Relationship between intracellular pH and proton mobility in rat and guinea-pig ventricular myocytes

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

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

Ultra-high dose-rate (FLASH) radiotherapy: Generation of early, transient, strongly acidic spikes in the irradiated tumor environment

Monte Carlo simulations of γ/fast electron-radiolysis of water show that the in situ formation of H₃O⁺ temporarily renders each "native" isolated spur/track region very acidic. For pulsed (FLASH) irradiation with high dose rate, this early time, transient "acid-spike" response is shown to extend evenly across the entire irradiated volume. Since pH controls many cellular processes, this study highlights the need to consider these spikes of acidity in understanding the fundamental mechanisms underlying FLASH radiotherapy.

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.

Red blood cell thickness is evolutionarily constrained by slow, hemoglobin-restricted diffusion in cytoplasm

During capillary transit, red blood cells (RBCs) must exchange large quantities of CO₂ and O₂ in typically less than one second, but the degree to which this is rate-limited by diffusion through cytoplasm is not known. Gas diffusivity is intuitively assumed to be fast and this would imply that the intracellular path-length, defined by RBC shape, is not a factor that could meaningfully compromise physiology. Here, we evaluated CO₂ diffusivity (D_CO2) in RBCs and related our results to cell shape. D_CO2 inside RBCs was determined by fluorescence imaging of [H⁺] dynamics in cells under superfusion. This method is based on the principle that H⁺ diffusion is facilitated by CO₂/HCO₃^- buffer and thus provides a read-out of D_CO2. By imaging the spread of H⁺ ions from a photochemically-activated source (6-nitroveratraldehyde), D_CO2 in human RBCs was calculated to be only 5% of the rate in water. Measurements on RBCs containing different hemoglobin concentrations demonstrated a halving of D_CO2 with every 75 g/L increase in mean corpuscular hemoglobin concentration (MCHC). Thus, to compensate for highly-restricted cytoplasmic diffusion, RBC thickness must be reduced as appropriate for its MCHC. This can explain the inverse relationship between MCHC and RBC thickness determined from >250 animal species.

Activity and distribution of intracellular carbonic anhydrase II and their effects on the transport activity of anion exchanger AE1/SLC4A1

We have investigated the previously published 'metabolon hypothesis' postulating that a close association of the anion exchanger 1 (AE1) and cytosolic carbonic anhydrase II (CAII) exists that greatly increases the transport activity of AE1. We study whether there is a physical association of and direct functional interaction between CAII and AE1 in the native human red cell and in tsA201 cells coexpressing heterologous fluorescent fusion proteins CAII-CyPet and YPet-AE1. In these doubly transfected tsA201 cells, YPet-AE1 is clearly associated with the cell membrane, whereas CAII-CyPet is homogeneously distributed throughout the cell in a cytoplasmic pattern. Förster resonance energy transfer measurements fail to detect close proximity of YPet-AE1 and CAII-CyPet. The absence of an association of AE1 and CAII is supported by immunoprecipitation experiments using Flag-antibody against Flag-tagged AE1 expressed in tsA201 cells, which does not co-precipitate native CAII but co-precipitates coexpressed ankyrin. Both the CAII and the AE1 fusion proteins are fully functional in tsA201 cells as judged by CA activity and by cellular HCO3(-) permeability (P(HCO3(-))) sensitive to inhibition by 4,4-Diisothiocyano-2,2-stilbenedisulfonic acid. Expression of the non-catalytic CAII mutant V143Y leads to a drastic reduction of endogenous CAII and to a corresponding reduction of total intracellular CA activity. Overexpression of an N-terminally truncated CAII lacking the proposed site of interaction with the C-terminal cytoplasmic tail of AE1 substantially increases intracellular CA activity, as does overexpression of wild-type CAII. These variously co-transfected tsA201 cells exhibit a positive correlation between cellular P(HCO3(-)) and intracellular CA activity. The relationship reflects that expected from changes in cytoplasmic CA activity improving substrate supply to or removal from AE1, without requirement for a CAII-AE1 metabolon involving physical interaction. A functional contribution of the hypothesized CAII-AE1 metabolon to erythroid AE1-mediated HCO3(-) transport was further tested in normal red cells and red cells from CAII-deficient patients that retain substantial CA activity associated with the erythroid CAI protein lacking the proposed AE1-binding sequence. Erythroid P(HCO3(-)) was indistinguishable in these two cell types, providing no support for the proposed functional importance of the physical interaction of CAII and AE1. A theoretical model predicts that homogeneous cytoplasmic distribution of CAII is more favourable for cellular transport of HCO3(-) and CO2 than is association of CAII with the cytoplasmic surface of the plasma membrane. This is due to the fact that the relatively slow intracellular transport of H(+) makes it most efficient to place the CA in the vicinity of the haemoglobin molecules, which are homogeneously distributed over the cytoplasm.

