Role of basolateral Na(+)-K(+)-Cl- cotransport in HCl secretion by amphibian gastric mucosa

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

Recent advances in the molecular and functional characterization of acid/base and electrolyte transporters in the basolateral membranes of gastric and duodenal epithelial cells

All segments of the gastrointestinal tract are comprised of an elaborately folded epithelium that expresses a variety of cell types and performs multiple secretory and absorptive functions. While the apical membrane expresses the electrolyte transporters that secrete or absorb electrolytes and water, basolateral transporters regulate the secretory or absorptive rates. During gastric acid formation, Cl⁻/HCO₃⁻ and Na(+) /H(+) exchange and other transporters secure Cl⁻ re-supply as well as pH and volume regulation. Gastric surface cells utilize ion transporters to secrete HCO₃⁻, maintain pH(i) during a luminal acid load and repair damaged surface areas during the process of epithelial restitution. Na(+)/H(+) exchange and Na(+)/HCO₃⁻ cotransport serve basolateral acid/base import for gastroduodenal HCO₃⁻ secretion. The gastric and duodenal epithelium also absorbs salt and water. Recent molecular information on novel ion transporters expressed in the gastric and duodenal epithelium has exploded; however, a function has not been found yet for all transporters. The purpose of this review is to summarize current knowledge on the molecular identity and cellular function of basolateral ion transporters in the gastric and duodenal epithelium.

Revisiting the parietal cell

The parietal cell is responsible for secreting concentrated hydrochloric acid into the gastric lumen. To fulfill this task, it is equipped with a broad variety of functionally coupled apical and basolateral ion transport proteins. The concerted scientific effort over the last years by a variety of researchers has provided us with the molecular identity of many of these transport mechanisms, thereby contributing to the clarification of persistent controversies in the field. This article will briefly review the current model of parietal cell physiology and ion transport in particular and will update the existing models of apical and basolateral transport in the parietal cell.

Expression of NKCC2 in the rat gastrointestinal tract

NKCC2, an isoform of Na+-K+-2Cl(-) cotransporter, is principally present in the kidney and plays a critical role in salt reabsorption. Expression of NKCC2 has been found in the apical membrane of intestinal epithelial cells in a number of marine fish, however, details for expression in the mammalian gastrointestinal tract are lacking. RT-PCR, Western blotting and immunohistochemistry were used to study the expression and localization of NKCC2 in the rat gastrointestinal tract. We found that mRNA transcripts, protein and immunoreactivity (IR) for NKCC2 were expressed in the stomach, small and large intestine of adult rats. NKCC2 IR was localized to the base of the gastric glands, intestinal epithelia, myenteric and submucosal plexuses. NKCC2 IR was expressed strongly in the apical membranes and weakly in the basolateral membranes of intestinal epithelial cells. In the enteric nervous system, NKCC2 IR was widely distributed and localized to enteric neurons with cholinergic, calretinin and nitrergic neuronal immunochemical codes in the myenteric plexus. It was localized to non-cholinergic secretomotor neurons in the submucosal plexus. In conclusion, this study for the first time clearly detected the expression of NKCC2 in the gastrointestinal tract of a mammalian species. Expression of NKCC2 in gastrointestinal epithelial cells suggested that this cation chloride cotransporter might be involved in gastrointestinal ion transport. Expression of NKCC2 in enteric neurons might contribute to the accumulation of Cl(-) and a more depolarized E(Cl)(-) in enteric neurons.

SLC26A7 can function as a chloride-loading mechanism in parietal cells

To date three potential candidates for parietal cell basolateral Cl(-) entry have been described: the highly 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS)-sensitive Cl(-)/HCO(3)(-) exchanger AE2, the HCO(3)(-) and lowly DIDS-sensitive SLC26A7 protein, and the Na(+)-2Cl(-)K(+) cotransporter (NKCC1). In this study we investigate the contribution of these pathways to secretagogue stimulated acid secretion. Individually hand-dissected rat gastric glands were microfluorimetrically monitored for Cl(-) influx and pH(i) changes. Transporter activity was determined by varying ion content and through the use of pharmacological inhibitors. Expression of SLC26A7 in rat parietal cells was shown by immunohistochemistry and Western blot. SLC26A7 was inhibited by 5-Nitro-2-(3-phenylpropyl-amino)benzoic acid (NPPB) (100 microM) in the Xenopus laevis oocyte expression system. Cl(-) influx in parietal cells was enhanced by histamine, depended partially on endogenous HCO(3)(-) synthesis and completely on extracellular Na(+). Removal and subsequent readdition of Cl(-) revealed a low and a high DIDS-sensitive HCO(3)(-) extrusion system contributing to Cl(-) uptake. At acidic pH(i), however, H(+) extrusion via the H(+),K(+)-ATPase depending on Cl(-) uptake was abolished only in the presence of 100 microM (NPPB) and at high (250 microM) DIDS concentration. There was no effect of the NKCC inhibitor bumetanide on stimulated H(+) extrusion. These results would be compatible with SLC26A7 as a Cl(-) uptake system under histamine stimulation.

