Proton permeation through the myocardial gap junction

Propagation of a rapid cell-to-cell H2O2 signal over long distances in a monolayer of cardiomyocyte cells

Cell-to-cell communication plays a cardinal role in the biology of multicellular organisms. H2O2 is an important cell-to-cell signaling molecule involved in the response of mammalian cells to wounding and other stimuli. We previously identified a signaling pathway that transmits wound-induced cell-to-cell H2O2 signals within minutes over long distances, measured in centimeters, in a monolayer of cardiomyocytes. Here we report that this long-distance H2O2 signaling pathway is accompanied by enhanced accumulation of cytosolic H2O2 and altered redox state in cells along its path. We further show that it requires the production of superoxide, as well as the function of gap junctions, and that it is accompanied by changes in the abundance of hundreds of proteins in cells along its path. Our findings highlight the existence of a unique and rapid long-distance H2O2 signaling pathway that could play an important role in different inflammatory responses, wound responses/healing, cardiovascular disease, and/or other conditions.

Distinct moieties underlie biphasic H+ gating of connexin43 channels, producing a pH optimum for intercellular communication

Most mammalian cells can intercommunicate via connexin-assembled, gap-junctional channels. To regulate signal transmission, connexin (Cx) channel permeability must respond dynamically to physiological and pathophysiological stimuli. One key stimulus is intracellular pH (pH_i), which is modulated by a tissue’s metabolic and perfusion status. Our understanding of the molecular mechanism of H⁺ gating of Cx43 channels—the major isoform in the heart and brain—is incomplete. To interrogate the effects of acidic and alkaline pH_i on Cx43 channels, we combined voltage-clamp electrophysiology with pH_i imaging and photolytic H⁺ uncaging, performed over a range of pH_i values. We demonstrate that Cx43 channels expressed in HeLa or N2a cell pairs are gated biphasically by pH_ivia a process that consists of activation by H⁺ ions at alkaline pH_i and inhibition at more acidic pH_i. For Cx43 channel–mediated solute/ion transmission, the ensemble of these effects produces a pH_i optimum, near resting pH_i. By using Cx43 mutants, we demonstrate that alkaline gating involves cysteine residues of the C terminus and is independent of motifs previously implicated in acidic gating. Thus, we present a molecular mechanism by which cytoplasmic acid–base chemistry fine tunes intercellular communication and establishes conditions for the optimal transmission of solutes and signals in tissues, such as the heart and brain.—Garciarena, C. D., Malik, A., Swietach, P., Moreno, A. P., Vaughan-Jones, R. D. Distinct moieties underlie biphasic H⁺ gating of connexin43 channels, producing a pH optimum for intercellular communication.

Pado, a fluorescent protein with proton channel activity can optically monitor membrane potential, intracellular pH, and map gap junctions

An in silico search strategy was developed to identify potential voltage-sensing domains (VSD) for the development of genetically encoded voltage indicators (GEVIs). Using a conserved charge distribution in the S2 α-helix, a single in silico search yielded most voltage-sensing proteins including voltage-gated potassium channels, voltage-gated calcium channels, voltage-gated sodium channels, voltage-gated proton channels, and voltage-sensing phosphatases from organisms ranging from mammals to bacteria and plants. A GEVI utilizing the VSD from a voltage-gated proton channel identified from that search was able to optically report changes in membrane potential. In addition this sensor was capable of manipulating the internal pH while simultaneously reporting that change optically since it maintains the voltage-gated proton channel activity of the VSD. Biophysical characterization of this GEVI, Pado, demonstrated that the voltage-dependent signal was distinct from the pH-dependent signal and was dependent on the movement of the S4 α-helix. Further investigation into the mechanism of the voltage-dependent optical signal revealed that inhibiting the dimerization of the fluorescent protein greatly reduced the optical signal. Dimerization of the FP thereby enabled the movement of the S4 α-helix to mediate a fluorescent response.

