Out-of-equilibrium pH transients in the guinea-pig ventricular myocyte

Intracellular pH regulation and the acid delusion

The hydrogen ion concentration ([H+]) in intracellular cytoplasmic fluid (ICF) must be maintained in a narrow range in all species for normal protein functions. Thus, mechanisms regulating ICF are of fundamental biological importance. Studies on the regulation of ICF [H+] have been hampered by use of pH notation, failure to consider the roles played by differences in the concentration of strong ions (strong ion difference, SID), the conservation of mass, the principle of electrical neutrality, and that [H+] and bicarbonate ions [HCO3-] are dependent variables. This argument is based on the late Peter Stewart's physical-chemical analysis of [H+] regulation reported in this journal nearly forty years ago (Stewart. 1983. Can. J. Physiol. Pharmacol. 61: 1444-1461. Doi:10.1139/y83-207). We start by outlining the principles of Stewart's analysis and then provide a general understanding of its significance for regulation of ICF [H+]. The system may initially appear complex, but it becomes evident that changes in SID dominate regulation of [H+]. The primary strong ions are Na+, K+, and Cl-, and a few organic strong anions. The second independent variable, partial pressure of carbon dioxide (PCO2), can easily be assessed. The third independent variable, the activity of intracellular weak acids ([Atot]), is much more complex but largely plays a modifying role. Attention to these principles will potentially provide new insights into ICF pH regulation.

The control of acidity in tumor cells: a biophysical model

Acidosis of the tumor microenvironment leads to cancer invasion, progression and resistance to therapies. We present a biophysical model that describes how tumor cells regulate intracellular and extracellular acidity while they grow in a microenvironment characterized by increasing acidity and hypoxia. The model takes into account the dynamic interplay between glucose and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {O}_2$$\end{document}O2 consumption with lactate and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {CO}_2$$\end{document}CO2 production and connects these processes to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {H}^+$$\end{document}H+ and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HCO}_3^-$$\end{document}HCO3- fluxes inside and outside cells. We have validated the model with independent experimental data and used it to investigate how and to which extent tumor cells can survive in adverse micro-environments characterized by acidity and hypoxia. The simulations show a dominance of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {H}^+$$\end{document}H+ exchanges in well-oxygenated regions, and of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HCO}_3^-$$\end{document}HCO3- exchanges in the inner hypoxic regions where tumor cells are known to acquire malignant phenotypes. The model also includes the activity of the enzyme Carbonic Anhydrase 9 (CA9), a known marker of tumor aggressiveness, and the simulations demonstrate that CA9 acts as a nonlinear \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {pH}_i$$\end{document}pHi equalizer at any \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {O}_2$$\end{document}O2 level in cells that grow in acidic extracellular environments.

Using bidirectional chemical exchange for improved hyperpolarized [13 C]bicarbonate pH imaging

PURPOSE

Rapid chemical exchange can affect SNR and pH measurement accuracy for hyperpolarized pH imaging with [¹³ C]bicarbonate. The purpose of this work was to investigate chemical exchange effects on hyperpolarized imaging sequences to identify optimal sequence parameters for high SNR and pH accuracy.

METHODS

Simulations were performed under varying rates of bicarbonate-CO₂ chemical exchange to analyze exchange effects on pH quantification accuracy and SNR under different sampling schemes. Four pulse sequences, including 1 new technique, a multiple-excitation 2D EPI (multi-EPI) sequence, were compared in phantoms using hyperpolarized [¹³ C]bicarbonate, varying parameters such as tip angles, repetition time, order of metabolite excitation, and refocusing pulse design. In vivo hyperpolarized bicarbonate-CO₂ exchange measurements were made in transgenic murine prostate tumors to select in vivo imaging parameters.

RESULTS

Modeling of bicarbonate-CO₂ exchange identified a multiple-excitation scheme for increasing CO₂ SNR by up to a factor of 2.7. When implemented in phantom imaging experiments, these sampling schemes were confirmed to yield high pH accuracy and SNR gains. Based on measured bicarbonate-CO₂ exchange in vivo, a 47% CO₂ SNR gain is predicted.

CONCLUSION

The novel multi-EPI pulse sequence can boost CO₂ imaging signal in hyperpolarized ¹³ C bicarbonate imaging while introducing minimal pH bias, helping to surmount a major hurdle in hyperpolarized pH imaging.

Evidence-based guidelines for controlling pH in mammalian live-cell culture systems

A fundamental variable in culture medium is its pH, which must be controlled by an appropriately formulated buffering regime, since biological processes are exquisitely sensitive to acid-base chemistry. Although awareness of the importance of pH is fostered early in the training of researchers, there are no consensus guidelines for best practice in managing pH in cell cultures, and reporting standards relating to pH are typically inadequate. Furthermore, many laboratories adopt bespoke approaches to controlling pH, some of which inadvertently produce artefacts that increase noise, compromise reproducibility or lead to the misinterpretation of data. Here, we use real-time measurements of medium pH and intracellular pH under live-cell culture conditions to describe the effects of various buffering regimes, including physiological CO2/HCO3- and non-volatile buffers (e.g. HEPES). We highlight those cases that result in poor control, non-intuitive outcomes and erroneous inferences. To improve data reproducibility, we propose guidelines for controlling pH in culture systems.

Recovery from acidosis is a robust trigger for loss of force in murine hypokalemic periodic paralysis

Hypokalemic periodic paralysis causes episodes of muscle weakness. Mi et al. investigate the rest-induced weakness that occurs after vigorous exercise and find that acidosis, as occurs with exercise, leads to accumulation of myoplasmic Cl⁻, which favors a depolarized resting potential when pH returns to normal.

Periodic paralysis is an ion channelopathy of skeletal muscle in which recurrent episodes of weakness or paralysis are caused by sustained depolarization of the resting potential and thus reduction of fiber excitability. Episodes are often triggered by environmental stresses, such as changes in extracellular K⁺, cooling, or exercise. Rest after vigorous exercise is the most common trigger for weakness in periodic paralysis, but the mechanism is unknown. Here, we use knock-in mutant mouse models of hypokalemic periodic paralysis (HypoKPP; Na_V1.4-R669H or Ca_V1.1-R528H) and hyperkalemic periodic paralysis (HyperKPP; Na_V1.4-M1592V) to investigate whether the coupling between pH and susceptibility to loss of muscle force is a possible contributor to exercise-induced weakness. In both mouse models, acidosis (pH 6.7 in 25% CO₂) is mildly protective, but a return to pH 7.4 (5% CO₂) unexpectedly elicits a robust loss of force in HypoKPP but not HyperKPP muscle. Prolonged exposure to low pH (tens of minutes) is required to cause susceptibility to post-acidosis loss of force, and the force decrement can be prevented by maneuvers that impede Cl⁻ entry. Based on these data, we propose a mechanism for post-acidosis loss of force wherein the reduced Cl⁻ conductance in acidosis leads to a slow accumulation of myoplasmic Cl⁻. A rapid recovery of both pH and Cl⁻ conductance, in the context of increased [Cl]_in/[Cl]_out, favors the anomalously depolarized state of the bistable resting potential in HypoKPP muscle, which reduces fiber excitability. This mechanism is consistent with the delayed onset of exercise-induced weakness that occurs with rest after vigorous activity.

Carbonic anhydrase inhibitors modify intracellular pH transients and contractions of rat middle cerebral arteries during CO2/HCO3- fluctuations

The CO₂/HCO₃^- buffer minimizes pH changes in response to acid-base loads, HCO₃^- provides substrate for Na⁺,HCO₃^--cotransporters and Cl^-/HCO₃^--exchangers, and H⁺ and HCO₃^- modify vasomotor responses during acid-base disturbances. We show here that rat middle cerebral arteries express cytosolic, mitochondrial, extracellular, and secreted carbonic anhydrase isoforms that catalyze equilibration of the CO₂/HCO₃^- buffer. Switching from CO₂/HCO₃^--free to CO₂/HCO₃^--containing extracellular solution results in initial intracellular acidification due to hydration of CO₂ followed by gradual alkalinization due to cellular HCO₃^- uptake. Carbonic anhydrase inhibition decelerates the initial acidification and attenuates the associated transient vasoconstriction without affecting intracellular pH or artery tone at steady-state. Na⁺,HCO₃^--cotransport and Na⁺/H⁺-exchange activity after NH₄⁺-prepulse-induced intracellular acidification are unaffected by carbonic anhydrase inhibition. Extracellular surface pH transients induced by transmembrane NH₃ flux are evident under CO₂/HCO₃^--free conditions but absent when the buffer capacity and apparent H⁺ mobility increase in the presence of CO₂/HCO₃^- even after the inhibition of carbonic anhydrases. We conclude that (a) intracellular carbonic anhydrase activity accentuates pH transients and vasoconstriction in response to acute elevations of pCO₂, (b) CO₂/HCO₃^- minimizes extracellular surface pH transients without requiring carbonic anhydrase activity, and (c) carbonic anhydrases are not rate limiting for acid–base transport across cell membranes during recovery from intracellular acidification.