Subtractive hybridization unravels a role for the ion cotransporter NKCC1 in the murine intestinal pacemaker

In the small intestine, interstitial cells of Cajal (ICC) surrounding the myenteric plexus generate the pacemaking slow waves that are essential for an efficient intestinal transit. The underlying molecular mechanisms of the slow wave are poorly known. Our aim was to identify ICC-specific genes and their function in the mouse jejunum. Suppression subtractive hybridization using two independent ICC-deficient mouse models identified 56 genes putatively downregulated in the muscularis propria compared with wild-type littermates. Differential expression was confirmed by real-time quantitative PCR for the tyrosine kinase receptor KIT, the established marker for ICC, and for the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1). Immunoreactivity for NKCC1 was detected in myenteric ICC but not in the ICC population located at the deep muscular plexus. NKCC1 was also expressed in enteric neurons and mucosal crypts. Bumetanide, an NKCC1 inhibitor, reversibly affected the shape, amplitude, and frequency of the slow waves. Similar alterations were observed in NKCC1 knockout mice. These data support the hypothesis that NKCC1 expressed in myenteric ICC is involved in the mechanism of slow waves in the murine jejunum.

Role of Na-K-2Cl cotransporter-1 in gastric secretion of nonacidic fluid and pepsinogen

Na-K-2Cl cotransporter-1 (NKCC) has been detected at exceptionally high levels in the gastric mucosa of several species, prompting speculation that it plays important roles in gastric secretion. To investigate this possibility, we 1) immunolocalized NKCC protein in the mouse gastric mucosa, 2) compared the volume and composition of gastric fluid from NKCC-deficient mice and their normal littermates, and 3) measured acid secretion and electrogenic ion transport by chambered mouse gastric mucosa. NKCC was localized to the basolateral margin of parietal cells, mucous neck cells, and antral base cells. In NKCC-deficient mice, gastric secretions of Na+, K+, Cl-, fluid, and pepsinogen were markedly impaired, whereas secretion of acid was normal. After stimulation with forskolin or 8-bromo-cAMP, chambered corpus mucosa vigorously secreted acid, and this was accompanied by an increase in transmucosal electrical current. Inhibition of NKCC with bumetanide reduced current to resting levels but had no effect on acid output. Although prominent pathways for basolateral Cl- uptake (NKCC) and apical Cl- exit [cystic fibrosis transmembrane conductance regulator (CFTR)] were found in antral base cells, no impairment in gastric secretion was detected in CFTR-deficient mice. Our results establish that NKCC contributes importantly to secretions of Na+, K+, Cl-, fluid, and pepsinogen by the gastric mucosa through a process that is electrogenic in character and independent of acid secretion. The probable source of the NKCC-dependent nonacidic electrogenic fluid secretion is the parietal cell. The observed dependence of pepsinogen secretion on NKCC supports the concept that a nonacidic secretory stream elaborated from parietal cells facilitates flushing of the proenzyme from the gastric gland lumen.

Mice with a targeted disruption of the AE2 Cl-/HCO3- exchanger are achlorhydric

The AE2 Cl-/HCO3- exchanger is expressed in numerous cell types, including epithelial cells of the kidney, respiratory tract, and alimentary tract. In gastric epithelia, AE2 is particularly abundant in parietal cells, where it may be the predominant mechanism for HCO3- efflux and Cl- influx across the basolateral membrane that is needed for acid secretion. To investigate the hypothesis that AE2 is critical for parietal cell function and to assess its importance in other tissues, homozygous null mutant (AE2(-/-)) mice were prepared by targeted disruption of the AE2 (Slc4a2) gene. AE2(-/-) mice were emaciated, edentulous (toothless), and exhibited severe growth retardation, and most of them died around the time of weaning. AE2(-/-) mice exhibited achlorhydria, and histological studies revealed abnormalities of the gastric epithelium, including moderate dilation of the gastric gland lumens and a reduction in the number of parietal cells. There was little evidence, however, that parietal cell viability was impaired. Ultrastructural analysis of AE2(-/-) gastric mucosa revealed abnormal parietal cell structure, with severely impaired development of secretory canaliculi and few tubulovesicles but normal apical microvilli. These results demonstrate that AE2 is essential for gastric acid secretion and for normal development of secretory canalicular and tubulovesicular membranes in mouse parietal cells.

