Hamster intestinal disaccharide absorption: extracellular hydrolysis precedes transport of the monosaccharide products

Functional characterization of a novel disaccharide transporter in lobster hepatopancreas

In animals, the accepted model of carbohydrate digestion and absorption involves reduction of disaccharides into the monosaccharides glucose, fructose, and galactose followed by their individual transmembrane transport into cells. In 2011, a gene for a distinct disaccharide sucrose transporter (SCRT) was found in Drosophila melanogaster and characterized in a yeast expression system. The purpose of the present investigation was to functionally identify and characterize a putative disaccharide transporter analog in the hepatopancreas of the American lobster, Homarus americanus. Purified hepatopancreatic brush-border membrane vesicles (BBMV) were used in transport experiments using ¹⁴C-sucrose and a Millipore filter isolation technique. In the absence of sodium, an external pH of 4 significantly stimulated the uptake of ¹⁴C-sucrose compared to that occurring at pH 5, 6, or 7. At pH 7, increasing external concentrations of sodium increased ¹⁴C-sucrose uptake by BBMV in a hyperbolic fashion and this stimulation was significantly reduced when the pH was changed to 4, suggesting that both protons and sodium ions were each capable of driving the uptake of the sugar. In experiments with a variety of monosaccharides, disaccharides, and trisaccharides, used as potential inhibitors of ¹⁴C-sucrose uptake, only maltose and trehalose inhibited carrier-mediated ¹⁴C-sucrose transport. An additional experiment showed that 20 mM maltose was a competitive inhibitor of ¹⁴C-sucrose uptake. The use of a putative lobster SCRT by both maltose and trehalose is nutritionally appropriate for lobsters as they commonly digest glycogen and chitin, polymers of maltose and trehalose, respectively. These findings suggest there is a brush-border proton- or sodium-dependent, hepatopancreatic carrier process, shared by sucrose, maltose, and trehalose, that may function to absorb disaccharides that are produced from digestion of naturally occurring dietary constituents.

Acute enterocyte adaptation to luminal glucose: a posttranslational mechanism for rapid apical recruitment of the transporter GLUT2

BACKGROUND

Glucose absorption postprandially increases markedly to levels far greater than possible by the classic glucose transporter sodium-glucose cotransporter 1 (SGLT1).

HYPOTHESIS

Luminal concentrations of glucose >50 mM lead to rapid, phenotypic, non-genomic adaptations by the enterocyte to recruit another transporter, glucose transporter 2 (GLUT2), to the apical membrane to increase glucose absorption.

METHODS

Isolated segments of jejunum were perfused in vivo with glucose-containing solutions in anesthetized rats. Carrier-mediated glucose uptake was measured in 10 and 100 mM glucose solutions (n = 6 rats each) with and without selective inhibitors of SGLT1 and GLUT2.

RESULTS

The mean rate of carrier-mediated glucose uptake increased in rats perfused with 100 mM versus 10 mM glucose to 13.9 ± 2.9 μmol from 2.1 ± 0.1 μmol, respectively (p < 0.0001). Using selective inhibitors, the relative contribution of GLUT2 to glucose absorption was 56% in the 100 mM concentration of glucose compared to the 10 mM concentration (27%; p < 0.01). Passive absorption accounted for 6% of total glucose absorption at 100 mM glucose.

CONCLUSION

A small amount of GLUT2 is active at the lesser luminal concentrations of glucose, but when exposed to concentrations of 100 mM, the enterocyte presumably changes its phenotype by recruiting GLUT2 apically to markedly augment glucose absorption.

Sugar absorption in the intestine: the role of GLUT2

Intestinal glucose absorption comprises two components. One is classical active absorption mediated by the Na+/glucose cotransporter. The other is a diffusive component, formerly attributed to paracellular flow. Recent evidence, however, indicates that the diffusive component is mediated by the transient insertion of glucose transporter type 2 (GLUT2) into the apical membrane. This apical GLUT2 pathway of intestinal sugar absorption is present in species from insect to human, providing a major route at high sugar concentrations. The pathway is regulated by rapid trafficking of GLUT2 to the apical membrane induced by glucose during assimilation of a meal. Apical GLUT2 is therefore a target for multiple short-term and long-term nutrient-sensing mechanisms. These include regulation by a newly recognized pathway of calcium absorption through the nonclassical neuroendocrine l-type channel Cav1.3 operating during digestion, activation of intestinal sweet taste receptors by natural sugars and artificial sweeteners, paracrine and endocrine hormones, especially insulin and GLP-2, and stress. Permanent apical GLUT2, resulting in increased sugar absorption, is a characteristic of experimental diabetes and of insulin-resistant states induced by fructose and fat. The nutritional consequences of apical and basolateral GLUT2 regulation are discussed in the context of Western diet, processed foods containing artificial sweeteners, obesity, and diabetes.