pH buffering of single rat skeletal muscle fibers in the in vivo environment

Homeostasis of intracellular pH (pHi) has a crucial role for the maintenance of cellular function. Several membrane transporters such as lactate/H(+) cotransporter (MCT), Na(+)/H(+) exchange transporter (NHE), and Na(+)/HCO3 (-) cotransporter (NBC) are thought to contribute to pHi regulation. However, the relative importance of each of these membrane transporters to the in vivo recovery from the low pHi condition is unknown. Using an in vivo bioimaging model, we pharmacologically inhibited each transporter separately and all transporters together and then evaluated the pHi recovery profiles following imposition of a discrete H(+) challenge loaded into single muscle fibers by microinjection. The intact spinotrapezius muscle of adult male Wistar rats (n = 72) was exteriorized and loaded with the fluorescent probe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-acetoxymethyl ester (10 μM). A single muscle fiber was then loaded with low-pH solution [piperazine-N,N'-bis(2-ethanesulfonic acid) buffer, pH 6.5, ∼2.33 × 10(-3) μl] by microinjection over 3 s. The rats were divided into groups for the following treatments: 1) no inhibitor (CONT), 2) MCT inhibition (by α-Cyano-4-hydroxyciannamic acid; 4 mM), 3) NHE inhibition (by ethylisopropyl amiloride; 0.5 mM), 4) NBC inhibition (by DIDS; 1 mM), and 5) MCT, NHE, and NBC inhibition (All blockade). The fluorescence ratio (F500 nm/F445 nm) was determined from images captured during 1 min (60 images/min) and at 5, 10, 15, and 20 min after injection. The pHi at 1-2 s after injection significantly decreased from resting pHi (ΔpHi = -0.73 ± 0.03) in CONT. The recovery response profile was biphasic, with an initial rapid and close-to-exponential pHi increase (time constant, τ: 60.0 ± 7.9 s). This initial rapid profile was not affected by any pharmacological blockade but was significantly delayed by carbonic anhydrase inhibition. In contrast, the secondary, more gradual, return toward baseline that restored CONT pHi to 84.2% of baseline was unimpeded by MCT, NHE, and NBC blockade separately but abolished by All blockade (ΔpHi = -0.60 ± 0.07, 72.8% initial pHi, P < 0.05 vs. CONT). After injection of H(+) into, or superfusion onto, an adjacent fiber pHi of the surrounding fibers decreased progressively for the 20-min observation period (∼7.0, P < 0.05 vs. preinjection/superfusion). In conclusion, these results support that, after an imposed H(+) load, the MCT, NHE, and NBC transporters are not involved in the initial rapid phase of pHi recovery. In contrast, the gradual recovery phase was abolished by inhibiting all three membrane transporter systems simultaneously. The alteration of pHi in surrounding fibers suggest that H(+) uptake by neighboring fibers can help alleviate the pH consequences of myocyte H(+) exudation.

Nuclear proton dynamics and interactions with calcium signaling

Biochemical signals acting on the nucleus can regulate gene expression. Despite the inherent affinity of nucleic acids and nuclear proteins (e.g. transcription factors) for protons, little is known about the mechanisms that regulate nuclear pH (pH_nuc), and how these could be exploited to control gene expression. Here, we show that pH_nuc dynamics can be imaged using the DNA-binding dye Hoechst 33342. Nuclear pores allow the passage of medium-sized molecules (calcein), but protons must first bind to mobile buffers in order to gain access to the nucleoplasm. Fixed buffering residing in the nucleus of permeabilized cells was estimated to be very weak on the basis of the large amplitude of pH_nuc transients evoked by photolytic H⁺-uncaging or exposure to weak acids/bases. Consequently, the majority of nuclear pH buffering is sourced from the cytoplasm in the form of mobile buffers. Effective proton diffusion was faster in nucleoplasm than in cytoplasm, in agreement with the higher mobile-to-fixed buffering ratio in the nucleus. Cardiac myocyte pH_nuc changed in response to maneuvers that alter nuclear Ca^2 + signals. Blocking Ca^2 + release from inositol-1,4,5-trisphosphate receptors stably alkalinized the nucleus. This Ca^2 +-pH interaction may arise from competitive binding to common chemical moieties. Competitive binding to mobile buffers may couple the efflux of Ca^2 +via nuclear pores with a counterflux of protons. This would generate a stable pH gradient between cytoplasm and nucleus that is sensitive to the state of nuclear Ca^2 + signaling. The unusual behavior of protons in the nucleus provides new mechanisms for regulating cardiac nuclear biology.

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Facilitated diffusion aboard mobile buffers is the only means by which protons enter the nucleus.

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The relative scarcity of fixed buffers residing in the nucleus accelerates proton diffusivity.

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Nuclear Ca^2 + signals can regulate nuclear pH and generate stable gradients relative to cytoplasm.

Intracellular carbonic anhydrase activity sensitizes cancer cell pH signaling to dynamic changes in CO2 partial pressure

Carbonic anhydrase (CA) enzymes catalyze the chemical equilibration among CO2, HCO3(-) and H(+). Intracellular CA (CAi) isoforms are present in certain types of cancer, and growing evidence suggests that low levels correlate with disease severity. However, their physiological role remains unclear. Cancer cell CAi activity, measured as cytoplasmic CO2 hydration rate (kf), ranged from high in colorectal HCT116 (∼2 s(-1)), bladder RT112 and colorectal HT29, moderate in fibrosarcoma HT1080 to negligible (i.e. spontaneous kf = 0.18 s(-1)) in cervical HeLa and breast MDA-MB-468 cells. CAi activity in cells correlated with CAII immunoreactivity and enzymatic activity in membrane-free lysates, suggesting that soluble CAII is an important intracellular isoform. CAi catalysis was not obligatory for supporting acid extrusion by H(+) efflux or HCO3(-) influx, nor for maintaining intracellular pH (pHi) uniformity. However, in the absence of CAi activity, acid loading from a highly alkaline pHi was rate-limited by HCO3(-) supply from spontaneous CO2 hydration. In solid tumors, time-dependence of blood flow can result in fluctuations of CO2 partial pressure (pCO2) that disturb cytoplasmic CO2-HCO3(-)-H(+) equilibrium. In cancer cells with high CAi activity, extracellular pCO2 fluctuations evoked faster and larger pHi oscillations. Functionally, these resulted in larger pH-dependent intracellular [Ca(2+)] oscillations and stronger inhibition of the mTORC1 pathway reported by S6 kinase phosphorylation. In contrast, the pHi of cells with low CAi activity was less responsive to pCO2 fluctuations. Such low pass filtering would "buffer" cancer cell pHi from non-steady-state extracellular pCO2. Thus, CAi activity determines the coupling between pCO2 (a function of tumor perfusion) and pHi (a potent modulator of cancer cell physiology).

Pumping Ca2+ up H+ gradients: a Ca2(+)-H+ exchanger without a membrane

Cellular processes are exquisitely sensitive to H+ and Ca2+ ions because of powerful ionic interactions with proteins. By regulating the spatial and temporal distribution of intracellular [Ca2+] and [H+], cells such as cardiac myocytes can exercise control over their biological function. A well-established paradigm in cellular physiology is that ion concentrations are regulated by specialized, membrane-embedded transporter proteins. Many of these couple the movement of two or more ionic species per transport cycle, thereby linking ion concentrations among neighbouring compartments. Here, we compare and contrast canonical membrane transport with a novel type of Ca(2+)-H+ coupling within cytoplasm, which produces uphill Ca2+ transport energized by spatial H+ ion gradients, and can result in the cytoplasmic compartmentalization of Ca2+ without requiring a partitioning membrane. The mechanism, demonstrated in mammalian myocytes, relies on diffusible cytoplasmic buffers, such as carnosine, homocarnosine and ATP, to which Ca2+ and H+ ions bind in an apparently competitive manner. These buffer molecules can actively recruit Ca2+ to acidic microdomains, in exchange for the movement of H+ ions. The resulting Ca2+ microdomains thus have the potential to regulate function locally. Spatial cytoplasmic Ca(2+)-H+ exchange (cCHX) acts like a 'pump' without a membrane and may be operational in many cell types.

