Characterization of Na-H exchange in apical membrane vesicles of rat colon

The Role of Plasma Membrane Sodium/Hydrogen Exchangers in Gastrointestinal Functions: Proliferation and Differentiation, Fluid/Electrolyte Transport and Barrier Integrity

The five plasma membrane Na⁺/H⁺ exchanger (NHE) isoforms in the gastrointestinal tract are characterized by distinct cellular localization, tissue distribution, inhibitor sensitivities, and physiological regulation. NHE1 (Slc9a1) is ubiquitously expressed along the gastrointestinal tract in the basolateral membrane of enterocytes, but so far, an exclusive role for NHE1 in enterocyte physiology has remained elusive. NHE2 (Slc9a2) and NHE8 (Slc9a8) are apically expressed isoforms with ubiquitous distribution along the colonic crypt axis. They are involved in pH_i regulation of intestinal epithelial cells. Combined use of a knockout mouse model, intestinal organoid technology, and specific inhibitors revealed previously unrecognized actions of NHE2 and NHE8 in enterocyte proliferation and differentiation. NHE3 (Slc9a3), expressed in the apical membrane of differentiated intestinal epithelial cells, functions as the predominant nutrient-independent Na⁺ absorptive mechanism in the gut. The new selective NHE3 inhibitor (Tenapanor) allowed discovery of novel pathophysiological and drug-targetable NHE3 functions in cystic-fibrosis associated intestinal obstructions. NHE4, expressed in the basolateral membrane of parietal cells, is essential for parietal cell integrity and acid secretory function, through its role in cell volume regulation. This review focuses on the expression, regulation and activity of the five plasma membrane Na⁺/H⁺ exchangers in the gastrointestinal tract, emphasizing their role in maintaining intestinal homeostasis, or their impact on disease pathogenesis. We point to major open questions in identifying NHE interacting partners in central cellular pathways and processes and the necessity of determining their physiological role in a system where their endogenous expression/activity is maintained, such as organoids derived from different parts of the gastrointestinal tract.

Antidiarrheal Action of Bacillus subtilis CU1 CNCM I-2745 and Lactobacillus plantarum CNCM I-4547 in Mice

Preventive actions of probiotics as antidiarrheal agents are well documented, but their mechanisms are poorly understood. Two selected probiotics, Bacillus subtilis CU1 and Lactobacillus plantarum CNCM I-4547, were tested in mouse experimental models of diarrhea and the possible mechanisms of action were investigated. Diarrhea was induced in mice by oral castor oil administration or by i.v. injection of lipopolysaccharide (LPS) of Salmonella enteritis. The antidiarrheal drug loperamide was used as control. Fecal water excretion was quantified for 2 h and paracellular permeability and electrical parameters of the colon were assessed in Ussing chambers. The expression of colonic exchangers or channels and of Toll-like receptor 4 (TLR4) was assessed by immunohistochemistry. Prophylactic treatment with B. subtilis CU1 or with L. plantarum CNCM I-4547 reduced LPS-induced diarrhea. The reduction of water excretion was in the same range as those induced by loperamide. In the castor oil model, this effect was only observed with B. subtilis CU1. The two probiotic treatments abolished the increase in paracellular permeability induced by LPS, but not by castor oil. However, only L. plantarum CNCM I-4547 treatment decreased the colonic expression of TLR-4. After B. subtilis CU1, colonic expression of cystic fibrosis transmembrane conductance regulator (CFTR) was reduced and that of Na⁺/H⁺ exchanger 3 (NHE3) increased. B. subtilis CU1 may increase the capacity of the colon to absorb excess of water in diarrheic conditions by acting on CFTR and NHE3 expression. The two probiotics strains showed an impact on diarrhea through limitation of water excretion that may involve paracellular permeability or electrolyte transport for L. plantarum CNCM I-4547 and B. subtilis CU1 respectively.

Bumetanide increases Cl--dependent short-circuit current in late distal colon: Evidence for the presence of active electrogenic Cl- absorption

Mammalian colonic epithelia consist of cells that are capable of both absorbing and secreting Cl^-. The present studies employing Ussing chamber technique identified two opposing short-circuit current (I_sc) responses to basolateral bumetanide in rat distal colon. Apart from the transepithelial Cl^--secretory I_sc in early distal colon that was inhibited by bumetanide, bumetanide also stimulated I_sc in late distal colon that had not previously been identified. Since bumetanide inhibits basolateral Na⁺-K⁺-2Cl^- cotransporter (NKCC) in crypt cells and basolateral K⁺-Cl^- cotransporter (KCC) in surface epithelium, we proposed this stimulatory I_sc could represent a KCC-mediated Cl^- absorptive current. In support of this hypothesis, ion substitution experiments established Cl^- dependency of this absorptive I_sc and transport inhibitor studies demonstrated the involvement of an apical Cl^- conductance. Current distribution and RNA sequencing analyses revealed that this Cl^- absorptive I_sc is closely associated with epithelial Na⁺ channel (ENaC) but is not dependent on ENaC activity. Thus, inhibition of ENaC by 10 μM amiloride or benzamil neither altered the direction nor its activity. Physiological studies suggested that this Cl^- absorptive I_sc senses dietary Cl^- content; thus when dietary Cl^- was low, Cl^- absorptive I_sc was up-regulated. In contrast, when dietary Cl^- was increased, Cl^- absorptive I_sc was down-regulated. We conclude that an active Cl^- extrusion mechanism exists in ENaC-expressing late distal colon and likely operates in parallel with ENaC to facilitate NaCl absorption.