Renal neutral alpha-D-glucosidase has no role in transport of D-glucose derived from maltose hydrolysis

To reinvestigate the "hydrolase-related transport" concept, neutral alpha-D-glucosidase, a membrane-bound disaccharidase of renal proximal tubule, was first purified from brush-border membranes and then asymmetrically reincorporated into egg phosphatidylcholine vesicles. Proteolytic treatments and immunotitration studies demonstrated that this enzyme was integrated in native and artificial membrane vesicles with a similar topology. The uptake of free glucose and glucose produced by maltose hydrolysis was studied using 1) proteoliposomes containing integrated neutral alpha-D-glucosidase, in combination with other membrane proteins, and 2) proteoliposomes containing only the other membrane proteins but incubated in a medium containing neutral alpha-D-glucosidase in its hydrophilic form. No modification was observed in the uptake of free D-glucose or D-glucose produced by maltose hydrolysis, regardless of enzyme localization. In contrast to previous findings on the hydrolase-related transport concept, these results rule out any participation of neutral alpha-D-glucosidase in the transport of free glucose or glucose produced by maltose hydrolysis. Hydrolytic activity and transmembrane transport appear to be two independent and sequential steps.

Paracellular transport of water and carbohydrates during intestinal perfusion of protamine in the rat

With these experiments, the authors' purpose was to determine whether the intestinal perfusion of protamine would successfully block paracellular transport without causing significant change in cardiovascular function. In anesthetized (50 mg x kg-1 sodium pentobarbital) rats (n=12), heart rate and mean arterial blood pressure were measured during perfusion (0.5 mL x min-1) of a carbohydrate-electrolyte solution through the small intestine. The carbohydrate-electrolyte solution contained 150 mM glucose, 150 mM fructose, 10 mM lactulose, 17 mEq sodium, 3 mEq potassium, and either 0.0, 0.1, 1.0, or 10 mg x mL-1 protamine. Osmolality of the 4 solutions ranged from 363 +/- 2 to 365 +/- 3 mOsm x kg-1. Core temperature was maintained at 37 degrees C in an environmental chamber. Heart rate and mean arterial blood pressure were constant during all intestinal perfusions. Forty-one percent of the perfused lactulose was absorbed. Absorption of glucose, fructose, and lactulose was significantly inhibited by 0.1 mg x mL-1 protamine, while water absorption was decreased 41 percent by 1.0 mg x mL-1 protamine. Water and lactulose absorption fell 75% with protamine, and glucose and fructose absorption fell 50%. Lactulose and fructose absorption did not decrease further when protamine dose rose to 10 mg x mL-1. These results indicate that 1) perfusion of protamine into the small intestine in doses that significantly affect intestinal transport does not significantly affect heart rate and mean arterial blood pressure; and 2) if the primary effect of protamine is to block paracellular movement of water and solute, the greater protamine inhibition of water and lactulose absorption is consistent with a greater paracellular transport of water and lactulose than for glucose and fructose.

Action of robenidine on the intestinal transport and digestion of nutrients in rabbit

Robenidine is an anticoccidial guanidine used as an additive in rabbit fodder. Because its action is restricted to the small intestine, the present work addresses the question whether robenidine affects the growth of the animals, sugar and amino acid intestinal transport and membrane-bound intestinal digestion. For this purpose we have determined the intestinal transport of the substrates, and the enzymatic activity of neutral aminopeptidase and sucrase. We have found that robenidine diminishes the tissue accumulation of L-leucine and D-galactose at long incubation times, and increases the transepithelial mucosal to serosal flux of both substrates. These results suggest that robenidine may stimulate the enterocyte basolateral membrane flux of sugars and neutral amino acids. These results have been corroborated by means of isolated brush border and basolateral membrane vesicles. Apart from these effects, robenidine has also been shown to increase the enzymatic activity of neutral aminopeptidase and sucrase and thus resulting in a better digestion of nutrients.