Facilitation by intracellular carbonic anhydrase of Na+ -HCO3- co-transport but not Na+ / H+ exchange activity in the mammalian ventricular myocyte

Carbonic anhydrase enzymes (CAs) catalyse the reversible hydration of CO2 to H+ and HCO3- ions. This catalysis is proposed to be harnessed by acid/base transporters, to facilitate their transmembrane flux activity, either through direct protein-protein binding (a 'transport metabolon') or local functional interaction. Flux facilitation has previously been investigated by heterologous co-expression of relevant proteins in host cell lines/oocytes. Here, we examine the influence of intrinsic CA activity on membrane HCO3- or H+ transport via the native acid-extruding proteins, Na+ -HCO3- cotransport (NBC) and Na+ / H+ exchange (NHE), expressed in enzymically isolated mammalian ventricular myocytes. Effects of intracellular and extracellular (exofacial) CA (CAi and CAe) are distinguished using membrane-permeant and -impermeant pharmacological CA inhibitors, while measuring transporter activity in the intact cell using pH and Na+ fluorophores. We find that NBC, but not NHE flux is enhanced by catalytic CA activity, with facilitation being confined to CAi activity alone. Results are quantitatively consistent with a model where CAi catalyses local H+ ion delivery to the NBC protein, assisting the subsequent (uncatalysed) protonation and removal of imported HCO3- ions. In well-superfused myocytes, exofacial CA activity is superfluous, most likely because extracellular CO2/HCO3- buffer is clamped at equilibrium. The CAi insensitivity of NHE flux suggests that, in the native cell, intrinsic mobile buffer-shuttles supply sufficient intracellular H+ ions to this transporter, while intrinsic buffer access to NBC proteins is restricted. Our results demonstrate a selective CA facilitation of acid/base transporters in the ventricular myocyte, implying a specific role for the intracellular enzyme in HCO3- transport, and hence pHi regulation in the heart.

Increased sarcolemmal Na(+)/H(+) exchange activity in hypertrophied myocytes from dogs with chronic atrioventricular block

Dogs with compensated biventricular hypertrophy due to chronic atrioventricular block (cAVB), are more susceptible to develop drug-induced Torsade-de-Pointes arrhythmias and sudden cardiac death. It has been suggested that the increased Na(+) influx in hypertrophied cAVB ventricular myocytes contribute to these lethal arrhythmias. The increased Na(+) influx was not mediated by Na(+) channels, in fact the Na(+) current proved reduced in cAVB myocytes. Here we tested the hypothesis that increased activity of the Na(+)/H(+) exchanger type 1 (NHE-1), commonly observed in hypertrophic hearts, causes the elevated Na(+) influx. Cardiac acid-base transport was studied with a pH-sensitive fluorescent dye in ventricular myocytes isolated from control and hypertrophied cAVB hearts; the H(+) equivalent flux through NHE-1, Na(+)-HCO(-) 3 cotransport (NBC), Cl(-)/OH(-) exchange (CHE), and Cl(-)/HCO(-) 3 exchange (AE) were determined and normalized per liter cell water and corrected for surface-to-volume ratio. In cAVB, sarcolemmal NHE-1 flux was increased by 65 ± 6.3% in the pH i interval 6.3-7.2 and NBC, AE, and CHE fluxes remained unchanged. Accordingly, at steady-state intracellular pH the total sarcolemmal Na(+) influx by NHE-1 + NBC increased from 8.5 ± 1.5 amol/μm(2)/min in normal myocytes to 15 ± 2.4 amol/μm(2)/min in hypertrophied cAVB myocytes. We conclude that compensated cardiac hypertrophy in cAVB dogs is accompanied with an increased sarcolemmal NHE-1 activity. This in conjunction with unchanged activity of the other acid-base transporters will raise the intracellular Na(+) in hypertrophied cAVB myocytes.

Genetically encoded pH-indicators reveal activity-dependent cytosolic acidification of Drosophila motor nerve termini in vivo

All biochemical processes, including those underlying synaptic function and plasticity, are pH sensitive. Cytosolic pH (pH(cyto)) shifts are known to accompany nerve activity in situ, but technological limitations have prevented characterization of such shifts in vivo. Genetically encoded pH-indicators (GEpHIs) allow for tissue-specific in vivo measurement of pH. We expressed three different GEpHIs in the cytosol of Drosophila larval motor neurons and observed substantial presynaptic acidification in nerve termini during nerve stimulation in situ. SuperEcliptic pHluorin was the most useful GEpHI for studying pH(cyto) shifts in this model system. We determined the resting pH of the nerve terminal cytosol to be 7.30 ± 0.02, and observed a decrease of 0.16 ± 0.01 pH units when the axon was stimulated at 40 Hz for 4 s. Realkalinization occurred upon cessation of stimulation with a time course of 20.54 ± 1.05 s (τ). The chemical pH-indicator 2,7-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein corroborated these changes in pH(cyto). Bicarbonate-derived buffering did not contribute to buffering of acid loads from short (≤ 4 s) trains of action potentials but did buffer slow (~60 s) acid loads. The magnitude of cytosolic acid transients correlated with cytosolic Ca(2+) increase upon stimulation, and partial inhibition of the plasma membrane Ca(2+)-ATPase, a Ca(2+)/H(+) exchanger, attenuated pH(cyto) shifts. Repeated stimulus trains mimicking motor patterns generated greater cytosolic acidification (~0.30 pH units). Imaging through the cuticle of intact larvae revealed spontaneous pH(cyto) shifts in presynaptic termini in vivo, similar to those seen in situ during fictive locomotion, indicating that presynaptic pH(cyto) shifts cannot be dismissed as artifacts of ex vivo preparations.

Extramitochondrial domain rich in carbonic anhydrase activity improves myocardial energetics

CO2 is produced abundantly by cardiac mitochondria. Thus an efficient means for its venting is required to support metabolism. Carbonic anhydrase (CA) enzymes, expressed at various sites in ventricular myocytes, may affect mitochondrial CO2 clearance by catalyzing CO2 hydration (to H(+) and HCO3(-)), thereby changing the gradient for CO2 venting. Using fluorescent dyes to measure changes in pH arising from the intracellular hydration of extracellularly supplied CO2, overall CA activity in the cytoplasm of isolated ventricular myocytes was found to be modest (2.7-fold above spontaneous kinetics). Experiments on ventricular mitochondria demonstrated negligible intramitochondrial CA activity. CA activity was also investigated in intact hearts by (13)C magnetic resonance spectroscopy from the rate of H(13)CO3(-) production from (13)CO2 released specifically from mitochondria by pyruvate dehydrogenase-mediated metabolism of hyperpolarized [1-(13)C]pyruvate. CA activity measured upon [1-(13)C]pyruvate infusion was fourfold higher than the cytoplasm-averaged value. A fluorescent CA ligand colocalized with a mitochondrial marker, indicating that mitochondria are near a CA-rich domain. Based on immunoreactivity, this domain comprises the nominally cytoplasmic CA isoform CAII and sarcoplasmic reticulum-associated CAXIV. Inhibition of extramitochondrial CA activity acidified the matrix (as determined by fluorescence measurements in permeabilized myocytes and isolated mitochondria), impaired cardiac energetics (indexed by the phosphocreatine-to-ATP ratio measured by (31)P magnetic resonance spectroscopy of perfused hearts), and reduced contractility (as measured from the pressure developed in perfused hearts). These data provide evidence for a functional domain of high CA activity around mitochondria to support CO2 venting, particularly during elevated and fluctuating respiratory activity. Aberrant distribution of CA activity therefore may reduce the heart's energetic efficiency.

Intracellular pH in the resistance vasculature: regulation and functional implications

Net acid extrusion from vascular smooth muscle (VSMCs) and endothelial cells (ECs) in the wall of resistance arteries is mediated by the Na(+),HCO(3)(-) cotransporter NBCn1 (SLC4A7) and the Na(+)/H(+) exchanger NHE1 (SLC9A1) and is essential for intracellular pH (pH(i)) control. Experimental evidence suggests that the pH(i) of VSMCs and ECs modulates both vasocontractile and vasodilatory functions in resistance arteries with implications for blood pressure regulation. The connection between disturbed pH(i) and altered cardiovascular function has been substantiated by a genome-wide association study showing a link between NBCn1 and human hypertension. On this basis, we here review the current evidence regarding (a) molecular mechanisms involved in pH(i) control in VSMCs and ECs of resistance arteries at rest and during contractions, (b) implications of disturbed pH(i) for resistance artery function, and (c) involvement of disturbed pH(i) in the pathogenesis of vascular disease. The current evidence clearly implies that pH(i) of VSMCs and ECs modulates vascular function and suggests that disturbed pH(i) either consequent to disturbed regulation or due to metabolic challenges needs to be taken into consideration as a mechanistic component of artery dysfunction and disturbed blood pressure regulation.

