Measuring and modeling chloride-hydroxyl exchange in the Guinea-pig ventricular myocyte. Biophys J. 2008;94:2385-2403. [PMID: 18055536 DOI: 10.1529/biophysj.107.118885]

Acidic environments trigger intracellular H+-sensing FAK proteins to re-balance sarcolemmal acid-base transporters and auto-regulate cardiomyocyte pH

AIMS

In cardiomyocytes, acute disturbances to intracellular pH (pHi) are promptly corrected by a system of finely tuned sarcolemmal acid-base transporters. However, these fluxes become thermodynamically re-balanced in acidic environments, which inadvertently causes their set-point pHi to fall outside the physiological range. It is unclear whether an adaptive mechanism exists to correct this thermodynamic challenge, and return pHi to normal.

METHODS AND RESULTS

Following left ventricle cryo-damage, a diffuse pattern of low extracellular pH (pHe) was detected by acid-sensing pHLIP. Despite this, pHi measured in the beating heart (13C NMR) was normal. Myocytes had adapted to their acidic environment by reducing Cl-/HCO3- exchange (CBE)-dependent acid-loading and increasing Na+/H+ exchange (NHE1)-dependent acid-extrusion, as measured by fluorescence (cSNARF1). The outcome of this adaptation on pHi is revealed as a cytoplasmic alkalinization when cells are superfused at physiological pHe. Conversely, mice given oral bicarbonate (to improve systemic buffering) had reduced myocardial NHE1 expression, consistent with a needs-dependent expression of pHi-regulatory transporters. The response to sustained acidity could be replicated in vitro using neonatal ventricular myocytes incubated at low pHe for 48 h. The adaptive increase in NHE1 and decrease in CBE activities was linked to Slc9a1 (NHE1) up-regulation and Slc4a2 (AE2) down-regulation. This response was triggered by intracellular H+ ions because it persisted in the absence of CO2/HCO3- and became ablated when acidic incubation media had lower chloride, a solution manoeuvre that reduces the extent of pHi-decrease. Pharmacological inhibition of FAK-family non-receptor kinases, previously characterized as pH-sensors, ablated this pHi autoregulation. In support of a pHi-sensing role, FAK protein Pyk2 (auto)phosphorylation was reduced within minutes of exposure to acidity, ahead of adaptive changes to pHi control.

CONCLUSIONS

Cardiomyocytes fine-tune the expression of pHi-regulators so that pHi is at least 7.0. This autoregulatory feedback mechanism defines physiological pHi and protects it during pHe vulnerabilities.

Protocol to record and quantify the intracellular pH in contracting cardiomyocytes

Intracellular pH (pHi) plays critical roles in the regulation of cardiac function. Methods and techniques for cardiac pHi measurement have continued to evolve since early 1960s. Fluorescent microscopy is the most recently developed technique with several advantages over other techniques including higher spatial and temporal resolutions, and feasibility for contracting cell measurement. Here, we describe detailed methods for mouse cardiomyocyte isolation, and simultaneous measurement and quantification of pHi and sarcomere length in contracting cardiomyocytes. For complete details on the use and execution of this protocol, please refer to Lyu et al. (2022).

Beat-to-beat dynamic regulation of intracellular pH in cardiomyocytes

The mammalian heart beats incessantly with rhythmic mechanical activities generating acids that need to be buffered to maintain a stable intracellular pH (pHi) for normal cardiac function. Even though spatial pHi non-uniformity in cardiomyocytes has been documented, it remains unknown how pHi is regulated to match the dynamic cardiac contractions. Here, we demonstrated beat-to-beat intracellular acidification, termed pHi transients, in synchrony with cardiomyocyte contractions. The pHi transients are regulated by pacing rate, Cl-/HCO3 - transporters, pHi buffering capacity, and β-adrenergic signaling. Mitochondrial electron-transport chain inhibition attenuates the pHi transients, implicating mitochondrial activity in sculpting the pHi regulation. The pHi transients provide dynamic alterations of H+ transport required for ATP synthesis, and a decrease in pHi may serve as a negative feedback to cardiac contractions. Current findings dovetail with the prevailing three known dynamic systems, namely electrical, Ca2+, and mechanical systems, and may reveal broader features of pHi handling in excitable cells.

