Membrane distribution of sodium-hydrogen and chloride-bicarbonate exchangers in crypt and villus cell membranes from rabbit ileum

Increased expression of Na+/H+ exchanger in the injured renal tissues of focal glomerulosclerosis in rats

The renal mRNA expression of Na+/H+ exchanger (NHE) and the effects of NHE inhibitor, amiloride, on renal injury were investigated in adriamycin (ADR)-induced glomerulosclerosis model in rats, which progressively developed extensive glomerulosclerosis and interstitial fibrosis. NHE-1 mRNA from the cortex of the ADR rats progressively increased at weeks 4 and 8 and then peaked at week 16, which paralleled with the degree of glomerular sclerosis and interstitial fibrosis. The interstitial fibrosis in the ADR-rats was prevented by a daily administration of amiloride. A simultaneous analysis of the effects of a high salt diet on NHE-1 mRNA expression or renal injury was performed in the ADR rats at weeks 2 and 8. Renal or glomerular hypertrophy was observed in the control or ADR rats fed an 8% NaCl diet at week 2 and 8 compared to a 1% NaCl diet, while the NHE-1 mRNA expression was not up-regulated by an 8% NaCl diet at week 2. At week 8, the NHE-1 mRNA expression or glomerulosclerosis and interstitial fibrosis were enhanced in the ADR rats fed an 8% NaCl diet compared to a 1% NaCl diet. This histological aggravation by an 8% NaCl diet was prevented by a daily administration of amiloride but not by furosemide. In conclusion, the increased NHE-1 mRNA expression and the preventive effects of amiloride on the renal lesions suggest a potential importance of NHE in the progressive process of ADR-nephropathy. The high salt diet had a hypertrophic and destructive effect on kidney or glomeruli in the ADR rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucocorticoids regulate Na+/H+ exchange expression and activity in region- and tissue-specific manner

Na+/H+ exchangers (NHEs) are integral membrane proteins that exchange Na+ for H+ across membranes. Four isoforms have been cloned (NHE-1-4). NHE-3 localizes to the apical domain, and its expression is increased in dexamethasone-treated rats by Northern analysis. This stimulatory effect on expression is region and tissue specific, being present in ileum and proximal colon, but not in jejunum, distal colon, or kidney. The increase in transcript expression in ileum correlates with an increase in protein expression by immunoblotting. Changes in apical Na+/H+ exchange activity, as measured by 22Na uptake into brush-border membrane vesicles, correlate with expression differences, with significant increases observed in ileum and proximal colon. In situ hybridization showed NHE-3 mRNA only in villus and absorptive cells of control rats, the pattern not being altered by dexamethasone treatment. This suggests that dexamethasone does not increase expression by inducing crypt cells to express NHE-3 prematurely. We conclude that glucocorticoids selectively increase intestinal NHE-3 activity in a region-specific manner and that this effect also appears to be tissue specific.

Localization of bicarbonate transport along the crypt-villus axis in rabbit ileum

BACKGROUND/AIMS

HCO3- can be absorbed as well as secreted in the rabbit ileum. With 25 mmol/L HCO3- on the serosal side only, a serosa-to-mucosa flux (Jsm) is found; with 25 mmol/L on the mucosal side only, epinephrine elicits a mucosa-to-serosa flux (Jms). This study aimed to localize these two processes along the crypt-villus axis.

METHODS

Excised ileal segments were exposed luminally to 2 mol/L Na2SO4 (hypertonic treatment) or to isotonic Ringer's solution for 15 minutes. Mucosa was then chamber-mounted, and measurements were made of Jsm or Jms and of short-circuit current (Isc) responses to glucose plus alanine and to either theophylline or epinephrine.

RESULTS

With HCO3-/CO2 added to the serosal side only, hypertonically treated tissues showed a 22% decline in Jsm; a 25% decline in Isc response to theophylline; and a 71% decline in Isc response to glucose plus alanine compared with control. With HCO3-/CO2 added to the mucosal side only, tissues showed 92% and 87% declines in Jms and Isc responses to epinephrine, respectively, and a 87% decline in Isc response to glucose plus alanine. Histological examination showed destruction of villus caused by hypertonic treatment but sparing of crypt cells.

CONCLUSIONS

Both HCO3- and Cl- are secreted mainly by crypt cells and absorbed mainly by villus cells.

Gradient for D-glucose and linoleic acid uptake along the crypt-villus axis of rabbit jejunal brush border membrane vesicles

Glucose uptake into jejunal brush border membrane (BBM) varies along the crypt-villus axis (CVA). In the present study, the question was addressed whether uptake of the essential long-chain fatty acid linoleic acid also varies along the CVA. Using agitation techniques, five jejunal enterocyte fractions were sequentially isolated from female New Zealand white rabbits. A sixth and final fraction of lower-villus/crypt cells was obtained by the scraping of the remaining jejunal mucosa. Cell fraction along the CVA was proven histologically, by noting decreasing alkaline phosphatase activities in sequentially isolated fractions, and by demonstrating [3H-methyl]thymidine uptake mainly in the final fraction of the lower villus/crypt cells. BBM vesicles were prepared from the upper, mid- and lower-villus/crypt enterocyte fractions, using differential centrifugation and divalent ion precipitation. D-Glucose uptake into each fraction showed an Na(+)-gradient dependent time-course "overshoot" with linear uptake to 15 s and a subsequent decline to a steady-state plateau. Varying D-glucose concentrations from 50-1000 microM demonstrated saturation kinetics of uptake, with maximal transport rates (Vmax) and Michaelis affinity constants (Km) varying between fractions; the Km and Vmax were both lowest in the upper-villus fraction. A linear relationship existed between linoleic acid concentration (25-200 microM) and uptake in each fraction. Linoleic acid uptake was equivalent in all fractions when expressed per mg protein, but when expressed in terms of the estimated minimal BBM, vesicle surface area uptake was greater in the upper- than in the lower-villus/crypt fractions. Thus, BBM vesicle uptake of both linoleic acid and glucose vary along the crypt-villus axis of the rabbit jejunum.

