The sodium dependent bicarbonate transporter NCBE/NBCn2 is predominantly expressed in the central nervous system (CNS). The highest protein concentrations are found in the choroid plexus. The primary function of this integral plasma membrane transport protein is to regulate intracellular neuronal pH and also probably to maintain the pH homeostasis across the blood-cerebrospinal fluid barrier. NCBE is predicted to contain at least 10 transmembrane helices. The N- and C- termini are both cytoplasmic, with a large N-terminal domain (Nt-NCBE) and a relatively small C-terminal domain (Ct-NCBE). The Nt-NCBE is likely to be involved in bicarbonate recognition and transport and contains key areas of regulation involving pH sensing and protein-protein interactions. Intrinsic disordered protein regions (IDPRs) are defined as protein regions having no rigid three-dimensional structure under physiological conditions. They are believed to be involved in signaling networks in which specific, low affinity, protein-protein interactions play an important role. We predict that NCBE and other SoLute Carrier 4 (SLC4) family members have a high level of intrinsic disorder in their cytoplasmic regions. To provide biophysical evidence for the IDPRs predicted in Nt-NCBE, we produced pure (>99%), recombinant Nt-NCBE using E. coli as the expression host. The protein was used to perform differential scanning fluorescence spectroscopy (DSF), in order to search for small molecules that would induce secondary or tertiary structure in the IDPRs. We expect this to assist the development of selective pharmaceutical compounds against individual SLC4 family members. We have also determined a low resolution (4 Å) X-ray crystal structure of the N-terminal core domain. The N-terminal cytoplasmic domain (cdb3) of anion exchanger 1 (AE1) shares a similar fold with the N-terminal core domain of NCBE. Crystallization conditions for the full-length N-terminal domain have been sought, but only the core domain yields diffracting crystals.
SLC4; intrinsic disorder; drug screen; NCBE; IDP; bicarbonate
NCBE (SLC4A10) is a member of the SLC4 family of bicarbonate transporters, several of which play important roles in intracellular-pH regulation and transepithelial HCO3− transport. Here we characterize a new antibody that was generated in rabbit against a fusion protein consisting of maltose-binding protein and the first 135 amino acids (aa) of the N-terminus of human NCBE. Western blotting—both of purified peptides representing the initial ~120aa of the transporters and of full-length transporters expressed in Xenopus oocytes—demonstrated that the antibody is specific for NCBE versus the two most closely related proteins, NDCBE (SLC4A8) and NBCn1 (SLC4A7). Western blotting of tissue in four regions of adult mouse brain indicates that NCBE is expressed most abundantly in cerebral cortex (CX), cerebellum (CB) and hippocampus (HC), and less so in subcortex (SCX). NCBE protein was present in CX, CB, and HC microdissected to avoid choroid plexus. Immunocytochemistry shows that NCBE is present at the basolateral membrane of E18 fetal and adult choroid plexus. NCBE protein is present by western blot and immunocytochemistry in cultured and freshly dissociated HC neurons but not astrocytes. By western blot, nearly all NCBE in mouse and rat brain is highly N-glycosylated (~150 kDa). PNGase F reduces the MW of natural NCBE in mouse brain or human NCBE expressed in oocytes to approximately the predicted MW of the unglycosylated protein. In oocytes, mutating any one of the three consensus N-glycosylation sites reduces glycosylation of the other two, and the triple mutant exhibits negligible functional expression.
slc4a10; central nervous system; brain; oocytes; glycosylation
The choroid plexus epithelium (CPE) is located in the ventricular system of the brain, where it secretes the majority of the cerebrospinal fluid (CSF) that fills the ventricular system and surrounds the central nervous system. The CPE is a highly vascularized single layer of cuboidal cells with an unsurpassed transepithelial water and solute transport rate. Several members of the slc4a family of bicarbonate transporters are expressed in the CPE. In the basolateral membrane the electroneutral Na+ dependent Cl−/HCO3− exchanger, NCBE (slc4a10) is expressed. In the luminal membrane, the electrogenic Na+:HCO3− cotransporter, NBCe2 (slc4a5) is expressed. The electroneutral Na+:HCO3− cotransporter, NBCn1 (slc4a7), has been located in both membranes. In addition to the bicarbonate transporters, the Na+/H+ exchanger, NHE1 (slc9a1), is located in the luminal membrane of the CPE. Genetically modified mice targeting slc4a2, slc4a5, slc4a7, slc4a10, and slc9a1 have been generated. Deletion of slc4a5, 7 or 10, or slc9a1 has numerous impacts on CP function and structure in these mice. Removal of the transporters affects brain ventricle size (slc4a5 and slc4a10) and intracellular pH regulation (slc4a7 and slc4a10). In some instances, removal of the proteins from the CPE (slc4a5, 7, and 10) causes changes in abundance and localization of non-target transporters known to be involved in pH regulation and CSF secretion. The focus of this review is to combine the insights gathered from these knockout mice to highlight the impact of slc4 gene deletion on the CSF production and intracellular pH regulation resulting from the deletion of slc4a5, 7 and 10, and slc9a1. Furthermore, the review contains a comparison of the described human mutations of these genes to the findings in the knockout studies. Finally, the future perspective of utilizing these proteins as potential targets for the treatment of CSF disorders will be discussed.