Fructose and related food carbohydrates. Sources, intake, absorption, and clinical implications

It is possible to point out subjects consuming considerable quantities of fructose and sorbitol, and the intake seems to be increasing both from added and natural sources. Studies of the absorption of fructose in animals are inconsistent, and the mechanisms of fructose uptake seem to vary in accordance with the species. In most species fructose absorption takes place by a specific carrier (facilitated transport), but it may be active in the rat. In vitro studies of human intestine are very scarce; there is no evidence of active intestinal fructose transport in the human intestine. By means of hydrogen breath tests, a very low absorption capacity for fructose given as the free monosaccharide has been found in humans. Fructose given as sucrose or in equimolar combinations with glucose is well absorbed, and only fructose in excess of glucose is malabsorbed. On this basis it is hypothesized that two different uptake mechanisms for fructose are present in the human intestine. One of these may be a disaccharidase-related uptake system. Sorbitol ingestion may aggravate malabsorption of fructose given as the monosaccharide; it is not known whether a specific mechanism is involved. In children and adults with functional bowel distress the absorption capacities for fructose may not differ from those of healthy individuals, but malabsorption of fructose and/or sorbitol may be the cause of or aggravate abdominal symptoms. Fructose polymers (fructans) are also subject to increasing nutritional interest. Fructans are not absorbed in the small intestine but are strongly fermented in the large bowel. Fructans may be of potential benefit for large-bowel function and blood glucose regulation.

Intestinal disaccharidases in five species of phyllostomoid bats

1. Intestinal disaccharidases were studied in nectarivorous (Leptonycteris curasoae and Glossophaga soricina), frugivorous (Artibeus jamaicensis and Sturnira lilium), and insectivorous (Pteronotus personatus) adult bats. 2. Adult bats lacked measurable lactase activity. With the exception of trehalase activity, which was present only in P. personatus, nectar- and fruit-eating bats exhibited higher disaccharidase activities standardized by intestinal nominal area than insect-eating P. personatus. 3. Maltase and sucrase activities were significantly linearly correlated. 4. Apparent affinity of sucrase varied almost 5-fold among species. This variation may reflect unstirred layer effects resulting from sucrase being a membrane bound enzyme rather than differences in the "true" affinity of sucrase in solution. 5. Passerine birds showed higher maltase activity per unit of sucrase activity than bats and hummingbirds. Maximal sucrase and maltase activities standardized per intestinal nominal area are 1.5-2 times higher in hummingbirds than in nectar-feeding bats.

The transport and metabolism of the uridine mononucleotides by rat jejunum in vitro

1. Both uridine 3'-monophosphate (3'-UMP) and uridine 5'-monophosphate (5'-UMP) when perfused through the lumen of isolated rat jejunum gave rise to uracil as the only transported pyrimidine appearing in the serosal medium; neither the nucleotide nor the nucleoside could be detected in the serosal fluid. 2. There was a low level of the nucleoside, uridine, in the luminal fluid after the nucleotide had passed through the jejunal segment. Luminal nucleoside appearance was more marked from the 3' form of the nucleotide. 3. The hydrolysis of the nucleotides to the nucleoside form occurred via a brush-border membrane enzyme, which had the same maximal velocity (Vmax) for the two nucleotides (699 +/- 35 and 747 +/- 10 nmol min-1 (mg protein)-1 for 3'-UMP and 5'-UMP, respectively) but a different Michaelis constant (Km) so that 3'-UMP (Km = 58 +/- 3 microM) hydrolysis is favoured over 5'-UMP hydrolysis (Km = 108 +/- microM) at lower concentrations. 4. At 0.05 mM, luminal 3'-UMP gave rise to a higher rate of serosal uracil appearance than luminal 5'-UMP, but at higher luminal concentrations (0.1-0.2 mM) the rate of serosal uracil appearance was the same from both nucleotides. 5. The transmural transport of uracil from the uridine mononucleotides is discussed with reference to the metabolism and compartmentalization of the small intestine responsible for the appearance of the free pyrimidine in the serosal fluid.

The transport of pyrimidines into tissue rings cut from rat small intestine

1. At low concentrations (0.1 mM) the transport of uracil, 5-fluorouracil and thymine into jejunal tissue rings is an active process. 2. The transport of 5-fluorouracil into tissue rings cut from the duodenum and jejunum was greater than the transport into rings cut from the ileum. This difference was abolished by starving the rats for 48 h before the experiment. 3. The active transport can be abolished by replacing the Na+ in the incubation medium with either K+ or mannitol, or by increasing the concentration of the pyrimidine to 1.0 mM. 4. The accumulation of uracil or 5-fluorouracil into the jejunal rings was identical when determined by radioactive tracer or by high-performance liquid chromatography. 5. The apparent Michaelis constant (Km) for 5-fluorouracil transport into jejunal rings was 0.074 mM in the standard Na+ bicarbonate Krebs-Ringer solution and 0.394 mM in the K+-substituted bicarbonate Krebs-Ringer solution. 6. Both thymine and uracil inhibited the transport of 5-fluorouracil into jejunal tissue rings: however, cytosine and orotic acid did not.