Imaging pH with hyperpolarized 13C

pH is a fundamental physiological parameter that is tightly controlled by endogenous buffers. The acid-base balance is altered in many disease states, such as inflammation, ischemia and cancer. Despite the importance of pH, there are currently no routine methods for imaging the spatial distribution of pH in humans. The enormous gain in sensitivity afforded by dynamic nuclear polarization (DNP) has provided a novel way in which to image tissue pH using MR, which has the potential to be translated into the clinic. This review explores the advantages and disadvantages of current pH imaging techniques and how they compare with DNP-based approaches for the measurement and imaging of pH with hyperpolarized (13)C. Intravenous injection of hyperpolarized (13)C-labeled bicarbonate results in the rapid production of hyperpolarized (13)CO(2) in the reaction catalyzed by carbonic anhydrase. As this reaction is close to equilibrium in the body and is pH dependent, the ratio of the (13)C signal intensities from H(13)CO(3)(-) and (13)CO(2), measured using MRS, can be used to calculate pH in vivo. The application of this technique to a murine tumor model demonstrated that it measured predominantly extracellular pH and could be mapped in the animal using spectroscopic imaging techniques. A second approach has been to use the production of hyperpolarized (13)CO(2) from hyperpolarized [1-(13)C]pyruvate to measure predominantly intracellular pH. In tissues with a high aerobic capacity, such as the heart, the hyperpolarized [1-(13)C]pyruvate undergoes rapid oxidative decarboxylation, catalyzed by intramitochondrial pyruvate dehydrogenase. Provided that there is sufficient carbonic anhydrase present to catalyze the rapid equilibration of the hyperpolarized (13)C label between CO(2) and bicarbonate, the ratio of their resonance intensities may again be used to estimate pH, which, in this case, is predominantly intracellular. As both pyruvate and bicarbonate are endogenous molecules they have the potential to image tissue pH in the clinic.

Proton-sensing Ca2+ binding domains regulate the cardiac Na+/Ca2+ exchanger

The cardiac Na(+)/Ca(2+) exchanger (NCX) regulates cellular [Ca(2+)](i) and plays a central role in health and disease, but its molecular regulation is poorly understood. Here we report on how protons affect this electrogenic transporter by modulating two critically important NCX C(2) regulatory domains, Ca(2+) binding domain-1 (CBD1) and CBD2. The NCX transport rate in intact cardiac ventricular myocytes was measured as a membrane current, I(NCX), whereas [H(+)](i) was varied using an ammonium chloride "rebound" method at constant extracellular pH 7.4. At pH(i) = 7.2 and [Ca(2+)](i) < 120 nM, I(NCX) was less than 4% that of its maximally Ca(2+)-activated value. I(NCX) increases steeply at [Ca(2+)](i) between 130-150 nM with a Hill coefficient (n(H)) of 8.0 ± 0.7 and K(0.5) = 310 ± 5 nM. At pH(i) = 6.87, the threshold of Ca(2+)-dependent activation of I(NCX) was shifted to much higher [Ca(2+)](i) (600-700 nM), and the relationship was similarly steep (n(H) = 8.0±0.8) with K(0.5) = 1042 ± 15 nM. The V(max) of Ca(2+)-dependent activation of I(NCX) was not significantly altered by low pH(i). The Ca(2+) affinities for CBD1 (0.39 ± 0.06 μM) and CBD2 (K(d) = 18.4 ± 6 μM) were exquisitely sensitive to [H(+)], decreasing 1.3-2.3-fold as pH(i) decreased from 7.2 to 6.9. This work reveals for the first time that NCX can be switched off by physiologically relevant intracellular acidification and that this depends on the competitive binding of protons to its C(2) regulatory domains CBD1 and CBD2.

Measuring intracellular pH in the heart using hyperpolarized carbon dioxide and bicarbonate: a 13C and 31P magnetic resonance spectroscopy study

Aims

Technological limitations have restricted in vivo assessment of intracellular pH (pH_i) in the myocardium. The aim of this study was to evaluate the potential of hyperpolarized [1-¹³C]pyruvate, coupled with ¹³C magnetic resonance spectroscopy (MRS), to measure pH_i in the healthy and diseased heart.

Methods and results

Hyperpolarized [1-¹³C]pyruvate was infused into isolated rat hearts before and immediately after ischaemia, and the formation of ¹³CO₂ and H¹³CO₃⁻ was monitored using ¹³C MRS. The HCO₃⁻/CO₂ ratio was used in the Henderson–Hasselbalch equation to estimate pH_i. We tested the validity of this approach by comparing ¹³C-based pH_i measurements with ³¹P MRS measurements of pH_i. There was good agreement between the pH_i measured using ¹³C and ³¹P MRS in control hearts, being 7.12 ± 0.10 and 7.07 ± 0.02, respectively. In reperfused hearts, ¹³C and ³¹P measurements of pH_i also agreed, although ¹³C equilibration limited observation of myocardial recovery from acidosis. In hearts pre-treated with the carbonic anhydrase (CA) inhibitor, 6-ethoxyzolamide, the ¹³C measurement underestimated the ³¹P-measured pH_i by 0.80 pH units. Mathematical modelling predicted that the validity of measuring pH_i from the H¹³CO₃⁻/¹³CO₂ ratio depended on CA activity, and may give an incorrect measure of pH_i under conditions in which CA was inhibited, such as in acidosis. Hyperpolarized [1-¹³C]pyruvate was also infused into healthy living rats, where in vivo pH_i from the H¹³CO₃⁻/¹³CO₂ ratio was measured to be 7.20 ± 0.03.

Conclusion

Metabolically generated ¹³CO₂ and H¹³CO₃⁻ can be used as a marker of cardiac pH_iin vivo, provided that CA activity is at normal levels.

Tumor-associated carbonic anhydrase 9 spatially coordinates intracellular pH in three-dimensional multicellular growths

CA9 is a membrane-tethered, carbonic anhydrase (CA) enzyme, expressed mainly at the external surface of cells, that catalyzes reversible CO(2) hydration. Expression is greatly enhanced in many tumors, particularly in aggressive carcinomas. The functional role of CA9 in tumors is not well established. Here we show that CA9, when expressed heterologously in cultured spheroids (0.5-mm diameter, ~25,000 cells) of RT112 cells (derived from bladder carcinoma), induces a near-uniform intracellular pH (pH(i)) throughout the structure. Dynamic pH(i) changes during displacements of superfusate CO(2) concentration are also spatially coincident (within 2 s). In contrast, spheroids of wild-type RT112 cells lacking CA9 exhibit an acidic core (~0.25 pH(i) reduction) and significant time delays (~9 s) for pH(i) changes in core versus peripheral regions. pH(i) non-uniformity also occurs in CA9-expressing spheroids after selective pharmacological inhibition of the enzyme. In isolated RT112 cells, pH(i) regulation is unaffected by CA9 expression. The influence of CA9 on pH(i) is thus only evident in multicellular tissue. Diffusion-reaction modeling indicates that CA9 coordinates pH(i) spatially by facilitating CO(2) diffusion in the unstirred extracellular space of the spheroid. We suggest that pH(i) coordination may favor survival and growth of a tumor. By disrupting spatial pH(i) control, inhibition of CA9 activity may offer a novel strategy for the clinical treatment of CA9-associated tumors.

S0859, an N-cyanosulphonamide inhibitor of sodium-bicarbonate cotransport in the heart

BACKGROUND AND PURPOSE

Intracellular pH (pH(i)) in heart is regulated by sarcolemmal H(+)-equivalent transporters such as Na(+)-H(+) exchange (NHE) and Na(+)-HCO(3) (-) cotransport (NBC). Inhibition of NBC influences pH(i) and can be cardioprotective in animal models of post-ischaemic reperfusion. Apart from a rabbit polyclonal NBC-antibody, a selective NBC inhibitor compound has not been studied. Compound S0859 (C(29)H(24)ClN(3)O(3)S) is a putative NBC inhibitor. Here, we provide the drug's chemical structure, test its potency and selectivity in ventricular cells and assess its suitability for experiments on cardiac contraction.

EXPERIMENTAL APPROACH

pH(i) recovery from intracellular acidosis was monitored using pH-epifluorescence (SNARF-fluorophore) in guinea pig, rat and rabbit isolated ventricular myocytes. Electrically evoked cell shortening (contraction) was measured optically. With CO(2)/HCO(3) (-)-buffered superfusates containing 30 muM cariporide (to inhibit NHE), pH(i) recovery is mediated by NBC.

KEY RESULTS

S0859, an N-cyanosulphonamide compound, reversibly inhibited NBC-mediated pH(i) recovery (K (i)=1.7 microM, full inhibition at approximately 30 microM). In HEPES-buffered superfusates, NHE-mediated pH(i) recovery was unaffected by 30 microM S0859. With CO(2)/HCO(3) (-) buffer, pH(i) recovery from intracellular alkalosis (mediated by Cl(-)/HCO(3) (-) and Cl(-)/OH(-) exchange) was also unaffected. Selective NBC-inhibition was not due to action on carbonic anhydrase (CA) enzymes, as 100 microM acetazolamide (a membrane-permeant CA-inhibitor) had no significant effect on NBC activity. pH(i) recovery from acidosis was associated with increased contractile-amplitude. The time course of recovery of pH(i) and contraction was slowed by S0859, confirming that NBC is a significant controller of contractility during acidosis.