Na-H Exchanger Isoform-2 (NHE2) Mediates Butyrate-dependent Na+ Absorption in Dextran Sulfate Sodium (DSS)-induced Colitis

Diarrhea associated with ulcerative colitis (UC) occurs primarily as a result of reduced Na(+) absorption. Although colonic Na(+) absorption is mediated by both epithelial Na(+) channels (ENaC) and Na-H exchangers (NHE), inhibition of NHE-mediated Na(+) absorption is the primary cause of diarrhea in UC. As there are conflicting observations reported on NHE expression in human UC, the present study was initiated to identify whether NHE isoforms (NHE2 and NHE3) expression is altered and how Na(+) absorption is regulated in DSS-induced inflammation in rat colon, a model that has been used to study UC. Western blot analyses indicate that neither NHE2 nor NHE3 expression is altered in apical membranes of inflamed colon. Na(+) fluxes measured in vitro under voltage clamp conditions in controls demonstrate that both HCO3 (-)-dependent and butyrate-dependent Na(+) absorption are inhibited by S3226 (NHE3-inhibitor), but not by HOE694 (NHE2-inhibitor) in normal animals. In contrast, in DSS-induced inflammation, butyrate-, but not HCO3 (-)-dependent Na(+) absorption is present and is inhibited by HOE694, but not by S3226. These observations indicate that in normal colon NHE3 mediates both HCO3 (-)-dependent and butyrate-dependent Na(+) absorption, whereas DSS-induced inflammation activates NHE2, which mediates butyrate-dependent (but not HCO3 (-)-dependent) Na(+) absorption. In in vivo loop studies HCO3 (-)-Ringer and butyrate-Ringer exhibit similar rates of water absorption in normal rats, whereas in DSS-induced inflammation luminal butyrate-Ringer reversed water secretion observed with HCO3 (-)-Ringer to fluid absorption. Lumen butyrate-Ringer incubation activated NHE3-mediated Na(+) absorption in DSS-induced colitis. These observations suggest that the butyrate activation of NHE2 would be a potential target to control UC-associated diarrhea.

Aldosterone induces active K⁺ secretion by enhancing mucosal expression of Kcnn4c and Kcnma1 channels in rat distal colon

Although both Kcnn4c and Kcnma1 channels are present on colonic mucosal membranes, only Kcnma1 has been suggested to mediate K(+) secretion in the colon. Therefore, studies were initiated to investigate the relative roles of Kcnn4c and Kcnma1 in mediating aldosterone (Na-free diet)-induced K(+) secretion. Mucosal to serosal (m-s), serosal to mucosal (s-m), and net (86)Rb(+) (K(+) surrogate) fluxes as well as short circuit currents (I(sc); measure of net ion movement) were measured under voltage clamp condition in rat distal colon. Active K(+) absorption, but not K(+) secretion, is present in normal, while aldosterone induces active K(+) secretion (1.04 ± 0.26 vs. -1.21 ± 0.15 μeq·h(-1)·cm(-2); P < 0.001) in rat distal colon. Mucosal VO(4) (a P-type ATPase inhibitor) inhibited the net K(+) absorption in normal, while it significantly enhanced net K(+) secretion in aldosterone animals. The aldosterone-induced K(+) secretion was inhibited by the mucosal addition of 1) either Ba(2+) (a nonspecific K(+) channel blocker) or charybdotoxin (CTX; a common Kcnn4 and Kcnma1 channel blocker) by 89%; 2) tetraethyl ammonium (TEA) or iberiotoxin (IbTX; a Kcnma1 channel blocker) by 64%; and 3) TRAM-34 (a Kcnn4 channel blocker) by 29%. TRAM-34, but not TEA, in the presence of IbTX further significantly inhibited the aldosterone-induced K(+) secretion. Thus the aldosterone-induced Ba(2+)/CTX-sensitive K(+) secretion consists of IbTX/TEA-sensitive (Kcnma1) and IbTX/TEA-insensitive fractions. TRAM-34 inhibition of the IbTX-insensitive fraction is consistent with the aldosterone-induced K(+) secretion being mediated partially via Kcnn4c. Western and quantitative PCR analyses indicated that aldosterone enhanced both Kcnn4c and Kcnma1α protein expression and mRNA abundance. In vitro exposure of isolated normal colonic mucosa to aldosterone also enhanced Kcnn4c and Kcnma1α mRNA levels, and this was prevented by exposure to actinomycin D (an RNA synthesis inhibitor). These observations indicate that aldosterone induces active K(+) secretion by enhancing mucosal Kcnn4c and Kcnma1 expression at the transcriptional level.

Increased colonic sodium absorption in rats with chronic renal failure is partially mediated by AT1 receptor agonism

To test the hypothesis that colonic Na(+) transport is altered in the 5/6 nephrectomized rat model of chronic renal failure (CRF), we measured Na(+) fluxes across distal colon from control (CON), CRF, and CRF rats treated with the angiotensin II (ANG II) receptor antagonist losartan (+LOS). We also evaluated overall fluid and Na(+) balance and compared colonic protein and mRNA expression profiles for electroneutral [sodium-hydrogen exchanger (NHE)] and electrogenic Na(+) transport [epithelial sodium channel (ENaC)] in these groups. Consistent with a 60% enhancement in colonic Na(+) absorption in CRF, urinary Na(+) excretion increased by about 50% while serum Na(+) homeostasis was maintained. These CRF-induced changes in Na(+) handling were normalized by treatment with LOS. Net Na(+) absorption was also stimulated in in vitro tissues from CON rats following acute serosal addition of ANG II (10(-7) M), and this increase was blocked by AT(1) antagonism but not by an AT(2) antagonist. In CRF, colonic protein and mRNA expression variably increased for apical NHE2, NHE3, and ENaC alpha-, beta-, gamma-subunits, whereas expression of basolateral NHE1 and Na(+)-K(+)-ATPase (alpha-isoform) remained unaltered. Upregulation of the ENaC subunit mRNA was attenuated somewhat by LOS treatment. Previously, we showed that colonic AT(1) receptor protein is upregulated twofold in CRF, and here we find that AT(1) and AT(2) mRNA and AT(2) protein abundance is unchanged in CRF. We conclude that Na(+) absorption in CRF rat distal colon is increased due to elevated expression of proteins mediating electroneutral and electrogenic uptake and that it is partially mediated by AT(1) receptors.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

MOLECULAR PHYSIOLOGY OF INTESTINAL N+/H+EXCHANGE

The sodium/hydrogen exchange (NHE) gene family plays an integral role in neutral sodium absorption in the mammalian intestine. The NHE gene family is comprised of nine members that are categorized by cellular localization (i.e., plasma membrane or intracellular). In the gastrointestinal (GI) tract of multiple species, there are resident plasma membrane isoforms including NHE1 (basolateral) and NHE2 (apical), recycling isoforms (NHE3), as well as intracellular isoforms (NHE6, 7, 9). NHE3 recycles between the endosomal compartment and the apical plasma membrane and functions in both locations. NHE3 regulation occurs during normal digestive processes and is often inhibited in diarrheal diseases. The C terminus of NHE3 binds multiple regulatory proteins to form large protein complexes that are involved in regulation of NHE3 trafficking to and from the plasma membrane, turnover number, and protein phosphorylation. NHE1 and NHE2 are not regulated by trafficking. NHE1 interacts with multiple regulatory proteins that affect phosphorylation; however, whether NHE1 exists in large multi-protein complexes is unknown. Although intestinal and colonic sodium absorption appear to involve at least NHE2 and NHE3, future studies are necessary to more accurately define their relative contributions to sodium absorption during human digestion and in pathophysiological conditions.

