Transport of thiamine in human intestine: mechanism and regulation in intestinal epithelial cell model Caco-2

A carrier-mediated mechanism for pyridoxine uptake by human intestinal epithelial Caco-2 cells: regulation by a PKA-mediated pathway

Vitamin B6 is essential for cellular functions and growth due to its involvement in important metabolic reactions. Humans and other mammals cannot synthesize vitamin B6 and thus must obtain this micronutrient from exogenous sources via intestinal absorption. The intestine, therefore, plays a central role in maintaining and regulating normal vitamin B6 homeostasis. Due to the water-soluble nature of vitamin B6 and the demonstration that transport of other water-soluble vitamins in intestinal epithelial cells involves specialized carrier-mediated mechanisms, we hypothesized that transport of vitamin B6 in these cells is also carrier mediated in nature. To test this hypothesis, we examined pyridoxine transport in a model system for human enterocytes, the human-derived intestinal epithelial Caco-2 cells. The results showed pyridoxine uptake to be 1) linear with time for up to 10 min of incubation and to occur with minimal metabolic alteration in the transported substrate, 2) temperature and energy dependent but Na+ independent, 3) pH dependent with higher uptake at acidic compared with alkaline pHs, 4) saturable as a function of concentration (at buffer pH 5.5 but not 7.4) with an apparent Michaelis-Menten constant (Km) of 11.99 +/- 1.41 microM and a maximal velocity (Vmax) of 67.63 +/- 3.87 pmol. mg protein-1. 3 min-1, 5) inhibited by pyridoxine structural analogs (at buffer pH 5.5 but not 7.4) but not by unrelated compounds, and 6) inhibited in a competitive manner by amiloride with an apparent inhibitor constant (Ki) of 0.39 mM. We also examined the possible regulation of pyridoxine uptake by specific intracellular regulatory pathways. The results showed that whereas modulators of PKC, Ca+2/calmodulin (CaM), and nitric oxide (NO)-mediated pathways had no effect on pyridoxine uptake, modulators of PKA-mediated pathway were found to cause significant reduction in pyridoxine uptake. This reduction was mediated via a significant inhibition in the Vmax, but not the apparent Km, of the pyridoxine uptake process. These results demonstrate, for the first time, the involvement of a specialized carrier-mediated mechanism for pyridoxine uptake by intestinal epithelial cells. This system is pH dependent and amiloride sensitive and appears to be under the regulation of an intracellular PKA-mediated pathway.

In vitro and in vivo characterization of the minimal promoter region of the human thiamin transporter SLC19A2

The molecular mechanisms involved in the regulation of thiamin transport in mammalian cells are poorly understood. Previous studies established that a human thiamin transporter, SLC19A2, plays a role in thiamin uptake in human tissues. We cloned the 5' regulatory region of the SLC19A2 gene, identified the minimal promoter required for basal activity, and located multiple putative cis elements. To further characterize the SLC19A2 promoter, we investigated, in the present study, the role of the putative cis elements in regulating the activity of the SLC19A2 promoter in vitro and confirmed the activity of the SLC19A2 promoter in vivo. In vitro studies demonstrated that mutation of specific cis elements in the SLC19A2 minimal promoter [Gut-enriched Krupple-like factor (GKLF), nuclear factor-1 (NF-1), and stimulating protein-1 (SP-1)] led to a decrease in activity. Using electrophoretic mobility shift assays, four specific DNA/protein complexes were identified. The interacting factors were determined by oligonucleotide competition and antibody supershift analysis and shown to be GKLF, NF-1, and SP-1. Cotransfection studies of the SLC19A2 promoter with an SP-1 containing vector in Drosophila SL2 cells further confirmed a role for SP-1 in regulating SLC19A2 promoter activity. In vivo studies using transgenic mice established the functionality of the full-length and minimal SLC19A2 promoters. Furthermore, our studies revealed that the pattern of expression of the SLC19A2 promoter-Luciferase constructs in transgenic mice was similar to the reported SLC19A2 RNA expression pattern in native human tissues. The results demonstrate the importance of GKLF, NF-1, and SP-1 in regulating the activity of the SLC19A2 promoter and provide direct in vivo confirmation of promoter activity.

