Psychological stress impairs Na+-dependent glucose absorption and increases GLUT2 expression in the rat jejunal brush-border membrane

Chronic psychological stress impacts many functions of the gastrointestinal tract. However, the effect of stress on nutrient absorption is poorly documented. This study was designed to investigate glucose transporters in rats submitted to different periods of water-avoidance stress (WAS). Rats were subjected to WAS (1 h/day) for 1, 5, or 10 consecutive days. Four hours after the last WAS session, rats were killed and segments of jejunum were mounted in Ussing chambers to study electrophysiological properties of the jejunum and Na+-dependent glucose absorption kinetics. Mucosa was obtained to prepare brush-border membrane vesicles (BBMV) used to measure [14C]fructose uptake as well as sodium-glucose transporter 1 (SGLT-1) and GLUT2 expression by Western blot analysis. Exposure of animals to WAS induced a decrease in Na+-dependent glucose absorption Vmax after 1, 5, and 10 days without any change in SGLT-1 expression. Potential difference across the jejunum was decreased for all stressed groups. Furthermore, we observed an increase in phloretin-sensitive uptake of [14C]fructose by BBMV after 1, 5, or 10 days of WAS, which was not present in control animals. This suggested the abnormal appearance of GLUT2 in the brush border, which was confirmed by Western blot analysis. We concluded that psychological stress induces major changes in glucose transport with a decrease in Na+-dependent glucose absorption and an increase in GLUT2 expression at the brush-border membrane level.     

Glucose and fructose uptake by <Emphasis Type="Italic">Limulus polyphemus</Emphasis> hepatopancreatic brush border and basolateral membrane vesicles: evidence for Na<Superscript>+</Superscript>-dependent sugar transport activity

[³H]-fructose and [³H]-glucose transport activities were determined in brush border membrane vesicles (BBMV) and basolateral membrane vesicles (BLMV) from Limulus polyphemus (horseshoe crab) hepatopancreas. Glucose transport was equilibrative in the absence of sodium and sodium dependent in the presence of sodium in BBMV, suggesting GLUT-like and SGLT-like transport activity. Glucose transport by BLMV was equilibrative and sodium independent. Fructose uptake by BBMV and BLMV was equilibrative in the absence of sodium and sodium dependent in the presence of sodium. Western blot analysis using a rabbit anti-mouse SGLT-1 polyclonal antibody indicated the presence of a cross-reacting horseshoe crab BBMV protein of similar molecular weight to the mammalian SGLT1. Sequence alignment of the mouse SGLT-4 and SGLT1 with a translated, horseshoe crab-expressed sequence tag also indicated significant identity between species. Fructose and glucose uptake in the absence and presence of sodium by hepatopancreas BBMV and BLMV indicated the presence of sodium-dependent transport activity for each sugar that may result from the presence of transporters similar to those described for other species.     

Ontogenic Changes of Villus Growth,Lactase Activity,and Intestinal Glucose Transporters in Preterm and Term Born Calves with or without Prolonged Colostrum Feeding

Oral glucose supply is important for neonatal calves to stabilize postnatal plasma glucose concentration. The objective of this study was to investigate ontogenic development of small intestinal growth, lactase activity, and glucose transporter in calves (n = 7 per group) that were born either preterm (PT; delivered by section 9 d before term) or at term (T; spontaneous vaginal delivery) or spontaneously born and fed colostrum for 4 days (TC). Tissue samples from duodenum and proximal, mid, and distal jejunum were taken to measure villus size and crypt depth, protein concentration of mucosa and brush border membrane vesicles (BBMV), total DNA and RNA concentration of mucosa, mRNA expression and activity of lactase, and mRNA expression of sodium-dependent glucose co-transporter-1 (SGLT1) and facilitative glucose transporter 2 (GLUT2) in mucosal tissue. Additionally, protein expression of SGLT1 in BBMV and GLUT2 in crude mucosal membranes and immunochemical localization of GLUT2 in the enterocytes were determined. Villus height in distal jejunum was lower in TC than in T. Crypt depth in all segments was largest and the villus height/crypt depth ratio in jejunum was smallest in TC calves. Concentration of RNA was highest in duodenal mucosa of TC calves, but neither lactase mRNA and activity nor SGLT1 and GLUT2 mRNA and protein expression differed among groups. Localization of GLUT2 in the apical membrane was greater, whereas in the basolateral membrane was lower in TC than in T and PT calves. Our study indicates maturation processes after birth for mucosal growth and trafficking of GLUT2 from the basolateral to the apical membrane. Minor differences of mucosal growth, lactase activity, and intestinal glucose transporters were seen between PT and T calves, pointing at the importance of postnatal maturation and feeding for mucosal growth and GLUT2 trafficking.     