The effect of intestinal hypoperfusion on intestinal absorption and permeability during cardiopulmonary bypass

BACKGROUND/AIMS

Mean arterial pressure is reduced during hypothermic cardiopulmonary bypass. The aim of this study was to assess whether this was associated with intestinal hypoperfusion and whether it affected intestinal absorption and permeability.

METHODS

Twenty-six patients undergoing coronary artery bypass grafting underwent an intestinal absorption-permeability test involving ingestion of 3-O-methyl-D-glucose, D-xylose, L-rhamnose, and lactulose. Ingestion took place 2 days before, within 3 hours, and 5 days after hypothermic cardiopulmonary bypass. Hemodynamic parameters and gastric mucosal laser Doppler blood flow were measured perioperatively in eight patients.

RESULTS

Hypothermic (28 degrees C), nonpulsatile cardiopulmonary bypass resulted in a 25% reduction in mean blood pressure, 10% reduction in cardiac index, and a 46% reduction in gastric mucosal laser Doppler blood flow. There was 85.4%, 85.5%, and 73.6% reduction (P < 0.01) in active (3-O-methyl-D-glucose) and passive (D-xylose) carrier-mediated transport and passive, nonmediated transcellular (L-rhamnose) transport in the immediate postoperative period, respectively. The differential urine excretion of lactulose/L-rhamnose increased sixfold. All parameters returned to control levels by the fifth postoperative day.

CONCLUSIONS

Cardiopulmonary bypass, while maintaining generally acceptable levels of hemodynamic performance, is associated with significant intestinal hypoperfusion and malabsorption of monosaccharides, which may have implications for enteral drug treatment in the immediate postoperative period.

Na+/H+ exchangers, NHE-1 and NHE-3, of rat intestine. Expression and localization

Na-H exchange (NHE) is one of the major non-nutritive Na absorptive pathways of the intestine and kidney. Of the four NHE isoforms that have been cloned, only one, NHE-3, appears to be epithelial specific. We have examined the regional and cellular expression of NHE-3 in the rat intestine. NHE-3 message in the small intestine was more abundant in the villus fractions of the small intestine than in the crypts. Analysis of NHE-3 mRNA distribution in the gut by in situ hybridization demonstrated epithelial cell specificity, as well as expression preferential to villus cells. NHE-1 message, in contrast, was ubiquitous, with slightly greater expression exhibited in the differentiating crypt and lower villus cells of the small intestine. Isoform-specific NHE-3 fusion protein antibody identified a 97-kD membrane protein in the upper villus cells of the small intestine, which was exclusively localized in the apical membrane. In contrast, antibody previously developed against the COOH-terminal region of human NHE-1 (McSwine, R. L., G. Babnigg, M. W. Musch, E. B. Chang, and M. L. Villereal, manuscript submitted for publication) identified a 110-kD basolateral membrane protein. These data suggest that unlike NHE-1, which probably serves a "housekeeping" function, NHE-3 may be involved in vectorial Na transport by the intestine.

Bicarbonate secretion in rabbit ileum: electrogenicity, ion dependence, and effects of cyclic nucleotides

BACKGROUND

Ileal HCO3- secretion is not well understood. The aim of this study was to examine its Na+ and Cl- dependencies, electrogenicity, and responses to amiloride, 4-acetamido-4'-isothiocyano-stilbene-2,2'-disulfonate (SITS), and cyclic nucleotides.

METHODS

The serosa to mucosa HCO3- flux (Jsm) across rabbit ileal mucosa mounted between HCO(3-)-free mucosal solution and HCO(3-)-containing serosal solutions was determined by titration.

RESULTS

In SO4(2-)-containing Ringer's solution, Jsm varied with [Na+] in two phases, one with a high and one with a low affinity for Na+; amiloride inhibited the high- and SITS inhibited the low-affinity phase. Switching from SO4(2-)- to Cl(-)-containing Ringer's solution caused a SITS-inhibitable 42% increase in Jsm. Changes in Jsm were coupled 3:2 with changes in short-circuit current. Cyclic nucleotide effects on Jsm were as follows. In SO4(2-)-containing Ringer's solution at 141 (but not 80) mmol/L Na+, theophylline caused equal increases in Jsm and short-circuit current that equaled the combined effects of 8-Br-5'-cyclic guanosine monophosphate (cGMP) and 8-Br-5'-cyclic adenosine monophosphate (cAMP). Serosal SITS blocked these effects, but amiloride did not. In Cl(-)-containing Ringer's solution, theophylline and bumetanide together (but not separately) increased Jsm.

CONCLUSIONS

(1) Basolateral HCO3- entry occurs via Na+/H exchange and a SITS-inhibitable process (Na(+)-HCO3- cotransport?). (2) Most HCO3- exit across the brush border occurs by a Cl(-)-independent process and some by Cl-/HCO3- exchange. (3) At low cellular [Cl-], HCO3- can be secreted via anion channels activated by cAMP and cGMP. (4) Ileal HCO3- secretion is electrogenic.

Chloride transport in brush-border membrane vesicles from chick jejunum

This study sought to characterize the mechanism(s) of Cl- transport across brush-border membrane vesicles (BBMV) isolated from chick jejunum. An inwardly directed proton gradient stimulated chloride (36Cl-) uptake. This uptake was inhibited by SITS and H2-DIDS. pH-gradient-stimulated Cl- uptake was electroneutral, since it was only slightly decreased by voltage clamping the BBMV with K+ and valinomycin. An outwardly directed HCO3- gradient significantly increased chloride uptake in the presence of a pH gradient. pH-driven chloride uptake was reduced by the presence of several anions in the uptake buffer. The rank order of potency for inhibition of pH-driven Cl- uptake was Cl- > SCN- > HCO3- > I- > Glu- > HPO4(2-). In the absence of a pH gradient, chloride was less concentrated inside the vesicles than outside. Chloride uptake under these conditions was stimulated by a positive electrical potential inside the vesicles. This stimulation was inhibited by the addition of several anions outside the vesicles. The order of inhibitory potency was SCN- > I- > Cl- > HCO3- > Glu- > HPO4(2-). The results are consistent with the presence of a Cl-/base exchanger and a chloride conductance pathway in the brush-border membrane of chick small intestine.