Coupled Ca2+/H+ transport by cytoplasmic buffers regulates local Ca2+ and H+ ion signaling

Ca(2+) signaling regulates cell function. This is subject to modulation by H(+) ions that are universal end-products of metabolism. Due to slow diffusion and common buffers, changes in cytoplasmic [Ca(2+)] ([Ca(2+)]i) or [H(+)] ([H(+)]i) can become compartmentalized, leading potentially to complex spatial Ca(2+)/H(+) coupling. This was studied by fluorescence imaging of cardiac myocytes. An increase in [H(+)]i, produced by superfusion of acetate (salt of membrane-permeant weak acid), evoked a [Ca(2+)]i rise, independent of sarcolemmal Ca(2+) influx or release from mitochondria, sarcoplasmic reticulum, or acidic stores. Photolytic H(+) uncaging from 2-nitrobenzaldehyde also raised [Ca(2+)]i, and the yield was reduced following inhibition of glycolysis or mitochondrial respiration. H(+) uncaging into buffer mixtures in vitro demonstrated that Ca(2+) unloading from proteins, histidyl dipeptides (HDPs; e.g., carnosine), and ATP can underlie the H(+)-evoked [Ca(2+)]i rise. Raising [H(+)]i tonically at one end of a myocyte evoked a local [Ca(2+)]i rise in the acidic microdomain, which did not dissipate. The result is consistent with uphill Ca(2+) transport into the acidic zone via Ca(2+)/H(+) exchange on diffusible HDPs and ATP molecules, energized by the [H(+)]i gradient. Ca(2+) recruitment to a localized acid microdomain was greatly reduced during intracellular Mg(2+) overload or by ATP depletion, maneuvers that reduce the Ca(2+)-carrying capacity of HDPs. Cytoplasmic HDPs and ATP underlie spatial Ca(2+)/H(+) coupling in the cardiac myocyte by providing ion exchange and transport on common buffer sites. Given the abundance of cellular HDPs and ATP, spatial Ca(2+)/H(+) coupling is likely to be of general importance in cell signaling.

Sarcolemmal localisation of Na+/H+ exchange and Na+-HCO3- co-transport influences the spatial regulation of intracellular pH in rat ventricular myocytes

Membrane acid extrusion by Na(+)/H(+) exchange (NHE1) and Na(+)-HCO3(-) co-transport (NBC) is essential for maintaining a low cytoplasmic [H(+)] (∼60 nm, equivalent to an intracellular pH (pHi) of 7.2). This protects myocardial function from the high chemical reactivity of H(+) ions, universal end-products of metabolism. We show here that, in rat ventricular myocytes, fluorescent antibodies map the NBC isoforms NBCe1 and NBCn1 to lateral sarcolemma, intercalated discs and transverse tubules (t-tubules), while NHE1 is absent from t-tubules. This unexpected difference matches functional measurements of pHi regulation (using AM-loaded SNARF-1, a pH fluorophore). Thus, myocyte detubulation (by transient exposure to 1.5 m formamide) reduces global acid extrusion on NBC by 40%, without affecting NHE1. Similarly, confocal pHi imaging reveals that NBC stimulation induces spatially uniform pHi recovery from acidosis, whereas NHE1 stimulation induces pHi non-uniformity during recovery (of ∼0.1 units, for 2-3 min), particularly at the ends of the cell where intercalated discs are commonly located, and where NHE1 immunostaining is prominent. Mathematical modelling shows that this induction of local pHi microdomains is favoured by low cytoplasmic H(+) mobility and long H(+) diffusion distances, particularly to surface NHE1 transporters mediating high membrane flux. Our results provide the first evidence for a spatial localisation of [H(+)]i regulation in ventricular myocytes, suggesting that, by guarding pHi, NHE1 preferentially protects gap junctional communication at intercalated discs, while NBC locally protects t-tubular excitation-contraction coupling.