CONCLUSIONS AND IMPLICATIONS

Compound S0859 is a selective, high-affinity generic NBC inhibitor, potentially important for probing the transporter's functional role in heart and other tissues.

Regulation of tumor pH and the role of carbonic anhydrase 9

The high metabolic rate required for tumor growth often leads to hypoxia in poorly-perfused regions. Hypoxia activates a complex gene expression program, mediated by hypoxia inducible factor 1 (HIF1alpha). One of the consequences of HIF1alpha activation is up-regulation of glycolysis and hence the production of lactic acid. In addition to the lactic acid-output, intracellular titration of acid with bicarbonate and the engagement of the pentose phosphate shunt release CO(2) from cells. Expression of the enzyme carbonic anhydrase 9 on the tumor cell surface catalyses the extracellular trapping of acid by hydrating cell-generated CO(2) into [see text] and H(+). These mechanisms contribute towards an acidic extracellular milieu favoring tumor growth, invasion and development. The lactic acid released by tumor cells is further metabolized by the tumor stroma. Low extracellular pH may adversely affect the intracellular milieu, possibly triggering apoptosis. Therefore, primary and secondary active transporters operate in the tumor cell membrane to protect the cytosol from acidosis. We review mechanisms regulating tumor intracellular and extracellular pH, with a focus on carbonic anhydrase 9. We also review recent evidence that may suggest a role for CA9 in coordinating pH(i) among cells of large, unvascularized cell-clusters.

Expression of membrane-bound carbonic anhydrases IV, IX, and XIV in the mouse heart

Expression of membrane-bound carbonic anhydrases (CAs) of CA IV, CA IX, CA XII, and CA XIV has been investigated in the mouse heart. Western blots using microsomal membranes of wild-type hearts demonstrate a 39-, 43-, and 54-kDa band representing CA IV, CA IX, and CA XIV, respectively, but CA XII could not be detected. Expression of CA IX in the CA IV/CA XIV knockout animals was further confirmed using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Cardiac cells were immunostained using anti-CA/FITC and anti-alpha-actinin/TRITC, as well as anti-CA/FITC and anti-SERCA2/TRITC. Subcellular CA localization was investigated by confocal laser scanning microscopy. CA localization in the sarcolemmal (SL) membrane was examined by double immunostaining using anti-CA/FITC and anti-MCT-1/TRITC. CAs showed a distinct distribution pattern in the sarcoplasmic reticulum (SR) membrane. CA XIV is predominantly localized in the longitudinal SR, whereas CA IX is mainly expressed in the terminal SR/t-tubular region. CA IV is present in both SR regions, whereas CA XII is not found in the SR. In the SL membrane, only CA IV and CA XIV are present. We conclude that CA IV and CA XIV are associated with the SR as well as with the SL membrane, CA IX is located in the terminal SR/t-tubular region, and CA XII is not present in the mouse heart. Therefore, the unique subcellular localization of CA IX and CA XIV in cardiac myocytes suggests different functions of both enzymes in excitation-contraction coupling.

pH-Regulated Na(+) influx into the mammalian ventricular myocyte: the relative role of Na(+)-H(+) exchange and Na(+)-HCO Co-transport

In the heart, intracellular Na(+) concentration (Na(+) (i)) is a controller of intracellular Ca(2+) signaling, and hence of key aspects of cell contractility and rhythm. Na(+) (i) will be influenced by variation in Na(+) influx. In the present work, we consider one source of Na(+) influx, sarcolemmal acid extrusion. Acid extrusion is accomplished by sarcolemmal H(+) and HCO(3) (-) transporters that import Na(+) ions while exporting H(+) or importing HCO(3) (-). The capacity of this system to import Na(+) is enormous, up to four times the maximum capacity of the Na(+)-K(+) ATPase to extrude Na(+) ions from the cell. In this review we consider the role of Na(+)-H(+) exchange (NHE) and Na(+)-HCO(3) (-)co-transport (NBC) in mediating Na(+) influx into cardiac myocytes. We consider, in particular, the role of NBC, as so little is known about Na(+) influx through this transporter. We show that both proteins mediate significant Na(+) influx and that although, in the ventricular myocyte, NBC-mediated Na(+) influx is less than through NHE, the proportions may be altered under a variety of conditions, including exposure to catecholamines, membrane depolarization, and interference with activity of the enzyme, carbonic anhydrase.

pH-Dependence of extrinsic and intrinsic H(+)-ion mobility in the rat ventricular myocyte, investigated using flash photolysis of a caged-H(+) compound

Passive H(+)-ion mobility within eukaryotic cells is low, due to H(+)-ion binding to cytoplasmic buffers. A localized intracellular acidosis can therefore persist for seconds or even minutes. Because H(+)-ions modulate so many biological processes, spatial intracellular pH (pH(i))-regulation becomes important for coordinating cellular activity. We have investigated spatial pH(i)-regulation in single and paired ventricular myocytes from rat heart by inducing a localized intracellular acid-load, while confocally imaging pH(i) using the pH-fluorophore, carboxy-SNARF-1. We present a novel method for localizing the acid-load. This involves the intracellular photolytic uncaging of H(+)-ions from a membrane-permeant acid-donor, 2-nitrobenzaldehyde. The subsequent spatial pH(i)-changes are consistent with intracellular H(+)-mobility and cell-to-cell H(+)-permeability constants measured using more conventional acid-loading techniques. We use the method to investigate the effect of reducing pH(i) on intrinsic (non-CO(2)/HCO(3)(-) buffer-dependent) and extrinsic (CO(2)/HCO(3)(-) buffer-dependent) components of H(i)(+)-mobility. We find that although both components mediate spatial regulation of pH within the cell, their ability to do so declines sharply at low pH(i). Thus acidosis severely slows intracellular H(+)-ion movement. This can result in spatial pH(i) nonuniformity, particularly during the stimulation of sarcolemmal Na(+)-H(+) exchange. Intracellular acidosis thus presents a window of vulnerability in the spatial coordination of cellular function.

Acidosis in models of cardiac ventricular myocytes

The effects of acidosis on cardiac electrophysiology and excitation-contraction coupling have been studied extensively. Acidosis decreases the strength of contraction and leads to altered calcium transients as a net result of complex interactions between protons and a variety of intracellular processes. The relative contributions of each of the changes under acidosis are difficult to establish experimentally, however, and significant uncertainties remain about the key mechanisms of impaired cardiac function. In this paper, we review the experimental findings concerning the effects of acidosis on the action potential and calcium handling in the cardiac ventricular myocyte, and we present a modelling study that establishes the contribution of the different effects to altered Ca2+ transients during acidosis. These interactions are incorporated into a dynamical model of pH regulation in the myocyte to simulate respiratory acidosis in the heart.

A dynamic model of excitation-contraction coupling during acidosis in cardiac ventricular myocytes

Acidosis in cardiac myocytes is a major factor in the reduced inotropy that occurs in the ischemic heart. During acidosis, diastolic calcium concentration and the amplitude of the calcium transient increase, while the strength of contraction decreases. This has been attributed to the inhibition by protons of calcium uptake and release by the sarcoplasmic reticulum, to a rise of intracellular sodium caused by activation of sodium-hydrogen exchange, decreased calcium binding affinity to Troponin-C, and direct effects on the contractile machinery. The relative contributions and concerted action of these effects are, however, difficult to establish experimentally. We have developed a mathematical model to examine altered calcium-handling mechanisms during acidosis. Each of the alterations was incorporated into a dynamical model of pH regulation and excitation-contraction coupling to predict the time courses of key ionic species during acidosis, in particular intracellular pH, sodium and the calcium transient, and contraction. This modeling study suggests that the most significant effects are elevated sodium, inhibition of sodium-calcium exchange, and the direct interaction of protons with the contractile machinery; and shows how the experimental data on these contributions can be reconciled to understand the overall effects of acidosis in the beating heart.