The gene for congenital chloride diarrhea maps close to but is distinct from the gene for cystic fibrosis transmembrane conductance regulator

Congenital chloride diarrhea (CLD) is characterized by watery stools with high chloride content beginning prenatally and is inherited as an autosomal recessive trait. Perfusion studies have established a basic defect in ileal and colonic Cl-/HCO3- transport, resulting in defective chloride absorption. The protein and its gene defects have, however, remained uncharacterized. We attempted to exclude candidate genes by considering linkage disequilibrium as well as genetic linkage in a small number of Finnish families. Initial results were suggestive of linkage between CLD and the cystic fibrosis transmembrane regulator gene (CFTR). Extended analysis in eight families confirmed close linkage to chromosome 7 markers proximal of CFTR, with maximum logarithm of odds scores of 5.11 and 5.06 for D7S501 and D7S496, respectively, at zero recombination. Allelic associations were observed that were striking between CLD and D7S496 and weaker between CLD and D7S501. Multipoint analyses mapped CLD unequivocally at D7S496 with a maximum logarithm of odds score of 9.33. We conclude that the CLD gene maps close to, but is distinct from, CFTR.

Segmental differences in short-chain fatty acid transport in rabbit colon: effect of pH and Na

Short-chain fatty acids (SCFAs) are the predominant luminal anion in the mammalian colon. Although they are rapidly absorbed in vivo, little is known about the mechanisms of transepithelial transport in vitro. Previous studies have suggested that SCFA transport may be linked to Na absorption or an anion exchange mechanism. We compared the transport of propionate under short-circuit conditions in rabbit proximal and distal colon to determine whether there were segmental differences, how SCFAs may be linked to either Na absorption or anion transport, and whether SCFAs, as weak electrolytes, may be affected by transepithelial pH gradients. In distal colon, propionate transport was not significantly altered by stimulation of electrogenic Na absorption, epinephrine or Cl removal. However, a modest transepithelial pH gradient (luminal 6.8/serosal 7.4) stimulated propionate absorption. In proximal colon, propionate transport was significantly altered by maneuvers that either stimulated (lowered [Na] in the bathing media) or inhibited (theophylline) apical Na-H exchange. Neither Cl removal, nor the anion exchange inhibitor DIDS, nor a transepithelial bicarbonate gradient, altered propionate transport. A transepithelial pH gradient inhibited propionate secretion, but not in a manner entirely consistent with the effect of pH on the distribution of a weak electrolyte. These results suggest that there is significant segmental heterogeneity in colonic SCFA transport; that transepithelial propionate fluxes are altered by changes in pH or electroneutral Na absorption (Na-H exchange), but not by chloride removal, bicarbonate gradients or electrogenic Na absorption. Regulation of SCFA transport may be an important factor in the physiology of colonic fluid balance.

Effect of short-chain fatty acids on cell volume and intracellular pH in rat distal colon

Superfusion of isolated crypts from the rat colon with sodium-butyrate-containing solutions induced an increase in the crypt diameter indicating a swelling of the crypt cells. The response to butyrate (50 mmol l-1) was not uniform along the crypt axis, the most pronounced swelling being observed in the upper third of the crypt. The butyrate effect was concentration-dependent and was completely suppressed by amiloride, suggesting that it is caused by activation of the Na+/H+ exchanger. Acetate, propionate and isobutyrate had a similar action. In HEPES-buffered solution the butyrate-induced change in cell volume was monophasic, i. e. only a swelling took place, whereas in HCO3- buffer it was biphasic, i. e. swelling was followed by a regulatory volume decrease. This decrease was suppressed by K+ and Cl- channel blockers as well as inhibitors of leukotriene synthesis. Measurements of intracellular pH with the fluorescent dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) revealed that butyrate induced an acidification of the cell, which was stronger in HEPES than in HCO3- buffer. Estimation of Na+/H+ exchange activity, tested as recovery of intracellular pH from an acid load via an NH4Cl prepulse, revealed a much lower Na+/H+ exchange activity in the fundus region compared to the upper third of the crypt. The smaller volume response evoked by butyrate in the fundus region probably reflects the smaller Na+/H+ activity compared to the more differentiated cells near the opening of the crypt. It is concluded that cell swelling caused by short-chain fatty acids is a physiological stimulus for volume regulation. This response is restricted to the more differentiated cells.

Characterization of chicken intestinal brush border membrane Ns/H exchange

1. Na/H exchange is the major pathway for Na uptake in brush border membrane vesicles from chicken small intestine. Hanes-Woolf analysis demonstrated that Na and H competed at the same extravesicular site. The KNa for Na+ at extravesicular pH 6.6 is 35 mM and at pH 7.4, 12 mM. 2. Similar to mammalian intestinal cells, the Na/H exchanger does not appear to have an internal proton modifier site. Varying intravesicular pH from 6.1 to 7.8 stimulates uptake, but a sigmoidal relationship is not observed. 3. The ability of several amiloride analogs to inhibit the exchanger was tested and the inhibitory profile was similar, but not identical to Na/H exchangers in mammalian tissues. The potency series (from most to least potent) is hexamethylamiloride approximately ethylisopropylamiloride > methylisobutylamiloride > dimethylamiloride >> amiloride.