Water metabolism in the eel acclimated to sea water: from mouth to intestine

Eels seem to be a suitable model system for analysing regulatory mechanisms of drinking behavior in vertebrates, since most dipsogens and antidipsogens in mammals influence the drinking rate in the seawater eels similarly. The drinking behavior in fishes consists of swallowing alone, since they live in water and water is constantly held in the mouth for respiration. Therefore, contraction of the upper esophageal sphincter (UES) muscle limits the drinking rate in fishes. The UES of the eel was innervated by the glossopharyngeal-vagal motor complex (GVC) in the medulla oblongata (MO). The GVC neurons were immunoreactive to an antibody raised against choline acetyltransferase (ChAT), an acetylcholine (ACh) synthesizing enzyme, indicating that the eel UES muscle is controlled cholinergically by the GVC. The neuronal activity of the GVC was inhibited by adrenaline or dopamine, suggesting catecholaminergic innervation to the GVC. The AP and the commissural nucleus of Cajal (NCC) in the MO projected to the GVC and were immunoreactive to an antibody raised against tyrosine hydroxylase (TH), rate limiting enzyme to produce catecholamines from tyrosine. Therefore, it is likely that activation in the AP or the NCC may inhibit the GVC and thus relaxes the UES muscle, which allows for water to enter into the esophagus. During passing through the esophagus, the imbibed sea water (SW) was desalted to approximately 1/2 SW, which was further diluted in the stomach and arrived at the intestine as approximately 1/3 SW, almost isotonic to the plasma. Finally, from the diluted SW, the eel intestine absorbed water following the Na(+)-K(+)-2Cl(-) cotransport (NKCC2) system. The NaCl and water absorption across the intestine was regulated by various factors, especially by peptides such as atrial natriuretic peptide (ANP) and somatostatin (SS-25 II). During desalination in the esophagus, however, excess salt enters into the blood circulation, which is liable to raise the plasma osmolarity. However, the eel heart was constricted powerfully by the hyperosmolarity, suggesting that the hyperosmolarity enhances the stroke volume to the gill, where excess salt was extruded powerfully via Na(+)-K(+)-2Cl(-) cotransport (NKCC1) system.

Two isoforms of the Na+/K+/2Cl- cotransporter are expressed in the European eel (Anguilla anguilla)

Two cDNA isoforms of the NKCC1 secretory cotransporter have been isolated from the European eel. The NKCC1a isoform exhibited mRNA expression in a wide range of tissues in a similar fashion to mammals, whereas NKCC1b was expressed primarily in the brain. The effect of freshwater (FW) to seawater (SW) transfer on NKCC1a expression was dependent on the developmental stage. In non-migratory yellow eels, NKCC1a mRNA expression in the gill was transiently up-regulated 4.3-fold after 2 days but also subsequently by 2.5-6-fold 3 weeks after SW transfer. Gill NKCC1a expression was localised mainly in branchial chloride cells of SW acclimated yellow eels. In contrast to yellow eels, NKCC1a mRNA abundance was not significantly different following SW acclimation in silver eel gill. NKCC1a mRNA abundance decreased in the kidney following SW acclimation and this may correlate with lower tubular ion/fluid secretion and urine flow rates in SW teleosts. Kidney NKCC1a mRNA expression in silver eels was also significantly lower than in yellow eels, suggesting some pre-acclimation of mRNA levels. NKCC1a mRNA was expressed at similar low levels in the middle intestine of FW- and SW-acclimated yellow or silver eels, suggesting the presence of an ion secretory mechanism in this gut segment.

Contribution of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) to transepithelial transport of H(+), NH(4)(+), K(+), and Na(+) in rat outer medullary collecting duct

In rat kidney, the "secretory" isoform of the Na-K-Cl cotransporter, NKCC1 (BSC-2), localizes to the basolateral membrane of the alpha intercalated cell, the acid secreting cell of the outer medullary collecting duct (OMCD). This laboratory has reported that NKCC1 mediates Cl(-) uptake across the basolateral membrane in series with Cl(-) secretion across the apical membrane in rat OMCD. NKCC1 transports NH(4)(+), K(+), and Na(+) as well as Cl(-); therefore, a role for the cotransporter in the process of HCl, NH(4)Cl, KCl, and NaCl secretion has been suggested. Thus, it was determined if bumetanide, an inhibitor of NKCC1, alters transepithelial cation transport in rat OMCD. OMCD tubules from deoxycorticosterone pivalate (DOCP)-treated rats were perfused in vitro. Hydration of CO(2), rather than NH(4)(+), provides the principle source of H(+) for net acid secretion. In HCO(3)(-)/CO(2)-buffered solutions, no effect of bumetanide on net K(+) flux was detected. Under some conditions, bumetanide addition resulted in a small reduction in secretion of net H(+) equivalents. Transepithelial Na(+) flux, J(Na), was -1.5 +/- 1.7 pmol/mm per min, values not different from zero. However, with the application of bumetanide to the bath, J(Na) was +5.2 +/- 1.3 pmol/mm per min (P < 0.05), which indicates net Na(+) absorption. In conclusion, inhibition of NKCC1 in rat OMCD changes transepithelial movement of Na(+) and Cl(-). The role of NKCC1 in the secretion of net H(+) equivalents is small.