Calcineurin homologous protein isoform 2 (CHP2), Na+/H+ exchangers-binding protein, is expressed in intestinal epithelium

The Na+/H+ exchangers (NHEs) comprise a family of membrane proteins that catalyze the electroneutral exchange of Na+ and H+. Calcineurin homologous protein (CHP) acts as a crucial cofactor for NHE activity through direct interaction with the carboxyl-terminal tail region of NHEs. We have cloned a new rat CHP isoform (rCHP2) and characterized the binding property to NHEs and the tissue distribution. rCHP2 binds to the juxtamembrane region of plasma membrane-type NHE isoforms (NHE1-5) in vivo and in vitro as well as rCHP1 (original rat CHP). Interestingly, CHP2 is predominantly expressed in the small and large intestine although rCHP1 shows relatively ubiquitous expression at both the mRNA and protein levels. In situ hybridization experiments demonstrated the abundant expression of CHP2 in the epithelial cell layer of villi of the small intestine in contrast with the expression of CHP1 in both the epithelial layer and connective tissues. These results suggest that CHP2 functions in the absorptive epithelium for the intestine with NHE(s).

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Characterization of apical membrane Cl-dependent Na/H exchange in crypt cells of rat distal colon

A novel Cl-dependent Na/H exchange (Cl-NHE) has been identified in apical membranes of crypt cells of rat distal colon. The presence of Cl is required for both outward proton gradient-driven Na uptake in apical membrane vesicles (AMV) and Na-dependent intracellular pH recovery from an acid load in the crypt gland. The present study establishes that Cl-dependent outward proton gradient-driven (22)Na uptake 1) is saturated with increasing extravesicular Na concentration with a Michaelis constant (K(m)) for Na of approximately 24.2 mM; 2) is saturated with increasing outward H concentration gradient with a hyperbolic curve and a K(m) for H of approximately 1.5 microM; 3) is inhibited by the Na/H exchange (NHE) inhibitors amiloride, ethylisopropylamiloride, and HOE-694 with an inhibitory constant (K(i)) of approximately 480.2, 1.1, and 9.5 microM, respectively; 4) is inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, an anion exchange inhibitor at low concentration and a Cl channel blocker at high dose, and by 5-nitro-2(3-phenylpropylamino)benzoic acid, a Cl channel blocker, with a K(i) of approximately 280.6 and 18.3 microM, respectively; and 5) substantially stimulated Cl-NHE activity by dietary Na depletion, which increases plasma aldosterone and inhibits NHE in surface cell AMV. These properties of Cl-NHE are distinct from those of NHE1, NHE2, and NHE3 isoforms that are present in colonic epithelial cells; thus these results suggest that the colonic crypt cell Cl-NHE is a novel NHE isoform.

Characterization and molecular localization of anion transporters in colonic epithelial cells

This study describes the identification and characterization of anion transporters in apical membrane (APM) and basolateral membrane (BLM) of rat distal colon. Cl-HCO3, Cl-OH, Cl-butyrate, and butyrate-HCO3 exchanges and Na-HCO3 cotransporter are present in rat distal epithelial cells. Cl-HCO3 exchange (1) is present only in APM from surface, but not from crypt cells; (2) is also present in BLM; and (3) of surface cell is encoded by anion exchange (AE)-1 isoform, whereas BLM Cl-HCO3 is encoded by AE2 isoform. Cl-OH exchange is present only in APM, but not in BLM from surface and crypt cells, and is responsible for regulation of cell functions (i.e., cell pH and cell volume regulation). Butyrate-HCO3 exchange (1) is also present in apical membrane vesicles (AMV) from surface, but not from crypt cells; (2) is present in BLM; and (3) is responsible for SCFA-dependent HCO3 secretion. By contrast, Cl-butyrate exchange: (1) is present in APM from both surface and crypt cells; (2) is not present in BLM; and (3) recycles butyrate by absorbing Cl. Na-HCO3 cotransport: (1) is present only in BLM; (2) is expressed predominantly in midcrypt regions; and (3) may be linked to HCO3 secretion. A mechanism for HCO3 movement across the crypt apical membrane has not as yet been identified.

Na(+)-dependent fluid absorption in intact perfused rat colonic crypts

BACKGROUND & AIMS

The traditional paradigm of fluid movement in the mammalian colon is that fluid absorption and secretion are present in surface and crypt cells, respectively. We have recently demonstrated Na(+)-dependent fluid absorption in isolated crypts that are devoid of neurohumoral stimulation. We now explore the mechanism of Na(+)-dependent fluid absorption in isolated rat colonic crypts.

METHODS

Net fluid absorption was determined using microperfusion techniques and methoxy[(3)H]inulin with ion substitutions and transport inhibitors.

RESULTS

Net fluid absorption was reduced but not abolished by substitution of either N-methyl-D-glucamine- Cl(-) or tetramethylammonium for Na(+) and by lumen addition of 5-ethylisopropyl amiloride, an amiloride analogue that selectively inhibits Na(+)-H(+) exchange. Net fluid absorption was also dependent on lumen Cl(-) because removal of lumen Cl(-) significantly (P < 0.001) reduced net fluid absorption. DIDS at 100 micromol/L, a concentration at which DIDS is an anion exchange inhibitor, minimally reduced net fluid absorption (P < 0.05). In contrast, either 500 micromol/L DIDS, a concentration at which DIDS is known to act as a Cl(-) channel blocker, or 10 micromol/L NPPB, a Cl(-) channel blocker, both substantially inhibited net fluid absorption (P < 0.001). Finally, both the removal of bath Cl(-) and addition of bath bumetanide, an inhibitor of Na-K-2Cl cotransport and Cl(-) secretion, resulted in a significant increase in net fluid absorption.

CONCLUSIONS

(1) Net Na(+)-dependent net fluid absorption in the isolated colonic crypt represents both a larger Na(+)-dependent absorptive process and a smaller secretory process; and (2) the absorptive process consists of a Na(+)-dependent, HCO(3)(-)-independent process and a Na(+)-independent, Cl(-)-dependent, HCO(3)(-)-dependent process. Fluid movement in situ represents these transport processes plus fluid secretion induced by neurohumoral stimulation.