Cloning and expression of a cAMP-activated Na+/H+ exchanger: evidence that the cytoplasmic domain mediates hormonal regulation

The ubiquitous plasma membrane Na+/H+ exchanger (termed NHE1) is activated by diverse hormonal signals, with the notable exception of hormones acting through cAMP as second messenger. Therefore, the Na+/H+ exchanger found in the nucleated trout red cell is of particular interest since it is activated by catecholamines, forskolin, and cAMP analogues. We report here that a cloned cDNA encoding the red cell exchanger restores functional Na+/H+ activity when transfected into Na+/H+ antiporter-deficient fibroblasts (i.e., it regulates intracellular pH in a Na-dependent and amiloride-sensitive manner). This red cell exchanger represents an additional form of Na+/H+ exchanger (termed beta NHE), which is characterized by a specific cytoplasmic domain involved in activation by the cAMP-dependent signaling pathway. After transfection in the same cellular context, beta NHE, but not NHE1, is activated by cAMP or by hormones that increase cAMP levels. Comparison of the amino acid sequences of exchangers shows that beta NHE, but not NHE1, contains two clustered consensus motifs for phosphorylation by a cAMP-dependent protein kinase (protein kinase A; PKA). A deletion mutant devoid of the C-terminal region of the cytoplasmic loop containing the two PKA sites restores Na+/H+ activity but is no longer activated by cAMP analogues or catecholamines. In red blood cells, the Na+/H+ exchanger is also activated by another pathway involving protein kinase C (PKC). Expression of beta NHE in fibroblasts shows that these two independent signaling pathways impinge on two distinct domains of the exchanger. The cytoplasmic segment containing PKA consensus sites, which is crucial for cAMP activation, is unnecessary for stimulation by PKC activators.

A gradient in expression of the Escherichia coli heat-stable enterotoxin receptor exists along the villus-to-crypt axis of rat small intestine

Binding of Escherichia coli heat-stable enterotoxin to its receptor is critical to the initiation of toxin-induced secretion and diarrheal disease; it is also likely, however, that this receptor binds an endogenous ligand. In order to characterize the expression of the heat-stable enterotoxin receptor in the small intestine, we isolated epithelial cells from villus tip to crypt in rat jejunum and ileum. Binding of radiolabeled toxin was maximal in the villus preparations and gradually decreased along the villus-to-crypt axis, paralleling the decline of sucrase activity. Northern blots of total RNA identified a single heat stable enterotoxin receptor transcript (3.8 kb), predominantly in the villus cell fractions. In situ hybridization demonstrated clear signal in the villus cells with no apparent signal in the crypt cells, lamina propria or muscularis. Expression of this receptor was greatest after enterocytes leave the proliferative cycle and enter villi. This pattern of gene and protein expression may reflect a role of this receptor in binding endogenous ligands which in turn may regulate intestinal ion flux along the villus-to-crypt axis.

Modulation of chloride secretion in the rat colon by intracellular bicarbonate

Extracellular HCO3- stimulates colonic net Cl- absorption in part by inhibiting basal Cl- secretion. This inhibition was investigated by measuring serosal-to-mucosal Cl- flux across short-circuited colonic segments from Sprague-Dawley rats. Mucosal intracellular pH and bicarbonate were estimated using the pH-sensitive dye BCECF. When extracellular [HCO3-] ([HCO3-]e) was increased from 0 to 39 mmol/L at PCO2 33 mm Hg, mucosal intracellular [HCO3-] ([HCO3-]i) increased to 25.3 mmol/L and serosal-to-mucosal Cl- flux decreased from 13.0 to 7.1 microEq.cm-2.h-1. When PCO2 was increased to 72 mm Hg at [HCO3-]e 39 mmol/L, [HCO3-]i increased to 29.8 mmol/L and serosal-to-mucosal Cl- flux decreased to 5.9 microEq.cm-2.h-1. In Ringer's solution containing 21 mmol/L HCO3- and 20 mmol/L Cl- (but not 100 mmol/L Cl-), increasing PCO2 from 21 to 70 mm Hg increased [HCO3-]i to 22.6 mmol/L and decreased serosal-to-mucosal Cl- flux from 3.0 to 1.7 microEq.cm-2.h-1. Overall, serosal-to-mucosal Cl- flux was inversely related to [HCO3-]i on either side of an [HCO3-]i plateau of 9-18 mmol/L at which flux was stable. These data suggest that [HCO3-]i is an important modulator of basal Cl- secretion in rat distal colon.

Chemical probes for anion transporters of mammalian cell membranes

Mammalian cell membranes harbor several types of chloride channels, chloride-cation symporters/cotransporters, and several classes of anion exchangers/antiporters. These transport systems subserve different cellular or organismic functions, depending on the nature of the cell, the spatial organization of transporters, and their functional interplay. Chemical probing has played a central role in the structural and functional delineation of the various anion transport systems. The design of specific probes or their selection from existing sources coupled with their judicious application to the most appropriate biological system had led to the identification of specific anion transporters and to the elucidation of the underlying molecular transport mechanism. In many instances, chemical probing has remained the major or exclusive analytical tool for the functional definition or identification of a given transport system, particularly for discerning among the various anion transporters which operate in highly heterogeneous cell membrane systems. This work critically reviews the present state of the chemical armamentarium available for the most common anion transporters found in mammalian cell membranes. It encompasses the description of the most useful or commonly used probes in terms of their chemical, biochemical, physiological, and pharmacological properties. The review deals primarily with what chemical probes tell about anion transporters and, most importantly, with the limitations inherent in the use of probes in transport studies.

Cl-/base exchange and cellular pH regulation in enterocytes isolated from chick small intestine

Intracellular pH (pHi) and Cl-/base exchange activity have been examined in isolated chicken enterocytes, both in the presence and absence of 25 mM HCO3-/5% CO2. Intracellular pH was measured with BCECF, a pH-sensitive carboxyfluorescein derivative. Under resting conditions pHi was 7.17 in Hepes and 7.12 in HCO3(-)-buffered solutions. Cells became more alkaline upon withdrawal of Cl-. Cells depleted of Cl- acidified upon reinstatement of Cl-. These changes were faster in the presence of HCO3- than in its absence. After an alkaline load (removal of HCO3- from the medium) pHi decreases towards base line in the presence of Cl-, but not in its absence. The Cl(-)-dependent pHi changes were prevented by H2DIDS and were unaffected by Na+. The Cl(-)-induced recovery from an alkaline load exhibited simple saturation kinetics, with an apparent Km of 12.5 mM Cl- and maximum velocity of approximately 0.20 pH units min-1. The Cl-/base exchange is functional under resting conditions, as shown by cell alkalinization on exposure to 0.5 mM H2DIDS, both in the presence and in the absence of HCO3-. It is concluded that Cl-/base exchange participates in setting the resting intracellular pH in isolated chicken enterocytes and helps recover from alkaline loads. The exchange operates both in the presence and in the absence of bicarbonate.