Pharmacological investigations of the anti-diabetic effect of Cortex Moutan and its active component paeonol

Cortex Moutan (CM, root bark of Paeonia suffruticosa Andr.) is one of the common herbs found in anti-diabetic traditional Chinese medicine formulae. To study the potential anti-diabetic mechanisms of CM, four in vitro models (intestinal brush border membrane vesicles (BBMV), rat hepatoma cell line H4IIE, human skin fibroblasts cell line Hs68 and mouse adipocytes 3T3-L1) were used. CM showed significant in vitro anti-diabetic effects by inhibiting glucose uptake of BBMV and enhancing glucose uptake into Hs68 and 3T3-L1 cells. Using bioassay-guided fractionation, paeonol was confirmed to be one of the active constituents for inhibiting BBMV glucose uptake. With neonatal-streptozotocin diabetic rats, paeonol (200 and 400mg/kgbody wt.) was found to improve oral glucose tolerance in vivo. To the best of our knowledge, this is the first report on the anti-diabetic effect of paeonol.     

Comparison of brush border membrane vesicles prepared by three methods from larval Manduca sexta midgut

Brush border membrane vesicles (BBMV) were prepared from freshly isolated posterior larval Manduca sexta midguts by differential calcium precipitation, differential magnesium precipitation and differential ultrasonication. BBMV were also prepared from frozen posterior larval M. sexta midguts by differential calcium precipitaion and differential magnesium precipitation. The yield of BBMV by both differential precipitation methods was 5–6 times greater than that by the differential ultrasonification method. Enrichments of the brush border membrane marker enzymes alkaline phosphatase, γ-glutamyl transferase, and aminopeptidase were similar in all preparations. The polypeptide composition of all preparations was also similar. The specific activity of mitochondrial and microsomal marker enzymes was higher in BBMV prepared from freshly isolated midguts by the differential precipitation methods than in BBMV prepared by the ultrasonication method. The specific activity of cytochrome-c oxidase was 2.5–7 times higher in BBMV prepared from frozen midguts than in BBMV prepared from fresh tissue.     

Multiple plasma membrane receptors but not NPC1L1 mediate high-affinity, ezetimibe-sensitive cholesterol uptake into the intestinal brush border membrane

We compared cholesterol uptake into brush border membrane vesicles (BBMV) made from the small intestines of either wild-type or Niemann-Pick C1-like 1 (NPC1L1) knockout mice to elucidate the contribution of NPC1L1 to facilitated uptake; this uptake involves cholesterol transport from lipid donor particles into the BBM of enterocytes. The lack of NPC1L1 in the BBM of the knockout mice had no effect on the rate of cholesterol uptake. It follows that NPC1L1 cannot be the putative high-affinity, ezetimibe-sensitive cholesterol transporter in the brush border membrane (BBM) as has been proposed by others. The following findings substantiate this conclusion: (I) NPC1L1 is not a brush border membrane protein but very likely localized to intracellular membranes; (II) the cholesterol absorption inhibitor ezetimibe and its analogues reduce cholesterol uptake to the same extent in wild-type and NPC1L1 knockout mouse BBMV. These findings indicate that the prevailing belief that NPC1L1 facilitates intestinal cholesterol uptake into the BBM and its interaction with ezetimibe is responsible for the inhibition of this process can no longer be sustained.     