Construction of a High-Density Genetic Map and Identification of Quantitative Trait Loci for Nitrite Tolerance in the Pacific White Shrimp (Litopenaeus vannamei)

Nitrite is a major environmental toxin in aquaculture systems that disrupts multiple physiological functions in aquatic animals. Although nitrite tolerance in shrimp is closely related to successful industrial production, few genetic studies of this trait are available. In this study, we constructed a high-density genetic map of Litopenaeus vannamei with 17,242 single nucleotide polymorphism markers spanning 6,828.06 centimorgans (cM), with an average distance of 0.4 cM between adjacent markers on 44 linkage groups (LGs). Using this genetic map, we identified two markers associated with nitrite tolerance. We then sequenced the transcriptomes of the most nitrite-tolerant and nitrite-sensitive individuals from each of four genetically distinct L. vannamei families (LV-I–4). We found 2,002, 1,983, 1,954, and 1,867 differentially expressed genes in families LV-1, LV-2, LV-3, and LV-4, respectively. By integrating QTL and transcriptomics analyses, we identified a candidate gene associated with nitrite tolerance. This gene was annotated as solute carrier family 26 member 6 (SLC26A6). RNA interference (RNAi) analysis demonstrated that SLC26A6 was critical for nitrite tolerance in L. vannamei. The present study increases our understanding of the molecular mechanisms underlying nitrite tolerance in shrimp and provides a basis for molecular-marker-assisted shrimp breeding.

Action Potential Shortening and Impairment of Cardiac Function by Ablation of Slc26a6

BACKGROUND

Intracellular pH (pH_i) is critical to cardiac excitation and contraction; uncompensated changes in pH_i impair cardiac function and trigger arrhythmia. Several ion transporters participate in cardiac pH_i regulation. Our previous studies identified several isoforms of a solute carrier Slc26a6 to be highly expressed in cardiomyocytes. We show that Slc26a6 mediates electrogenic Cl^-/HCO₃^- exchange activities in cardiomyocytes, suggesting the potential role of Slc26a6 in regulation of not only pH_i, but also cardiac excitability.

METHODS AND RESULTS

To test the mechanistic role of Slc26a6 in the heart, we took advantage of Slc26a6 knockout (Slc26a6^-/- ) mice using both in vivo and in vitro analyses. Consistent with our prediction of its electrogenic activities, ablation of Slc26a6 results in action potential shortening. There are reduced Ca²⁺ transient and sarcoplasmic reticulum Ca²⁺ load, together with decreased sarcomere shortening in Slc26a6^-/- cardiomyocytes. These abnormalities translate into reduced fractional shortening and cardiac contractility at the in vivo level. Additionally, pH_i is elevated in Slc26a6^-/- cardiomyocytes with slower recovery kinetics from intracellular alkalization, consistent with the Cl^-/HCO₃^- exchange activities of Slc26a6. Moreover, Slc26a6^-/- mice show evidence of sinus bradycardia and fragmented QRS complex, supporting the critical role of Slc26a6 in cardiac conduction system.

CONCLUSIONS

Our study provides mechanistic insights into Slc26a6, a unique cardiac electrogenic Cl^-/HCO₃^- transporter in ventricular myocytes, linking the critical roles of Slc26a6 in regulation of pH_i, excitability, and contractility. pH_i is a critical regulator of other membrane and contractile proteins. Future studies are needed to investigate possible changes in these proteins in Slc26a6^-/- mice.