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

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.

Production of transforming growth factor-alpha by normal rat small intestine

Transforming growth factor-alpha (TGF-alpha) and epidermal growth factor (EGF) are similar in structure and biological activity. In the present study, the distributions of TGF-alpha mRNA, TGF-alpha immunoreactivity, and TGF-alpha-EGF receptor mRNA were examined in epithelial and nonepithelial compartments of the jejunum, and the effect of TGF-alpha on growth of a jejunal crypt cell line (IEC-6) was determined. Epithelial cells eluted from the rat jejunal cryptvillus axis expressed TGF-alpha mRNA at twofold higher levels in the villus tip than in the crypt and EGF receptor mRNA at sevenfold higher levels in the villus tip. Expression of these two mRNA transcripts in the subepithelium was low. Immunohistochemical staining showed TGF-alpha immunoreactivity predominantly in the epithelium and muscularis. Immunostaining of villus cells was uniform, whereas crypt cells did not stain. IEC-6 cells bound 125I-EGF to a single class of high-affinity (dissociation constant = 833 pM) receptors. EGF and TGF-alpha (10 ng/ml) only modestly stimulated IEC-6 cell growth in the presence of 5% serum but increased expression of the protooncogenes c-jun and c-myc threefold over control cells. These findings suggest that, among the potential physiological roles for TGF-alpha produced by the jejunal epithelium, promotion of cell migration and modulation of fluid and electrolyte transport may be as relatively important as stimulation of cell proliferation.

Mechanism of intestinal secretion: effect of cyclic AMP on rabbit ileal crypt and villus cells

Cyclic AMP-dependent secretagogues such as cholera toxin inhibit the coupled absorption of Na+ and Cl- and stimulate the secretion of HCO3- and Cl- in the ileum. Aside from Cl- secretion, little is known about the mechanism of these cyclic AMP-mediated effects. We therefore determined the effect of forskolin, an agent known to increase intracellular cyclic AMP by stimulation of adenylyl cyclase, on Na+/H+ and Cl-/HCO3- exchange in isolated crypt and villus cells from rabbit ileum. Forskolin increased cyclic AMP in the villus cells and decreased intracellular pH. The effect of forskolin on pH in villus cells was HCO3- independent, Na+ dependent, and amiloride sensitive. Further, the rate of recovery from an acid load was decreased by forskolin. These data suggest that increasing cyclic AMP inhibits Na+/H+ exchange in villus cells. In crypt cells also, forskolin increased cyclic AMP; however, forskolin increased intracellular pH in these cells. The effect of forskolin in crypt cells was also HCO3- independent, Na+ dependent, and amiloride sensitive. However, the rate of recovery from an acid load was increased by forskolin, the opposite effect of that seen in villus cells. These data suggest that increasing cyclic AMP in crypt cells stimulates Na+/H+ exchange. Inhibition of Na+/H+ exchange on the brush border membrane in villus cells would be expected to inhibit coupled NaCl absorption (which occurs by coupling of Na+/H+ and Cl-/HCO3- exchange). Stimulation of Na+/H+ exchange in crypt cells, present only on the basolateral membrane, alkalinizes the cell, which would be expected to stimulate HCO3- secretion by stimulating the Cl-/HCO3- exchanger on the brush border membrane. Thus, these results provide a mechanism for some of the previously unexplained in vivo and in vitro effects of cyclic AMP on ileal electrolyte transport.

On the mechanisms of the basal alkaline secretion in the rat ileum in vivo

Basal alkaline secretion of the denervated rat ileum was monitored by a pH-stat method. Changes of transepithelial electrical potential difference (PD) were also continuously registered. In other experiments net fluid transport was measured with a gravimetric method. The importance of the enteric nervous system for the recorded variables was investigated by giving i.v. hexamethonium, neuropeptide Y (NPY) or methionine-enkephalin or by stimulating electrically the mesenteric nerves surrounding the superior mesenteric artery. Alkaline secretion was inhibited by about 20% by mesenteric nerve stimulation or by neuropeptide Y (NPY) or met-enkephalin i.v. A somewhat greater inhibition (approximately 30%) of transepithelial electrical potential difference was elicited by the mesenteric nerve stimulation and NPY whereas met-enkephalin did not cause any transepithelial electrical potential difference change. Net fluid absorption was markedly diminished (by approximately 65-75%) by met-enkephalin but not by NPY. The cellular mechanisms underlying alkaline secretion were investigated by means of amiloride, SITS and acetazolamide. The basal alkaline secretion and transepithelial electrical potential difference were not influenced by 10(-3)M or 10(-4) SITS. In contrast 10(-3) M amiloride caused a significant increase of alkaline secretion but not of transepithelial electrical potential difference. A 35% reduction in the alkaline secretion but not transepithelial electrical potential difference was observed after acetazolamide had been given intravenously. A similar decrease was observed after giving hexamethonium. We conclude: (1) Enteric nerves are of comparatively small importance in controlling the ileal alkaline secretion recorded during basal conditions; (2) About 35% of the basal ileal alkaline secretion is carbonic anhydrase dependent. This mechanism is not influenced by nicotinic receptor blockade; (3) Under the present experimental conditions there may be an alkaline secretion which is concealed by a simultaneously operating Na+/H+ exchanger and; (4) No consistent quantitative correlation exists between alkaline secretion, transepithelial electrical potential difference and net field transport in the denervated rat ileum.