Immunohistochemical localization of Na+-dependent glucose transporter in the rat digestive tract

Summary Glucose is actively absorbed in the intestine by the action of the Na⁺-dependent glucose transporter. Using an antibody against the rabbit intestinal Na⁺-dependent glucose transporter (SGLT1), we examined the localization of SGLT1 immunohistochemically along the rat digestive tract (oesophagus, stomach, duodenum, jejunum, ileum, colon and rectum). SGLT1 was detected in the small intestine (duodenum, jejunum and ileum), but not in the oesophagus, stomach, colon or rectum. SGLT1 was localized at the brush border of the absorptive epithelium cells in the small intestine. Electron microscopical examination showed that SGLT1 was localized at the apical plasma membrane of the absorptive epithelial cells. SGLT1 was not detected at the basolateral plasma membrane. Along the crypt-villus axis, all the absorptive epithelial cells in the villus were positive for SGLT1, whose amount increased from the bottom of the villus to its tip. On the other hand, cells in the crypts exhibited little or no staining for SGLT1. Goblet cells scattered throughout the intestinal epithelium were negative for SGLT1. These observations show that SGLT1 is specific to the apical plasma membrane of differentiated absorptive epithelial cells in the small intestine, and suggest that active uptake of glucose occurs mainly in the absorptive epithelial cells in the small intestine.     

Stereoselective renal secretion of carbenicillin in rabbits: Role of the organic anion transporter at the renal brush border membrane

Stereoselectivity in the renal secretion of carbenicillin (CBPC) was studied in rabbits. Significant renal secretion of CBPC was observed in vivo, with the secretion of the S-epimer being greater than that of the R-epimer. Stereoselective transport of CBPC was further studied in vitro using basolateral and brush border membrane vesicles prepared from rabbit kidneys. The transport of CBPC by the organic anion transporter into the basolateral membrane vesicles (BLMV) was not stereoselective. In contrast, a distinct stereoselectivity was observed in the transport of CBPC by the organic anion transporter into the brush border membrane vesicles (BBMV), with the transport of the S-epimer being more favorable. Significant epimer-epimer interactions were also observed in the transport into BBMV. The stereoselectivity of the transport of CBPC was calculated from the kinetic parameters with consideration of epimer-epimer interactions and was similar to that observed in vivo. It was concluded that the observed stereoselectivity in the renal secretion of CBPC in vivo reflected that of transport via the organic anion transporter located at the brush border membrane. Chirality 10:349–357, 1998. © 1998 Wiley-Liss, Inc.     

Na-glucose and Na-neutral amino acid cotransport are uniquely regulated by constitutive nitric oxide in rabbit small intestinal villus cells

Na-nutrient cotransport processes are not only important for the assimilation of essential nutrients but also for the absorption of Na in the mammalian small intestine. The effect of constitutive nitric oxide (cNO) on Na-glucose (SGLT-1) and Na-amino acid cotransport (NAcT) in the mammalian small intestine is unknown. Inhibition of cNO synthase with N(G)-nitro-l-arginine methyl ester (L-NAME) resulted in the inhibition of Na-stimulated (3)H-O-methyl-D-glucose uptake in villus cells. However, Na-stimulated alanine uptake was not affected in these cells. The L-NAME-induced reduction in SGLT-1 in villus cells was not secondary to an alteration in basolateral membrane Na-K-ATPase activity, which provides the favorable Na gradient for this cotransport process. In fact, SGLT-1 was inhibited in villus cell brush-border membrane (BBM) vesicles prepared from animals treated with L-NAME. Kinetic studies demonstrated that the mechanism of inhibition of SGLT-1 was secondary to a decrease in the affinity for glucose without a change in the maximal rate of uptake of glucose. Northern blot studies demonstrated no change in the mRNA levels of SGLT-1. Western blot studies demonstrated no significant change in the immunoreactive protein levels of SGLT-1 in ileal villus cell BBM from L-NAME-treated rabbits. These studies indicate that inhibition of cNO production inhibits SGLT-1 but not NAcT in the rabbit small intestine. Therefore, whereas cNO promotes Na-glucose cotransport, it does not affect NAcT in the mammalian small intestine.     