cerebrospinal fluid; brain pH; knockout mice; membrane transporters; epithelial physiology
The choroid plexus epithelium (CPE) has served as a model-epithelium for cell polarization and transport studies and plays a crucial role for cerebrospinal fluid (CSF) production. The normal luminal membrane expression of Na+,K+-ATPase, aquaporin-1 and Na+/H+ exchanger 1 in the choroid plexus is severely affected by deletion of the slc4a10 gene that encodes the bicarbonate transporting protein Ncbe/NBCn2. The causes for these deviations from normal epithelial polarization and redistribution following specific gene knockout are unknown, but may be significant for basic epithelial cell biology. Therefore, a more comprehensive analysis of cell polarization in the choroid plexus is warranted. We find that the cytoskeleton in the choroid plexus contains αI-, αII-, βI-, and βII-spectrin isoforms along with the anchoring protein ankyrin-3, most of which are mainly localized in the luminal membrane domain. Furthermore, we find α-adducin localized near the plasma membranes globally, but with only faint expression in the luminal membrane domain. In slc4a10 knockout mice, the abundance of β1 Na+,K+-ATPase subunits in the luminal membrane is markedly reduced. Anion exchanger 2 abundance is increased in slc4a10 knockout and its anchor protein, α-adducin is almost exclusively found near the basolateral domain. The αI- and βI-spectrin abundances are also decreased in the slc4a10 knockout, where the basolateral domain expression of αI-spectrin is exchanged for a strictly luminal domain localization. E-cadherin expression is unchanged in the slc4a10 knockout, while small decreases in abundance are observed for its probable adaptor proteins, the catenins. Interestingly, the abundance of the tight junction protein claudin-2 is significantly reduced in the slc4a10 knockouts, which may critically affect paracellular transport in this epithelium. The observations allow the generation of new hypotheses on basic cell biological paradigms that can be tested experimentally in future studies.
sodium hydrogen exchanger; sodium bicarbonate cotransporter; epithelial polarization; cytoskeleton; choroid plexus; cerebrospinal fluid
The sodium-driven chloride/bicarbonate exchanger (NDCBE), a member of the SLC4 family of bicarbonate transporters, was recently found to modulate excitatory neurotransmission in hippocampus. By using light and electron microscopic immunohistochemistry, we demonstrate here that NDCBE is expressed throughout the adult rat brain, and selectively concentrates in presynaptic terminals, where it is closely associated with synaptic vesicles. NDCBE is in most glutamatergic axon terminals, and is also present in the terminals of parvalbumin-positive γ-aminobutyric acid (GABA)ergic cells. These findings suggest that NDCBE can regulate glutamatergic transmission throughout the brain, and point to a role for NDCBE as a possible regulator of GABAergic neurotransmission.
synaptic vesicle; glutamate; GABA; pH; VGLUT; VGAT; SLC4A8
Whereas activation of GABAA receptors by GABA usually results in a hyperpolarizing influx of chloride into the neuron, the reversed chloride driving force in the immature nervous system results in a depolarizing efflux of chloride. This GABAergic depolarization is deemed to be important for the maturation of the neuronal network. The concept of a developmental GABA switch has mainly been derived from in vitro experiments and reliable in vivo evidence is still missing. As GABAA receptors are permeable for both chloride and bicarbonate, the net effect of GABA also critically depends on the distribution of bicarbonate. Whereas chloride can either mediate depolarizing or hyperpolarizing currents, bicarbonate invariably mediates a depolarizing current under physiological conditions. Intracellular bicarbonate is quickly replenished by cytosolic carbonic anhydrases. Intracellular bicarbonate levels also depend on different bicarbonate transporters expressed by neurons. The expression of these proteins is not only developmentally regulated but also differs between cell types and even subcellular regions. In this review we will summarize current knowledge about the role of some of these transporters for brain development and brain function.