Phorbol ester inhibition of chicken intestinal brush-border sodium-proton exchange

Phorbol esters, specific activators of protein kinase C, inhibit amiloride-sensitive Na uptake from the mucosal medium in intact intestinal mucosa as well as in isolated chicken villus enterocytes. In isolated cells, maximal inhibition is observed at 60 s, and influx returns to control values within 15 min. This effect can be measured either as initial 22Na influx rates or by following changes in intracellular pH using the pH-sensitive fluorescent dye 5,6-carboxyfluorescein. The effects of amiloride and phorbol esters were not additive, suggesting inhibition of a common transport system, i.e., Na-H exchange. In brush-border membrane vesicles (BBMV) made from villus enterocytes, amiloride-sensitive Na-H exchange activity was significantly inhibited in phorbol ester-treated cells. The degree of inhibition of 22Na uptake by BBMV had the same time course and dose-effect relationship as phorbol ester-inhibited cellular Na uptake. Similarly, the time course of protein kinase C translocation from cytosol to particulate or brush-border membrane fractions correlated with Na uptake measurements made in whole cells and BBMV. These results suggest that protein kinase C activation in chicken villus enterocytes inhibits brush-border membrane Na-H exchange activity.

Correction of enhanced Na(+)-H+ exchange of rat small intestinal brush-border membranes in streptozotocin-induced diabetes by insulin or 1,25-dihydroxycholecalciferol

Diabetes was induced in rats by administration of a single i.p. injection of streptozotocin (50 mg/kg body wt). After 7 d, diabetic rats were further treated with insulin or 1,25-dihydroxycholecalciferol [1,25(OH)2D3] for an additional 5-7 d. Control, diabetic, diabetic + insulin, and diabetic + 1,25(OH)2D3 rats were then killed, their proximal small intestines were removed, and villus-tip epithelial cells were isolated and used to prepare brush-border membrane vesicles. Preparations from each of these groups were then analyzed and compared with respect to their amiloride-sensitive, electroneutral Na(+)-H+ exchange activity, using 22Na uptake as well as acridine orange techniques. The results of these experiments demonstrated that (a) H+ gradient-dependent 22Na uptake as well as Na+ gradient-dependent transmembrane H+ fluxes were significantly increased in diabetic vesicles compared to their control counterparts, (b) kinetic studies demonstrated that this enhanced 22Na uptake in diabetes was a result of increased maximal velocity (Vmax) of this exchanger with no change in apparent affinity (Km) for Na+, (c) serum levels of 1,25(OH)2D3 were significantly lower in diabetic animals compared with their control counterparts; and (d) insulin or 1,25(OH)2D3 treatment restored the Vmax alterations to control values, without any significant changes in Km, concomitant with significantly increasing the serum levels of 1,25(OH)2D3 in diabetic animals. These results indicate that Na(+)-H+ activity is significantly increased in proximal small intestinal luminal membranes of streptozotocin-induced diabetic rats. Moreover, alterations in the serum levels of 1,25(OH)2D3 may, at least in part, explain this enhanced antiporter activity and its correction by insulin.

Increased Na(+)-H+ antiporter activity in apical membrane vesicles from mutant LLC-PK1 cells

In whole cell experiments, the PKE20 mutant of the renal epithelial cell line LLC-PK1 displays a severalfold elevation of Na(+)-H+ antiporter activity at the apical surface (J.G. Haggerty, N. Agarwal, R.F. Reilly, E. A. Adelberg, and C.W. Slayman. Proc. Natl. Acad. Sci. USA 85: 6797-6801, 1988). The present study was undertaken to explore the properties of the mutant at the membrane level. Apical membrane vesicles were prepared by the magnesium-aggregation technique, with a similar enrichment (ca. 10-fold) of the marker enzyme gamma-glutamyltranspeptidase in vesicles from parent and mutant cell lines. In both cases, 22Na influx was stimulated by an inside-acid pH gradient, inhibited by ethylisopropylamiloride (EIPA), and unaffected by valinomycin, indicating that it was mediated by Na(+)-H+ antiport. Quantitatively, PKE20 vesicles showed a 4.2-fold increase in the maximal velocity of Na(+)-H+ antiporter activity compared with the parent, with only minor increases in the activity of two other Na(+)-dependent transporters (14-56% for alpha-methylglucoside and L-glutamate). Dose-response curves for EIPA indicated that the increased Na(+)-H+ antiport activity in PKE20 vesicles was due to an increased activity of the relatively amiloride-resistant form of the Na(+)-H+ antiporter with little or no change in the amiloride-sensitive form. No differences in polypeptide composition of the two vesicle preparations could be detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Taken together, the results indicate that the mutation in PKE20 is expressed at the membrane level and is specific for the relatively amiloride-resistant Na(+)-H+ antiporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of intestinal secretion. Effect of serotonin on rabbit ileal crypt and villus cells

To determine the mechanism of action of an intestinal secretagogue, serotonin, we have isolated crypt and villus cells and demonstrated Na:H and Cl:HCO3 exchange activity using the intracellular pH-sensitive fluorescent dye, 2,7-bis (carboxy-ethyl)-5,6-carboxy-fluorescein. Serotonin alkalinized both crypt and villus cells. Alkalinization in villus cells was HCO3 dependent and Na independent. In contrast, alkalinization in crypt cells was HCO3 independent and Na dependent. In villus cells, recovery from an alkaline load induced by Cl removal, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid or propionate pulse, known to occur via the Cl:HCO3 exchange, is inhibited by serotonin. In contrast, in crypt cells, recovery from an acid load induced by Na removal, amiloride and NH4Cl pulse, known to occur via Na:H exchange, is stimulated by serotonin. These data suggest that serotonin is inhibiting Cl:HCO3 exchange in villus cells and stimulating Na:H exchange in crypt cells. These effects of serotonin would be expected to inhibit coupled Na and Cl absorption by villus cells and stimulate HCO3 secretion by crypt cells in the intact ileum.