pH-dependent modulation of connexin-based gap junctional uncouplers

Gap junction (GJ) channels formed from connexin (Cx) proteins provide a direct pathway for electrical and metabolic cell–cell communication exhibiting high sensitivity to intracellular pH (pH(i)). We examined pH(i)-dependent modulation of junctional conductance (g(j)) of GJs formed of Cx26, mCx30.2, Cx36, Cx40, Cx43, Cx45, Cx46, Cx47 and Cx50 by reagents representing several distinct groups of uncouplers, such as long carbon chain alkanols (LCCAs), arachidonic acid, carbenoxolone, isoflurane, flufenamic acid and mefloquine. We demonstrate that alkalization by NH4Cl to pH ∼8 increased g(j) in cells expressing mCx30.2 and Cx45, yet did not affect g(j) of Cx26, Cx40, Cx46, Cx47 and Cx50 and decreased it in Cx43 and Cx36 GJs. Unexpectedly, cells expressing Cx45, but not other Cxs, exhibited full coupling recovery after alkalization with NH4Cl under the continuous presence of LCCAs, isoflurane and mefloquine. There was no coupling recovery by alkalization in the presence of arachidonic acid, carbenoxolone and flufenamic acid. In cells expressing Cx45, IC50 for octanol was 0.1, 0.25 and 2.68 mm at pH(i) values of 6.9, 7.2 and 8.1, respectively. Histidine modification of Cx45 protein by N-bromosuccinimide reduced the coupling-promoting effect of NH4Cl as well as the uncoupling effect of octanol. This suggests that LCCAs and some other uncouplers may act through the formation of hydrogen bonds with the as-of-yet unidentified histidine/s of the Cx45 GJ channel protein.

The role of carbonic anhydrase 9 in regulating extracellular and intracellular ph in three-dimensional tumor cell growths

We have studied the role of carbonic anhydrase 9 (CA9), a cancer-associated extracellular isoform of the enzyme carbonic anhydrase in multicellular spheroid growths (radius of approximately 300 microm) of human colon carcinoma HCT116 cells. Spheroids were transfected with CA9 (or empty vector) and imaged confocally (using fluorescent dyes) for both intracellular pH (pH(i)) and pH in the restricted extracellular spaces (pH(e)). With no CA9 expression, spheroids developed very low pH(i) (approximately 6.3) and reduced pH(e) (approximately 6.9) at their core, associated with a diminishing gradient of acidity extending out to the periphery. With CA9 expression, core intracellular acidity was less prominent (pH(i) = approximately 6.6), whereas extracellular acidity was enhanced (pH(e) = approximately 6.6), so that radial pH(i) gradients were smaller and radial pH(e) gradients were larger. These effects were reversed by eliminating CA9 activity with membrane-impermeant CA inhibitors. The observation that CA9 activity reversibly reduces pH(e) indicates the enzyme is facilitating CO(2) excretion from cells (by converting vented CO(2) to extracellular H(+)), rather than facilitating membrane H(+) transport (such as H(+) associated with metabolically generated lactic acid). This latter process requires titration of exported H(+) ions with extracellular HCO(3)(-), which would reduce rather than increase extracellular acidity. In a multicellular structure, the net effect of CA9 on pH(e) will depend on the cellular CO(2)/lactic acid emission ratio (set by local oxygenation and membrane HCO(3)(-) uptake). Our results suggest that CO(2)-producing tumors may express CA9 to facilitate CO(2) excretion, thus raising pH(i) and reducing pH(e), which promotes tumor proliferation and survival. The results suggest a possible basis for attenuating tumor development through inhibiting CA9 activity.

Gating, permselectivity and pH-dependent modulation of channels formed by connexin57, a major connexin of horizontal cells in the mouse retina

Mouse connexin57 (Cx57) is expressed most abundantly in horizontal cells of the retina, and forms gap junction (GJ) channels, which constitute a structural basis for electrical and metabolic intercellular communication, and unapposed hemichannels (UHCs) that are involved in an exchange of ions and metabolites between the cytoplasm and extracellular milieu. By combining fluorescence imaging and dual whole-cell voltage clamp methods, we showed that HeLa cells expressing Cx57 and C-terminally fused with enhanced green fluorescent protein (Cx57-EGFP) form junctional plaques (JPs) and that only cell pairs exhibiting at least one JP demonstrate cell-to-cell electrical coupling and transfer of negatively and positively charged dyes with molecular mass up to approximately 400 Da. The permeability of the single Cx57 GJ channel to Alexa fluor-350 is approximately 90-fold smaller than the permeability of Cx43, while its single channel conductance (57 pS) is only 2-fold smaller than Cx43 (110 pS). Gating of Cx57-EGFP/Cx45 heterotypic GJ channels reveal that Cx57 exhibit a negative gating polarity, i.e. channels tend to close at negativity on the cytoplasmic side of Cx57. Alkalization of pH(i) from 7.2 to 7.8 increased gap junctional conductance (g(j)) of approximately 100-fold with pK(a) = 7.41. We show that this g(j) increase was caused by an increase of both the open channel probability and the number of functional channels. Function of Cx57 UHCs was evaluated based on the uptake of fluorescent dyes. We found that under control conditions, Cx57 UHCs are closed and open at [Ca(2+)](o) = approximately 0.3 mm or below, demonstrating that a moderate reduction of [Ca(2+)](o) can facilitate the opening of Cx57 UHCs. This was potentiated with intracellular alkalization. In summary, our data show that the open channel probability of Cx57 GJs can be modulated by pH(i) with very high efficiency in the physiologically relevant range and may explain pH-dependent regulation of cell-cell coupling in horizontal cell in the retina.