GABA; pH; chloride; bicarbonate; ion transporter
The addition of ammonium ions to the external medium results in an inhibition of the sodium influx and net uptake in Carassius auratus, while intraperitoneal injection of ammonium produces the opposite effect. The simultaneous chloride balance is not significantly affected by these treatments. The addition of bicarbonate ions to the external medium results in a reduction of the influx and net flux of chloride, while injection of bicarbonate produces the opposite effect. The simultaneous sodium balance is not significantly altered. The effects of the external additions are reversible after elimination of the excess ammonium or bicarbonate ions by rinsing. Inhibition of carbonic anhydrase in the gill by injection of acetazoleamide produces a simultaneous inhibition of both sodium and chloride exchanges. These results confirm the hypothesis of an exchange of sodium for ammonium, and of bicarbonate for chloride across the gill. A tentative schematic representation of the ionic absorption mechanisms in the branchial cell of the fresh-water teleosts is given. Similarities with other biological membranes and especially with the renal tubule are pointed out.
Using a triple-lumen constant perfusion system, the following observations were made in normal subjects. First, chloride, bicarbonate, and sodium were found to exhibit net movement across ileal mucosa against electrochemical gradients. Second, during perfusion with a balanced electrolyte solution simulating plasma, the ileum generally absorbed, but sometimes secreted fluid. A reciprocal net movement of chloride and bicarbonate was noted when sodium movement was zero. Increasing rates of sodium absorption were associated with decreasing bicarbonate secretion rates and finally bicarbonate absorption. Even when bicarbonate was absorbed ileal contents were alkalinized (by contraction of luminal volume). Third, net chloride movement was found to be sensitive to bicarbonate concentration in ileal fluid. For instance, chloride was absorbed from solutions containing 14 or 44 mEq/liter of bicarbonate, but was secreted when ileal fluid contained 87 mEq/liter of bicarbonate. Fourth, when chloridefree (sulfate) solutions were infused, the ileum absorbed sodium bicarbonate and the ileal contents were acidified. Fifth, when plasma-like solutions were infused, the potential difference (PD) between skin and ileal lumen was near zero and did not change when chloride was replaced by sulfate in the perfusion solution.
These results suggest that ileal electrolyte transport occurs via a simultaneous double exchange, Cl/HCO2 and Na/H. In this model neither the anion nor the cation exchange causes net ion movement; net movement results from the chemical reaction between hydrogen and bicarbonate. No other unitary model explains all of the following observations: (a) human ileal transport in vivo is essentially nonelectrogenic even though Na, Cl, and HCO3 are transported against electrochemical gradients, (b) the ileum can secrete as well as absorb, (c) ileal contents are alkalinized during absorption of or during secretion into a plasma-like solution, and (d) the ileum acidifies its contents when sulfate replaces chloride. Data obtained with a carbonic anhydrase inhibitor support the proposed model.
Chloride absorption and bicarbonate secretion are vital functions of epithelia1–6, as highlighted by cystic fibrosis and diseases associated with mutations in members of the SLC26 chloride-bicarbonate exchangers. Many SLC26 transporters (SLC26T) are expressed in the luminal membrane together with CFTR7, which activates electrogenic chloride-bicarbonate exchange by SLC26T8. However, the ability of SLC26T to regulate CFTR and the molecular mechanism of their interaction are not known. We report here a reciprocal regulatory interaction between the SLC26T DRA, SLC26A6 and CFTR. DRA markedly activates CFTR by increasing its overall open probablity (NPo) sixfold. Activation of CFTR by DRA was facilitated by their PDZ ligands and binding of the SLC26T STAS domain to the CFTR R domain. Binding of the STAS and R domains is regulated by PKA-mediated phosphorylation of the R domain. Notably, CFTR and SLC26T co-localize in the luminal membrane and recombinant STAS domain activates CFTR in native duct cells. These findings provide a new understanding of epithelial chloride and bicarbonate transport and may have important implications for both cystic fibrosis and diseases associated with SLC26T.