Effects of pH and cyclic adenosine monophosphate on ileal electrolyte transport in the rat and rabbit

Alterations in extracellular pH cause reciprocal changes in NaCl absorption in the rat and rabbit ileum. The presence of cholera toxin-induced secretion does not affect pH action measured by in vivo perfusion of the rat ileum. We examined the interaction of pH and cyclic adenosine monophosphate-induced secretion in the rabbit ileum. We found that alterations in arterial pH did not affect ileal absorption in the rabbit in the presence of cholera toxin-induced secretion. This was true whether transport was studied during in vivo ileal perfusion of anesthetized rabbits or by measuring Na+ and Cl- fluxes across isolated, short-circuited tissues in the Ussing chamber. The effects of pH also were blocked when normal rabbit ileum was exposed to 1 mmol/L dibutyryl cyclic adenosine monophosphate in vitro. By contrast, alterations in bathing solution pH affected ileal absorption in the rat in the presence and absence of cyclic adenosine monophosphate. Similarly, exposure to cyclic adenosine monophosphate did not affect the response of the rat colon to PCO2. These findings suggest that the apparently independent effects of pH and cyclic adenosine monophosphate in the rat ileum are not universal. In tissues such as the rabbit ileum, the mechanisms of pH and cyclic adenosine monophosphate action may have biochemical or physiological pathways in common.

NaCl absorption in the rabbit ileum. Effect of acid-base variables

In vivo and in vitro studies suggest that acid-base variables regulate ion transport in the rabbit ileum. The relative importance of these variables on active Na+ and Cl- absorption has not been defined. Isolated, stripped ileal segments were studied under short-circuited conditions in the Ussing flux chamber. Unidirectional 22Na and 36Cl fluxes were measured after changes in bathing solution pH, PCO2, and/or [HCO3-]. When pH was decreased from 7.6 to 7.1, net flux of Na+ increased from 0.1 +/- 0.7 to 2.6 +/- 0.7 mu Eq/cm2 per hour and net flux of Cl- increased from -2.0 +/- 0.9 to 1.3 +/- 0.6 mu Eq/cm2 per hour. These changes were rapid in onset, completely reversible, and accounted for by changes in mucosal-to-serosal fluxes of these ions. They were accompanied by small decreases in short-circuit current, but there were no changes in residual flux. These pH effects were not inhibited by the presence of luminal bumetanide (1 mmol/L), furosemide (1 mmol/L), hydroflumethiazide (1 mmol/L), or 4,4'-diisothiocyanostilbene-2,2'-disulfonate (1 mmol/L), or by the carbonic anhydrase inhibitor methazolamide (1 mmol/L). When data from all combinations of acid-base conditions were combined and analyzed by linear regression, pH was the only variable that correlated with mucosal-to-serosal fluxes (r = -0.84) and net flux (r = -0.85) for Na+, mucosal-to-serosal fluxes (r = -0.96) and net flux (r = -0.99) for Cl-, and short-circuit current (r = 0.97). These findings suggest that extracellular pH modulates active Na+ and Cl- absorption in the rabbit ileum.

Inhibition of Na/H exchange in avian intestine by atrial natriuretic factor

Effects of 8-bromo-cGMP (8-Br-cGMP) and synthetic rat atriopeptin III (APIII) on sodium absorption by isolated chicken villus enterocytes and intact chicken ileal mucosa were determined. In isolated cells, both agents significantly decreased initial rates of influx of 22Na and caused a persistent decrease in intracellular pH (pHi); effects that are not additive to those caused by amiloride (10(-3) M). The ED50 for APIII was 0.3 nM. In intact mucosa, both 8-Br-cGMP (10(-4) M) and 5-(N-methyl-N-isobutyl)amiloride (MIBA) (10(-5) M) reduced JNams and JNa.net, their effects were not additive. APIII (10(-7) M) significantly increased cellular cGMP but not cAMP. Both 8-Br-cGMP (10(-4) M) and APIII (10(-7) M) stimulated a persistent increase in cytosolic calcium (Cai), which could be prevented by pretreating the cells with the cytosolic calcium buffering agent MAPTAM or with H-8, an inhibitor of cyclic nucleotide-dependent protein kinases. Furthermore, pretreatment of cells with H-8 or the calmodulin inhibitor, calmidazolium (CM), prevented the effects of 8-Br-cGMP and APIII on pHi. However, the pHi response to subsequent addition of the calcium-ionophore ionomycin was blocked only by CM and not by H-8. These data suggest that APIII and 8-Br-cGMP inhibit amiloride-sensitive Na/H exchange by increasing Cai, an event requiring activation of cGMP-dependent protein kinase.

Characterization of basolateral membrane Na/H antiport in rat jejunum

Na uptake studies were performed in order to examine the activity of a Na/H exchanger in basolateral membrane vesicles isolated from rat jejunum. Experiments were carried out under voltage-clamped conditions in order to avoid electrodiffusional ionic movements. 1 mM Na uptake was found to be enhanced by an outward proton gradient and its initial rate was further increased by the presence of monensin or nigericin. The pH gradient-driven Na uptake was inhibited by 2 mM amiloride and unaffected by 0.1 mM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. The initial rate of the proton gradient-induced Na uptake was saturable with respect to external Na, with a Km of 13.6 +/- 1.4 mM and a Vmax of 35.4 +/- 2.2 nmol/mg protein per min. Li competed with Na for the exchange process, whereas K, Rb, Cs, tetramethylammonium had no effect. We conclude that rat jejunal basolateral membrane contains a Na/H exchanger whose properties are similar to those of the antiporter identified in the brush-border membrane.