Instantaneous current-voltage relationships during the course of the human cardiac ventricular action potential: new computational insights into repolarization dynamics

AIMS

To adopt a novel three-dimensional (3D) representation of cardiac action potential (AP) to compactly visualize dynamical properties of human cellular ventricular repolarization.

METHODS AND RESULTS

We have recently established a novel 3D representation of cardiac AP, which is based on the iterative measurement of instantaneous ion current-voltage profiles during the course of an AP. Such an approach has been originally developed on real patch-clamped ventricular cells, and subsequently improved in silico on several cardiac ventricular AP models of different mammals, and on models of different AP types of the human heart. We apply it here on two different models of human ventricular AP, and show that it compactly provides further insights into repolarization dynamics. The 3D representation of the AP includes equilibrium points during repolarization, and can be screened in terms of what we have shown to be a region, during late repolarization, when membrane conductance becomes negative and repolarization therefore auto-regenerative. We have called this time window auto-regenerative-repolarization-phase (ARRP).

CONCLUSION

In addition to previous findings obtained through the same procedure, we show here that 3D current-voltage-time representations of human ventricular AP allow compact visualization of dynamical properties, which are relevant for the physiology and pathology of ventricular repolarization. In particular, we suggest that the volume under the current surface corresponding to the ARRP might be used as a predictor of safety of repolarization, in single cells and during AP conduction in cell pairs.

Imaging an optogenetic pH sensor reveals that protons mediate lateral inhibition in the retina

The reciprocal synapse between photoreceptors and horizontal cells underlies lateral inhibition and establishes the antagonistic center-surround receptive fields of retinal neurons to enhance visual contrast. Despite decades of study, the signal mediating the negative feedback from horizontal cells to cones has remained under debate because the small, invaginated synaptic cleft has precluded measurement. Using zebrafish retinas, we show that light elicits a change in synaptic proton concentration with the correct magnitude, kinetics and spatial dependence to account for lateral inhibition. Light, which hyperpolarizes horizontal cells, causes synaptic alkalinization, whereas activating an exogenously expressed ligand-gated Na(+) channel, which depolarizes horizontal cells, causes synaptic acidification. Whereas acidification was prevented by blocking a proton pump, re-alkalinization was prevented by blocking proton-permeant ion channels, suggesting that distinct mechanisms underlie proton efflux and influx. These findings reveal that protons mediate lateral inhibition in the retina, raising the possibility that protons are unrecognized retrograde messengers elsewhere in the nervous system.

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.

Endothelial alkalinisation inhibits gap junction communication and endothelium-derived hyperpolarisations in mouse mesenteric arteries

Abstract  Gap junctions mediate intercellular signalling in arteries and contribute to endothelium-dependent vasorelaxation, conducted vascular responses and vasomotion. Considering its putative role in vascular dysfunction, mechanistic insights regarding the control of gap junction conductivity are required. Here, we investigated the consequences of endothelial alkalinisation for gap junction communication and endothelium-dependent vasorelaxation in resistance arteries. We studied mesenteric arteries from NMRI mice by myography, confocal fluorescence microscopy and electrophysiological techniques. Removing CO2/HCO3(-), reducing extracellular [Cl(-)] or adding 4,4-diisothiocyanatostilbene-2,2-disulphonic acid inhibited or reversed Cl(-)/HCO3(-) exchange, alkalinised the endothelium by 0.2-0.3 pH units and inhibited acetylcholine-induced vasorelaxation. NO-synthase-dependent vasorelaxation was unaffected by endothelial alkalinisation whereas vasorelaxation dependent on small- and intermediate-conductance Ca(2+)-activated K(+) channels was attenuated by ∼75%. The difference in vasorelaxation between arteries with normal and elevated endothelial intracellular pH (pHi) was abolished by the gap junction inhibitors 18β-glycyrrhetinic acid and carbenoxolone while other putative modulators of endothelium-derived hyperpolarisations - Ba(2+), ouabain, iberiotoxin, 8Br-cAMP and polyethylene glycol catalase - had no effect. In the absence of CO2/HCO3(-), addition of the Na(+)/H(+)-exchange inhibitor cariporide normalised endothelial pHi and restored vasorelaxation to acetylcholine. Endothelial hyperpolarisations and Ca(2+) responses to acetylcholine were unaffected by omission of CO2/HCO3(-). By contrast, dye transfer between endothelial cells and endothelium-derived hyperpolarisations of vascular smooth muscle cells stimulated by acetylcholine or the proteinase-activated receptor 2 agonist SLIGRL-amide were inhibited in the absence of CO2/HCO3(-). We conclude that intracellular alkalinisation of endothelial cells attenuates endothelium-derived hyperpolarisations in resistance arteries due to inhibition of gap junction communication. These findings highlight the role of pHi in modulating vascular function.