Neuronal activity results in significant pH shifts in neurons, glia, and interstitial space. Several transport mechanisms are involved in the fine-tuning and regulation of extra- and intracellular pH. The sodium-independent electroneutral anion exchangers (AEs) exchange intracellular bicarbonate for extracellular chloride and thereby lower the intracellular pH. Recently, a significant association was found with the variant Ala867Asp of the anion exchanger AE3, which is predominantly expressed in brain and heart, in a large cohort of patients with idiopathic generalized epilepsy. To analyze a possible involvement of AE3 dysfunction in the pathogenesis of seizures, we generated an AE3-knockout mouse model by targeted disruption of Slc4a3. AE3-knockout mice were apparently healthy, and neither displayed gross histological and behavioral abnormalities nor spontaneous seizures or spike wave complexes in electrocorticograms. However, the seizure threshold of AE3-knockout mice exposed to bicuculline, pentylenetetrazole, or pilocarpine was reduced, and seizure-induced mortality was significantly increased compared to wild-type littermates. In the pyramidal cell layer of the hippocampal CA3 region, where AE3 is strongly expressed, disruption of AE3 abolished sodium-independent chloride-bicarbonate exchange. These findings strongly support the hypothesis that AE3 modulates seizure susceptibility and, therefore, are of significance for understanding the role of intracellular pH in epilepsy.
An investigation of small intestinal electrolyte transport was performed in a subject with congenital chloridorrhoea using a constant perfusion technique. The results indicate that the diarrhoea was not due to an abnormally high rate of secretion of fluid into the duodenum, and transport of electrolytes and glucose was normal in the jejunum. In contrast there was a marked abnormality of electrolyte transport in the ileum, chloride, sodium, and water entering the lumen and bicarbonate being absorbed in the absence of any concentration gradients for these ions. This is clearly different from the finding in normal subjects of sodium chloride and water absorption and bicarbonate secretion.
It is suggested that the likely prime mechanism for these abnormalities is a chloride/bicarbonate exchange acting in the direction of chloride secretion, that is, in the opposite direction to the normal anion exchange. The hydrogen ion gradient set up by this exchange could have induced a secondary reversal of the normal sodium/hydrogen exchange so that hydrogen was absorbed and sodium secreted.
From an analysis of the stool electrolyte concentrations and the rectal electrical potential difference it is suggested that in the colon chloride was secreted and bicarbonate absorbed against electrochemical gradients. Here too a reversed chloride/bicarbonate exchange is possibly responsible for the composition of the faecal electrolytes. As evidenced by the low faecal sodium concentration and the normal rectal potential difference, sodium transport is probably normal in the colon.
The existence of chloride/bicarbonate exchange across the basolateral membrane and its physiologic significance were examined in rabbit proximal tubules. S2 segments of the proximal straight tubule were perfused in vitro and changes in intracellular pH (pHi) and chloride activity (aCli) were monitored by double-barreled microelectrodes. Total peritubular chloride replacement with gluconate increased pHi by 0.8, and this change was inhibited by a pretreatment with an anion transport inhibitor, SITS. Peritubular bicarbonate reduction increased aCli, and most of this increase was lost when ambient sodium was totally removed. The reduction rates of pHi induced by a peritubular bicarbonate reduction or sodium removal were attenuated by 20% by withdrawal of ambient chloride. SITS application to the bath in the control condition quickly increased pHi, but did not change aCli. However, the aCli slightly decreased in response to SITS when the basolateral bicarbonate efflux was increased by reducing peritubular bicarbonate concentration. It is concluded that sodium coupled chloride/bicarbonate exchange is present in parallel with sodium-bicarbonate cotransport in the basolateral membrane of the rabbit proximal tubule, and it contributes to the basolateral bicarbonate and chloride transport.
The inhibitory neurotransmitter gamma-amino-butyric acid (GABA) not only modulates excitability in the mature nervous system but also regulates neuronal differentiation and circuit development. Horizontal cells, a subset of interneurons in the outer retina, are transiently GABAergic during the period of cone photoreceptor synaptogenesis. In rodents, both horizontal cells and cone axonal terminals express GABAA receptors. To explore the possibility that transient GABA expression in mouse neonatal horizontal cells influences the structural development of synaptic connectivity in the outer retina, we examined a mutant in which expression of GAD67, the major synthesizing enzyme for GABA, is selectively knocked out in the retina.