Critical role of bicarbonate and bicarbonate transporters in cardiac function

Bicarbonate is one of the major anions in mammalian tissues and extracellular fluids. Along with accompanying H⁺, HCO₃^- is generated from CO₂ and H₂O, either spontaneously or via the catalytic activity of carbonic anhydrase. It serves as a component of the major buffer system, thereby playing a critical role in pH homeostasis. Bicarbonate can also be utilized by a variety of ion transporters, often working in coupled systems, to transport other ions and organic substrates across cell membranes. The functions of HCO₃^- and HCO₃^--transporters in epithelial tissues have been studied extensively, but their functions in heart are less well understood. Here we review studies of the identities and physiological functions of Cl^-/HCO₃^- exchangers and Na⁺/HCO₃^- cotransporters of the SLC4A and SLC26A families in heart. We also present RNA Seq analysis of their cardiac mRNA expression levels. These studies indicate that slc4a3 (AE3) is the major Cl^-/HCO₃^- exchanger and plays a protective role in heart failure, and that Slc4a4 (NBCe1) is the major Na⁺/HCO₃^- cotransporter and affects action potential duration. In addition, previous studies show that HCO₃^- has a positive inotropic effect in the perfused heart that is largely independent of effects on intracellular Ca²⁺. The importance of HCO₃^- in the regulation of contractility is supported by experiments showing that isolated cardiomyocytes exhibit sharply enhanced contractility, with no change in Ca²⁺ transients, when switched from Hepes-buffered to HCO₃^-- buffered solutions. These studies demonstrate that HCO₃^- and HCO₃^--handling proteins play important roles in the regulation of cardiac function.

Potential role of cardiac chloride channels and transporters as novel therapeutic targets

The heart and blood vessels express a range of anion currents (e.g. ICl.PKA) and symporter/antiporters (e.g. Cl(-)/HCO3(-) exchanger) that translocate chloride (Cl(-)). They have been proposed to contribute to a variety of physiological processes including cellular excitability, cell volume homeostasis and apoptosis. Additionally there is evidence that Cl(-) currents or transporters may play a role in cardiac pathophysiology. Arrhythmogenesis, the process of cardiac ischaemic preconditioning, and the adaptive remodelling process in myocardial hypertrophy and heart failure have all been linked to such channels or transporters. We have explored the possibility that selective targeting of one or more of these may provide benefit in cardiovascular disease. Existing evidence points to an emerging role of cardiac cell anion channels as potential therapeutic targets, the 'disease-specificity' of which may represent a substantial improvement on current targets. However, the limitations of current techniques hitherto applied (such as developmental compensation in gene-modified animals) and pharmacological agents (which do not at present possess sufficient selectivity for the adequate probing of function) have thus far hindered translation to the introduction of new therapy.

Functional interaction between bicarbonate transporters and carbonic anhydrase modulates lactate uptake into mouse cardiomyocytes

Blood-derived lactate is a precious energy substrate for the heart muscle. Lactate is transported into cardiomyocytes via monocarboxylate transporters (MCTs) together with H(+), which couples lactate uptake to cellular pH regulation. In this study, we have investigated how the interplay between different acid/base transporters and carbonic anhydrases (CA), which catalyze the reversible hydration of CO2, modulates the uptake of lactate into isolated mouse cardiomyocytes. Lactate transport was estimated both as lactate-induced acidification and as changes in intracellular lactate levels measured with a newly developed Förster resonance energy transfer (FRET) nanosensor. Recordings of intracellular pH showed an increase in the rate of lactate-induced acidification when CA was inhibited by 6-ethoxy-2-benzothiazolesulfonamide (EZA), while direct measurements of lactate flux demonstrated a decrease in MCT transport activity, when CA was inhibited. The data indicate that catalytic activity of extracellular CA increases lactate uptake and counteracts intracellular lactate-induced acidification. We propose a hypothetical model, in which HCO3 (-), formed from cell-derived CO2 at the outer surface of the cardiomyocyte plasma membrane by membrane-anchored, extracellular CA, is transported into the cell via Na(+)/HCO3 (-) cotransport to counteract intracellular acidification, while the remaining H(+) stabilizes extracellular pH at the surface of the plasma membrane during MCT activity to enhance lactate influx into cardiomyocytes.

Resistance to cardiomyocyte hypertrophy in ae3-/- mice, deficient in the AE3 Cl-/HCO3- exchanger

Background

Cardiac hypertrophy is central to the etiology of heart failure. Understanding the molecular pathways promoting cardiac hypertrophy may identify new targets for therapeutic intervention. Sodium-proton exchanger (NHE1) activity and expression levels in the heart are elevated in many models of hypertrophy through protein kinase C (PKC)/MAPK/ERK/p90^RSK pathway stimulation. Sustained NHE1 activity, however, requires an acid-loading pathway. Evidence suggests that the Cl⁻/HCO₃⁻ exchanger, AE3, provides this acid load. Here we explored the role of AE3 in the hypertrophic growth cascade of cardiomyocytes.