Ouabain-sensitive H,K-ATPase functions as Na,K-ATPase in apical membranes of rat distal colon

Na,K-ATPase activity has been identified in the apical membrane of rat distal colon, whereas ouabain-sensitive and ouabain-insensitive H,K-ATPase activities are localized solely to apical membranes. This study was designed to determine whether apical membrane Na,K-ATPase represented contamination of basolateral membranes or an alternate mode of H,K-ATPase expression. An antibody directed against the H, K-ATPase alpha subunit (HKcalpha) inhibited apical Na,K-ATPase activity by 92% but did not alter basolateral membrane Na,K-ATPase activity. Two distinct H,K-ATPase isoforms exist; one of which, the ouabain-insensitive HKcalpha, has been cloned. Because dietary sodium depletion markedly increases ouabain-insensitive active potassium absorption and HKcalpha mRNA and protein expression, Na, K-ATPase and H,K-ATPase activities and protein expression were determined in apical membranes from control and sodium-depleted rats. Sodium depletion substantially increased ouabain-insensitive H, K-ATPase activity and HKcalpha protein expression by 109-250% but increased ouabain-sensitive Na,K-ATPase and H,K-ATPase activities by only 30% and 42%, respectively. These studies suggest that apical membrane Na,K-ATPase activity is an alternate mode of ouabain-sensitive H,K-ATPase and does not solely represent basolateral membrane contamination.

Cyclic nucleotide-gated cation channels mediate sodium and calcium influx in rat colon

We found mRNA for the three isoforms of the cyclic nucleotide-gated nonselective cation channel expressed in the mucosal layer of the rat intestine from the duodenum to the colon and in intestinal epithelial cell lines in culture. Because these channels are permeable to sodium and calcium and are stimulated by cGMP or cAMP, we measured 8-bromo-cGMP-stimulated sodium-mediated short-circuit current (I(sc)) in proximal and distal colon and unidirectional (45)Ca(2+) fluxes in proximal colon to determine whether these channels could mediate transepithelial sodium and calcium absorption across the colon. Sodium-mediated I(sc), stimulated by 8-bromo-cGMP, were inhibited by dichlorobenzamil and l-cis-diltiazem, blockers of cyclic nucleotide-gated cation channels, suggesting that these ion channels can mediate transepithelial sodium absorption. Sodium-mediated I(sc) and net transepithelial (45)Ca(2+) absorption were stimulated by heat-stable toxin from Escherichia coli that increases cGMP. Addition of l-cis-diltiazem inhibited the enhanced transepithelial absorption of both ions. These results suggest that cyclic nucleotide-gated cation channels simultaneously increase net sodium and calcium absorption in the colon of the rat.

Differential regulation of NHE isoforms by sodium depletion in proximal and distal segments of rat colon

Dietary sodium depletion has multiple diverse effects on ion transport in the rat colon, including both the induction and inhibition of electroneutral NaCl absorption in proximal and distal colon of rat, respectively. To establish the mechanism of the differential regulation of Na+ absorption by sodium depletion, this study utilized 1) HOE-694, a dose-dependent inhibitor of Na+/H+ exchanger (NHE) isoforms, in studies of proton gradient-driven 22Na uptake (i.e., Na+/H+ exchange) by apical membrane vesicles (AMV); 2) Northern blot analyses of NHE isoform-specific mRNA abundance; and 3) Western blot analyses of NHE isoform-specific protein expression. HOE-694 inhibition studies establish that 25 microM HOE-694-sensitive (NHE2) and 25 microM HOE-694-insensitive (NHE3) Na+/H+ exchange activities are present in AMV of both proximal and distal colon of normal rats. In proximal colon, dietary sodium depletion enhanced both NHE2 and NHE3 isoform-specific Na+/H+ exchange activities, protein expression, and mRNA abundance. In contrast, in distal colon both NHE2 and NHE3 isoform-specific Na+/H+ exchange activities, protein expression, and mRNA abundance were inhibited by sodium depletion. NHE1 isoform-specific mRNA abundance in proximal or distal colon was not altered by sodium depletion. Differential effects by sodium depletion on Na+/H+ exchange in rat colon are tissue specific and isoform specific; sodium depletion both induces and inhibits apical Na+/H+ exchange at a pretranslational level.

Distribution and regulation of apical Cl/anion exchanges in surface and crypt cells of rat distal colon

Na depletion inhibits electroneutral Na-Cl absorption in intact tissues and Na/H exchange in apical membrane vesicles (AMV) of rat distal colon. Two anion (Cl/HCO3 and Cl/OH) exchanges have been identified in AMV from surface cells of rat distal colon. To determine whether Cl/HCO3 and/or Cl/OH exchange is responsible for vectorial Cl movement, this study examined the spatial distribution and the effect of Na depletion on anion-dependent 36Cl uptake by AMV in rat distal colon. These studies demonstrate that HCO3 concentration gradient-driven 36Cl uptake (i.e., Cl/HCO3 exchange) is 1) primarily present in AMV from surface cells and 2) markedly reduced by Na depletion. In contrast, OH concentration gradient-driven 36Cl uptake (i.e., Cl/OH exchange) present in both surface and crypt cells is not affected by Na depletion. In Na-depleted animals HCO3 also stimulates 36Cl via Cl/OH exchange with low affinity. These results suggest that Cl/HCO3 exchange is responsible for vectorial Cl absorption, whereas Cl/OH exchange is involved in cell volume and/or cell pH homeostasis.

Role of Cl channels in Cl-dependent Na/H exchange

A novel Na/H exchange activity that requires Cl was recently identified in the apical membrane of crypt cells of the rat distal colon. This study explores the nature of the coupling of Cl and Na/H exchange. A concentration of 100 microM 5-nitro-2-(3-phenylpropylamino)benzoic acid, a Cl channel blocker, inhibited the Cl dependence of both proton gradient-driven 22Na uptake from crypt cell apical membrane vesicles and Na-dependent intracellular pH recovery from an acid load during microperfusion of the crypt lumen. Cl-dependent proton gradient-driven 22Na uptake was inhibited by 94% by 500 microM DIDS but only by 1% by 10 microM DIDS, an anion exchange inhibitor at low concentrations but a Cl channel blocker at high concentrations. In addition, a polyclonal antibody to the cystic fibrosis transmembrane conductance regulator (CFTR) inhibited Cl-dependent proton gradient-driven 22Na uptake by 38%. These results indicate that the Cl dependence of Na/H exchange in the colonic crypt apical membrane involves a Cl channel and not a Cl/anion exchange and permit the speculation that this Cl channel activity represents both CFTR and the outward rectifying Cl conductance.