Contribution of the Na+-K+-2Cl- cotransporter NKCC1 to Cl- secretion in rat OMCD

In rat kidney the "secretory" isoform of the Na+-K+-2Cl- cotransporter (NKCC1) localizes to the basolateral membrane of the alpha-intercalated cell. The purpose of this study was to determine whether rat outer medullary collecting duct (OMCD) secretes Cl- and whether transepithelial Cl- transport occurs, in part, through Cl- uptake across the basolateral membrane mediated by NKCC1 in series with Cl- efflux across the apical membrane. OMCD tubules from rats treated with deoxycorticosterone pivalate were perfused in vitro in symmetrical HCO/CO2-buffered solutions. Cl- secretion was observed in this segment, accompanied by a lumen positive transepithelial potential. Bumetanide (100 microM), when added to the bath, reduced Cl- secretion by 78%, although the lumen positive transepithelial potential and fluid flux were unchanged. Bumetanide-sensitive Cl- secretion was dependent on extracellular Na+ and either K+ or NH, consistent with the ion dependency of NKCC1-mediated Cl- transport. In conclusion, OMCD tubules from deoxycorticosterone pivalate-treated rats secrete Cl- into the luminal fluid through NKCC1-mediated Cl- uptake across the basolateral membrane in series with Cl- efflux across the apical membrane. The physiological role of NKCC1-mediated Cl- uptake remains to be determined. However, the role of NKCC1 in the process of fluid secretion could not be demonstrated.

Luminal ammonia retards restitution of guinea pig injured gastric mucosa in vitro

The present study was conducted to elucidate the mechanisms by which Helicobacter pylori (HP)-derived ammonia causes gastric mucosal injury. Intact sheets of guinea pig gastric fundic mucosae were incubated in Ussing chambers. Both the luminal and the serosal pH were kept at 7.4. Transmucosal potential difference (PD) and electrical resistance (R) were monitored as indices of mucosal integrity. Restitution was evaluated by recovery of PD, R, and transmucosal [(3)H]mannitol flux after Triton X-100-induced mucosal injury. The effects of luminal or serosal NH(4)Cl on function and morphology of uninjured or injured mucosae were examined. In uninjured mucosae, serosal NH(4)Cl induced more profound decreases in PD and R and more prominent vacuolation in gastric epithelial cells than did luminal NH(4)Cl. In contrast, luminal NH(4)Cl markedly inhibited restitution in injured mucosae and caused an extensive vacuolation in gastric epithelial cells, as did serosal NH(4)Cl. Transmucosal ammonia flux was greater in the injured than in the uninjured mucosae. These results suggest that 1) basolateral membrane of gastric epithelial cells is more permeable to ammonia than apical membrane and 2) luminal ammonia, at concentrations detected in HP-infected gastric lumen, retards restitution in injured mucosae.