Immunocytochemistry and electron microscopy revealed that the assembly of triad synapses involving cone axonal pedicles and the dendrites of horizontal and bipolar cells is unaffected in the mutant retina. Moreover, loss of GABA synthesis in the outer retina did not perturb the spatial distributions and cell densities of cones and horizontal cells. However, there were some structural alterations at the cellular level: the average size of horizontal cell dendritic clusters was larger in the mutant, and there was also a small but significant increase in cone photoreceptor pedicle area. Moreover, metabotropic glutamate receptor 6 (mGluR6) receptors on the dendrites of ON bipolar cells occupied a slightly larger proportion of the cone pedicle in the mutant.
Together, our analysis shows that transient GABA synthesis in horizontal cells is not critical for synapse assembly and axonal and dendritic lamination in the outer retina. However, pre- and postsynaptic structures are somewhat enlarged in the absence of GABA in the developing outer retina, providing for a modest increase in potential contact area between cone photoreceptors and their targets. These findings differ from previous results in which pharmacological blockade of GABAA receptors in the neonatal rabbit retina caused a reduction in cone numbers and led to a grossly disorganized outer retina.
Using a constant perfusion technique, sodium and bicarbonate absorption was studied in human subjects.
The following observations were made on sodium absorption from saline solution: (a) the rate of sodium absorption is markedly influenced by bulk water flow, (b) when net water flow is zero, sodium absorption is zero if there are no concentration gradients between plasma and lumen that favor net NaCl diffusion; and (c) the PD between abraded skin and jejunal lumen is near zero when saline is perfused and does not change with partial substitution of sulfate or bicarbonate for chloride. Based on these observations, we conclude that sodium absorption from saline is entirely passive in the human jejunum. On the other hand, in the presence of bicarbonate sodium is absorbed actively against electrochemical gradients.
The mechanism of the link between bicarbonate and sodium absorption was studied in normal subjects and in 11 patients with pernicious anemia; the latter were chosen because they do not secrete gastric acid which can react with bicarbonate in the jejunal lumen. We observed that bicarbonate absorption (a) occurs against steep electrochemical gradients, (b) does not generate a potential difference between abraded skin and jejunal lumen, (c) is inhibited by acetazolamide, and (d) generates a high CO2 tension in jejunal fluid. These observations suggest that bicarbonate absorption is mediated by active hydrogen secretion, rather than by bicarbonate ion transport per se, and that the link between sodium and bicarbonate transport is best explained by a sodium-hydrogen exchange process.
A key principle of retinal organization is that distinct ON and OFF channels are relayed by separate populations of bipolar cells to different sublaminae of the inner plexiform layer (IPL). ON bipolar cell axons have been thought to synapse exclusively in the inner IPL (the ON sublamina) onto dendrites of ON-type amacrine and ganglion cells. However, M1 melanopsin-expressing ganglion cells and dopaminergic amacrine (DA) cells apparently violate this dogma. Both are driven by ON bipolar cells, but their dendrites stratify in the outermost IPL, within the OFF sublamina. Here, in the mouse retina, we show that some ON cone bipolar cells make ribbon synapses in the outermost OFF sublayer, where they costratify with and contact the dendrites of M1 and DA cells. Whole-cell recording and dye filling in retinal slices indicate that Type 6 ON cone bipolars provide some of this ectopic ON channel input. Imaging studies in dissociated bipolar cells show that these ectopic ribbon synapses are capable of vesicular release. There is thus an accessory ON sublayer in the outer IPL.
retina; bipolar cell; ON channel circuitry; dopaminergic amacrine cell; melanopsin ganglion cell
Electrophysiological conversion of chloride-dependent synapses from inhibitory to excitatory function, as a result of aberrant neuronal chloride homeostasis, is a known mechanism for the genesis of neuropathic pain. This paper examines theoretically how this type of synaptic conversion can disrupt circuit logic in spinal nociceptive circuits. First, a mathematical scaling factor is developed to represent local aberration in chloride electrochemical driving potential. Using this mathematical scaling factor, electrophysiological symbols are developed to represent the magnitude of synaptic conversion within nociceptive circuits. When inserted into a nociceptive circuit diagram, these symbols assist in understanding the generation of neuropathic pain associated with the collapse of transmembrane chloride gradients. A more generalized scaling factor is also derived to represent the interplay of chloride and bicarbonate driving potentials on the function of GABAergic and glycinergic synapses. These mathematical and symbolic representations of synaptic conversion help illustrate the critical role that anion driving potentials play in the transduction of pain. Using these representations, we discuss ramifications of glial-mediated synaptic conversion in the genesis, and treatment, of neuropathic pain.