Regulation of intracellular pH gradients by identified Na/H exchanger isoforms and a short-chain fatty acid

Colonic luminal short-chain fatty acids (SCFA) stimulate electroneutral sodium absorption via activation of apical Na/H exchange. HT29-C1 cells were used previously to demonstrate that transepithelial SCFA gradients selectively activate polarized Na/H exchangers. Fluorometry and confocal microscopy (with BCECF and carboxy SNARF-1, respectively) are used to measure intracellular pH (pHi) in HT29-C1 cells, to find out which Na/H exchanger isoforms are expressed and if results are due to pHi gradients. Inhibition of Na/H exchange by HOE-694 identified 1) two inhibitory sites [50% inhibitory dose (ID50) = 1.6 and 0.05 microM] in suspended cells and 2) one inhibitory site each in the apical and basolateral membranes of filter-attached cells (apical ID50 = 1.4 microM, basolateral ID50 = 0.3 microM). RT-PCR detected mRNA of Na/H exchanger isoforms NHE1 and NHE2 but not of NHE3. Confocal microscopy of filter-attached cells reported HOE-694-sensitive pHi recovery in response to luminal or serosal 130 mM propionate. Confocal analysis along the apical-to-basal axis revealed that 1) luminal or serosal propionate establishes transcellular pHi gradients and 2) the predominant site of pHi acidification and pHi recovery is the apical portion of cells. Luminal propionate produced a significantly greater acidification of the apical vs. basal portion of the cell (compared with serosal propionate), but no other dependence on the orientation of the SCFA gradient was observed. Results provide direct evidence for a subcellular response that assures robust activation of apical NHE2 and dampening of basolateral NHE1 during pHi regulation.

Characterization of Mg2+ transport in brush border membrane vesicles of rabbit ileum studied with mag-fura-2

Mg2+ transport in rabbit ileal brush border membrane vesicles (BBMV) was characterized by means of a modified mag-fura-2 technique. In the presence of an i>o Na+ gradient, BBMV showed a saturable Mg2+ uptake with a Km of 1.64 mmol l-1. There was no evidence of an overshoot. K+, Li+, and choline+ were as effective as Na+ in stimulating Mg2+ transport. In contrast, only a small amount of Mg2+ transport was observed in the presence either of an o>i Na+ gradient, or in an Na+ equilibrium or in the absence of Na+. Moreover, the findings that Na+ efflux was not stimulated but inhibited by outside Mg2+ and that the nonfluorescent amiloride-analogues DMA and EIPA did not affect Mg2+ transport do not favour the idea of an Mg2+/Na+ antiport system. At Cl- equilibrium, independent of the Na+ gradient, the rate of Mg2+ transport was markedly suppressed compared with the transport rate noted in the presence of an i>o Cl- gradient. The stimulating effect of inside anions could be enhanced by SCN- and decreased by SO2-4. Furthermore, nonfluorescent anion transport antagonist H2-DIDS stimulated Mg2+ transport. These findings indicate that Mg2+ transport can be modulated by inside anions. Mg2+ transport appeared to be electroneutral because it was not dependent on membrane potential. Mg2+ transport was neither stimulated by Bay K8644, a Ca2+ channel agonist, nor inhibited by verapamil, diltiazem, nifedipine and imipramine, the Ca2+ channel antagonists. It, therefore, seems unlikely that Mg2+ uses the Ca2+ transport system.

5-(N,N-Dimethyl)-amiloride to discriminate the Unidirectional electrolyte transports in rat small intestine and proximal colon in vivo

The effect of dimethyl-amiloride (DMA), a selective Na+/H+ exchange blocker, was studied on electrolyte net fluxes and unidirectional fluxes of Na and Cl at four levels of rat intestine in vivo in basal conditions. DMA was applied intraluminally at concentrations of 10(-4) and 10(-3) M in the model of ligated loops prepared from duodenum, proximal jejunum, distal ileum and ascending colon in fasted Sprague Dawley rats. Two iso-osmotic test solutions were used: (1) hypo-ionic: Na+ 80 mM and (2) iso-ionic: Na+ 148 mM, pH 8.2. 22Na was placed in the loop and 36Cl was given by intravenous route at the beginning of the experiment. Na+/H+ was calculated by two different means, one was based on pH variation following amiloride inhibition of Na influx, the other on the calculation of the passive Na transport. The quantitative evaluation shows that Na/H exchange largely contributes to the electroneutral absorption and luminal pH regulation. The exchanger activity decreases from duodenum, jejunum, ileum and colon where it is completed by K/H exchange to assure low colon luminal pH.

Novel transport properties of colonic crypt cells: fluid absorption and Cl-dependent Na-H exchange

Colonic ion transport is heterogeneous including the long-accepted spatial separation of absorptive and secretory processes between surface and crypt cells. We recently described the isolation of individual crypts from the rat distal colon that were studied using microperfusion technology. Na-dependent fluid absorption was consistently demonstrated in these crypts during perfusion with a Ringer-like solution; dibutyryl cyclic AMP, VIP and acetylcholine, when added to the bath solution, all induced net fluid secretion. As several morphologic techniques, including immunocytochemistry, failed to provide evidence for the presence of myofibroblasts in the isolated crypt preparation, we propose that a Na-dependent absorptive process is a constitutive transport mechanism in crypt cells, while secretory processes are regulated by the release of one or more neurohumoral agonists from lamina propria cells including myofibroblasts. The mechanism of Na-dependent fluid movement was also studied by determining [H] gradient stimulation of 22Na uptake in isolated apical membrane vesicles (AMV) from crypt cells. In contrast to Na-H exchange in surface cell AMV, Na-H exchange in crypt cells is Cl-dependent. Intracellular pH determined in crypt cells using video-imaging fluorescence microscopy established that the response to an acid load requires both lumen Na and Cl. As a result, these studies have identified a novel Cl-dependent Na-H exchange in crypt AMV that may mediate apical membrane Na uptake and regulate pHi.