Methods

AE3-deficient (ae3^−/−) mice were compared to wildtype (WT) littermates to examine the role of AE3 protein in the development of cardiomyocyte hypertrophy. Mouse hearts were assessed by echocardiography. As well, responses of cultured cardiomyocytes to hypertrophic stimuli were measured. pH regulation capacity of ae3^−/− and WT cardiomyocytes was assessed in cultured cells loaded with the pH-sensitive dye, BCECF-AM.

Results

ae3^−/− mice were indistinguishable from wild type (WT) mice in terms of cardiovascular performance. Stimulation of ae3^−/− cardiomyocytes with hypertrophic agonists did not increase cardiac growth or reactivate the fetal gene program. ae3^−/− mice are thus protected from pro-hypertrophic stimulation. Steady state intracellular pH (pH_i) in ae3^−/− cardiomyocytes was not significantly different from WT, but the rate of recovery of pH_i from imposed alkalosis was significantly slower in ae3^−/− cardiomyocytes.

Conclusions

These data reveal the importance of AE3-mediated Cl⁻/HCO₃⁻ exchange in cardiovascular pH regulation and the development of cardiomyocyte hypertrophy. Pharmacological antagonism of AE3 is an attractive approach in the treatment of cardiac hypertrophy.

The chemistry, physiology and pathology of pH in cancer

Cell survival is conditional on the maintenance of a favourable acid-base balance (pH). Owing to intensive respiratory CO2 and lactic acid production, cancer cells are exposed continuously to large acid-base fluxes, which would disturb pH if uncorrected. The large cellular reservoir of H(+)-binding sites can buffer pH changes but, on its own, is inadequate to regulate intracellular pH. To stabilize intracellular pH at a favourable level, cells control trans-membrane traffic of H(+)-ions (or their chemical equivalents, e.g. ) using specialized transporter proteins sensitive to pH. In poorly perfused tumours, additional diffusion-reaction mechanisms, involving carbonic anhydrase (CA) enzymes, fine-tune control extracellular pH. The ability of H(+)-ions to change the ionization state of proteins underlies the exquisite pH sensitivity of cellular behaviour, including key processes in cancer formation and metastasis (proliferation, cell cycle, transformation, migration). Elevated metabolism, weakened cell-to-capillary diffusive coupling, and adaptations involving H(+)/H(+)-equivalent transporters and extracellular-facing CAs give cancer cells the means to manipulate micro-environmental acidity, a cancer hallmark. Through genetic instability, the cellular apparatus for regulating and sensing pH is able to adapt to extracellular acidity, driving disease progression. The therapeutic potential of disturbing this sequence by targeting H(+)/H(+)-equivalent transporters, buffering or CAs is being investigated, using monoclonal antibodies and small-molecule inhibitors.

The SLC26 gene family of anion transporters and channels

The phylogenetically ancient SLC26 gene family encodes multifunctional anion exchangers and anion channels transporting a broad range of substrates, including Cl(-), HCO3(-), sulfate, oxalate, I(-), and formate. SLC26 polypeptides are characterized by N-terminal cytoplasmic domains, 10-14 hydrophobic transmembrane spans, and C-terminal cytoplasmic STAS domains, and appear to be homo-oligomeric. SLC26-related SulP proteins of marine bacteria likely transport HCO3(-) as part of oceanic carbon fixation. SulP genes present in antibiotic operons may provide sulfate for antibiotic biosynthetic pathways. SLC26-related Sultr proteins transport sulfate in unicellular eukaryotes and in plants. Mutations in three human SLC26 genes are associated with congenital or early onset Mendelian diseases: chondrodysplasias for SLC26A2, chloride diarrhea for SLC26A3, and deafness with enlargement of the vestibular aqueduct for SLC26A4. Additional disease phenotypes evident only in mouse knockout models include oxalate urolithiasis for Slc26a6 and Slc26a1, non-syndromic deafness for Slc26a5, gastric hypochlorhydria for Slc26a7 and Slc26a9, distal renal tubular acidosis for Slc26a7, and male infertility for Slc26a8. STAS domains are required for cell surface expression of SLC26 proteins, and contribute to regulation of the cystic fibrosis transmembrane regulator in complex, cell- and tissue-specific ways. The protein interactomes of SLC26 polypeptides are under active investigation.