Spontaneous cholesterol movement between lipid vesicles and monkey small intestinal brush border membrane

[14C]Cholesterol movement between egg phosphatidylcholine-cholesterol lipid vesicles and vesicles prepared from monkey small intestinal brush border membrane (BBMV) was studied in physiological buffer at 37 degrees C. The rate of cholesterol transfer from sonicated unilamellar vesicles (ULV) to BBMV follows apparently first-order kinetics. Intermembrane cholesterol movement was strikingly similar in both the directions. However, from BBMV to ULV, the transfer rate was three times faster than that of ULV to brush border membrane (BBM). Similarity in the rate constant was observed when cholesterol transfer was studied using either large multilamellar lipid vesicles or ULV as the donor and BBMV as the acceptor membrane. Rate constant was also the same when the acceptor membrane used was either intact BBMV or ULV prepared from BBM lipids. The rate of transfer of label was not affected even when the acceptor vesicle concentration was increased over fivefold, indicating the first-order nature of the reaction. Transfer of cholesterol from ULV to BBMV was accelerated by the presence of acetone, dimethyl sulfoxide (DMSO), deoxycholate, and papain. Partially purified nonspecific lipid-exchange protein increased the rate of cholesterol transfer by about threefold. Reduction in BBM cholesterol and phospholipid content was noted by DMSO, acetone, and deoxycholate, while papain caused a small depletion of membrane protein. Cholesterol transfer is temperature dependent with an activation energy of 31 kJ X mol-1, which is almost identical in the presence or absence of nonspecific lipid-exchange protein. The molecular mechanism of intermembrane cholesterol movement is discussed in view of the kinetic data obtained.     

Immunohistochemical localization of Na+-dependent glucose transporter in rat jejunum

Summary Glucose is actively absorbed via a Na⁺-dependent active glucose transporter (Na-GT) in the small intestine. We raised a polyclonal antibody against the peptide corresponding to amino acids 564–575 of rabbit intestinal Na-GT, and localized it immunohistochemically in the rat jejunum. By means of immunofluorescence staining, Na-GT was located at the brush border of the absorptive epithelial cells of the intestinal villi. Electron-microscopic examination showed that Na-GT was localized at the plasma membrane of the apical microvilli of these cells. Little Na-GT was found at the basolateral plasma membrane. Along the crypt-villus axis, all of the absorptive epithelial cells in the villus were positive for Na-GT. In addition to the brush border staining, the supranuclear positive staining, which was shown to be the Golgi apparatus by use of electron microscopy, was seen in cells located between the base to the middle of the villus. Cells in crypts exhibited little or no staining for Na-GT. Goblet cells scattered in the intestinal epithelium were negative for Na-GT staining. These observations show that Na-GT is specific to the apical plasma membrane of the absorptive epithelial cells, and that the onset of Na-GT synthesis may occur near the crypt-villus junction.     

Epidermal growth factor binding, stimulation of phosphorylation, and inhibition of gluconeogenesis in rat proximal tubule