Mosquito larvae exhibit luminal pH extremes along the axial length of their alimentary canal that range from very alkaline (pH > 10) in the anterior midgut to slightly acid in the hindgut. The principal buffer in the system is thought to be bicarbonate and/or carbonate, because the lumen is known to contain high levels of bicarbonate/carbonate and is surrounded by various epithelial cell types which express a variety of carbonic anhydrases. However, the precise mechanisms responsible for the transport of bicarbonate/carbonate into and out of the lumen are unclear. In the present study, we test the hypothesis that SLC4-like anion transporters play a role in bicarbonate/carbonate accumulation in the larval mosquito alimentary canal. Molecular, physiological and immnuohistochemical characterizations of Slc4-like transporters in the gut of larval mosquitoes (Aedes aegypti and Anopheles gambiae) demonstrate the presence of both a Na+-independent chloride/bicarbonate anion exchanger (AE) as well as a Na+-dependent anion exchanger (NDAE). Notably, immunolocalization experiments in Malpighian tubules show that the two proteins can be located in the same tissue, but to different cell types. Immunolabeling experiments in the gastric caecae show that the two proteins can be found in the same cells, but on opposite sides (basal vs. apical). In summary, our results indicate that the alimentary canal of larval mosquitoes exhibits robust expression of two SLC4-like transporters in locations that are consistent with a role in the regulation of luminal pH. The precise physiological contributions of each transporter remain to be determined.
Anion exchangers; mosquito larvae; alimentary canal; Malpighian tubule; Gastric caeca
Using continuous microperfusion techniques, we studied the load dependence of bicarbonate reabsorption along cortical distal tubules of the rat kidney and their bicarbonate permeability. Net bicarbonate transport was evaluated from changes in tracer inulin concentrations and total CO2 measurements by microcalorimetry. Bicarbonate permeability was estimated from the flux of total CO2 along known electrochemical gradients into bicarbonate-and chloride-free perfusion solution containing 10(-4) M acetazolamide. Transepithelial potential differences were measured with conventional glass microelectrodes. Significant net bicarbonate reabsorption occurred at luminal bicarbonate levels from 5 to 25 mM, and at perfusion rates from 5 to 30 nl/min. Bicarbonate reabsorption increased in a load-dependent manner, both during increments in luminal bicarbonate concentration or perfusion rate, reaching saturation at a load of 250 pmol/min with a maximal reabsorption rate of approximately 75 pmol/min.mm. Rate of bicarbonate reabsorption was flow dependent at luminal concentrations of 10 but not at 25 mM. During chronic metabolic alkalosis, maximal rates of reabsorption were significantly reduced to 33 pmol/min.mm. The bicarbonate permeability was 2.32 +/- 0.13 x 10(-5) cm/s in control rats, and 2.65 +/- 0.26 x 10(-5) cm/s in volume-expanded rats. Our data indicate that at physiological bicarbonate concentrations in the distal tubule passive bicarbonate fluxes account for only 16-21% of net fluxes. At high luminal bicarbonate concentrations, passive bicarbonate reabsorption contributes moderately to net reabsorption of this anion.
During perfusion of a plasma-like solution, colonic absorption rate of chloride was much higher than the secretion rate of bicarbonate (34 vs. 3.5 meq/h, respectively). This might suggest that anion exchange (Cl/HCO3) accounts for only a small fraction of total chloride absorption. However, if the colon absorbs as well as secretes bicarbonate, this reasoning would underestimate the magnitude of the anion exchange. To see if the colon absorbs bicarbonate, we perfused a chloride-free solution (which would eliminate bicarbonate secretion via (Cl/HCO3 exchange) and found that the colon absorbed bicarbonate at a rate of 5.1 meq/h. Calculation of electrochemical gradients and measurement of luminal fluid PCO2 indicated that this bicarbonate absorption was mediated passively in response to electrical gradients, rather than via reversed Cl/HCO3 exchange or acid secretion. The combined results of the plasma-like and chloride-free perfusion experiments suggest Cl/HCO3 exchange at a rate of 8.6 meq/h (the sum of bicarbonate movements, 3.5 and 5.1 meq/h, observed in the two experiments). To obtain a second estimate under different experimental conditions, a choline chloride-choline bicarbonate (sodium-free) solution was perfused; with this solution, chloride and bicarbonate absorption dependent on active sodium transport should be eliminated or markedly reduced, and the magnitude of Cl/HCO3 exchange should be revealed. This experiment suggested a Cl/HCO3 exchange rate of 9.3 meq/h, similar to the first estimate. As chloride was absorbed at a rate of 34 meq/h during perfusion of the plasma-like solution, the Cl/HCO3 exchange provides for approximately one-fourth of total chloride absorption.