Human-induced pluripotent stem cell-derived cardiomyocytes for studies of cardiac ion transporters

Human-induced pluripotent stem cells (hiPSCs) can differentiate into functional cardiomyocytes (iCell Cardiomyocytes) with ion channel activities that are remarkably similar to adult cardiomyocytes. Here, we extend this characterization to cardiac ion transporters. Additionally, we document facile molecular biological manipulation of iCell Cardiomyocytes to overexpress and knockdown transporters and regulatory proteins. Na/Ca exchange (NCX1) and Na/K pump currents were recorded via patch clamp, and Na/H and Cl/OH exchanges were recorded via oscillating proton-selective microelectrodes during patch clamp. Flux densities of all transport systems are similar to those of nonrodent adult cardiomyocytes. NCX1 protein and NCX1 currents decline after NCX1 small interfering (si)RNA transfection with similar time courses (τ ≈ 2 days), and an NCX1-Halo fusion protein is internalized after its extracellular labeling by AlexaFluor488 Ligand with a similar time course. Loss of the cardiac regulatory protein phospholemman (PLM) occurs over a longer time course (τ ≈ 60 h) after PLM small interfering RNA transfection. Similar to multiple previous reports for adult cardiomyocytes, Na/K pump currents in iCell Cardiomyocytes are not enhanced by activating cAMP production with either maximal or submaximal cytoplasmic Na and using either forskolin or isoproterenol to activate adenylate cyclases. Finally, we describe Ca influx-dependent changes of iCell Cardiomyocyte capacitance (Cm). Large increases of Cm occur during Ca influx via NCX1, thereby documenting large internal membrane reserves that can fuse to the sarcolemma, and subsequent declines of Cm document active endocytic processes. Together, these results document a great potential of iCell Cardiomyocytes for both short- and long-term studies of cardiac ion transporters and their regulation.

Regulation of ion gradients across myocardial ischemic border zones: a biophysical modelling analysis

The myocardial ischemic border zone is associated with the initiation and sustenance of arrhythmias. The profile of ionic concentrations across the border zone play a significant role in determining cellular electrophysiology and conductivity, yet their spatial-temporal evolution and regulation are not well understood. To investigate the changes in ion concentrations that regulate cellular electrophysiology, a mathematical model of ion movement in the intra and extracellular space in the presence of ionic, potential and material property heterogeneities was developed. The model simulates the spatial and temporal evolution of concentrations of potassium, sodium, chloride, calcium, hydrogen and bicarbonate ions and carbon dioxide across an ischemic border zone. Ischemia was simulated by sodium-potassium pump inhibition, potassium channel activation and respiratory and metabolic acidosis. The model predicted significant disparities in the width of the border zone for each ionic species, with intracellular sodium and extracellular potassium having discordant gradients, facilitating multiple gradients in cellular properties across the border zone. Extracellular potassium was found to have the largest border zone and this was attributed to the voltage dependence of the potassium channels. The model also predicted the efflux of [Formula: see text] from the ischemic region due to electrogenic drift and diffusion within the intra and extracellular space, respectively, which contributed to [Formula: see text] depletion in the ischemic region.

Effect of nitric oxide donors S-nitroso-N-acetyl-DL-penicillamine, spermine NONOate and propylamine propylamine NONOate on intracellular pH in cardiomyocytes

