and explains the confusion surrounding these two distinct transport systems.

Short-term effect of epidermal growth factor on glucose uptake in endoscopic biopsies

Epidermal growth factor (EGF) up-regulation of glucose absorption via increased Na+/glucose co-transporter (SGLT-1) activity has previously been described in rabbit jejunal brush-border membrane and in differentiated Caco-2 cells. The goal of the present study was to assess the in vitro effect of EGF (200 ng/ml) on glucose uptake in human mucosal specimens, and we describe a simple procedure that uses endoscopic biopsies for short-term gludose uptake measurements. Uptake values for the EGF-treated biopsies ranged from 2.7 to 29.0, with a mean uptake of 10.65, while uptake values for the untreated biopsies ranged from 0.9 to 17.5, with a mean uptake of 7.99 (P < 0.05, paired t test). This early effect of EGF on human enterocytes may have important therapeutic implications. A role in increasing the rate of internal rehydration is suggested.

Comparative aspects of chloride-dependent amino acid transport across the brush-border membrane of mammalian small intestine

Chloride-dependent amino acid transport has been described in several tissues. This article briefly reviews the evidence of cotransport of chloride and amino acids across the brush-border membrane of rabbit distal ileum. On the basis of amino acid carriers described in the rabbit and the surveys of chloride-dependence reported, a comparison of amino acid carriers in the mammalian small intestine is performed. Additional characteristics of the carriers in the different species are included in the discussion when necessary. From this comparison the rabbit distal ileum and the pig small intestine emerge as the best models of amino acid transport in the human small intestine.

Experimental studies of intestinal ion and water transport

A major advance in transport physiology was H. H. Ussing's development of the voltage-clamp method, and later the Koefoed-Johnsen-Ussing model for Na+ transport. In the same decade, J. C. Skou identified the Na(+)-K(+)-ATPase, which maintains the Na+ and K+ gradients that drive most epithelial transport processes. With this foundation, Danish scientists have pursued the mechanism of ion transport and the resulting solute-linked water flow. Recent contributions have been on isosmotic transport, suggesting solute recycling, and KCl-water cotransport in the basolateral epithelial cell membrane. Efficient small intestinal nutrient absorption is dependent on coupling to the Na+ gradient. Cotransport of Na+ and glucose is quantitatively the most important absorptive mechanism in the small intestine, as illustrated by the success of oral rehydration solutions in diarrhoea. The majority of amino acids are likewise transported by Na+ dependent carriers, but recent experiments have identified a concomitant Cl- dependency for some. Regulation of intestinal secretion, both under normal digestive processes, and in response to enterotoxins, has turned out to be very complex. It involves local and central neuronal regulation through an array of neurotransmitters and local actions of gastrointestinal hormones. Major effectors are the submucosal neurons and the main transmitters serotonin, vasoactive intestinal peptide, acetylcholine, substance P, and neurotensin. Development of antisecretagogues is impeded by the existence of several receptor subtypes and significant species differences. The Na+ and water-conserving properties of the large intestine have been shown to be regulated by adrenocortical hormones, with aldosterone as a potent stimulator of colonic Na+ absorption. A major colonic function is the symbiosis with the anaerobic bacterial population. The fermentation of carbohydrate to short-chain fatty acids, which can be absorbed, supplements small intestinal digestive function.

beta-Amino acid transport in pig small intestine in vitro by a high-affinity, chloride-dependent carrier

This study describes unidirectional influx of amino acids and D-glucose across the small intestinal brush-border membrane of fully weaned eight week old pigs. Influx is minimal in the duodenum and maximal in the distal and/or mid small intestine. Influx of beta-alanine, taurine and N-methyl-aminoisobutyric acid is chloride-dependent. The activation stoichiometry for taurine influx is 1.0 +/- 0.2 chloride/2.4 +/- 0.3 sodium/1 taurine. Influx of D-glucose, lysine, glycine and glutamate is chloride-independent. An ABC test demonstrates a common beta-amino acid carrier: (a) the apparent affinity constant K1/2Taurine is 44 +/- 13 microM (means +/- S.D.) and the inhibitory constant (KiTaurine) against beta-alanine influx is 41 +/- 5 microM (means +/- S.E.). (b) K1/2beta-alanine is 97 +/- 23 microM and Kibeta-alanine against taurine influx is 160 +/- 22 microM. (c) KiHypotaurine against taurine and beta-alanine influx is 43 +/- 4 (n = 7) and 22 +/- 5 microM (n = 7), respectively. In conclusion, a high affinity, low capacity, sodium- and chloride-dependent carrier of beta-amino acids is present in pig small intestine.

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.