Folate uptake in the human intestine: promoter activity and effect of folate deficiency

The intestinal folate absorption process occurs via a specialized mechanism that involves the reduced folate carrier (RFC). In humans, multiple variants of the hRFC (driven by multiple promoters) have been identified with variant I being the prominent form expressed in the intestine. While it is known that promoter B (pB) of hRFC drives the expression of this variant, little is known about the minimal region required for basal activity of this promoter in human intestinal epithelial cells. Also not known is whether folate absorption in the human intestine is up-regulated during folate deficiency (as occur in animal studies), and if so, whether transcriptional mechanisms via activation of hRFC pB are involved in such regulation. To address these issues, we have used deletion constructs of the hRFC pB and determined their activity in two human intestinal epithelial cell lines: the colon-derived Caco-2 cells, and the duodenum-derived HuTu-80 cells. Our results showed that activity of hRFC pB to be significantly higher in Caco-2 cells compared to HuTu-80 cells, a finding that corresponds with a higher level of folate uptake and endogenous hRFC mRNA levels in the former compared to the latter cell type. The minimal region required for basal activity of hRFC pB in Caco-2 cells was found to be encoded in a sequence between -1088 and -1043, while in HuTu-80 cells it was encoded in a sequence between -1431 and -1088. Growing Caco-2 cells in a folate deficient medium led to a significant and specific up-regulation in folate uptake. This up-regulation was associated with a parallel increase in hRFC protein and mRNA levels, and in the activity of hRFC pB. The most responsive sequence of pB to the effect of folate deficiency was found to be encoded in a sequence between -2016 and -1431, i.e., outside the minimal region of the pB. These results show that different minimal regions for hRFC pB are utilized by different intestinal epithelial cells. In addition, folate-deficiency was found to up-regulate folate uptake by human intestinal epithelial cells and that this regulation involves activation of hRFC pB.

Choline uptake in human intestinal Caco-2 cells is carrier-mediated

The objective of the current investigation was to examine the transport characteristics of choline, an endogenous quaternary ammonium compound, into human intestinal Caco-2 cells; the transport of choline has not been characterized in human intestine. The cellular accumulation of choline was independent of an inwardly directed Na(+) gradient and demonstrated temperature dependence and saturability. Using the initial uptake rates, choline accumulation was best characterized by a Michaelis-Menten equation and a diffusion component with a K(m) and V(max) of 110 +/- 3 micro mol/L and 2800 +/- 250 pmol/(mg protein. 10 min), respectively. Choline uptake was significantly inhibited by an excess of choline itself and by hemicholinium-3, a structural analog of choline. However other hydrophilic organic cations, such as tetraethylammonium (TEA) and N-methylnicotinamide (NMN), did not affect choline uptake in Caco-2 cells. Additionally, two typical p-glycoprotein substrates, daunomycin and verapamil, both inhibited choline accumulation. However the opposite was not true: choline did not inhibit DNM accumulation in Caco-2 cells. These results indicate the presence of a carrier-mediated transport system for choline in Caco-2 cells. The substrate specificity of this carrier is unlike that seen in the rat intestinal epithelium, and the human transport protein is distinct from those for TEA and NMN. P-glycoprotein substrates may inhibit choline uptake through specific or nonspecific interactions with the choline transporter.