1. Previous studies suggest that exogenous nitric oxide (NO) and NO-dependent signalling pathways modulate intracellular pH (pH(i)) in different cell types, but the role of NO in pH(i) regulation in the heart is poorly understood. Therefore, in the present study we investigated the effect of the NO donors S-nitroso-N-acetyl-DL-penicillamine, spermine NONOate and propylamine propylamine NONOate on pH(i) in rat isolated ventricular myocytes. 2. Cells were isolated from the hearts of adult Wistar rats and pH(i) was monitored using the pH-sensitive fluorescent indicator 5-(and-6)-carboxy seminaphtharhodafluor (SNARF)-1 (10 μmol/L) and a confocal microscope. To test the effect of NO donors on the Na⁺/H⁺ exchanger (NHE), basal pH(i) in Na⁺-free buffer and pH(i) recovery from intracellular acidosis after an ammonium chloride (10 mmol/L) prepulse were monitored. The role of carbonic anhydrase was tested using acetazolamide (50 μmol/L). 4,4-Diisothiocyanatostilbene-2,2'-disulphonic acid (0.5 mmol/L; DIDS) was used to inhibit the Cl⁻/OH⁻ and Cl⁻/HCO₃-exchangers. Acetazolamide and DIDS were applied via the superfusion system 1 and 5 min before the NO donors. 3. All three NO donors acutely decreased pH(i) and this effect persisted until the NO donor was removed. In Na⁺-free buffer, the decrease in basal pH(i) was increased, whereas inhibition of carbonic anhydrase and Cl⁻/OH⁻ and Cl⁻/HCO₃⁻ exchangers did not alter the effects of the NO donors on pH(i). After an ammonium preload, pH(i) recovery was accelerated in the presence of the NO donors. 4. In conclusion, exogenous NO decreases basal pH(i), leading to increased NHE activity. Carbonic anhydrase and chloride-dependent sarcolemmal HCO₃⁻ and OH⁻ transporters are not involved in the NO-induced decrease in pH(i) in rat isolated ventricular myocytes.

Carbonic anhydrase XIV in the normal and hypertrophic myocardium

Two AE3 transcripts, full-length (AE3fl) and cardiac (AE3c) are expressed in the heart. AE3 catalyzes electroneutral Cl(-)/HCO(3)(-) exchange across cardiomyocyte sarcolemma. AE proteins associate with carbonic anhydrases (CA), including CAII and CAIV, forming a HCO(3)(-) transport metabolon (BTM), increasing HCO(3)(-) fluxes and regulating cardiomyocytes pH. CAXIV, which is also expressed in the heart's sarcolemma, is a transmembrane enzyme with an extracellular catalytic domain. Herein, AE3/CAXIV physical association was examined by coimmunoprecipitation using rodent heart lysates. CAXIV immunoprecipitated with anti-AE3 antibody and both AE3fl and AE3c were reciprocally immunoprecipitated using anti-CAXIV antibody, indicating AE3fl-AE3c/CAXIV interaction in the myocardium. Coimmunoprecipitation experiments on heart lysates from a mouse with targeted disruption of the ae3 gene, failed to pull down AE3 with the CAXIV antibody. Confocal images demonstrated colocalization of CAXIV and AE3 in mouse ventricular myocytes. Functional association of AE3fl and CAXIV was examined in isolated hypertrophic rat cardiomyocytes, using fluorescence measurements of BCECF to monitor cytosolic pH. Hypertrophic cardiomyocytes of spontaneously hypertensive rats (SHR) presented elevated myocardial AE-mediated Cl(-)/HCO(3)(-) exchange activity (J(HCO3-) mM.min(-1)) compared to normal (Wistar) rats (7.5±1.3, n=4 versus 2.9±0.1, n=6, respectively). AE3fl, AE3c, CAII, CAIV, and CAIX protein expressions were similar in SHR and Wistar rat hearts. However, immunoblots revealed a twofold increase of CAXIV protein expression in the SHR myocardium compared to normal hearts (n=11). Furthermore, the CA-inhibitor, benzolamide, neutralized the stimulatory effect of extracellular CA on AE3 transport activity (3.7±1.5, n=3), normalizing AE3-dependent HCO(3)(-) fluxes in SHR. CAXIV/AE3 interaction constitutes an extracellular component of a BTM which potentiates AE3-mediated HCO(3)(-) transport in the heart. Increased CAXIV expression and consequent AE3/CAXIV complex formation would render AE3 hyperactive in the SHR heart.