Involvement of AE3 isoform of Na(+)-independent Cl(-)/HCO(3)(-) exchanger in myocardial pH(i) recovery from intracellular alkalization

Myocardial pH(i) recovery from intracellular alkalization results in part from the acid load (-J(H+)) carried by Cl(-)/HCO(3)(-) anion-exchangers (AE). Three AE isoforms, AE1, AE2 and AE3, have been identified in cardiac membranes, but the function of each isoform on pH(i) homeostasis is still under investigation. This work explored, by means of specific antibodies, the role of AE3 isoform in myocardial pH(i) regulation. We developed rabbit polyclonal antibodies against the extracellular "loops": one connecting the fifth to sixth and the other one the seventh to eighth transmembrane domains (loops 3 and 4, respectively) of AE3, and their effect on pH(i) regulation was studied in rat papillary muscles. The anti-AE3 loop 3 antibody decreased -J(H+) in response to myocardial alkalization (from a mean control value of 1.06+/-0.26 to 0.32+/-0.13 mmol/L/min, n=7, P<0.05) without affecting the baseline pH(i) (7.22+/-0.03 vs. 7.21+/-0.04). The anti-AE3 loop 4 antibody did not modify either pH(i) recovery or baseline pH(i). Under control conditions, endothelin-1 (ET-1) increased -J(H+) in response to myocardial alkalization from 1.30+/-0.18 to 2.01+/-0.33 mmol/L /min (n=5, P<0.05). This effect of ET-1 on -J(H+) was abolished by anti-AE3 loop 3 antibody. In addition, the MgATP-induced stimulation of AE activity was reduced by the anti-AE3 loop 3 antibody. These data support the key role of the AE3 isoform in myocardial pH(i) recovery from alkaline loads and also in the stimulatory effect of ET-1 on AE activity. To a lesser extent, it may also contribute to the effect of MgATP on pH(i).

Involvement of the Na+-independent Cl-/HCO3- exchange (AE) isoform in the compensation of myocardial Na+/H+ isoform 1 hyperactivity in spontaneously hypertensive rats

Enhanced activity of Na+/H+ isoform 1 (NHE-1) and the Na+-independent Cl-/HCO3- exchange (AE) is a feature of the hypertrophied myocardium in spontaneously hypertensive rats (SHR). The present study explored the possibility that sustained intracellular acidosis due to increased myocardial acid loading through AE causes NHE-1 enhancement. To this aim, SHR were treated for 2 weeks with a rabbit polyclonal antibody against an AE3 isoform that was recently developed and proven to have inhibitory effects on myocardial AE activity. We then compared the AE activity in the left ventricle papillary muscles isolated from untreated SHR with antiAE3-treated SHR; AE activity was measured in terms of the rate of intracellular pH recovery after an intracellular alkali load was introduced. AE activity was diminished by approximately 70% in SHR treated with the antiAE3 antibody, suggesting that the AE3 isoform is a major carrier of acid-equivalent influx in the hypertrophied myocardium. However, the antibody treatment failed to normalize NHE-1 activity that remained elevated in the myocardium of normotensive rats. The data therefore rule out the possibility that NHE-1 hyperactivity in hypertensive myocardium was due to sustained intracellular acidosis induced by increased AE activity that characterizes SHR myocardial tissue.

Use of flow cytometry and SNARF to calibrate and measure intracellular pH in NS0 cells

BACKGROUND

Two calibration methods have been proposed for determining the relation between the fluorescence ratio of a pH-sensitive fluorescent indicator and intracellular pH (pHi). The first method uses nigericin to clamp pHi to external pH (pHe) and the second is the null point method. We compared these different calibration methods, solution conditions, and temperatures by using flow cytometry and the fluorescent dye 1,5- (and-6)-carboxy seminaphtorhodafluor-1-acetoxymethyl ester with an NS0 cell line.

METHODS

The nigericin method was performed in glucose solutions supplemented with KCl and 2-(N-morpholino)ethane sulphonic acid plus tris(hydroxymethyl)aminomethane (solution 1A), a mixture of K2HPO4/KH2PO4 in glucose-solution supplemented solutions (solution 2A), or bicarbonate buffered growth medium supplemented with K2HPO4/KH2PO4 (solution 2B); this allowed a range of pHe values to be used. The effect of temperature (22 degrees C or 37 degrees C) on the nigericin calibration curve was also investigated. The null point method was performed by using a series of solutions with a mixture of weak acid and base with a known pHi response.

RESULTS

Using solution 1A as the calibration solution resulted in acidic values of pHi for cells cultured in medium as compared with the values achieved with solution 2A. Using solution 2B did not affect the calibration curve. For the temperatures considered in this study, there was no affect on the calibration curve, but temperature did affect the pHi value of cells in phosphate buffered saline. The pseudo-null point method used with flow cytometry resulted in a calibration curve that was significantly different (P<0.05) from that achieved using the nigericin method.

CONCLUSIONS

Our data indicates that the choice of calibration solution can affect the reported pHi value; therefore, careful choice of solution is important.

Experimental generation and computational modeling of intracellular pH gradients in cardiac myocytes

It is often assumed that pH(i) is spatially uniform within cells. A double-barreled microperfusion system was used to apply solutions of weak acid (acetic acid, CO(2)) or base (ammonia) to localized regions of an isolated ventricular myocyte (guinea pig). A stable, longitudinal pH(i) gradient (up to 1 pH(i) unit) was observed (using confocal imaging of SNARF-1 fluorescence). Changing the fractional exposure of the cell to weak acid/base altered the gradient, as did changing the concentration and type of weak acid/base applied. A diffusion-reaction computational model accurately simulated this behavior of pH(i). The model assumes that H(i)(+) movement occurs via diffusive shuttling on mobile buffers, with little free H(+) diffusion. The average diffusion constant for mobile buffer was estimated as 33 x 10(-7) cm(2)/s, consistent with an apparent H(i)(+) diffusion coefficient, D(H)(app), of 14.4 x 10(-7) cm(2)/s (at pH(i) 7.07), a value two orders of magnitude lower than for H(+) ions in water but similar to that estimated recently from local acid injection via a cell-attached glass micropipette. We conclude that, because H(i)(+) mobility is so low, an extracellular concentration gradient of permeant weak acid readily induces pH(i) nonuniformity. Similar concentration gradients for weak acid (e.g., CO(2)) occur across border zones during regional myocardial ischemia, raising the possibility of steep pH(i) gradients within the heart under some pathophysiological conditions.

Modelling intracellular H(+) ion diffusion

Intracellular pH, an important modulator of cell function, is regulated by plasmalemmal proteins that transport H(+), or its equivalent, into or out of the cell. The pH(i) is also stabilised by high-capacity, intrinsic buffering on cytoplasmic proteins, oligopeptides and other solutes, and by the extrinsic CO(2)/HCO(3)(-) (carbonic) buffer. As mobility of these buffers is lower than for the H(+) ion, they restrict proton diffusion. In this paper we use computational approaches, based on the finite difference and finite element methods (FDM and FEM, respectively), for analysing the spatio-temporal behaviour of [H(+)] when it is locally perturbed. We analyse experimental data obtained for various cell-types (cardiac myocytes, duodenal enterocytes, molluscan neurons) where pH(i) has been imaged confocally using intracellular pH-sensitive dyes. We design mathematical algorithms to generate solutions for two-dimensional diffusion that fit data in terms of an apparent intracellular H(+) diffusion coefficient, D(H)(app). The models are used to explore how the spatial distribution of [H(+)](i) is affected by membrane H(+)-equivalent transport and by cell geometry. We then develop a mechanistic model, describing spatio-temporal changes of [H(+)](i) in a cardiac ventricular myocyte in terms of H(+)-shuttling on mobile buffers and H(+)-anchoring on fixed buffers. We also discuss how modelling may include the effects of extrinsic carbonic-buffering. Overall, our computational approach provides a framework for future analyses of the physiological consequences of pH(i) non-uniformity.

Intracellular proton mobility and buffering power in cardiac ventricular myocytes from rat, rabbit, and guinea pig

Intracellular pH (pHi) is an important modulator of cardiac function. The spatial regulation of pH within the cytoplasm depends, in part, on intracellular H+ (Hi+) mobility. The apparent diffusion coefficient for Hi+, DHapp, was estimated in single ventricular myocytes isolated from the rat, guinea pig, and rabbit. DHapp was derived by best-fitting predictions of a two-dimensional model of H+ diffusion to the local rise of intracellular [H+], recorded confocally (ratiometric seminaphthorhodafluor fluorescence) downstream from an acid-filled, whole cell patch pipette. Under CO2/HCO3--free conditions, DHapp was similar in all three species (mean values: 8-12.5 x 10-7 cm2/s) and was over 200-fold lower than that for H+ in water. In guinea pig myocytes, DHapp was increased 2.5-fold in the presence of CO2/HCO3- buffer, in agreement with previous observations in rabbit myocytes. Hi+ mobility is therefore low in cardiac cells, a feature that may predispose them to the generation of pHi gradients in response to sarcolemmal acid/base transport or local cytoplasmic acid production. Low Hi+ mobility most likely results from H+ shuttling among cytoplasmic mobile and fixed buffers. This hypothesis was explored by comparing the pHi dependence of intrinsic, intracellular buffering capacity, measured for all three species, and subdividing buffering into mobile and fixed fractions. The proportion of buffer that is mobile will be the main determinant of DHapp. At a given pHi, this proportion appeared to be similar in all three species, consistent with a common value for DHapp. Over the pHi range of 6.0-8.0, the proportion is expected to change, predicting that DHapp may display some pHi sensitivity.