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.

pH-dependent channel gating in connexin26 hemichannels involves conformational changes in N-terminus

Connexin (Cx) hemichannels controlling an exchange of ions and metabolites between the cytoplasm and extracellular milieu can be modulated by the variation of intracellular pH during physiological and pathological conditions. To address the mechanism by which the pH exerts its effect on hemichannels, we have performed two 100-ns molecular dynamics simulations of the Cx26 channel in both acidic and neutral states. The results show that: 1) transmembrane domains undergo clockwise motions around the pore axis under both acidic and neutral conditions, while extracellular segments keep stable. 2) Under neutral condition, Cx26 has a tightly closed configuration that occurs through the assembly of N-terminal helix (NTH) region. This shows a constriction formed by the interhelical interactions of Asp2 and Met1 from neighboring NTH, which shapes the narrowest segment (pore radius<2Å) of the pore, preventing the passage of ions from the extracellular side. This indicates that Asp2 may act as a channel gate. 3) Under the acidic condition, the constriction is relieved by the protonation of Asp2 causing interruption of interhelical interactions, Cx26 has a flexibly opening pore (pore radius>4.5Å) around NTH region, allowing the passage of chloride ions unimpeded by the side-chain Asp2. While in the extracellular part two chloride ions interact with the side-chain Lys41 from three subunits. Finally, we provide a plausible mechanism of pH-dependent gating of hemichannel that involves protonation of the aspartic residues, suggesting that the pH sensitivity of hemichannel permeability is a sophisticated mechanism for cell regulating ion permeation.

Extracellular proton depression of peak and late Na⁺ current in the canine left ventricle

Cardiac ischemia reduces excitability in ventricular tissue. Acidosis (one component of ischemia) affects a number of ion currents. We examined the effects of extracellular acidosis (pH 6.6) on peak and late Na(+) current (I(Na)) in canine ventricular cells. Epicardial and endocardial myocytes were isolated, and patch-clamp techniques were used to record I(Na). Action potential recordings from left ventricular wedges exposed to acidic Tyrode solution showed a widening of the QRS complex, indicating slowing of transmural conduction. In myocytes, exposure to acidic conditions resulted in a 17.3 ± 0.9% reduction in upstroke velocity. Analysis of fast I(Na) showed that current density was similar in epicardial and endocardial cells at normal pH (68.1 ± 7.0 vs. 63.2 ± 7.1 pA/pF, respectively). Extracellular acidosis reduced the fast I(Na) magnitude by 22.7% in epicardial cells and 23.1% in endocardial cells. In addition, a significant slowing of the decay (time constant) of fast I(Na) was observed at pH 6.6. Acidosis did not affect steady-state inactivation of I(Na) or recovery from inactivation. Analysis of late I(Na) during a 500-ms pulse showed that the acidosis significantly reduced late I(Na) at 250 and 500 ms into the pulse. Using action potential clamp techniques, application of an epicardial waveform resulted in a larger late I(Na) compared with when an endocardial waveform was applied to the same cell. Acidosis caused a greater decrease in late I(Na) when an epicardial waveform was applied. These results suggest acidosis reduces both peak and late I(Na) in both cell types and contributes to the depression in cardiac excitability observed under ischemic conditions.