Modulation of uptake of organic cationic drugs in cultured human colon adenocarcinoma Caco-2 cells by an ecto-alkaline phosphatase activity

Alkaline phosphatase (ALP) refers to a group of nonspecific phosphomonoesterases located primarily in cell plasma membrane. It has been described in different cell lines that ecto-ALP is directly or indirectly involved in the modulation of organic cation transport. We aimed to investigate, in Caco-2 cells, a putative modulation of 1-methyl-4-phenylpyridinium (MPP(+)) apical uptake by an ecto-ALP activity. Ecto-ALP activity and (3)H-MPP(+) uptake were evaluated in intact Caco-2 cells (human colon adenocarcinoma cell line), in the absence and presence of a series of drugs. The activity of membrane-bound ecto-ALP expressed on the apical surface of Caco-2 cells was studied at physiological pH using p-nitrophenylphosphate as substrate. The results showed that Caco-2 cells express ALP activity, characterized by an ecto-oriented active site functional at physiological pH. Genistein (250 micro M), 3-isobutyl-1-methylxanthine (1 mM), verapamil (100 micro M), and ascorbic acid (1 mM) significantly increased ecto-ALP activity and decreased (3)H-MPP(+) apical transport in this cell line. Orthovanadate (100 micro M) showed no effect on (3)H-MPP(+) transport and on ecto-ALP activity. On the other hand, okadaic acid (310 nM) and all trans-retinoic acid (1 micro M) significantly increased (3)H-MPP(+) uptake and inhibited ecto-ALP activity. There is a negative correlation between the effect of drugs upon ecto-ALP activity and (3)H-MPP(+) apical transport (r = -0.9; P = 0.0014). We suggest that apical uptake of organic cations in Caco-2 cells is affected by phosphorylation/dephosphorylation mechanisms, and that ecto-ALP activity may be involved in this process.

Classification structure-activity relations (C-SAR) in prediction of human intestinal absorption

AB/HIA is a "soft" filter for identifying compounds with poor intestinal membrane permeability. The analyzed data set included over 1000 drug-like compounds with experimental human intestinal absorption (HIA) values. A sequence of recursive partitioning analyses based on multiple physicochemical and structural descriptors led to the derivation of the rule-based algorithm (filter). The obtained rules reveal a simple physicochemical model of intestinal permeability; they also account for the specific effects caused by quarternary nitrogens and biphosphonate groups. Comparison of the observed and predicted values revealed very low percent of disagreement (15% false-positives and 3% false-negatives). The unusual absorption of compounds that deviated from the predicted values was explained in terms of active transport, efflux, chemical stability, chelating ability, and solubility. Most of these effects can be accounted for by new, substructure-specific rules that can be added into the existing filter. This can lead to the development of a reliable theoretical model for predicting human intestinal absorption. If combined with other models for predicting first pass metabolism, the updated AB/HIA filter can be very useful in predicting oral bioavailability.

Effect of intravenous infusions of thiamine on the disposition kinetics of thiamine and its pyrophosphate

BACKGROUND

Thiamine supplementation is necessary in patients with thiamine deficiency syndromes. Experimental evidence suggests that tissue uptake and the elimination of thiamine are dose-dependent.

AIM

The aim of the present study was to investigate the effect of different i.v. infusion rates of thiamine on blood concentrations of thiamine and its active metabolite thiamine pyrophosphate (TPP) and on renal excretion of thiamine.

METHODS

Twelve healthy subjects received in a two-period block randomized study 150 mg thiamine intravenously over either 1 or 24 h.

RESULTS

The maximum blood concentrations (Cmax) of thiamine were significantly higher after the more rapid infusion (RI; 2300 ng/mL) than after the slower infusion (SI; 177 ng/mL). The AUC of thiamine was identical after both infusion protocols. There was a slightly (10%) increased AUC of TPP (P < 0.08) after SI, whereas C(max) values were comparable. Urinary excretion of thiamine was significantly decreased from 83.6% of the applied dose after RI to 57.6% after the SI.

CONCLUSIONS

Our data suggest an increased tissue uptake of thiamine when it is given as an SI compared with a RI of the same dose. It is concluded, therefore, that an SI of thiamine may be superior to RI or bolus injections to treat severe deficiency syndromes.