Transepithelial SCFA gradients regulate polarized Na/H exchangers and pH microdomains in colonic epithelia

Short chain fatty acids (SCFAs) stimulate electroneutral sodium absorption by activation of apical Na/H exchange in colonocytes. It is often assumed that activation of Na/H exchange is via an intracellular acidification caused by SCFA uptake. These lecture notes review shortcomings in this model of SCFA-stimulated sodium absorption, revealed by recent reports in the literature. This is supplemented by information generated in our laboratory using both a tissue culture model of colonocytes (HT29-C1 cells) and a native tissue preparation (mouse distal colonic mucosa). In both preparations, evidence suggests that physiologic SCFA gradients may generate pH heterogeneity in aqueous microdomains near the plasma membrane of colonocytes. Finally, direct observation of such extracellular microdomains with confocal microscopy is used to support a new model, in which pH microdomains play an important role in regulating both SCFA fluxes and sodium absorption.

Effect of SCFA on intracellular pH and intracellular pH regulation of guinea-pig caecal and colonic enterocytes and of HT29-19a monolayers

The response of the intracellular pH (pHi, measured with BCECF) of the caecal and distal colonic epithelium of guinea pig and of monolayers of HT29 clone 19a cells on the addition of short-chain fatty acids (SCFA) was assessed. Addition of SCFA to the luminal side of these cells had no major effect on pHi, independent of whether the apical Na+/H+ exchange or the apical K+/H+ ATPase was inhibited or not. Addition of SCFA to the serosal side, on the other hand, caused a marked decrease of pHi, followed by an effective regulation back to basal values, and after removal of the acid, the cells became alkalinized. Intracellular pH is mainly regulated by mechanisms in the basolateral membrane. The basolateral Na+/H+ exchanger and the Cl-/HCO3- exchanger were mainly responsible for pHi regulation. Inhibition studies are consistent with a NHE-1 type Na+/H+ exchanger in the basolateral membranes. The apical Na+/H+ exchanger of caecal enterocytes and in HT29 cells, and the apical K+/H+ ATPase in the apical membrane of the distal colon have no or little influence on pHi regulation. The comparison shows that the HT29-19a cell line is an adequate model for studying pHi phenomena of hind gut epithelial cells.

Chloride-dependent Na-H exchange. A novel mechanism of sodium transport in colonic crypts

The mechanism of sodium movement across apical membrane of colonic crypt cells of rat distal colon was examined in studies of both 22Na uptake by apical membrane vesicles (AMV) and the rate of intracellular pH (pHi) recovery from an acid load by the addition of lumen sodium. In the presence of chloride but not in its absence, 22Na uptake in crypt AMV was stimulated by an outward gradient of either [H+] or [Na+]. 22Na uptake stimulated by an outward [Na+] gradient was also observed in the presence of other halides in the order of chloride > bromide > fluoride > iodide. pHi recovery from an acid load was both lumen sodium- and chloride-dependent, and the rate of pHi recovery by lumen sodium in the presence of chloride was 65-fold greater than that in the absence of chloride (dpH/dt is 655.4 and 10.2 in the presence and absence of chloride, respectively). One mM amiloride inhibited both [H+] gradient-stimulated 22Na uptake in the presence of chloride in crypt AMV (80%) and lumen sodium- and chloride-dependent pHi recovery in crypt cells (96%). [H+] gradient stimulation of 22Na uptake by crypt AMV in the presence of chloride was less sensitive to amiloride than amiloride inhibition of Na-H exchange in colonic surface AMV. These studies provide compelling evidence that a chloride-dependent Na-H exchange that is relatively amiloride-resistant is present in the apical membrane of colonic crypt cells. As prior studies have not identified a chloride-dependent Na-H exchange, the molecular and functional basis of this novel transport process is not known.

Na(+)-H(+) exchange activity in brush-border membrane vesicles isolated from chick small intestine

This study was undertaken to investigate the presence of a Na(+)-H(+) antiporter in brush-border membrane vesicles (BBMV) isolated from chick small intestine. An outwardly directed proton gradient (pH 5.5 inside, 7.5 outside) stimulated Na+ uptake into BBMV and resulted in a transient accumulation. No accumulation was observed in the absence of a proton gradient. Voltage clamping the membrane with K+ and valinomycin decreased the Na+ overshoot. Amiloride inhibited pH gradient-driven Na+ uptake in a dose-dependent manner with an IC50 of 44 microM. The relationship between pH gradient-driven Na+ uptake and external Na+ concentration followed simple, saturating Michaelis-Menten kinetics. Eadie-Hofstee analysis of the pH gradient-driven Na+ uptake indicated a single transport system with a Vmax of 33 nmol/mg protein per 15 s and a Km for Na+ of 12 mM. The initial rate of pH-driven Na+ uptake increased as the intravesicular pH decreased, with a Hill coefficient close to 1. These findings indicate that BBMV isolated from chicken small intestine possess a Na(+)-H(+) exchanger. This exchanger does not appear to be the one involved in cell pH regulation.

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)

Mechanisms of Na+ transport in human distal colonic apical membrane vesicles

Apical membrane vesicles purified from mucosal scrapings obtained from distal segments of organ donor colons and a 22Na-uptake technique were used to characterize the mechanism(s) of Na+ transport into these vesicles. An outwardly directed H+ gradient (pH 5.5in/7.5out) markedly increased uptake of 22Na into these vesicles. Osmolarity studies demonstrated that 22Na was taken up into the intravesicular space with minimal binding observed to the surface of the vesicles. Voltage clamping in the presence of K+/valinomycin reduced the H+ gradient-dependent 22Na uptake into these vesicles by approximately 45% and generation of an inside negative membrane potential significantly increased 22Na uptake. Under non voltage clamped conditions, H+ gradient-dependent 22Na uptake into these vesicles was significantly inhibited by specific inhibitors of Na(+)-H+ exchange (DMA, HMA and EIPA) as well as by inhibitor of epithelial Na+ channels (phenamil). Under voltage clamped conditions, H+ gradient-dependent 22Na uptake, however, was unaffected by phenamil (20 microM), but was almost completely inhibited by DMA, HMA and EIPA (20 microM each). The mechanism of amiloride inhibition of electroneutral Na(+)-H+ exchange was noncompetitive with a Ki for amiloride of 340 microM. Electroneutral 22Na uptake exhibited saturation kinetics with an apparent Km for Na+ of 8.7 +/- 1.7 mM and a Vmax of 2.02 +/- 0.45 nmol/mg per 5 s. The Na(+)-H+ exchange demonstrated cation specificity similar to the Na(+)-H+ exchangers described in other epithelia. These studies demonstrate for the first time that Na+ transport across the apical membranes of human distal colon involves both conductive Na+ uptake and an electroneutral Na(+)-H+ exchange process.