A novel K(ATP) current in cultured neonatal rat atrial appendage cardiomyocytes

The functional and pharmacological properties of ATP-sensitive K(+) (K(ATP)) channels were studied in primary cultured neonatal rat atrial appendage cardiomyocytes. Activation of a whole-cell inward rectifying K(+) current depended on the pipette ATP concentration and correlated with a membrane hyperpolarization close to the K(+) equilibrium potential. The K(ATP) current could be activated either spontaneously or by a hypotonic stretch of the membrane induced by lowering the osmolality of the bathing solution from 290 to 260 mOsm/kg H(2)O or by the K(+) channel openers diazoxide and cromakalim with EC(50) approximately 1 and 10 nmol/L, respectively. The activated atrial K(ATP) current was highly sensitive to glyburide, with an IC(50) of 1.22+/-0.15 nmol/L. Recorded in inside-out patches, the neonatal atrial K(ATP) channel displayed a conductance of 58.0+/-2.2 pS and opened in bursts of 133.8+/-20.4 ms duration, with an open time duration of 1.40+/-0.10 ms and a close time duration of 0.66+/-0.04 ms for negative potentials. The channel had a half-maximal open probability at 0.1 mmol/L ATP, was activated by 100 micromol/L diazoxide, and was inhibited by glyburide, with an IC(50) in the nanomolar range. Thus, pending further tests at low concentrations of K(ATP) channel openers, the single-channel data confirm the results obtained with whole-cell recordings. The neonatal atrial appendage K(ATP) channel thus shows a unique functional and pharmacological profile resembling the pancreatic beta-cell channel for its high affinity for glyburide and diazoxide and for its conductance, but also resembling the ventricular channel subtype for its high affinity for cromakalim, its burst duration, and its sensitivity to ATP. Reverse transcriptase-polymerase chain reaction experiments showed the expression of Kir6.1, Kir6.2, SUR1A, SUR1B, SUR2A, and SUR2B subunits, a finding supporting the hypothesis that the neonatal atrial K(ATP) channel corresponds to a novel heteromultimeric association of K(ATP) channel subunits.

Mice lacking the basolateral Na-K-2Cl cotransporter have impaired epithelial chloride secretion and are profoundly deaf

In chloride-secretory epithelia, the basolateral Na-K-2Cl cotransporter (NKCC1) is thought to play a major role in transepithelial Cl(-) and fluid transport. Similarly, in marginal cells of the inner ear, NKCC1 has been proposed as a component of the entry pathway for K(+) that is secreted into the endolymph, thus playing a critical role in hearing. To test these hypotheses, we generated and analyzed an NKCC1-deficient mouse. Homozygous mutant (Nkcc1(-/-)) mice exhibited growth retardation, a 28% incidence of death around the time of weaning, and mild difficulties in maintaining their balance. Mean arterial blood pressure was significantly reduced in both heterozygous and homozygous mutants, indicating an important function for NKCC1 in the maintenance of blood pressure. cAMP-induced short circuit currents, which are dependent on the CFTR Cl(-) channel, were reduced in jejunum, cecum, and trachea of Nkcc1(-/-) mice, indicating that NKCC1 contributes to cAMP-induced Cl(-) secretion. In contrast, secretion of gastric acid in adult Nkcc1(-/-) stomachs and enterotoxin-stimulated fluid secretion in the intestine of suckling Nkcc1(-/-) mice were normal. Finally, homozygous mutants were deaf, and histological analysis of the inner ear revealed a collapse of the membranous labyrinth, consistent with a critical role for NKCC1 in transepithelial K(+) movements involved in generation of the K(+)-rich endolymph and the endocochlear potential.

Parietal cells express high levels of Na-K-2Cl cotransporter on migrating into the gastric gland neck

Na-K-2Cl cotransport and Cl/HCO3 exchange are prominent mechanisms for Cl- uptake in Cl--secreting epithelial cells. We used immunofluorescence microscopy to delineate the distributions of Na-K-2Cl cotransporter-1 (NKCC1) and anion exchanger-2 (AE2) proteins in rat gastric mucosa (zymogenic zone). Parietal cells (PCs) above the neck of the gastric gland contained abundant AE2 but little or no NKCC1, whereas those in the neck and base contained high NKCC1 but diminished AE2. Lower levels of NKCC1 were detected in surface mucous cells and in cells comprising the blind ends of all glands. Pulse labeling of proliferating cells with bromodeoxyuridine indicated that new PCs originate in the isthmus with scant NKCC1; the subset of PCs that migrate downward expresses NKCC1 abruptly on entering the neck, within 7 days of cell division. Our results suggest that downwardly migrating PCs replace one mechanism for Cl- entry (Cl/HCO3 exchange) with another (Na-K-2Cl cotransport).

Characteristics of sodium uptake across the basolateral membrane of oxyntic cells

To characterize further serosal Na uptake into gastric oxyntic cells under resting conditions, cellular element concentrations were determined in isolated frog (Rana temporaria) gastric mucosae using electron microprobe analysis. The epithelia were kept short circuited in Ussing-type chambers, and element analysis was performed on freeze-dried cryosections. After ouabain (10(-4) M), the [Na] in oxyntic cells increased within 30 to 60 minutes from approximately 25 to 100 mmol/kg wet wt, and [K] decreased similarly (from 100 to 25 mmol/kg wet wt). These changes occurred regardless of whether the basolateral incubation medium contained HCO3 or N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) as buffers. When, prior to the addition of ouabain, 10(-3) M amiloride was applied to the serosal side to inhibit the Na-H antiporter, the ouabain-induced increase in cellular [Na] was prevented completely in HEPES-, but not in HCO3-Ringer. The data are compatible with the notion that Na is taken up by a Na-H antiporter and a Na-HCO3 symporter. At least under these experimental conditions, these transporters seem to contribute substantially to basolateral Na uptake in oxyntic cells.