Loss of the AE3 anion exchanger in a hypertrophic cardiomyopathy model causes rapid decompensation and heart failure

The AE3 Cl(-)/HCO(3)(-) exchanger is abundantly expressed in the sarcolemma of cardiomyocytes, where it mediates Cl(-)-uptake and HCO(3)(-)-extrusion. Inhibition of AE3-mediated Cl(-)/HCO(3)(-) exchange has been suggested to protect against cardiac hypertrophy; however, other studies indicate that AE3 might be necessary for optimal cardiac function. To test these hypotheses we crossed AE3-null mice, which appear phenotypically normal, with a hypertrophic cardiomyopathy mouse model carrying a Glu180Gly mutation in α-tropomyosin (TM180). Loss of AE3 had no effect on hypertrophy; however, survival of TM180/AE3 double mutants was sharply reduced compared with TM180 single mutants. Analysis of cardiac performance revealed impaired cardiac function in TM180 and TM180/AE3 mutants. TM180/AE3 double mutants were more severely affected and exhibited little response to β-adrenergic stimulation, a likely consequence of their more rapid progression to heart failure. Increased expression of calmodulin-dependent kinase II and protein phosphatase 1 and differences in methylation and localization of protein phosphatase 2A were observed, but were similar in single and double mutants. Phosphorylation of phospholamban on Ser16 was sharply increased in both single and double mutants relative to wild-type hearts under basal conditions, leading to reduced reserve capacity for β-adrenergic stimulation of phospholamban phosphorylation. Imaging analysis of isolated myocytes revealed reductions in amplitude and decay of Ca(2+) transients in both mutants, with greater reductions in TM180/AE3 mutants, consistent with the greater severity of their heart failure phenotype. Thus, in the TM180 cardiomyopathy model, loss of AE3 had no apparent anti-hypertrophic effect and led to more rapid decompensation and heart failure.

New insights into the physiological role of carbonic anhydrase IX in tumour pH regulation

In this review, we discuss the role of the tumour-associated carbonic anhydrase isoform IX (CAIX) in the context of pH regulation. We summarise recent experimental findings on the effect of CAIX on cell growth and survival, and present a diffusion-reaction model to help in the assessment of CAIX function under physiological conditions. CAIX emerges as an important facilitator of acid diffusion and acid transport, helping to overcome large cell-to-capillary distances that are characteristic of solid tumours. The source of substrate for CAIX catalysis is likely to be CO₂, generated by adequately oxygenated mitochondria or from the titration of metabolic acids with HCO₃⁻ taken up from the extracellular milieu. The relative importance of these pathways will depend on oxygen and metabolite availability, the spatiotemporal patterns of the cell's exposure to hypoxia and on the regulation of metabolism by genes. This is now an important avenue for further investigation. The importance of CAIX in regulating tumour pH highlights the protein as a potential target for cancer therapy.

A model of Na+/H+ exchanger and its central role in regulation of pH and Na+ in cardiac myocytes

A new kinetic model of the Na(+)/H(+) exchanger (NHE) was developed by fitting a variety of major experimental findings, such as ion-dependencies, forward/reverse mode, and the turnover rate. The role of NHE in ion homeostasis was examined by implementing the NHE model in a minimum cell model including intracellular pH buffer, Na(+)/K(+) pump, background H(+), and Na(+) fluxes. This minimum cell model was validated by reconstructing recovery of pH(i) from acidification, accompanying transient increase in [Na(+)](i) due to NHE activity. Based on this cell model, steady-state relationships among pH(i), [Na(+)](I), and [Ca(2+)](i) were quantitatively determined, and thereby the critical level of acidosis for cell survival was predicted. The acidification reported during partial blockade of the Na(+)/K(+) pump was not attributed to a dissipation of the Na(+) gradient across the membrane, but to an increase in indirect H(+) production. This NHE model, though not adapted to the dimeric behavioral aspects of NHE, can provide a strong clue to quantitative prediction of degree of acidification and accompanying disturbance of ion homeostasis under various pathophysiological conditions.

Ion transport in the intestine

PURPOSE OF REVIEW

In recent years, the field of intestinal physiology has witnessed significant progress in our understanding of the expression and function of ion transport proteins and their genes under physiological and pathophysiological conditions. This review will present some of these most recent advances in the small intestinal ion transport mechanisms.

RECENT FINDINGS

One of the new and exciting aspects of this field has been the integration of function and structure of several intestinal transport processes. This is well exemplified by the discussed intricacies of intestinal bicarbonate secretion as well as the role of scaffolding PDZ proteins interacting with several transporters. We also discuss some of the most recent data pointing to the role of ion transporters in the pathogenesis of inflammation-associated diarrhea and their potential role in the maintenance of epithelial integrity.