Image analysis software based on color segmentation for characterization of viability and physiological activity of biofilms

The novel image analysis software package bioImage_L was tested to calculate biofilm structural parameters in oral biofilms stained with dual-channel fluorescent markers. By identifying color tonalities in situ, the software independently processed the color subpopulations and characterized the viability and metabolic activity of biofilms.

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.

Ca2+-mobility in the sarcoplasmic reticulum of ventricular myocytes is low

The sarcoplasmic reticulum (SR) in ventricular myocytes contains releasable Ca(2+) for activating cellular contraction. Recent measurements of intra-SR (luminal) Ca(2+) suggest a high diffusive Ca(2+)-mobility constant (D(CaSR)). This could help spatially to unify SR Ca(2+)-content ([Ca(2+)](SRT)) and standardize Ca(2+)-release throughout the cell. But measurements of localized depletions of luminal Ca(2+) (Ca(2+)-blinks), associated with local Ca(2+)-release (Ca(2+)-sparks), suggest D(CaSR) may actually be low. Here we describe a novel method for measuring D(CaSR). Using a cytoplasmic Ca(2+)-fluorophore, we estimate regional [Ca(2+)](SRT) from localized, caffeine-induced SR Ca(2+)-release. Caffeine microperfusion of one end of a guinea pig or rat myocyte diffusively empties the whole SR at a rate indicating D(CaSR) is 8-9 microm(2)/s, up to tenfold lower than previous estimates. Ignoring background SR Ca(2+)-leakage in our measurement protocol produces an artifactually high D(CaSR) (>40 microm(2)/s), which may also explain the previous high values. Diffusion-reaction modeling suggests that a low D(CaSR) would be sufficient to support local SR Ca(2+)-signaling within sarcomeres during excitation-contraction coupling. Low D(CaSR) also implies that [Ca(2+)](SRT) may readily become spatially nonuniform, particularly under pathological conditions of spatially nonuniform Ca(2+)-release. Local control of luminal Ca(2+), imposed by low D(CaSR), may complement the well-established local control of SR Ca(2+)-release by Ca(2+)-channel/ryanodine receptor couplons.

Measuring and modeling chloride-hydroxyl exchange in the Guinea-pig ventricular myocyte

Protons are powerful modulators of cardiac function. Their intracellular concentration is regulated by sarcolemmal ion transporters that export or import H+-ions (or their ionic equivalent: HCO3-, OH-). One such transporter, which imports H+-equivalents, is a putative Cl-/OH- exchanger (CHE). A strong candidate for CHE is SLC26A6 protein, a product of the SLC26A gene family of anion transporters, which has been detected in murine heart. SLC26A6 protein is suggested to be an electrogenic 1Cl-/2OH-(2HCO3-) exchanger. Unfortunately, there is insufficient characterization of cardiac CHE against which the properties of heterologously expressed SLC26A6 can be matched. We therefore investigated the proton, Cl-, and voltage dependence of CHE activity in guinea-pig ventricular myocytes, using voltage-clamp, intracellular pH fluorescence, and mathematical modeling techniques. We find that CHE activity is tightly regulated by intracellular and extracellular pH, is voltage-insensitive over a wide range (+/-80 mV), and displays substrate dependence suggestive of electroneutral 1Cl-/1OH- exchange. These properties exclude electrogenic SLC26A6 as sole contributor to CHE. Either the SLC26A6 product in heart is electroneutral, or CHE comprises at least two transporters with oppositely balanced voltage sensitivity. Alternatively, CHE may comprise an H+-Cl- coinflux system, which cannot be distinguished kinetically from an exchanger. Irrespective of ionic mechanism, CHE's pH sensitivity helps to define resting intracellular pH, and hence basal function in the heart.

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.