Active sodium-urea counter-transport is inducible in the basolateral membrane of rat renal initial inner medullary collecting ducts

Rat inner medullary collecting ducts (IMCD3s) possess a luminal Na+-dependent, active urea secretory transport process, which is upregulated by water diuresis. In this study of perfused IMCDs microdissected from base (IMCD1), middle (IMCD2), or tip (IMCD3) of the inner medulla, we tested whether furosemide diuresis alters active urea transport. Rats received furosemide (10 mg/d s.c. for 3-4 d) and were compared with pair-fed control rats. Furosemide significantly decreased urine osmolality and urea clearance, and increased blood urea nitrogen. IMCD3s from furosemide-treated rats had significantly lower rates of active urea secretion than IMCD3s from control rats. IMCD2s showed no active urea transport in control or furosemide-treated rats. IMCD1s from control rats had no active urea transport, but IMCD1s from furosemide-treated rats expressed significant rates of active urea reabsorption. In IMCD1s, this active urea reabsorptive transport process was inhibited by: (i) 0. 25 mM phloretin (bath); (ii) 1 mM ouabain (bath); and (iii) replacing bath Na+ with NMDG+; it was stimulated by 10 nM bumetanide (bath). In summary, we found that furosemide decreased active urea secretion in IMCD3s and induced active urea reabsorption in IMCD1s. The new Na+- dependent, active urea reabsorptive transport process may be a basolateral Na+-urea antiporter.

Selective increase in gastric mucosal mRNA encoding basolateral Na-K-2C1 cotransporter following ileostomy in the rat

Results of previous studies suggest that major surgical resections or reconstructions of the distal small intestine can alter morphologic and functional properties of the stomach. Little is known about the effect of lesser surgical alterations such as construction of an ileostomy, on the morphology and transport properties of the gastric mucosa. To evaluate the effects of ileostomy, Sprague-Dawley rats underwent sham laparotomy (n = 10) or loop ileostomy construction (n = 10). After body weights had stabilized ( approximately 21 days) the animals were killed. Gastric mucosal scrapings were prepared for Northern blot analysis of messenger RNA levels for (1) H/K ATPase, found in parietal cells; (2) Na-K-2C1 cotransporter, found in both parietal and surface cells; and (3)Na/K ATPase, found in all gastric mucosal cells. Gastric mucosa from ileostomy animals was visibly hypertrophied compared to sham-operated animals. There was a 145% increase in the mRNA levels of the Na-K-2Cl cotransporter in gastric mucosa of the ileostomy group but no significant changes in H/K ATPase or Na/K ATPase mRNA levels. Construction of an ileostomy selectively enhances expression of the Na-K-C1 cotransporter in the gastric mucosa. Further studies are required to understand the neurohumoral stimuli underlying this selective response.

Evidence for a physiological role of NH4+ transport on the secretory Na(+)-K(+)-2Cl- cotransporter

The secretory Na(+)-K(+)-2Cl- cotransporter in salivary acinar cells is responsible for driving the transepithelial Cl- fluxes that give rise to fluid secretion. We demonstrate that the application of the muscarinic agonist carbachol to rat parotid acini results in an intracellular acid load that can be blocked by bumetanide, a specific inhibitor of the cotransporter. One component of this bumetanide-sensitive acid load is ouabain-sensitive while a second is dependent on the presence of sub-millimolar concentrations of NH4+ in our media. Our data indicate that this latter effect arises from NH4+ entry on the cotransporter operating in a Na(+)-NH4(+)-2Cl- cotransport mode and that at physiological NH4+ levels in the rat (approximately 0.1 mM), 10-15% of the acinar Cl- entry occurs via this route. We suggest that Na(+)-NH4(+)-2Cl- cotransport may also play a significant physiological role in other cell types and that this mode of operation of the secretory Na(+)-K(+)-2Cl- cotransporter could account for the currently unexplained presence of this protein in a number of tissues.

