Effect of repetitive activity upon intracellular pH, sodium and contraction in sheep cardiac Purkinje fibres

Alkaline nucleoplasm facilitates contractile gene expression in the mammalian heart

Cardiac contractile strength is recognised as being highly pH-sensitive, but less is known about the influence of pH on cardiac gene expression, which may become relevant in response to changes in myocardial metabolism or vascularization during development or disease. We sought evidence for pH-responsive cardiac genes, and a physiological context for this form of transcriptional regulation. pHLIP, a peptide-based reporter of acidity, revealed a non-uniform pH landscape in early-postnatal myocardium, dissipating in later life. pH-responsive differentially expressed genes (pH-DEGs) were identified by transcriptomics of neonatal cardiomyocytes cultured over a range of pH. Enrichment analysis indicated "striated muscle contraction" as a pH-responsive biological process. Label-free proteomics verified fifty-four pH-responsive gene-products, including contractile elements and the adaptor protein CRIP2. Using transcriptional assays, acidity was found to reduce p300/CBP acetylase activity and, its a functional readout, inhibit myocardin, a co-activator of cardiac gene expression. In cultured myocytes, acid-inhibition of p300/CBP reduced H3K27 acetylation, as demonstrated by chromatin immunoprecipitation. H3K27ac levels were more strongly reduced at promoters of acid-downregulated DEGs, implicating an epigenetic mechanism of pH-sensitive gene expression. By tandem cytoplasmic/nuclear pH imaging, the cardiac nucleus was found to exercise a degree of control over its pH through Na+/H+ exchangers at the nuclear envelope. Thus, we describe how extracellular pH signals gain access to the nucleus and regulate the expression of a subset of cardiac genes, notably those coding for contractile proteins and CRIP2. Acting as a proxy of a well-perfused myocardium, alkaline conditions are permissive for expressing genes related to the contractile apparatus.

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.

The Control of Diastolic Calcium in the Heart: Basic Mechanisms and Functional Implications

Normal cardiac function requires that intracellular Ca²⁺ concentration be reduced to low levels in diastole so that the ventricle can relax and refill with blood. Heart failure is often associated with impaired cardiac relaxation. Little, however, is known about how diastolic intracellular Ca²⁺ concentration is regulated. This article first discusses the reasons for this ignorance before reviewing the basic mechanisms that control diastolic intracellular Ca²⁺ concentration. It then considers how the control of systolic and diastolic intracellular Ca²⁺ concentration is intimately connected. Finally, it discusses the changes that occur in heart failure and how these may result in heart failure with preserved versus reduced ejection fraction.

Modulation of the cardiac Na+-Ca2+ exchanger by cytoplasmic protons: Molecular mechanisms and physiological implications

A precise temporal and spatial control of intracellular Ca²⁺ concentration is essential for a coordinated contraction of the heart. Following contraction, cardiac cells need to rapidly remove intracellular Ca²⁺ to allow for relaxation. This task is performed by two transporters: the plasma membrane Na⁺-Ca²⁺ exchanger (NCX) and the sarcoplasmic reticulum (SR) Ca²⁺-ATPase (SERCA). NCX extrudes Ca²⁺ from the cell, balancing the Ca²⁺entering the cytoplasm during systole through L-type Ca²⁺ channels. In parallel, following SR Ca²⁺ release, SERCA activity replenishes the SR, reuptaking Ca²⁺ from the cytoplasm. The activity of the mammalian exchanger is fine-tuned by numerous ionic allosteric regulatory mechanisms. Micromolar concentrations of cytoplasmic Ca²⁺ potentiate NCX activity, while an increase in intracellular Na⁺ levels inhibits NCX via a mechanism known as Na⁺-dependent inactivation. Protons are also powerful inhibitors of NCX activity. By regulating NCX activity, Ca²⁺, Na⁺ and H⁺ couple cell metabolism to Ca²⁺ homeostasis and therefore cardiac contractility. This review summarizes the recent progress towards the understanding of the molecular mechanisms underlying the ionic regulation of the cardiac NCX with special emphasis on pH modulation and its physiological impact on the heart.