A mathematical model of the slow force response to stretch in rat ventricular myocytes

We developed a model of the rat ventricular myocyte at room temperature to predict the relative effects of different mechanisms on the cause of the slow increase in force in response to a step change in muscle length. We performed simulations in the presence of stretch-dependent increases in flux through the Na(+)-H(+) exchanger (NHE) and Cl(-)-HCO(3)(-) exchanger (AE), stretch-activated channels (SAC), and the stretch-dependent nitric oxide (NO) induced increased open probability of the ryanodine receptors to estimate the capacity of each mechanism to produce the slow force response (SFR). Inclusion of stretch-dependent NHE & AE, SACs, and stretch-dependent NO effects caused an increase in tension following 15 min of stretch of 0.87%, 32%, and 0%, respectively. Comparing [Ca(2+)](i) dynamics before and after stretch in the presence of combinations of the three stretch-dependent elements, which produced significant SFR values (>20%), showed that the inclusion of stretch-dependent NO effects produced [Ca(2+)](i) transients, which were not consistent with experimental results. Further simulations showed that in the presence of SACs and the absence of stretch-dependent NHE & AE inhibition of NHE attenuated the SFR, such that reduced SFR in the presence of NHE blockers does not indicate a stretch dependence of NHE. Rather, a functioning NHE is responsible for a portion of the SFR. Based on our simulations we estimate that in rat cardiac myocytes at room temperature SACs play a significant role in producing the SFR, potentially in the presence of stretch-dependent NHE & AE and that NO effects, if any, must involve more mechanisms than just increasing the open probability of ryanodine receptors.

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.

Extracellular pH changes activate the p38-MAPK signalling pathway in the amphibian heart

We investigated the activation of the p38-MAPK signalling pathway during extracellular pH changes in the isolated perfused amphibian heart. Extracellular alkalosis (pH 8.5 or 9.5) maximally activated p38-MAPK within 2 min (4.17- and 3.20-fold, respectively) and this effect was reversible since the kinase phosphorylation levels decreased upon reperfusing the heart with normal Tris–Tyrode's buffer. Extracellular acidosis also activated p38-MAPK moderately, but persistently (1.65-fold, at 1 min and 1.91-fold, at 60 min). The alkalosis-induced p38-MAPK activation depended upon the Na+/H+ exchanger (NHE) and Na+/K+-ATPase, because it was abolished when the NHE inhibitors amiloride and HOE642 and the Na+/K+-ATPase inhibitor, ouabain, were used. Our studies also showed that extracellular alkalosis (pH 8.5) induced MAPKAPK2 phosphorylation (2.59-fold, 2 min) and HSP27 phosphorylation (5.33-fold, 2 min) in a p38-MAPK-dependent manner, as it was inhibited with 1 μmol l–1 SB203580. Furthermore,immunohistochemical studies of the phosphorylated forms of p38-MAPK and HSP27 revealed that these proteins were localised in the perinuclear region and dispersedly in the cytoplasm of ventricular cells during alkalosis. Finally,alkalosis induced the increase of HSP70 protein levels (1.52-fold, 5 min), but independently of p38-MAPK activation. These data indicate that the p38-MAPK signalling pathway is activated by extracellular pH changes and in the case of alkalosis this activation may have a protective role.

The effect of neuronal morphology and membrane-permeant weak acid and base on the dissipation of depolarization-induced pH gradients in snail neurons

Neuronal depolarization causes larger intracellular pH (pH(i)) shifts in axonal and dendritic regions than in the cell body. In this paper, we present evidence relating the time for collapse of these gradients to neuronal morphology. We have used ratiometric pH(i) measurements using 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) in whole-cell patch-clamped snail neurons to study the collapse of longitudinal pH gradients. Using depolarization to open voltage-gated proton channels, we produced alkaline pH(i) microdomains. In the absence of added mobile buffers, facilitated H(+) diffusion down the length of the axon plays a critical role in determining pH(i) microdomain lifetime, with axons of approximately 100 microm allowing pH differences to be maintained for >60 s. An application of mobile, membrane-permeant pH buffers accelerated the collapse of the alkaline-pH gradients but, even at 30 mM, was unable to abolish them. Modeling of the pH(i) dynamics showed that both the relatively weak effect of the weak acid/base on the peak size of the pH gradient and the accelerated collapse of the pH gradient could be due to the time taken for equilibration of the weak acid and base across the cell. We propose that appropriate weak acid/base mixes may provide a simple method for studying the role of local pH(i) signals without perturbing steady-state pH(i). Furthermore, an extrapolation of our in vitro data to longer and thinner neuronal structures found in the mammalian nervous system suggests that dendritic and axonal pH(i) are likely to be dominated by local pH(i)-regulating mechanisms rather than simply following the soma pH(i).