Na-K-Cl cotransporter expression in the developing and senescent gerbil cochlea

Changes in the cellular expression pattern of the Na-K-Cl cotransporter (NKCC) were investigated during postnatal development and with advancing age in the gerbil cochlea. At birth, faint immunostaining for NKCC was discernable in the developing stria vascularis (StV), Reissner's membrane, interdental cells and some relatively undifferentiated cells lining the cochlear partition. Between 2 and 4 days after birth (DAB) immunostaining persisted and increased in the future interdental, inner and outer sulcus and claudius cells but then disappeared from these sites by 8 DAB. In contrast, NKCC immunoreactivity in the StV increased progressively during development and approached adult levels by 12 DAB. Immunostaining for NKCC in subpopulations of fibrocytes in the inferior portion of the spiral ligament, the suprastrial region and the spiral limbus was first detectable between 10 and 12 DAB and staining intensity reached adult levels around 16 DAB. Changes in NKCC expression with advancing age generally mimicked those previously observed for Na,K-ATPase in focal regions of atrophic lateral wall. Diminished immunostaining was first seen in the StV, presumably associated with the involution of the marginal cell's basolateral processes. Further atrophy culminated in complete loss of immunostaining in the StV and an associated down-regulation of NKCC expression in spiral ligament transport fibrocytes. The marked similarities in the developmental and age-related expression patterns of NKCC and Na,K-ATPase point to a high level of functional cooperativity between these two ion transport mediators, which together provide an efficient mechanism for generating and maintaining high K+ levels in endolymph and the endocochlear potential.

Targeted disruption of the murine Na+/H+ exchanger isoform 2 gene causes reduced viability of gastric parietal cells and loss of net acid secretion

Multiple isoforms of the Na+/H+ exchanger (NHE) are expressed at high levels in gastric epithelium, but the physiological role of individual isoforms is unclear. To study the function of NHE2, which is expressed in mucous, zymogenic, and parietal cells, we prepared mice with a null mutation in the NHE2 gene. Homozygous null mutants exhibit no overt disease phenotype, but the cellular composition of the oxyntic mucosa of the gastric corpus is altered, with parietal and zymogenic cells reduced markedly in number. Net acid secretion in null mutants is reduced slightly relative to wild-type levels just before weaning and is abolished in adult animals. Although mature parietal cells are observed, and appear morphologically to be engaged in active acid secretion, many of the parietal cells are in various stages of degeneration. These results indicate that NHE2 is not required for acid secretion by the parietal cell, but is essential for its long-term viability. This suggests that the unique sensitivity of NHE2 to inhibition by extracellular H+, which would allow upregulation of its activity by the increased interstitial alkalinity that accompanies acid secretion, might enable this isoform to play a specialized role in maintaining the long-term viability of the parietal cell.

Partial cloning and characterization of Slc12a2: the gene encoding the secretory Na+-K+-2Cl- cotransporter

The Slc12a2 gene encodes a widely expressed bumetanide-sensitive Na+-K+-2Cl- cotransporter that participates in various functions such as Cl- secretion and cell volume regulation. We isolated and characterized 75 kilobases of the murine gene encoding the cotransporter. The cotransport protein is encoded by 27 exons. Ribonuclease protection assay and primer extension demonstrated tissue-specific transcription initiation sites located within 270 base pairs upstream of the start codon. Nucleotide sequence analysis of the proximal 5'-flanking region revealed the presence of a weak TATA box, multiple Sp1/GC consensus sites, and the consensus sequence of a putative transcriptional initiator. Transfection of luciferase reporter gene constructs in mouse inner medullary collecting duct (mIMCD-3) cells confirmed the location of the minimal promoter within a 120-base pair fragment upstream of the cDNA. We also report the identification of an alternatively spliced variant of the cotransporter, expressed primarily in brain. This new spliced variant lacks exon 21, which encodes a 16-amino acid peptide located in the COOH-terminal tail of the protein. The absence of this exon causes the loss of the single protein kinase A consensus site of the cotransport protein.

A plant cation-chloride co-transporter promoting auxin-independent tobacco protoplast division

Although auxin plays a central role in plant development, little is known about the signal transduction pathways triggered by auxin regulating cell elongation, division and differentiation. We describe the molecular analysis of the mutant tobacco line axi 4/1, which was regenerated from an auxin-independent callus created by activation T-DNA tagging. Transcriptional enhancer-mediated deregulated expression of the tagged plant gene axi 4 uncouples division of axi 4/1 protoplasts from external auxin stimuli, whereas in untransformed protoplasts expression of axi 4 and cell division are auxin dependent. axi 4 encodes a 109 kDa protein with significant homology to a family of electroneutral cation-chloride co-transporters (CCCs). We show that overexpression of AXI 4 or another member of the CCC family, the Na+/K+/2Cl--co-transporter from shark, triggers auxin-independent growth of tobacco protoplasts. We suggest that Na+/K+/2Cl--co-transporters may play a role in signalling cell division and that this function is highly conserved between AXI 4 and the shark Na+/K+/2Cl--co-transporter. We also demonstrate that a C-terminal fragment of AXI 4 is sufficient to promote auxin-independent cell division, showing that the C-termini of CCCs are functional subunits triggering cell division. This may allow a molecular dissection of this process not only in plants but also in animal cells.