Functional role of specific amino acid residues in human thiamine transporter SLC19A2: mutational analysis

SLC19A2 is a membrane thiamine transporter expressed in a variety of human tissues, including the gastrointestinal tract. Little is currently known about the structure/function relationship of SLC19A2. We examined the effect of introducing mutations in SLC19A2 identical to those found in thiamine-responsive megaloblastic anemia syndrome (TRMA), on functional activity and membrane expression of the transporter. We also examined the effect of mutating the only conserved anionic residue (E138) in the transmembrane (TM) domains of the SLC19A2 and that of the putative glycosylation sites (N63, N314). Northern blot analysis showed SLC19A2 mRNA was expressed at the same level in HeLa cells transfected with wild-type or mutated SLC19A2. Introducing the clinically relevant mutations (D93H, S143F, G172D) or mutation at the conserved anionic residue (E138A) of SLC19A2 led to a significant (P < 0.01) inhibition of thiamine uptake. Mutations of the two potential N-linked glycosylation sites (N63Q, N314Q) of SLC19A2 did not affect functional activity; they did, however, lead to a noticeable reduction in apparent molecular weight of protein. Western blot analysis showed all proteins (except D93H) were expressed in the membrane (not the cytoplasmic) fraction of HeLa cells. These results provide direct confirmation that clinically relevant mutations in SLC19A2 observed in TRMA cause malfunctioning of the transporter and/or a defect in its translation/stability. Results also show conserved TM anionic residue of the SLC19A2 protein is critical for its function. Furthermore, native SLC19A2 is glycosylated, but this is not important for its function.

Guanidine transport across the apical and basolateral membranes of human intestinal Caco-2 cells is mediated by two different mechanisms

The functional characteristics of the intestinal absorption and secretion of guanidine as a model of a nutritionally and metabolically essential organic cation were examined in the Caco-2 human intestinal cell line. Both apical and basolateral transport of [14C]-guanidine were studied using Caco-2 cells grown on polycarbonate permeable membranes. The basolateral-to-apical flux of [14C]-guanidine (i.e., its secretion) was quantitatively higher than the apical-to-basolateral transport (i.e., its absorption). When Na+ was replaced by K+ or Li+, both apical and basolateral accumulation were significantly inhibited. Studies using the cell monolayers and apical membrane vesicles obtained from Caco-2 cells showed a potential-independent mechanism of guanidine apical uptake and efflux. Conversely, basolateral uptake and efflux were membrane potential dependent. Kinetic analysis revealed that both saturable and nonsaturable mechanisms accounted for the apical and basolateral accumulations. The [14C]-guanidine efflux from cells through the apical and basolateral membranes was significantly reduced at 4 degrees C, suggesting carrier-mediated mechanisms. Moreover, the apical efflux was stimulated by an inwardly directed H+ gradient. Influx and efflux of [14C]-guanidine were unaffected by the presence of tetraethylammonium, cimetidine or decynium-22 in the donor compartment. Only quinine significantly reduced [14C]-guanidine entrance through apical and basolateral membranes and its exit through the basolateral membrane. In conclusion, our results suggest that the influx and the efflux through the apical membrane is mediated by different transporters, whereas transport across the basolateral membrane is mediated by a member of the organic cation transporter family with high affinity for guanidine.