Epidermal growth factor and insulin share many biological activities, including stimulation of cell proliferation, ion flux, glycolysis, fatty acid and glycogen synthesis, and activation of receptor-linked tyrosine kinase activity. In the kidney, insulin has been shown to regulate transport processes and inhibit gluconeogenesis in the proximal tubule. Since the kidney represents a major source of EGF, the present studies investigated whether proximal tubule contained EGF receptors, whether EGF receptors were localized to apical or basolateral membranes, and whether EGF receptor activation participated in the regulation of an important proximal tubule function, gluconeogenesis. Specific EGF receptors were demonstrated in the basolateral membrane of proximal tubule. Following incubation with 125I EGF, basolateral membranes demonstrated equilibrium binding at 4 degrees C and 23 degrees C. There was 78 +/- 2% specific binding (n = 13). The dissociation constant (Kd) was 1.5 x 10(-9) M and maximal binding was 44 fmol/mg protein. There was ninefold more specific binding to proximal tubule basolateral membrane than to brush border membrane. In basolateral, but not brush border membranes, EGF induced phosphorylation of the tyrosine residues of intrinsic membrane proteins, including a 170 kDa protein, corresponding to the EGF receptor. In the presence of the gluconeogenic substrates, alanine, lactate, and succinate, proximal tubule suspensions synthesized glucose. EGF inhibited glucose production in a concentration-dependent manner over a concentration range of 3 x 10(-11) to 3 x 10(-9) M. In addition, EGF inhibited angiotensin II-stimulated glucose production in the proximal tubule suspensions. EGF did not significantly increase net glucose metabolism nor decrease cellular ATP concentrations. Therefore, these studies demonstrated that rat proximal tubule contained specific receptors for EGF that were localized to the basolateral membrane and linked to tyrosine kinase activity. EGF significantly inhibited proximal tubule glucose production without significantly increasing net glucose consumption.     

Streptozotocin diabetes and sugar transport by rat ileal enterocytes: evidence for adaptation caused by an increased luminal nutrient load.

Preparations of villus enterocytes and brush border membrane vesicles have been used to study the effects of streptozotocin-induced diabetes mellitus in rats on sugar transport across the brush border and basolateral membranes of ileal epithelial cells. In isolated cells, diabetes increased Na(+)-dependent galactose transport across the brush border of mid-villus but not upper villus cells. Galactose transport across the basolateral membrane was, however, enhanced by diabetes in both cell populations. Kinetic analysis of vesicle data suggested the presence of two transporters for Na(+)-dependent glucose transport. Diabetes induced a 5-fold increase in both KT and Vmax of the high-affinity/low-capacity system together with a 2-fold increase in the Vmax of the low-affinity/high-capacity transporter. Glucose was almost undetectable in the lumen of the upper and lower ileum in control animals but was present at high levels (26.1 +/- 4.3 mM and 6.5 +/- 1.3 mM) in diabetic rats. The possible significance of these changes in luminal sugar concentration in relation to the adaptation of transport across ileal enterocytes is discussed.     

Phospholipid effects on SGLT1-mediated glucose transport in rabbit ileum brush border membrane vesicles

In small intestine, sodium-glucose cotransporter SGLT1 provides the main mechanism for sugar uptake. We investigated the effect of membrane phospholipids (PL) on this transport in rabbit ileal brush border membrane vesicles (BBMV). For this, PL of different charge, length, and saturation were incorporated into BBMV. Transport was measured related to (i) membrane surface charge (membrane-bound MC540 fluorescence), (ii) membrane thickness (PL incorporation of different acyl chain length), and (iii) membrane fluidity (r_12AS, fluorescence anisotropy of 12-AS).Compared to phosphatidylcholine (PC) carrying a neutral head group, inhibition of SGLT1 increased considerably with the acidic phosphatidic acid (PA) and phosphatidylinositol (PI) that increase membrane negative surface charge. The order of PL potency was PI>PA > PE = PS > PC. Inhibition by acidic PA-oleate was 5-times more effective than with neutral PE (phosphatidylethanolamine)-oleate. Lineweaver-Burk plot indicated uncompetitive inhibition of SGLT1 by PA.When membrane thickness was increased by neutral PC of varying acyl chain length, transport was increasingly inhibited by 16:1 PC to 22:1 PC. Even more pronounced inhibition was observed with mono-unsaturated instead of saturated acyl chains which increased membrane fluidity (indicated by decreased r_12AS).In conclusion, sodium-dependent glucose transport of rabbit ileal BBMV is modulated by (i) altered membrane surface charge, (ii) length of acyl chains via membrane thickness, and (iii) saturation of PL acyl chains altering membrane fluidity. Transport was attenuated by charged PL with longer and unsaturated acyl residues. Alterations of PL may provide a principle for attenuating dietary glucose uptake.     