SUMMARY

Mouse models deficient in some of the key genes encoding ion transporters and their adapter proteins continue to provide important clues into intestinal transport processes. Several of the new in-vivo findings revise or complement past paradigms, many of which were derived from in-vitro approaches. New data on the interdependent functions of multiple transporters, as exemplified here by intestinal bicarbonate secretion, increase the complexity of the intestinal ion transport mechanisms and continue to contribute to a more integrated view of the transport phenomena in the gut. Data from patients and mouse models of intestinal inflammation also increase our understanding of the pathophysiology of inflammation-associated diarrhea.

A meta-analysis of cardiac electrophysiology computational models

Computational models of cardiac electrophysiology are exemplar demonstrations of the integration of multiple data sets into a consistent biophysical framework. These models encapsulate physiological understanding to provide quantitative predictions of function. The combination or extension of existing models within a common framework allows integrative phenomena in larger systems to be investigated. This methodology is now routinely applied, as demonstrated by the increasing number of studies which use or extend previously developed models. In this study, we present a meta-analysis of this model re-use for two leading models of cardiac electrophysiology in the form of parameter inheritance trees, a sensitivity analysis and a comparison of the functional significance of the sodium potassium pump for defining restitution curves. These results indicate that even though the models aim to represent the same physiological system, both the sources of parameter values and the function of equivalent components are significantly different.

Putative re-entrant loop 1 of AE2 transmembrane domain has a major role in acute regulation of anion exchange by pH

Normal pH sensitivity of the SLC4A2/AE2 anion exchanger requires transmembrane domain (TMD) amino acid (aa) residues not conserved in the homologous but relatively pH-insensitive SLC4A1/AE1 polypeptide. We tested the hypothesis that the nonconserved aa cluster 1075DKPK1078 within the first putative re-entrant loop (RL1) of AE2 TMD contributes to pH sensor function by studying anion exchange function of AE2 mutants in which these and other RL1 aa were systematically substituted with corresponding RL1 aa from AE1. Regulation of Cl-/Cl- and Cl-/HCO(-)3 exchange by intracellular pH (pHi) or extracellular pH (pHo) was measured as 4,4'-di-isothiocyanatostilbene-2,2' disulfonic acid-sensitive 36Cl- efflux from Xenopus oocytes. AE2 RL1 mutants 1075AAAQ1078 and 1075AAAQN1079 showed reduced pHi sensitivity and pHo sensitivity was acid-shifted by approximately 1 pH unit. Individual mutants D1075A and P1077A exhibited moderately altered pH sensitivity, whereas a range of substitutions at conserved AE2 Ile-1079 substantially altered sensitivity to pHo and/or pHi. Substitution of the complete AE1 RL1 with AE2 RL1 failed to confer AE2-like pH sensitivity onto AE1. Replacement, however, of AE1 RL1 763SGPGAAAQ770 with AE2 1071VAPGDKPK1078 restored pHi sensitivity to the chimera AE2(1-920)/AE1(613-929) without affecting its low sensitivity to pHo. The results show that acute regulation of AE2 by pH requires RL1 of the TMD. We propose that critical segments of RL1 constitute part of an AE2 pH sensor that, together with residues within the N-terminal half of the TMD, constrain the AE2 polypeptide in a conformation required for regulation of anion exchange by pHi.

Intracellular pH regulation in heart

Intracellular pH (pHi) is an important modulator of cardiac excitation and contraction, and a potent trigger of electrical arrhythmia. This review outlines the intracellular and membrane mechanisms that control pHi in the cardiac myocyte. We consider the kinetic regulation of sarcolemmal H+, OH- and HCO3- transporters by pH, and by receptor-coupled intracellular signalling systems. We also consider how activity of these pHi effector proteins is coordinated spatially in the myocardium by intracellular mobile buffer shuttles, gap junctional channels and carbonic anhydrase enzymes. Finally, we review the impact of pHi regulatory proteins on intracellular Ca2+ signalling, and their participation in clinical disorders such as myocardial ischaemia, maladaptive hypertrophy and heart failure. Such multiple effects emphasise the fundamental role that pHi regulation plays in the heart.