The role of volume-sensitive Cl- channels in the stimulation of chloride absorption by short-chain fatty acids in the rat colon

The short-chain fatty acids acetate, propionate and butyrate induced a concentration-dependent decrease in short-circuit current (Isc) of the rat colon in vitro. The decrease in Isc, being more pronounced in the distal than in the proximal colon, was dependent on the presence of Cl- ions and partly on the presence of HCO3-. In the distal colon, the fall in Isc could be inhibited by amiloride, indicating that the activity of the Na+/H+ exchanger is necessary for the induction of this response. The decrease in Isc was diminished by the Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoate, and the lipoxygenase inhibitor, nordihydroguaiaretic acid. In contrast, inhibitors of the leukotriene pathway or a Cl- channel blocker did not affect the Isc response in the proximal colon. Measurements of unidirectional fluxes revealed that butyrate caused a stimulation of the mucosa to serosa fluxes (Fms) of Na+ and Cl- in the distal, but only of FNams in the proximal colon. Unidirectional Rb+ fluxes were not altered. The stimulation of Fclms correlated with the degree of metabolism of the short-chain fatty acid. The increase in FClms was most pronounced for butyrate, smaller for acetate and not observed with the poorly metabolizable short-chain fatty acid, isobutyrate. Consequently, two factors seem to be responsible for the stimulation of Cl- absorption by short-chain fatty acids in the distal colon: (1) the intracellular production of HCO3- during the oxidation of short-chain fatty acids as substrate for the apical Cl-/HCO3- exchanger, and (2) the activation of volume-sensitive basolateral Cl- channels.

Characterization of Na(+)-H+ antiporter activity associated with human cheek epithelial cells

Na+ transport activity was characterized in human cheek epithelial cells obtained from normotensive adult subjects. The cells were isolated using a mouth-wash procedure and assayed for Na+ uptake using a radioactive (22Na+) rapid filtration assay. Cheek cells displayed proton-dependent Na+ uptake activity that was dependent on the magnitude of the externally directed proton gradient measured using the fluorescent probe 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein to determine intracellular pH. Amiloride, ethylisopropylamiloride (EIPA), 5-(N,N-dimethyl)-amiloride, 5-(N-methyl-N-isobutyl)-amiloride (MIA), and 5-(N,N-hexamethylene)-amiloride (NNHA) all inhibited proton-dependent Na+ uptake, with MIA, EIPA, and NNHA being the most potent. The Michaelis constant (Km) for extracellular Na+ was 5.7 mM, while the maximum velocity for Na(+)-H+ antiporter activity was 4.3 nmol Na+.mg protein-1.30s-1. The Km for intracellular H+ was 0.17 microM, with a Hill coefficient of 0.7. Stimulation by ouabain and inhibition by bumetanide of cheek cell proton-dependent Na+ uptake indicated only relatively low activities of Na(+)-K(+)-ATPase and Na(+)-K(+)-2Cl- cotransport, respectively. These results are consistent with the presence of Na(+)-H+ antiporter activity in cheek cells. Cheek cells therefore provide a convenient, relatively noninvasive source of tissue for examining Na(+)-H+ antiporter activity in human subjects.

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.

Mechanisms of sodium and chloride transport across isolated sheep reticulum

1. 22Na+ and 36Cl- fluxes across isolated reticular epithelium of sheep were measured by using the Ussing-chamber technique. 2. Net NaCl absorption driven by Na(+)-K(+)-ATPase was observed under short-circuit conditions. 3. Evaluation of fluxes measured under voltage-clamp conditions indicated that Na+ absorption is mainly electroneutral. 4. Mucosal application of bumetanide, hydrochlorothiazide, or low dose amiloride (10(-4) M) produced no changes in Na+ transport whereas addition of higher doses of amiloride (> or = 10(-3) M) led to a reduction in net Na+ transport. Short chain fatty acids (SCFA) enhanced the amiloride-sensitive Na+ transport. 5. Alterations of JmsNa induced by inhibitors or by SCFA were always accompanied by qualitatively similar changes of JsmNa. Amiloride-sensitive JsmNa was also decreased at low mucosal Na+ concentration. 6. DIDS, SITS, and nitrate reduced both JmsCl and JsmCl. SCFA did not influence chloride transport. 7. It is concluded that Na+ transport is mediated by Na(+)-H+ exchange and by transport processes operating as Na+ self-exchange. Mucosal-to-serosal chloride transport seems partly to depend on anion exchange systems.

Mucosal ouabain and Na+ inhibit active Rb+(K+) absorption in normal and sodium-depleted rat distal colon

To determine the effect of mucosal sodium and mucosal ouabain on active Rb+(K+) absorption, unidirectional and net 86Rb+ fluxes were measured under voltage-clamp conditions in the distal colon of normal and sodium-depleted rats. The role of mucosal sodium (independent of serosal sodium) was evaluated in a model of Rb+(K+) absorption in which serosal ouabain markedly enhanced active Rb+(K+) absorption. In normal rats, mucosal sodium was a competitive inhibitor of Rb+(K+) absorption, and Rb+(K+) absorption consisted of a mucosal sodium-sensitive component and a mucosal sodium-insensitive component. Further, mucosal ouabain almost completely inhibited the mucosal sodium-insensitive component but did not affect the mucosal sodium-sensitive component. In sodium-depleted rats, both mucosal sodium-sensitive and mucosal sodium-insensitive fractions of Rb+(K+) absorption were also identified. Aldosterone markedly stimulated the mucosal sodium-sensitive component (1.68 +/- 0.15 vs. 0.60 +/- 0.10 muEq.h-1.cm-2) but not the sodium-insensitive component (0.88 +/- 0.09 vs. 0.64 +/- 0.06 muEq.h-1.cm-2) component of Rb+(K+) absorption; however, in contrast to normal animals, mucosal sodium in sodium-depleted animals was a noncompetitive inhibitor of Rb+(K+) absorption. The mucosal sodium-insensitive component of Rb+(K+) absorption in sodium-depleted animals was substantially inhibited by mucosal ouabain, but the mucosal sodium-sensitive component, unlike that in normal animals, was partially inhibited by mucosal ouabain. These studies indicate that the characteristics of the Rb+(K+) absorptive process in sodium-depleted animals differ significantly from those present in normal animals, suggesting that aldosterone induces an Rb+(K+) absorptive mechanism not present in normal animals.