Ion transporters in secretory and cyclically modulating ameloblasts: a new hypothesis for cellular control of preeruptive enamel maturation

Mature enamel consists of densely packed and highly organized large hydroxyapatite crystals. The molecular machinery responsible for the formation of fully matured enamel is poorly described but appears to involve oscillative pH changes at the enamel surface. We conducted an immunohistochemical investigation of selected transporters and related proteins in the multilayered rat incisor enamel organ. Connexin 43 (Cx-43) is found in papillary cells and ameloblasts, whereas Na+-K+-ATPase is heavily expressed during maturation in the papillary cell layer only. Given the distribution of Cx-43 channels and Na+-K+-ATPase, we suggest that ameloblasts and the papillary cell layer act as a functional syncytium. During enamel maturation ameloblasts undergo repetitive cycles of modulation between ruffle-ended (RA) and smooth-ended (SA) ameloblast morphologies. Carbonic anhydrase II and vacuolar H+-ATPase are expressed simultaneously at the beginning of the maturation stage in RA cells. The proton pumps are present in the ruffled border of RA and appear to be internalized during the SA stage. Both papillary cells and ameloblasts express plasma membrane acid/base transporters (AE2, NBC, and NHE1). AE2 and NHE1 change position relative to the enamel surface as localization of the tight junctions changes during ameloblast modulation cycles. We suggest that the concerted action of the papillary cell layer and the modulating ameloblasts regulates the enamel microenvironment, resulting in oscillating pH fluctuations. The pH fluctuations at the enamel surface may be required to keep intercrystalline spaces open in the surface layers of the enamel, enabling degraded enamel matrix proteins to be removed while hydroxyapatite crystals grow as a result of influx of calcium and phosphate ions.

Cardiac cell modelling: observations from the heart of the cardiac physiome project

In this manuscript we review the state of cardiac cell modelling in the context of international initiatives such as the IUPS Physiome and Virtual Physiological Human Projects, which aim to integrate computational models across scales and physics. In particular we focus on the relationship between experimental data and model parameterisation across a range of model types and cellular physiological systems. Finally, in the context of parameter identification and model reuse within the Cardiac Physiome, we suggest some future priority areas for this field.

Regulation of intracellular pH during oocyte growth and maturation in mammals

Regulation of intracellular pH (pH(i)) is a fundamental homeostatic process essential for the survival and proliferation of virtually all cell types. The mammalian preimplantation embryo, for example, possesses Na(+)/H(+) and HCO(3)(-)/Cl(-) exchangers that robustly regulate against acidosis and alkalosis respectively. Inhibition of these transporters prevents pH corrections and, perhaps unsurprisingly, leads to impaired embryogenesis. However, recent studies have revealed that the role and regulation of pH(i) is somewhat more complex in the case of the developing and maturing oocyte. Small meiotically incompetent growing oocytes are apparently incapable of regulating their own pH(i), and instead rely upon the surrounding granulosa cells to correct ooplasmic pH, until such a time that the oocyte has developed the capacity to regulate its own pH(i). Later, during meiotic maturation, pH(i)-regulating activities that were developed during growth are inactivated, apparently under the control of MAPK signalling, until the oocyte is successfully fertilized. Here, we will discuss pH homeostasis in early mammalian development, focussing on recent developments highlighting the unusual and unexpected scenario of pH regulation during oocyte growth and maturation.

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.