Dog red blood cells (RBC) lack a ouabain-sensitive sodium pump, and yet they are capable of volume regulation in vivo. The present study was designed to find in vitro conditions under which dog RBC could transport sodium outward, against an electrochemical gradient. Cells were first loaded with sodium chloride and water by preincubation in hypertonic saline. They were then incubated at 37°C in media containing physiologic concentrations of sodium, potassium, chloride, bicarbonate, glucose, and calcium. The cells returned to a normal salt and water content in 16–20 h. Without calcium in the medium the cells continued slowly to accumulate sodium. Removal of glucose caused rapid swelling and lysis, whether or not calcium was present. The net efflux of sodium showed a close relationship to medium calcium over a concentration range from 0 to 5 mM. Extrusion of salt and water was also demonstrated in fresh RBC (no hypertonic preincubation) when calcium levels in the media were sufficiently raised. The ion and water movements in these experiments were not influenced by ouabain or by removal of extracellular potassium. Magnesium could not substitute for calcium. It is concluded that dog RBC have an energy-dependent mechanism for extruding sodium chloride which requires external calcium and is quite distinct from the sodium-potassium exchange pump.
The renal handling of salt and protons and bicarbonate are intricately linked through shared transport mechanisms for sodium, chloride, protons, and bicarbonate. In the collecting duct, the regulated fine-tuning of salt and acid-base homeostasis is achieved by a series of transport proteins located in different cell types, intercalated and principal cells. Intercalated cells are considered to be of less importance for salt handling but recent evidence has suggested that the anion exchanger pendrin may participate in salt reabsorption and blood pressure regulation. Here, we examined the regulated expression of two functionally related but differentially expressed anion exchangers, AE1 and pendrin, by dietary electrolyte intake and aldosterone. Cortical expression of pendrin was regulated on mRNA and protein level. The combination of NaHCO3 and DOCA enhanced pendrin mRNA and protein levels, whereas DOCA or NaHCO3 alone had no effect. NaCl or KHCO3 increased pendrin mRNA, KCl decreased its mRNA abundance. On protein level, NH4Cl, NaCl, and KCl reduced pendrin expression, the other treatments were without effect. In contrast, AE1 mRNA or protein expression in kidney cortex was regulated by none of these treatments. In kidney medulla, NaHCO3/DOCA or NaHCO3 alone enhanced AE1 mRNA levels. AE1 protein abundance was increased by NH4Cl, NaHCO3/DOCA, and NaCl. Immunolocalization showed that during NH4Cl treatment the relative number of AE1 positive cells was increased and pendrin expressing cells reduced. Thus, pendrin and AE1 are differentially regulated with distinct mechanisms that separately affect mRNA and protein levels. Pendrin is regulated by acidosis and chloride intake, whereas AE1 is enhanced by acidosis, NaCl, and the combination of DOCA and NaHCO3.
The unidirectional fluxes of sodium, chloride, and of the bicarbonate and CO2 pair were determined across the isolated large intestine of the bullfrog, Rana catesbiana. The isolated large intestine of the frog is characterized by a mean transmembrane potential of 45 mv., serosal surface positive with respect to mucosal. The unidirectional sodium flux from mucosal to serosal surface was found to be equal to the short-circuit current, thus the net flux was less than the simultaneous short-circuit current. This discrepancy between active sodium transport and short-circuit current can be attributed to the active transport of cation in the same direction as sodium and/or the active transport of anion in the opposite direction. The unidirectional fluxes of chloride and the bicarbonate and CO2 pair revealed no evidence for active transport of either anion. A quantitative study of chloride fluxes at 45 mv. revealed a flux ratio of 1.8 which is considerably less than a ratio of 6 expected for free passive diffusion. It was concluded that a considerable proportion of the isotopic transfer of chloride could be attributed to "exchange diffusion." Study of the electrical properties of the isolated frog colon reveals that it can be treated as a simple D. C. resistance over the range of -20 to +95 mv.