Regulation of apical transporter of L-DOPA in human intestinal Caco-2 cells

The present study examined the nature of the apical inward L-3,4-dihydroxyphenylalanine (L-DOPA) transporter in human intestinal epithelial Caco-2 cells, and whether protein kinases modulate the activity of this transporter. The apical inward transfer of L-DOPA was promoted through an energy-dependent and sodium-insensitive transporter (Km=33 microM; Vmax=2932 pmol/mg protein/6 min). This transporter was insensitive to N-(methylamino)-isobutyric acid, but competitively inhibited by 2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid (BCH; IC50=83 microM). The organic cation inhibitor decynium 24 failed to affect the accumulation of L-DOPA, whereas the organic anion inhibitor 4,4'-diisothiocynatostilbene-2,2'-disulphonic acid (DIDS) competitively inhibited L-DOPA uptake (IC50=83 microM). However, the apical-to-basal and basal-to-apical transepithelial transport and the cell accumulation of [3H]-PAH was close to that of [14C]-sorbitol and insensitive to DIDS (300 microM). Modulators of protein kinase A (PKA) [cyclic adenosine monophosphate (cAMP), forskolin, H-89 and cholera toxin], protein kinase G (PKG) [cyclic guanosine monophosphate (GMP), zaprinast, LY 83583 and sodium nitroprusside] and protein kinase C (PKC) (phorbol 12,13-dibutirate and chelerythrine) failed to affect the accumulation of L-DOPA. The Ca2+/calmodulin inhibitors calmidazolium and trifluoperazine inhibited L-DOPA uptake (IC50s of 53 and 252 microM, respectively), but the rise of intracellular Ca2+ by A23187 (1 microM) and thapsigargin (1 microM) played no role on L-DOPA uptake. It is concluded that Caco-2 cells take up L-DOPA over the apical cell border through the sodium-independent and pH-sensitive L-type amino acid transporter.

Uptake of (3)H-1-methyl-4-phenylpyridinium ((3)H-MPP(+)) by human intestinal Caco-2 cells is regulated by phosphorylation/dephosphorylation mechanisms

Several transmembrane transporters of organic compounds are regulated by phosphorylation/dephosphorylation mechanisms. The aim of this study was to investigate the possible regulation of the intestinal uptake of organic cations by these mechanisms. The intestinal apical uptake of 1-methyl-4-phenylpyridinium (MPP(+)) was studied by incubating Caco-2 cells at 37 degrees for 5 min with 200 nM (3)H-MPP(+). Uptake of (3)H-MPP(+) by Caco-2 cells was not affected by activators of protein kinase G, and was not affected or slightly reduced (by 15-20%) by activators of protein kinase A or protein kinase C. Uptake of (3)H-MPP(+) by Caco-2 cells was reduced in a concentration-dependent manner by non-selective phosphodiesterase inhibitors (3-isobutyl-1-methylxanthine (IBMX), caffeine, teophylline). The IC(50) of IBMX was found to be 119 microM (102-138; n=9). Uptake of (3)H-MPP(+) by Caco-2 cells was not affected by inhibition of protein tyrosine kinase, but it was concentration-dependently reduced in the presence of inhibitors of mitogen-activated protein kinase. Uptake of (3)H-MPP(+) by Caco-2 cells was strongly reduced by Ca(2+)/calmodulin-mediated pathway inhibitors, but it was not dependent on extracellular Ca(2+). Our results suggest that the intestinal apical uptake of MPP(+) is regulated by phosphorylation/dephosphorylation mechanisms, being most probably active in the dephosphorylated state. Moreover, uptake of (3)H-MPP(+) by Caco-2 cells and by the extraneuronal monoamine transporter (EMT) are regulated in a very similar manner, suggesting an important participation of EMT in the intestinal uptake of this compound.

Expression and promoter analysis of SLC19A2 in the human intestine

The molecular mechanism and regulation of the intestinal uptake process of dietary thiamine is not well understood. Previous studies have established the involvement of a carrier-mediated system for thiamine uptake in the human intestine. Recently a human thiamine transporter, SLC19A2, was cloned from a number of human tissues. Little, however, is known about expression of the SLC19A2 message along the native human gastrointestinal tract, and no analysis of its promoter in intestinal tissue is available. Therefore, the current studies were aimed at investigating the expression of SLC19A2 in the human gastrointestinal tract and at analyzing the promoter of this potential intestinal thiamine transporter. First we cloned SLC19A2 cDNA from a human intestinal cell line (Caco-2) by reverse transcriptase-polymerase chain reaction, then used this cDNA as a probe in Northern blot analysis. SLC19A2 message was found to be expressed in all gastrointestinal tissues in the following order: liver>stomach>duodenum>jejunum>colon>cecum>rectum>ileum. SLC19A2 was also expressed at the protein level in Caco-2 cells and in native human small intestine by Western blot analysis. We also cloned the 5'-regulatory region of the SLC19A2 gene and confirmed activity of its promoter following transfection into intestinal epithelial Caco-2 cells. Furthermore, we identified the minimal promoter region required for basal activity of SLC19A2 in these cells which was found to be mainly encoded in a sequence between -356 and -36, and included multiple cis-regulatory elements. Transcription initiation sites of the SLC19A2 gene in intestinal epithelial Caco-2 cells were also identified by 5'-rapid amplification of cDNA ends. These results demonstrate that SLC19A2 is expressed in various regions of the human gastrointestinal tract. In addition, the results provide the first characterization of the SLC19A2 promoter. These findings raise the possibility that SLC19A2 may play a role in the normal intestinal thiamine absorption process.