Granulosa cells regulate oocyte intracellular pH against acidosis in preantral follicles by multiple mechanisms

Mammalian oocytes grow within ovarian follicles in which the oocyte is coupled to surrounding granulosa cells by gap junctions. We report here that growing oocytes isolated from mouse preantral follicles are incapable of recovering from an experimentally induced acidosis, and that oocytes acquire the ability to manage acid loads by activating Na(+)/H(+) exchange during growth. By contrast, granulosa cells from similar preantral follicles possess substantial Na(+)/H(+) exchange capacity, which is attributable to the simultaneous action of two Na(+)/H(+) exchanger isoforms: NHE1 and NHE3. Granulosa cells were also found to possess a V-type H(+)-ATPase that drives partial acidosis recovery when Na(+)/H(+) exchange is inactivated. By monitoring intracellular pH (pH(i)) in small follicle-enclosed oocytes, we found that the oocyte has access to each of these acidosis-correcting activities, such that small follicle-enclosed oocytes readily recover from acidosis in a manner resembling granulosa cells. However, follicle-enclosed oocytes are unable to access these activities if gap-junction communication within the follicle is inhibited. Together, these experiments identify the NHE isoforms involved in regulating oocyte pH(i), indicate that gap junctions allow granulosa cells to exogenously regulate oocyte pH(i) against acidosis until the oocyte has acquired endogenous pH(i) regulation, and reveal that granulosa cells possess multiple mechanisms for carrying out this function.

Microfluidic systems to examine intercellular coupling of pairs of cardiac myocytes

In this paper we describe a microfluidic environment that enables us to explore cell-to-cell signalling between longitudinally linked primary heart cells. We have chosen to use pairs (or doublets) of cardiac myocyte as a model system, not only because of the importance of cell-cell signalling in the study of heart disease but also because the single cardiomyocytes are both mechanically and electrically active and their synchronous activation due to the intercellular coupling within the doublet can be readily monitored on optical and electrical recordings. Such doublets have specialised intercellular contact structures in the form of the intercalated discs, comprising the adhesive junction (fascia adherens and macula adherens or desmosome) and the connecting junction (known as gap junction). The latter structure enables adjacent heart cells to share ions, second messengers and small metabolites (<1 kDa) between them and thus provides the structural basis for the synchronous (syncytical) behaviour of connected cardiomyocytes. Using the unique environment provided by the microfluidic system, described in this paper, we explore the local ionic conditions that enable the propagation of Ca(2+) waves between two heart cells. We observe that the ability of intracellular Ca(2+) waves to traverse the intercalated discs is dependent on the relative concentrations of diastolic Ca(2+) in the two adjacent cells. These experiments rely upon our ability to independently control both the electrical stimulation of each of the cells (using integrated microelectrodes) and to rapidly change (or switch) the local concentrations of ions and drugs in the extracellular buffer within the microfluidic channel (using a nanopipetting system). Using this platform, it is also possible to make simultaneous optical recordings (including fluorescence and cell contraction) to explore the effect of drugs on one or both cells, within the doublet.

H + Ion Activation and Inactivation of the Ventricular Gap Junction

H(+) ions are powerful modulators of cardiac function, liberated during metabolic activity. Among their physiological effects is a chemical gating of cell-to-cell communication, caused by H(+)-mediated closure of connexin (Cx) channels at gap junctions. This protects surrounding tissue from the damaging effects of local intracellular acidosis. Cx proteins (largely Cx-43 in ventricle) form multimeric pores between cells, permitting translocation of ions and other solutes up to approximately 1 kDa. The channels are essential for electrical and metabolic coordination of a tissue. Here we demonstrate that, contrary to expectation, H(+) ions can induce an increase of gap-junctional permeability. This occurs during modest intracellular acid loads in myocyte pairs isolated from mammalian ventricle. We show that the increase in permeability allows a local rise of [H(+)](i) to dissipate into neighboring myocytes, thereby providing a mechanism for spatially regulating intracellular pH (pH(i)). During larger acid loads, the increased permeability is overridden by a more familiar H(+)-dependent inhibition (H(+) inactivation). This restricts cell-to-cell H(+) movement, while allowing sarcolemmal H(+) transporters such as Na(+)/H(+) exchange, to extrude the acid from the cell. The H(+) sensitivity of Cx channels therefore defines whether junctional or sarcolemmal mechanisms are selected locally for the removal of an acid load. The bell-shaped pH dependence of permeability suggests that, in addition to H(+) inactivation, an H(+) activation process regulates the ensemble of Cx channels open at the junction. As well as promoting spatial pH(i) regulation, H(+) activation of junctional permeability may link increased metabolic activity to improved myocardial coupling, the better to meet mechanical demand.