Na(+)-H+ exchanger subtypes: a predictive review

In recent years, many reports have appeared describing distinct heterogeneity of proteins that heretofore were considered to be a single species or type. The division of proteins into different classes or subtypes is aided by pharmacological tools such as selective ligands, functional measurements such as those examining kinetic or regulatory differences, and molecular biological approaches that have identified distinct genes coding for similar yet distinguishable gene products. Currently, much effort is directed toward understanding the significance of these sometimes subtle differences in terms of functional consequences for the cells in which they exist. Although most reports to date involve hormone and neurotransmitter receptor subtypes, it is also possible that other cell surface molecules such as ion transporters exist as multiple subtypes. In this paper we review the current evidence that Na(+)-H+ exchange activity is mediated by different Na(+)-H+ exchanger subtypes. Although subtypes have not been identified with certainty, we can predict certain distinguishing characteristics that these putative subtypes may have that may be of value in correlating predicted gene products obtained from cDNA cloning with previously characterized Na(+)-H+ exchangers.

Mechanism of short-chain fatty acid uptake by apical membrane vesicles of rat distal colon

In this study, the presence of a bicarbonate gradient-dependent, carrier-mediated anion exchange process for butyrate (a representative short-chain fatty acid) uptake in apical membrane vesicles isolated from rat distal colon is described. An outward gradient of both butyrate- and bicarbonate-stimulated [14C]butyrate uptake and resulted in transient accumulation (an "overshoot" phenomenon). Butyrate gradient-stimulated [14C]butyrate uptake was not altered either by an imposed pH gradient or at different pH values. In contrast, bicarbonate gradient-stimulated [14C]butyrate uptake was stimulated severalfold by an additional imposition of an outward pH gradient (pHi = 7.5; pH0 = 6.0). This bicarbonate- and pH gradient-stimulated butyrate uptake was not inhibited by either voltage clamping, with equimolar intravesicular and extravesicular K+ and valinomycin, or 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an anion-exchange inhibitor. Increasing butyrate concentrations saturated the bicarbonate- and pH gradient-stimulated butyrate uptake with a half-maximal concentration (Km) of 26.9 +/- 1.6 mmol/L. Butyrate uptake was substantially inhibited by 20 mmol/L propionate (45%) and acetate (60%) but was not inhibited by oxalate, inorganic anions (SO4(2-) and NO3-), and transport inhibitors (amiloride, acetazolamide, furosemide, and ouabain). It is concluded from these results that bicarbonate gradient-stimulated butyrate uptake in apical membrane vesicles of rat distal colon occurs via a carrier-mediated anion-exchange process that differs from other DIDS-sensitive anion exchanges [e.g., the Cl- -OH- (HCO3-) process].

Segmental heterogeneity of the rat colon in the response to activators of secretion on the cAMP-, the cGMP- and the Ca(2+)-pathway

The electrolyte transport was compared in proximal and distal segments of the rat colon under control conditions and after induction of secretion on the cAMP-, the cGMP- and the Ca(2+)-pathway. Baseline short-circuit current was decreased by indomethacin and tetrodotoxin in the distal colon, indicating a spontaneous production of neuronally acting prostaglandins. In contrast, baseline short-circuit current in the proximal colon was decreased only by indomethacin, but not by tetrodotoxin. Unidirectional flux measurements revealed that in the distal colon sodium and chloride were absorbed, while the proximal colon secreted chloride. A morphological comparison between the distal and proximal epithelium revealed that the zonulae occludentes and the microvilli were longer in the distal colon. The size of the Golgi apparatus was several times larger in the crypt than in the surface region without differences between proximal and distal colon. Distal segments were more sensitive to an activator of the Ca(2+)-pathway, carbachol, or activators of the cAMP-pathway such as forskolin and a cAMP-analogue. In contrast, the activation of the cGMP-pathway by a cGMP-analogue or by the heat-stable enterotoxin of E. coli (STa) was more effective in the proximal colon. The results give evidence for a segmental specificity with regard to the intracellular pathways responsible for the activation of secretion.

Lipophilicity influence on conjunctival drug penetration in the pigmented rabbit: a comparison with corneal penetration

The influence of lipophilicity on the conjunctival penetration of beta blockers in the pigmented rabbit was investigated and compared with that on corneal penetration. The beta blockers were hydrophilic sotalol, atenolol, nadolol, pindolol, and acebutolol; lipophilic metoprolol, timolol, oxprenolol, levobunolol, labetalol, and alprenolol; and the very lipophilic propranolol and betaxolol. Drug penetration was evaluated by using the isolated pigmented rabbit conjunctiva and cornea in the modified Ussing chamber and was monitored by reversed phase HPLC. The conjunctiva was more permeable to all the beta blockers than was the cornea. A sigmoidal relationship, rather than the familiar parabolic relationship, best described the influence of lipophilicity on both conjunctival and corneal drug penetration. The ratio of corneal to conjunctival permeability coefficients was most sensitive to changes in log PC within the region of 1.5 and 2.5. Outside of this region, the ratio was relatively independent of changes in lipophilicity. For several beta blockers, their intrinsic sympathomimetic activity may play a minor role in influencing their conjunctival and corneal penetration.

Characterization of butyrate-dependent electroneutral Na-Cl absorption in the rat distal colon

Recent studies have established that mucosal butyrate stimulates electroneutral sodium-chloride (Na-Cl) absorption in the distal colon of the rat and a model in which Na-hydrogen (H) and Cl-butyrate exchanges are coupled has been proposed as the mechanism of butyrate-dependent electroneutral Na-Cl absorption. These studies were designed to examine butyrate-dependent electroneutral Na-Cl absorption in experimental conditions in which HCO3-dependent electroneutral Na-Cl absorption is inhibited: in Na-depleted (aldosterone-treated) animals and in the presence of increased mucosal cyclic adenosine monophosphate (AMP). Butyrate-dependent electroneutral Na-Cl absorption was markedly reduced in Na-depleted rats. In contrast, the inhibition of both net Na and net Cl absorption by 5 mM serosal theophylline was significantly less in butyrate-containing, HCO3-free Ringer solution than in butyrate-free- HCO3-containing Ringer solution. These studies indicate that cyclic AMP does not inhibit butyrate-dependent electroneutral Na-Cl absorption and we propose that the mechanism of cyclic AMP inhibition of HCO3-dependent electroneutral Na-Cl absorption may be a result of its inhibition of Cl-HCO3, not Na-H exchange.