The kinetics of bicarbonate-chloride exchange across the human red cell membrane was studied by following the time course of extracellular pH in a stopped-flow rapid-reaction apparatus during transfer of H+ into the cell by the CO2 hydration-dehydration cycle, under conditions where the rate of the process was determined by HCO3--Cl- exchange flux across the membrane. The flux of bicarbonate increased linearly with [HCO3-] gradient from 0.6 to 20 mM across the red cell membrane at both 37 degrees C and 2 degrees C, and decreased as transmembrane potential was increased by decreasing extracellular [Cl-]. An Arrhenius plot of the rate constants for the exchange indicates that the Q10 is strongly dependent on temperature, being about 1.7 between 24 degrees C and 42 degrees C and about 7 between 2 degrees C and 12 degrees C. These data agree well with the published values for Q10 of 1.2 between 24 degrees C and 40 degrees C and of 8 between 0 degrees C and 10 degrees C. The results suggest that different processes may determine the rate of HCO3- -Cl- exchange at low vs. physiological temperatures, and that the functional (and/or structural) properties of the red cell membrane vary markedly with temperature.
To better understand how nitric oxide (NO) alters the function of the nonpigmented ciliary epithelium (NPE), studies were performed to determine the influence of NO on sodium-hydrogen exchanger (NHE) activity.
Cytoplasmic pH (pHi) was measured in cultured porcine NPE loaded with BCECF (2′,7′-bis(2-carboxyl)-5(6)-carboxyfluorescein-acetoxyethyl ester). Na-H exchanger (NHE) was examined by immunolocalization.
In cells acidified by 5 minutes of exposure to 20 mM ammonium chloride, pHi recovery was partially inhibited by sodium nitroprusside (SNP), an NO donor, and l-arginine, the endogenous substrate for NO synthase. SNP and dimethyl amiloride (DMA), an NHE inhibitor, inhibited pHi recovery to a similar degree. In bicarbonate-free buffer SNP+DMA elicited no additional change in pHi recovery beyond that elicited by DMA alone. This suggests that SNP causes NHE inhibition. the SNP's effect on pHi recovery was mimicked by 8-pCPT-cGMP but suppressed by ODQ and H-8. Ouabain alone reduced pHi recovery, but SNP+ouabain caused significant further reduction. Immunolocalization studies revealed NHE1 and -4 in native and cultured NPE.
NHE1 and -4 are expressed at the NPE basolateral margin. The findings suggest the NHE is inhibited by NO which acts via a cGMP and protein kinase G signaling pathway. The NHE response does not appear to be the consequence of NO-induced Na,K-ATPase inhibition. Because NO synthases are expressed in porcine NPE, NO could act as an autocrine regulator of NHE activity. Although NHE inhibitors are known to lower intraocular pressure (IOP), further studies are needed to understand whether changes in NHE activity contribute to the IOP-lowering effect of NO donors.
Two of the gamma-aminobutyric acid (GABA) receptors, GABAA and GABAC, are ligand-gated chloride channels expressed by neurons in the retina and throughout the central nervous system. The different subunit composition of these two classes of GABA receptor result in very different physiological and pharmacological properties. Although little is known at the molecular level as to the subunit composition of any native GABA receptor, it is thought that GABAC receptors are homomeric assemblies of rho-subunits. However, we found that the kinetic and pharmacological properties of homomeric receptors formed by each of the rho-subunits cloned from perch retina did not resemble those of the GABAC receptors on perch bipolar cells. Because both GABAA and GABAC receptors are present on retinal bipolar cells, we attempted to determine whether subunits of these two receptor classes are capable of interacting with each other. We report here that, when coexpressed in Xenopus oocytes, heteromeric (rho 1B gamma 2) receptors formed by coassembly of the rho 1B-subunit with the gamma 2-subunit of the GABAA receptor displayed response properties very similar to those obtained with current recordings from bipolar cells. In addition to being unresponsive to bicuculline and diazepam, the time-constant of deactivation, and the sensitivities to GABA, picrotoxin and zinc closely approximated the values obtained from the native GABAC receptors on bipolar cells. These results provide the first direct evidence of interaction between GABA rho and GABAA receptor subunits. It seems highly likely that coassembly of GABAA and rho-subunits contributes to the molecular organization of GABAC receptors in the retina and perhaps throughout the nervous system.