Modeling Total Heart Function

Computational models of the electrical and mechanical function of the heart are reviewed. These models attempt to explain the integrated function of the heart in terms of ventricular anatomy, the structure and material properties of myocardial tissue, the membrane ion channels, and calcium handling and myofilament mechanics of cardiac myocytes. The models have established the computational framework for linking the structure and function of cardiac cells and tissue to the integrated behavior of the intact heart, but many more aspects of physiological function, including metabolic and signal transduction pathways, need to be included before significant progress can be made in understanding many disease processes.

Role of bicarbonate in the regulation of intracellular pH in the mammalian ventricular myocyte

Bicarbonate is important for pHi control in cardiac cells. It is a major part of the intracellular buffer apparatus, it is a substrate for sarcolemmal acid-equivalent transporters that regulate intracellular pH, and it contributes to the pHo sensitivity of steady-state pHi, a phenomenon that may form part of a whole-body response to acid/base disturbances. Both bicarbonate and H+/OH- transporters participate in the sarcolemmal regulation of pHi, namely Na(+)-HCO3-cotransport (NBC), Cl(-)-HCO3- exchange (i.e., anion exchange, AE), Na(+)-H+ exchange (NHE), and Cl(-)-OH- exchange (CHE). These transporters are coupled functionally through changes of pHi, while pHi is linked to [Ca2+]i through secondary changes in [Na+] mediated by NBC and NHE. Via such coupling, decreases of pHo and pHi can ultimately lead to an elevation of [Ca2+]i, thereby influencing cardiac contractility and electrical rhythm. Bicarbonate is also an essential component of an intracellular carbonic buffer shuttle that diffusively couples cytoplasmic pH to the sarcolemma and minimises the formation of intracellular pH microdomains. The importance of bicarbonate is closely linked to the activity of the enzyme carbonic anhydrase (CA). Without CA activity, intracellular bicarbonate-dependent buffering, membrane bicarbonate transport, and the carbonic shuttle are severely compromised. There is a functional partnership between CA and HCO3- transport. Based on our observations on intracellular acid mobility, we propose that one physiological role for CA is to act as a pH-coupling protein, linking bulk pH to the allosteric H+ control sites on sarcolemmal acid/base transporters.

Functional and molecular determination of carbonic anhydrase levels in bovine and cultured human chondrocytes

In this study, bovine articular and human chondrocytes from the C-20/A4 cell line were tested for the functional activity and molecular presence of the enzyme carbonic anhydrase. This enzyme is classically considered to be important in the maintenance of high cellular buffering capacity by catalysing the slow attainment of equilibrium between CO(2) and HCO(3)(-). The first functional assay measured the rate of pH equilibration after administration of a fixed dose of CO(2) solution to cell lysates. Compared to positive controls (human erythrocytes, murine M1 cells and purified carbonic anhydrase), chondrocyte lysates attained equilibrium at a significantly slower rate, similar to the rate obtained with a negative control (Xenopus oocytes). A second functional assay studied CO(2) hydration kinetics in intact C-20/A4 cells, using a pH-sensitive fluorescent dye, as the CO(2) content of the extracellular solution was changed. It was shown that C-20/A4 cells accelerate hydration only to a small degree. Hydration kinetics were reduced to the spontaneous rate in the presence of acetazolamide. Western immunoblotting with isoform-nonspecific antibodies to carbonic anhydrase demonstrated weak staining in both bovine and human chondrocytes.

Facilitation of intracellular H(+) ion mobility by CO(2)/HCO(3)(-) in rabbit ventricular myocytes is regulated by carbonic anhydrase

Intracellular H(+) mobility was estimated in the rabbit isolated ventricular myocyte by diffusing HCl into the cell from a patch pipette, while imaging pH(i) confocally using intracellular ratiometric SNARF fluorescence. The delay for acid diffusion between two downstream regions approximately 40 microm apart was reduced from approximately 25 s to approximately 6 s by replacing Hepes buffer in the extracellular superfusate with a 5 % CO(2)/HCO(3)(-) buffer system (at constant pH(o) of 7.40). Thus CO(2)/HCO(3)(-) (carbonic) buffer facilitates apparent H(+)(i) mobility. The delay with carbonic buffer was increased again by adding acetazolamide (ATZ), a membrane permeant carbonic anhydrase (CA) inhibitor. Thus facilitation of apparent H(+)(i) mobility by CO(2)/HCO(3)(-) relies on the activity of intracellular CA. By using a mathematical model of diffusion, the apparent intracellular H(+) equivalent diffusion coefficient (D(H)(app)) in CO(2)/HCO(3)(-)-buffered conditions was estimated to be 21.9 x 10(-7) cm(2) s(-1), 5.8 times faster than in the absence of carbonic buffer. Facilitation of H(+)(i) mobility is discussed in terms of an intracellular carbonic buffer shuttle, catalysed by intracellular CA. Turnover of this shuttle is postulated to be faster than that of the intrinsic buffer shuttle. By regulating the carbonic shuttle, CA regulates effective H(+)(i) mobility which, in turn, regulates the spatiotemporal uniformity of pH(i). This is postulated to be a major function of CA in heart.

Effect of S20787, a novel Cl--HCO3- exchange inhibitor, on intracellular pH regulation in guinea pig ventricular myocytes

S20787 has recently been proposed to be a selective Cl--HCO3- anion exchange (AE) inhibitor in rat cardiomyocytes. The AE transporter mediates sarcolemmal acid influx but is only one part of the cardiac cell's dual acid loading mechanism, the other part being a sarcolemmal Cl--OH- exchanger (CHE). We have therefore (1) investigated the differential effects of S20787 on the AE and CHE transporters in isolated guinea pig ventricular myocytes and (2) re-examined the influence of the drug on other sarcolemmal acid transporters by monitoring its effect on intracellular pH (pH(i)) recovery from alkali or acid loads. The pH(i) was measured using microspectrofluorimetry (carboxy-SNARF-1). The results indicate that CHE activity was unaffected by the drug (1-20 microM), whereas up to 78% of AE activity was blocked (K(i) = 3.9 microM). Thus, S20787 targets only the AE component of the dual acid influx system. Activities of other acid-transporting carriers, such as Na+-H+ exchange, Na+-HCO3- co-transport and the monocarboxylic acid transporter, were unaffected by the drug. The inhibitory efficacy of S20787 for AE in guinea pig cardiomyocytes appears to be considerably higher (approximately 78%) than proposed previously for rat cardiomyocytes (50%). This is most likely because, in both cells, a significant fraction (20-30%) of acid influx is mediated through the S20787-insensitive CHE transporter. Previous studies made no allowance for the CHE component, which would result in an underestimation. S20787 is thus a highly selective AE inhibitor which may be useful as an experimental tool and a potential cardiac protective agent in the heart.

The modelling of a primitive 'sustainable' conservative cell

The simple sustainable or 'eternal' cell model, assuming preservation of all proteins, is designed as a building block, a primitive element upon which one can build more complete functional cell models of various types, representing various species. In the modelling we emphasize the electrophysiological aspects, in part because these are a well-developed component of cell models and because membrane potentials and their fluctuations have been generally omitted from metabolically oriented cell models in the past. Fluctuations in membrane potential deserve heightened consideration because probably all cells have negative intracellular potentials and most cells demonstrate electrical activity with vesicular extrusion, receptor occupancy, as well as with stimulated excitation resulting in regenerative depolarization. The emphasis is on the balances of mass, charge, and of chemical species while accounting for substrate uptake, metabolism and metabolite loss from the cell. By starting with a primitive representation we emphasize the conservation ideas. As more advanced models are generated they must adhere to the same basic principles as are required for the most primitive incomplete model.

In situ calibration and [H+] sensitivity of the fluorescent Na+indicator SBFI

Despite the popularity of Na+-binding benzofuran isophthalate (SBFI) to measure intracellular free Na+ concentrations ([Na+]i), the in situ calibration techniques described to date do not favor the straightforward determination of all of the constants required by the standard equation (Grynkiewicz G, Poenie M, and Tsien RY. J Biol Chem 260: 3440–3450, 1985) to convert the ratiometric signal into [Na+]. We describe a simple method in which SBFI ratio values obtained during a “full” in situ calibration are fit by a three-parameter hyperbolic equation; the apparent dissociation constant ( K d) of SBFI for Na+ can then be resolved by means of a three-parameter hyperbolic decay equation. We also developed and tested a “one-point” technique for calibrating SBFI ratios in which the ratio value obtained in a neuron at the end of an experiment during exposure to gramicidin D and 10 mM Na+is used as a normalization factor for ratios obtained during the experiment; each normalized ratio is converted to [Na+]i using a modification of the standard equation and parameters obtained from a full calibration. Finally, we extended the characterization of the pH dependence of SBFI in situ. Although the K d of SBFI for Na+ was relatively insensitive to changes in pH in the range 6.8–7.8, acidification resulted in an apparent decrease, and alkalinization in an apparent increase, in [Na+]i values. The magnitudes of the apparent changes in [Na+]ivaried with absolute [Na+]i, and a method was developed for correcting [Na+]i values measured with SBFI for changes in intracellular pH.