Mechanism and regulation of vitamin B(6) uptake by renal tubular epithelia: studies with cultured OK cells

The kidneys play an important role in regulating vitamin B(6) body homeostasis, but limited information exists regarding the mechanism of pyridoxine uptake by renal epithelial cells, and no study exists on its regulation. To address these issues, we used the renal opossum-derived tubular epithelial (opossum kidney; OK) cells and found pyridoxine uptake to 1) be temperature and energy dependent, 2) be pH dependent, with a higher uptake at alkaline or neutral buffer pH compared with acidic pH, 3) be Na(+) independent, 4) involve a saturable component (apparent Michaelis- Menten constant of 2.40 +/- 0.23 microM), 5) be inhibited by structural analogs, and 6) be amiloride sensitive. Maintaining OK cells in a vitamin B(6)-deficient growth medium (for 48 h) led to a significant upregulation of pyridoxine uptake. This upregulation was found to be specific for pyridoxine, inhibited by cyclohexamide and actinomycin D, reversible, and mediated via an increase in maximal velocity. Pretreating OK cells with modulates of a Ca(2+)/calmodulin-mediated pathway led to a significant downregulation in pyridoxine uptake via inhibition of maximal velocity. These results demonstrate that pyridoxine uptake by renal tubular epithelial OK cells is via a specialized pH-sensitive carrier-mediated mechanism. This mechanism appears to be regulated by extracellular vitamin B(6) levels and an intracellular Ca(2+)/calmodulin-mediated pathway.

Mechanism of thiamine uptake by human jejunal brush-border membrane vesicles

Thiamine, a water-soluble vitamin, is essential for normal cellular functions, growth and development. Thiamine deficiency leads to significant clinical problems and occurs under a variety of conditions. To date, however, little is known about the mechanism of thiamine absorption in the native human small intestine. The objective of this study was, therefore, to characterize the mechanism of thiamine transport across the brush-border membrane (BBM) of human small intestine. With the use of purified BBM vesicles (BBMV) isolated from the jejunum of organ donors, thiamine uptake was found to be 1) independent of Na(+) but markedly stimulated by an outwardly directed H(+) gradient (pH 5.5(in)/pH 7.5(out)); 2) competitively inhibited by the cation transport inhibitor amiloride (inhibitor constant of 0.12 mM); 3) sensitive to temperature and osmolarity of the incubation medium; 4) significantly inhibited by thiamine structural analogs (amprolium, oxythiamine, and pyrithiamine), but not by unrelated organic cations (tetraethylammonium, N-methylnicotinamide, or choline); 5) not affected by the addition of ATP to the inside and outside of the BBMV; 6) potential insensitive; and 7) saturable as a function of thiamine concentration with an apparent Michaelis-Menten constant of 0.61 +/- 0.08 microM and a maximal velocity of 1.00 +/- 0.47 pmol. mg protein(-1). 10 s(-1). Carrier-mediated thiamine uptake was also found in BBMV of human ileum. These data demonstrate the existence of a Na(+)-independent, pH-dependent, amiloride-sensitive, electroneutral carrier-mediated mechanism for thiamine absorption in native human small intestinal BBMV.