Binding analyses of Bacillus thuringiensis Cry delta-endotoxins using brush border membrane vesicles of Ostrinia nubilalis

Transgenic corn expressing the Bacillus thuringiensis Cry1Ab gene is highly insecticidal to Ostrinia nubilalis (European corn borer) larvae. We ascertained whether Cry1F, Cry9C, or Cry9E recognizes the Cry1Ab binding site on the O. nubilalis brush border by three approaches. An optical biosensor technology based on surface plasmon resonance measured binding of brush border membrane vesicles (BBMV) injected over a surface of immobilized Cry toxin. Preincubation with Cry1Ab reduced BBMV binding to immobilized Cry1Ab, whereas preincubation with Cry1F, Cry9C, or Cry9E did not inhibit BBMV binding. BBMV binding to a Cry1F-coated surface was reduced when vesicles were preincubated in Cry1F or Cry1Ab but not Cry9C or Cry9E. A radioligand approach measured 125I-Cry1Ab toxin binding to BBMV in the presence of homologous (Cry1Ab) and heterologous (Cry1Ac, Cry1F, Cry9C, or Cry9E) toxins. Unlabeled Cry1Ac effectively competed for 125I-Cry1Ab binding in a manner comparable to Cry1Ab itself. Unlabeled Cry9C and Cry9E toxins did not inhibit (125)I-Cry1Ab binding to BBMV. Cry1F inhibited (125)I-Cry1Ab binding at concentrations greater than 500 nM. Cry1F had low-level affinity for the Cry1Ab binding site. Ligand blot analysis identified Cry1Ab, Cry1Ac, and Cry1F binding proteins in BBMV. The major Cry1Ab signals on ligand blots were at 145 kDa and 154 kDa, but a strong signal was present at 220 kDa and a weak signal was present at 167 kDa. Cry1Ac and Cry1F binding proteins were detected at 220 and 154 kDa. Anti-Manduca sexta aminopeptidase serum recognized proteins of 145, 154, and 167 kDa, and anti-cadherin serum recognized the 220 kDa protein. We speculate that isoforms of aminopeptidase and cadherin in the brush border membrane serve as Cry1Ab, Cry1Ac, and Cry1F binding proteins.     

Sodium-coupled sugar and amino acid transport in an acidic microenvironment

1. Nutrient transport mechanisms of lobster hepatopancreatic epithelial brush border membrane vesicles (BBMV) are strongly influenced by the acidic nature of the tubular lumen. 2. Sodium-dependent glucose uptake by BBMV was electrogenic and was stimulated at low pH by reducing sugar transport Ki, without affecting JM. 3. Glutamate was largely transported in zwitterionic form at pH 4.0 by an electrically silent cotransport mechanism with both Na and Cl. 4. Increased H+ concentration tripled the apparent membrane permeability to glutamate as well as the amino acid transport JM. 5. At pH 4.0 leucine was transported as a cation by two dissimilar carrier systems: a Na-independent process shared by polar amino acids, and an electroneutral Na-2Cl-dependent mechanism shared with non-polar amino acids. 6. A model is proposed for hepatopancreatic BBMV at acidic pH which employs ionic chemical gradients and membrane potential as nutrient transport driving forces.     

Altered Glycosylation of 63- and 68-kilodalton microvillar proteins in Heliothis virescens correlates with reduced Cry1 toxin binding,decreased pore formation,and increased resistance to Bacillus thuringiensis Cry1 toxins