Intracellular pH regulation in ventral horn neurones cultured from embryonic rat spinal cord

Intracellular pH was measured with the pH-sensitive fluorescent probe BCECF in spinal cord neurones cultured from rat embryos. At an external pH of 7.3, the average steady-state pHi was 7.18 +/- 0.03 (SEM, n = 97) and 7.02 +/- 0.01 (n = 221) in HEPES-buffered and in bicarbonate-buffered medium, respectively. In both external media, pHi was strongly dependent on external pH (pHe). In HEPES-buffered medium, pHi recovery following an acid load induced by transient application of ammonium required external Na+ and was inhibited by amiloride, indicating the presence of a Na+/H+ exchange. Na(+)- and HCO3(-)-dependent, DIDS-sensitive alkalinizing mechanisms also contributed to pHi regulation in CO2/bicarbonate-buffered medium. The presence of an electrogenic Na(+)-HCO3- cotransporter was confirmed by the alkalinizing effect of KCl application. The fact that pHi is lower in CO2/bicarbonate- than in HEPES-buffered medium and the alkalinization observed upon suppression of external Cl- suggest that the acidifying Cl-/HCO3- transporter plays an important role in defining pHi.

Characterization of intracellular pH regulation in the guinea-pig ventricular myocyte

1. Intracellular pH was recorded fluorimetrically by using carboxy-SNARF-1, AM-loaded into superfused ventricular myocytes isolated from guinea-pig heart. Intracellular acid and base loads were induced experimentally and the changes of pHi used to estimate intracellular buffering power (beta). The rate of pHi recovery from acid or base loads was used, in conjunction with the measurements of beta, to estimate sarcolemmal transporter fluxes of acid equivalents. A combination of ion substitution and pharmacological inhibitors was used to dissect acid effluxes carried on Na+-H+ exchange (NHE) and Na+-HCO3- cotransport (NBC), and acid influxes carried on Cl--HCO3- exchange (AE) and Cl--OH- exchange (CHE). 2. The intracellular intrinsic buffering power (betai), estimated under CO2/HCO3--free conditions, varied inversely with pHi in a manner consistent with two principal intracellular buffers of differing concentration and pK. In CO2/HCO3--buffered conditions, intracellular buffering was roughly doubled. The size of the CO2-dependent component (betaCO2) was consistent with buffering in a cell fully open to CO2. Because the full value of betaCO2 develops slowly (2.5 min), it had to be measured under equilibrium conditions. The value of betaCO2 increased monotonically with pHi. 3. In 5 % CO2/HCO3--buffered conditions (pHo 7.40), acid extrusion on NHE and NBC increased as pHi was reduced, with the greater increase occurring through NHE at pHi < 6.90. Acid influx on AE and CHE increased as pHi was raised, with the greater increase occurring through AE at pHi > 7.15. At resting pHi (7.04-7.07), all four carriers were activated equally, albeit at a low rate (about 0.15 mM min-1). 4. The pHi dependence of flux through the transporters, in combination with the pHi and time dependence of intracellular buffering (betai + betaCO2), was used to predict mathematically the recovery of pHi following an intracellular acid or base load. Under several conditions the mathematical predictions compared well with experimental recordings, suggesting that the model of dual acid influx and acid efflux transporters is sufficient to account for pHi regulation in the cardiac cell. Key properties of the pHi control system are discussed.

A novel role for carbonic anhydrase: cytoplasmic pH gradient dissipation in mouse small intestinal enterocytes

1. The spatial and temporal distribution of intracellular H+ ions in response to activation of a proton-coupled dipeptide transporter localized at the apical pole of mouse small intestinal isolated enterocytes was investigated using intracellular carboxy-SNARF-1 fluorescence in combination with whole-cell microspectrofluorimetry or confocal microscopy. 2. In Hepes-buffered Tyrode solution, application of the dipeptide Phe-Ala (10 mM) to a single enterocyte reduced pHi locally in the apical submembranous space. After a short delay (8 s), a fall of pHi occurred more slowly at the basal pole. 3. In the presence of CO2/HCO3--buffered Tyrode solution, the apical and basal rates of acidification were not significantly different and the time delay was reduced to 1 s or less. 4. Following application of the carbonic anhydrase inhibitor acetazolamide (100 microM) in the presence of CO2/HCO3- buffer, addition of Phe-Ala once again produced a localized apical acidification that took 5 s to reach the basal pole. Basal acidification was slower than at the apical pole. 5. We conclude that acid influx due to proton-coupled dipeptide transport can lead to intracellular pH gradients and that intracellular carbonic anhydrase activity, by facilitating cytoplasmic H+ mobility, limits their magnitude and duration.

Modelling myocardial ischaemia and reperfusion

Substrate depletion and increased intracellular acidity are believed to underlie clinically important manifestations of myocardial ischaemia. Recent advances in measuring ion concentrations and metabolite changes have provided a wealth of detail on the processes involved. Coupled with the rapid increase in computing power, this has allowed the development of a mathematical model of cardiac metabolism in normal and ischaemic conditions. Pre-existing models of cardiac cells such as Oxsoft HEART contain highly developed dynamic descriptions of cardiac electrical activity. While biophysically detailed, these models do not yet incorporate biochemical changes. Modelling of bioenergetic changes was based and verified against whole heart NMR spectroscopy. In the model, ATP hydrolysis and generation are calculated simultaneously as a function of [Pi]i. Simulation of pH regulation was based on the pHi dependency of acid efflux, examined in time-course studies of pHi recovery (measured in myocytes with the fluorophore carboxy-SNARF-1) from imposed acid and alkali loads. The force-[Ca2+]i relationship of myofibrils was used as the basis of modelling H+ competition with Ca2+, and thus of pH effects on contraction. This complex description of biochemically important changes in myocardial ischaemia was integrated into the OXSOFT models. The model is sufficiently complete to simulate calcium-overload arrhythmias during ischaemia and reperfusion-induced arrhythmias. The timecourse of both metabolite and pH changes correlates well with clinical and experimental studies. The model possesses predictive power, as it aided the identification of electrophysiological effects of therapeutic interventions such as Na(+)-H+ block. It also suggests a strategy for the control of cardiac arrhythmias during calcium overload by regulating sodium-calcium exchange. In summary, we have developed a biochemically and biophysically detailed model that provides a novel approach to studying myocardial ischaemia and reperfusion.

Sarcolemmal mechanisms for pHi recovery from alkalosis in the guinea-pig ventricular myocyte

1. The mechanism of pHi recovery from an intracellular alkali load (induced by acetate prepulse or by reduction/removal of ambient PCO2) was investigated using intracellular SNARF fluorescence in the guinea-pig ventricular myocyte. 2. In Hepes buffer (pHo 7.40), pHi recovery was inhibited by removal of extracellular Cl-, but not by removal of Na+o or elevation of K+o. Recovery was unaffected by the stilbene drug DIDS (4,4-diisothiocyanatostilbene-disulphonic acid), but was slowed dose dependently by the stilbene drug DBDS (dibenzamidostilbene-disulphonic acid). 3. In 5 % CO2/HCO3- buffer (pHo 7.40), pHi recovery was faster than in Hepes buffer. It consisted of an initial rapid recovery phase followed by a slow phase. Much of the rapid phase has been attributed to CO2-dependent buffering. The slow phase was inhibited completely by Cl-o removal but not by Na+o removal or K+o elevation. 4. At a test pHi of 7.30 in CO2/HCO3- buffer, the slow phase was inhibited 70 % by DIDS. The mean DIDS-inhibitable acid influx was equivalent in magnitude to the HCO3--stimulated acid influx. Similarly, the DIDS-insensitive influx was equivalent to that estimated in Hepes buffer. 5. We conclude that two independent sarcolemmal acid-loading carriers are stimulated by a rise of pHi and account for the slow phase of recovery from an alkali load. The results are consistent with activation of a DIDS-sensitive Cl--HCO3- anion exchanger (AE) to produce HCO3- efflux, and a DIDS-insensitive Cl--OH- exchanger (CHE) to produce OH- efflux. H+-Cl- co-influx as the alternative configuration for CHE is not, however, excluded. 6. The dual acid-loading system (AE plus CHE), previously shown to be activated by a fall of extracellular pH, is thus activated by a rise of intracellular pH. Activity of the dual-loading system is therefore controlled by pH on both sides of the cardiac sarcolemma.