Expression of the mouse Na-K-2Cl cotransporter, mBSC2, in the terminal inner medullary collecting duct, the glomerular and extraglomerular mesangium, and the glomerular afferent arteriole

Na-K-Cl cotransport plays an important role in the kidney in NaCl reabsorption in the thick ascending limb of Henle and a less well defined role in the inner medullary collecting duct (IMCD). Two Na-K-Cl cotransporters encoded by different genes have been identified in the mammalian kidney: BSC1/NKCC2 which localizes to the apical thick ascending limb of Henle and BSC2/NKCC1 which was isolated from a mouse IMCD cell line (mIMCD-3) but its localization has not been determined. In this study we generated a polyclonal antibody (anti-mBSC2) against the mouse BSC2/NKCC1 protein in order to characterize and localize this protein in mouse kidney. Western blot analysis with affinity-purified anti-mBSC2 showed a protein doublet of 140 and 150 kD which was most abundant in the renal papilla but also seen in cortex and outer medulla. The 140-150-kD bands were not seen with preimmune serum or with anti-mBSC2 preabsorbed with specific antigen. Immunolocalization confirmed expression of mBSC2 protein on the basolateral surface of terminal IMCD segments and demonstrated expression in the papillary surface epithelium. Immunofluorescence also revealed the unexpected presence of the BSC2 protein at the juxtaglomerular afferent arteriole, in a juxtaglomerular structure probably representing the extraglomerular mesangium, and throughout the glomerular mesangium.

Pentagastrin selectively modulates levels of mRNAs encoding apical H/K adenosine triphosphatase and basolateral Na-K-Cl cotransporter in rat gastric fundic mucosa

BACKGROUND

Gastrin regulates gastric acid secretion and gastric mucosal cell proliferation. We hypothesized that pentagastrin administration would affect mRNA levels of two membrane proteins that are important during stimulated states of HCl secretion, the basolateral Na-K-Cl cotransporter (BSC) and the apical H/K adenosine triphosphatase (H/K).

METHODS

Two groups of Fischer rats received intraperitoneal injections of pentagastrin (2.5 or 25 micrograms/kg) every 8 hours for three doses. A third group served as controls. An additional group received pentagastrin plus the gastrin receptor antagonist (GRA) L740,093. Fundic mucosae were subjected to semiquantitative Northern analysis of mRNAs encoding H/K and BSC. The mRNA for Na/K adenosine triphosphatase (Na/K), a transport protein not involved directly in acid secretion, also was evaluated.

RESULTS

Administration of pentagastrin caused dose-dependent increases in levels of mRNAs encoding H/K and BSC but had no significant effect on levels of Na/K mRNA. Administration of GRA prevented the pentagastrin-induced changes in mRNA levels for these transporters.

CONCLUSIONS

Pentagastrin administration selectively up-regulates levels of mRNA encoding membrane proteins involved in acid secretion. The up-regulation of the mRNAs encoding BSC during pentagastrin stimulation indicates that regulation of basolateral Cl- movement may be as important as the regulation of apical H+ movement under stimulated states.

Effects of eel neuropeptide Y on ion transport across the seawater eel intestine

A neuropeptide Y (eNPY) was isolated from the intestinal extract of eels. This peptide enhanced significantly the serosa-negative transepithelial potential difference (PD) and short-circuit current (Isc) across the intestine of the seawater eel after pretreatment with isobutylmethylxanthine, serotonin and methacholine. The effects of eNPY on the Isc were concentration-dependent with a threshold concentration of 3 x 10(-9) M and a maximal effect at 3 x 10(-7) M. Similar concentration-response curve was obtained by porcine peptide YY (pPYY). Since 9 amino acid residues are replaced in the pPYY, this result indicates that these substitutions do not change the potency and the efficacy. These stimulatory actions of eNPY were not blocked by tetrodotoxin, an inhibitor of neural firing, or yohimbine, an alpha 2-adrenoceptor antagonist, indicating that eNPY acts without enteric neural firing or catecholamine release. When eNPY and adrenaline (AD) were applied simultaneously, the effects were additive only at lower dosage (3 x 10(-8) M for eNPY, 3 x 10(-8) M for AD), but not at high dosage (10(-6) M eNPY, 10(-7) M AD). The ceiling effect at high dosage suggests that these two regulators act through common signal transduction systems and affect the Na(+)-K(+)-Cl- cotransport system, since both effects were completely blocked by bumetanide, a specific inhibitor of Na(+)-K(+)-Cl- cotransporter.