The binding and pore formation abilities of Cry1A and Cry1Fa Bacillus thuringiensis toxins were analyzed by using brush border membrane vesicles (BBMV) prepared from sensitive (YDK) and resistant (YHD2) strains of Heliothis virescens. 125I-labeled Cry1Aa, Cry1Ab, and Cry1Ac toxins did not bind to BBMV from the resistant YHD2 strain, while specific binding to sensitive YDK vesicles was observed. Binding assays revealed a reduction in Cry1Fa binding to BBMV from resistant larvae compared to Cry1Fa binding to BBMV from sensitive larvae. In agreement with this reduction in binding, neither Cry1A nor Cry1Fa toxin altered the permeability of membrane vesicles from resistant larvae, as measured by a light-scattering assay. Ligand blotting experiments performed with BBMV and 125I-Cry1Ac did not differentiate sensitive larvae from resistant larvae. Iodination of BBMV surface proteins suggested that putative toxin-binding proteins were exposed on the surface of the BBMV from resistant insects. BBMV protein blots probed with the N-acetylgalactosamine-specific lectin soybean agglutinin (SBA) revealed altered glycosylation of 63- and 68-kDa glycoproteins but not altered glycosylation of known Cry1 toxin-binding proteins in YHD2 BBMV. The F1 progeny of crosses between sensitive and resistant insects were similar to the sensitive strain when they were tested by toxin-binding assays, light-scattering assays, and lectin blotting with SBA. These results are evidence that a dramatic reduction in toxin binding is responsible for the increased resistance and cross-resistance to Cry1 toxins observed in the YHD2 strain of H. virescens and that this trait correlates with altered glycosylation of specific brush border membrane glycoproteins.     

Kinetics of D-glucose and L-leucine transport into sheep and pig intestinal brush border membrane vesicles

The kinetic parameters (Vmax, Kt) of Na+-dependent D-glucose transport into brush border membrane vesicles (BBMV) from sheep and pig jejunum were determined. Due to the fermentation of ingested carbohydrates in the rumen the small intestine of ruminants (sheep) has to absorb much less glucose than the small intestine of monogastric omnivores (pigs) or herbivores. Kinetic analysis of the concentration dependence of D-glucose transport revealed a ten-fold smaller Vmax value combined with a five times lower Kt value in sheep BBMV compared with pig BBMV. The Vmax value for L-leucine transport did not differ between the two species investigated, whereas the Kt value in the sheep exceeded that in the pig. It is concluded from these results that the mechanism for Na+-dependent D-glucose transport in ruminants is adapted to the small amounts of carbohydrates reaching the small intestine.     

Increased Na+-taurine symporter in rat renal brush border membranes: preformed or newly synthesized?

The renal adaptive response to a varied intake of sulfur amino acids is demonstrated by an increase in the initial rate of Na+-taurine symport (cotransport) by rat renal brush border membrane vesicles (BBMVs) after 8-14 days of a low methionine diet. A high (3%) taurine diet reduces Na+-taurine symport. Fasting for 3 days, which depletes renal tubule cell taurine content, also enhances Na+-taurine symport both initially (15 s) and throughout the overshoot. In this study we examine the possibility that a rapid-onset adaptive response is expressed in BBMV, with the increased Na+-taurine symport reflecting the incorporation of preformed symporter into membranes rather than new synthesis. Rats fed the low methionine diet for 14 days were placed on the high taurine diet for 12-18 h; Na+-taurine symport activity fell by 40%. Fasting for 4 h restored low methionine diet levels of Na+-taurine symport activity (92 pmol.mg protein-1.15 s-1), defining a rapidly induced rise in uptake. Colchicine (0.6 mg) was injected prior to fasting in a group of rats because it blocks the incorporation (import) of preformed symporter into the membrane. Animals injected with colchicine had a pattern of BBMV uptake similar to that found in animals switched to the high taurine diet for 18 h. This agent blocked the rapidly induced rise in uptake. Feeding with the high taurine diet for 4 h caused a fall in uptake of 16.5%; colchicine blocked this reduction in uptake. These results indicate that the nephron can respond rapidly to changes in the intake of amino acids, conserving taurine in periods of nutrient lack and excreting excess taurine within 4 h in periods of surfeit. This rapid response is expressed at the brush border surface. The use of cholchicine indicates that the increase or reduction in Na+-taurine symport activity is due to incorporation (import) of transporter into the BBMV rather than to de novo synthesis.