Mechanism of thiamine uptake by human colonocytes: studies with cultured colonic epithelial cell line NCM460

Thiamine (vitamin B(1)) is essential for normal cellular functions and growth. Mammals cannot synthesize thiamine and thus must obtain the vitamin via intestinal absorption. The intestine is exposed to a dietary thiamine source and a bacterial source in which the vitamin is synthesized by the normal microflora of the large intestine. Very little is known about thiamine uptake in the large intestine. The aim of this study was, therefore, to address this issue. Our results with human-derived colonic epithelial NCM460 cells as a model system showed thiamine uptake to be 1) temperature- and energy dependent, 2) Na(+) independent, 3) increased with increasing buffer pH from 5 to 8 and after cell acidification but inhibited by amiloride, 4) saturable as a function of concentration, 5) inhibited by thiamine structural analogs but not by unrelated organic cations, and 6) inhibited by modulators of a Ca(2+)/calmodulin-mediated pathway. NCM460 cells and native human colonic mucosa expressed the recently cloned human thiamine transporter THTR-1 (product of the SLC19A2 gene) at both mRNA and protein levels. These results demonstrate for the first time that human NCM460 colonocytes possess a specific carrier-mediated system for thiamine uptake that appears to be under the regulation of an intracellular Ca(2+)/calmodulin-mediated pathway. It is suggested that bacterially synthesized thiamine in the large intestine may contribute to thiamine nutrition of the host, especially toward cellular nutrition of the local colonocytes.

Nutrient absorption

Some interesting advances in mechanisms and regulation of nutrient absorption were reported last year. Further evidence was obtained that the rate-limiting step in triacylglycerol absorption, especially with large doses of lipid, is transport of prechylomicrons from the endoplasmic reticulum to the Golgi apparatus. Targeted disruption of the adenosine triphosphate-binding cassette transporter in mice produced changes similar to human Tangier disease and suggested that this mouse may be a model for studying intestinal high-density lipoprotein assembly and secretion. A new mechanism for carbohydrate malabsorption was discovered: in sucrase-isomaltase deficiency, the enzyme fails to anchor in the brush border membrane and so is secreted into the lumen, where it is ineffective. Glycosylating insulin at B1 phenylalanine permitted it to bind to the brush border membrane and greatly enhanced its hypoglycemic activity when given orally. CaCo-2 cells and normal human enterocytes were shown to have two variants of the human sodium-dependent vitamin C transporter, hSVCT1; one is active and the other is an inactive splice variant. In rats, the divalent metal ion transporter, DMT1, appeared to be important for regulation of both absorption of iron and its movement into the liver.

Expression of Fos immunoreactivity in some catecholaminergic brainstem neurons in rats following high-altitude exposure

This study examined the response of neurons of the cardiorespiratory centers, i.e., the nucleus tractus solitarius and the ventrolateral medulla as well as the area postrema in adult and postnatal rats subjected to high-altitude exposure at 4,000 m and 8,000 m. In adult control rats, sporadic Fos-positive neurons were detected in the above-mentioned areas. On exposure to 4,000 m altitude, the number of Fos-positive neurons was noticeably increased. At 8,000 m, the incidence of labeled cells was markedly increased, with many of them doubly labeled for tyrosine hydroxylase. In postnatal rats, Fos expression was not detected in these areas in either control rats or rats exposed to 4,000 m altitude. Fos-positive cells, however, were observed in the these areas in postnatal rats exposed to 8,000 m. In the latter, tyrosine hydroxylase labeling was observed in some Fos-positive cells in the nucleus tractus solitarius and ventrolateral medulla. In rats killed at 24 hr after exposure to high altitude, Fos expression in both the adult and the postnatal rats was comparable to that in their corresponding control rats. Present results suggest that Fos expression in various brainstem areas was induced by reduced oxygen tension in the ambient air at high altitude. Double labeling of some Fos-positive neurons with tyrosine hydroxylase indicates an increased sympathetic activation, which may be involved in the mediation of cardiorespiratory responses to hypoxia. This, however, was less evident in the postnatal animals. It is possible that the peripheral chemoreceptors or the regulation of autonomic functions is not fully developed in this age group.