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.

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.

Granulosa cells regulate intracellular pH of the murine growing oocyte via gap junctions: development of independent homeostasis during oocyte growth

Oocytes grow within ovarian follicles in which the oocyte is coupled to the surrounding granulosa cells by gap junctions. It was previously found that small growing oocytes isolated from juvenile mice and freed of their surrounding granulosa cells (denuded) lacked the ability to regulate their intracellular pH (pH(i)), did not exhibit the pH(i)-regulatory HCO(3)(-)/Cl(-) and Na(+)/H(+) exchange activities found in fully-grown oocytes, and had low pH(i). However, both exchangers became active as oocytes grew near to full size, and, simultaneously, oocyte pH(i) increased by approximately 0.25 pH units. Here, we show that, in the more physiological setting of the intact follicle, oocyte pH(i) is instead maintained at approximately 7.2 throughout oocyte development, and the growing oocyte exhibits HCO(3)(-)/Cl(-) exchange, which it lacks when denuded. This activity in the oocyte requires functional gap junctions, as gap junction inhibitors eliminated HCO(3)(-)/Cl(-) exchange activity from follicle-enclosed growing oocytes and substantially impeded the recovery of the oocyte from an induced alkalosis, implying that oocyte pH(i) may be regulated by pH-regulatory exchangers in granulosa cells via gap junctions. This would require robust HCO(3)(-)/Cl(-) exchange activity in the granulosa cells, which was confirmed using oocytectomized (OOX) cumulus-oocyte complexes. Moreover, in cumulus-oocyte complexes with granulosa cells coupled to fully-grown oocytes, HCO(3)(-)/Cl(-) exchange activity was identical in both compartments and faster than in denuded oocytes. Taken together, these results indicate that growing oocyte pH(i) is controlled by pH-regulatory mechanisms residing in the granulosa cells until the oocyte reaches a developmental stage where it becomes capable of carrying out its own homeostasis.

Novel method for measuring junctional proton permeation in isolated ventricular myocyte cell pairs

Partial exposure of single ventricular myocytes to membrane-permeant weak acids or bases, using a dual-microperfusion technique, generates large and stable intracellular pH (pHi) gradients. In this study, we have investigated the feasibility of using the technique to estimate junctional proton permeability. This was done by recording the pHi gradient developed across the junctional region of a pair of conjoined ventricular myocytes, isolated enzymically from a guinea pig heart when one of the cells was partially exposed to acetate or ammonium. We show that under HEPES-buffered conditions, the junctional discontinuity in the pHi profile can be used to derive an apparent proton permeability coefficient (PHapp). The mean PHapp obtained was 4.45 +/- 0.21.10(-4) cm/s (n=43) at an average junctional pHi of 7.04 +/- 0.02. In the presence of the junctional inhibitor alpha-glycyrrhetinic acid, exposure of the proximal cell to weak acid or base produced no pHi change in the distal cell, confirming that distal changes were normally caused by acid-base flux through connexons assembled into junctional channels. The validity of the dual-microperfusion method was tested further by using a diffusion-permeation-reaction model for intracellular protons, designed to highlight possible errors in the estimates of PHapp. Our technique for measuring PHapp provides a useful alternative to the previous, more invasive technique of locally loading acid through a cell-attached patch pipette. The technique may provide a simple method for investigating the factors regulating cell-to-cell proton transmission.