Acid-base regulation of ion transport in rabbit ileum in vitro

Changes in acid-base balance have a major influence on ion transport in the ileum. The goals of the present study were to delineate (a) the specific transport processes most affected by changes in acid-base metabolism, (b) the individual roles of pH, PCO2, and concentration of HCO3- in modulating ion transport, and (c) the relationship between acid-base sensitive and other ion-transport systems. Ion transport and electrical parameters were measured in rabbit ilea in vitro under short-circuit conditions with systematic variations of pH, PCO2, and concentrations of HCO3-. Increasing HCO3- concentrations, with constant PCO2 and increasing pH, caused a decrease in electroneutral Na+ and Cl- absorption. At 5 mmol/L HCO3-, net fluxes of Na+ and Cl- were 5.9 +/- 1.4 and 4.5 +/- 1.1 microEq.cm-2.h-1, while at 50 mmol/L HCO3-, net Na+ and Cl- fluxes were 0.7 +/- 0.7 and 0.2 +/- 0.6 microEq.cm-2.h-1. Transepithelial HCO3(-)-gradient experiments suggested that serosal HCO3- was the principal factor. Changes in PCO2 showed a complex biphasic response, increasing net Cl- flux as PCO2 increased from 11-30 mm Hg in 5 mmol/L HCO3-; net Na+ flux increased as PCO2 was changed from 36 to greater than 100 mm Hg in 22 mmol/L HCO3-. In contrast, increasing pH in a bicarbonate-free N-2-hydroxyethylpiperazine-N'-2 = ethane sulfonic acid buffer did not significantly alter Na+ transport. Acid-base stimulated Na+ absorption was inhibited by 10(-3) mol/L amiloride, but not by bumetanide, consistent with the involvement of Na(+)-H+ exchange rather than Na(+)-Cl- cotransport. Epinephrine did not increase Na+ absorption under acid-base stimulated conditions, but glucose did, suggesting that the rate-limiting step for electroneutral absorptive processes under these conditions occurs at the apical rather than the basolateral membrane. Combining all experiments, a significant correlation existed between net flux of Na+ and HCO3- concentration (r = -0.72, P less than 0.05) and between net flux of Na+ and pH (r = -0.68, P less than 0.01). Chloride absorption was correlated with pH (r = 0.72, P less than 0.01). These results suggest a profound regulatory role for acid-base balance in electroneutral Na(+)-Cl- transport in rabbit ileum in vitro.

H+ and HCO3- flux across apical surface of rat distal colon

Colonic ion transport is postulated to occur via simultaneous operation of Na(+)-H+ exchange and Cl(-)-HCO3- exchange. Accordingly H+ and HCO3- should be transported simultaneously by the colon. To assess simultaneous H+ and HCO3- transport, net acid-base flux was measured in isolated segments of rat distal colon. When both tissue surfaces were bathed in symmetrical solutions containing Cl-, net base was secreted (-1.0 +/- 0.1 mu eq.cm-2.h-1). Cl- substitution with gluconate in the mucosal medium caused net base flux to switch from secretion to absorption (2.0 +/- 0.2 mu eq.cm-2.h-1). To evaluate whether base absorption was dependent on H+ secretion via Na(+)-H+ exchange, mucosal Na+ was substituted with N-methylglucamine, and amiloride, an inhibitor of Na(+)-H+ exchange, was applied. Na+ substitution and 1 mM amiloride inhibited base absorption by 37 and 38%, respectively, suggesting operation of Na(+)-H+ exchange. Because base absorption persisted, an additional mechanism was considered, HCO3- absorption via Cl(-)-HCO3- exchange. This was evaluated with an inhibitor of Cl(-)-HCO3- exchange 4-acetamido-4'-isothiostilbene-2,2'-disulfonic acid (SITS). SITS (1 mM) inhibited HCO3- absorption by 40%. The effects of amiloride and SITS were additive, suggesting that the Na(+)-H+ and Cl(-)-HCO3- exchangers operate simultaneously. Amiloride also inhibited H+ secretion when net HCO3- was secreted, suggesting that the direction of HCO3- movement does not influence Na(+)-H+ exchange activity. These data suggest that the colon transports both H+ and HCO3- across the apical surface via Na(+)-H+ exchange and Cl(-)-HCO3- exchange; H+ is secreted via Na(+)-H+ exchange, whereas HCO3- can be secreted or absorbed via Cl(-)-HCO3- exchange.

Glucose transport and microvillus membrane physical properties along the crypt-villus axis of the rabbit

Both transport function and microvillus membrane physical properties evolve as the enterocyte matures and migrates up the crypt-villus axis. We isolated enriched fractions of villus tip, mid-villus, and crypt enterocytes from which microvillus membrane vesicles were prepared. Using this material we characterized the alterations that occur in microvillus membrane fluidity as the rabbit enterocyte matures and correlated these with kinetic studies of glucose transport. With increasing maturity the microvillus membrane becomes more rigid due to both an increase in the cholesterol/phospholipid ratio and alterations in individual phospholipid subclasses. Maximal rates of glucose transport were greatest in microvillus membrane vesicles prepared from mature cells. However, the glucose concentration producing half-maximal rates of transport (Km) was significantly lower in crypt microvillus membrane vesicles, suggesting that a distinct glucose transporter existed in crypt enterocytes. This distinction disappeared when differences between membrane lipid environments were removed. By fluidizing villus-tip microvillus membrane vesicles, in vitro, to levels seen in the crypt microvillus membrane, we observed a reduction in the Km of this transport system. These data suggest that the kinetic characteristics of the sodium-dependent glucose transporter are dependent upon its local membrane environment.

Autoradiographic localization of Na(+)-dependent L-valine uptake by the jejunum of streptozotocin-diabetic rats

The mechanisms involved in the increased Na(+)-dependent nutrient uptake across intestine of diabetic animals are poorly understood. Here we have studied the effect of acute (7d) and chronic (30-40d) diabetes on the autoradiographic localization of 3H-L-valine accumulation by rat jejunal villi and on enterocyte migration rate. In control rats, Na(+)-dependent valine uptake was confined to enterocytes on the upper 20-23% of the villus. In intestine from diabetic rats, however, this area was extended to occupy the upper 42-45% of an enlarged villus surface. Hyperphagia was not responsible for the expanded functional surface and systemic factors are therefore implicated in the adaptive response. Enterocyte migration rate was found to be unaffected by diabetes but an increased villus height in this condition resulted in an additional 13.5 h in enterocyte lifespan. These data are compatible with the hypothesis that during diabetes the earlier maturation of enterocyte absorptive function produces an epithelial surface containing a higher proportion of mature enterocytes.