pH recovery from a proton load in rat cardiomyocytes: effects of chronic exercise

The ability of cardiomyocytes to recover from a proton load was examined in the hearts of exercise-trained and sedentary control rats in CO₂/[Formula: see text]-free media. Acidosis was created by the NH₄Cl prepulse technique, and intracellular pH (pH_i) was determined using fluorescence microscopy on carboxy-SNARF-1 AM-loaded isolated cardiomyocytes. CO₂-independent pH_i buffering capacity (β_i) was measured by incrementally reducing the extracellular NH₄Cl concentration in steps of 50% from 20 to 1.25 mM. β_i increased as pH_i decreased in both exercise-trained and sedentary control groups. However, the magnitude of increase in β_i as a function of pH_i was found to be significantly ( P < 0.001) greater in the exercise-trained group compared with the sedentary control group. The rate of pH_i recovery from an imposed proton load was found to not be different between the exercise-trained and control groups. The Na⁺/H⁺ exchanger-dependent H⁺ extrusion rate during the recovery from an imposed proton load, however, was found to be significantly greater in the exercise-trained group compared with the control group. By increasing β_i and subsequently the Na⁺/H⁺ exchanger-dependent H⁺ extrusion rate, exercise training may provide cardiomyocytes with the ability to better handle an intracellular excess of H⁺ generated during hypoxia/ischemic insults and may serve in a cardioprotective role. These data may be predictive of two positive outcomes: 1) increased exercise tolerance by the heart and 2) a protective mechanism that limits the degree of myocardial acidosis and subsequent damage that accompanies ischemia-reperfusion stress. NEW & NOTEWORTHY The enhanced ability to deal with acidosis conferred by exercise training is likely to improve exercise tolerance and outcomes in response to myocardial ischemia-reperfusion injury.

Mapping of intracellular pH in the in vivo rodent heart using hyperpolarized [1-13C]pyruvate

Purpose

To demonstrate the feasibility of mapping intracellular pH within the in vivo rodent heart. Alterations in cardiac acid‐base balance can lead to acute contractile depression and alterations in Ca²⁺ signaling. The transient reduction in adenosine triphosphate (ATP) consumption and cardiac contractility may be initially beneficial; however, sustained pH changes can be maladaptive, leading to myocardial damage and electrical arrhythmias.

Methods

Spectrally selective radiofrequency (RF) pulses were used to excite the HCO3− and CO₂ resonances individually while preserving signal from the injected hyperpolarized [1‐¹³C]pyruvate. The large flip angle pulses were placed within a three‐dimensional (3D) imaging acquisition, which exploited CA‐mediated label exchange between HCO3− and CO₂. Images at 4.5 × 4.5 × 5 mm³ resolution were obtained in the in vivo rodent heart. The technique was evaluated in healthy rodents scanned at baseline and during high cardiac workload induced by dobutamine infusion.

Results

The intracellular pH was measured to be 7.15 ± 0.04 at baseline, and decreased to 6.90 ± 0.06 following 15 min of continuous β‐adrenergic stimulation.

Conclusions

Volumetric maps of intracellular pH can be obtained following an injection of hyperpolarized [1‐¹³C]pyruvate. The new method is anticipated to enable assessment of stress‐inducible ischemia and potential ventricular arrythmogenic substrates within the ischemic heart. Magn Reson Med 77:1810–1817, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Constitutive Intracellular Na+ Excess in Purkinje Cells Promotes Arrhythmogenesis at Lower Levels of Stress Than Ventricular Myocytes From Mice With Catecholaminergic Polymorphic Ventricular Tachycardia

Supplemental Digital Content is available in the text.

Background—

In catecholaminergic polymorphic ventricular tachycardia (CPVT), cardiac Purkinje cells (PCs) appear more susceptible to Ca²⁺ dysfunction than ventricular myocytes (VMs). The underlying mechanisms remain unknown. Using a CPVT mouse (RyR2^{R4496C+/Cx40eGFP}), we tested whether PC intracellular Ca²⁺ ([Ca²⁺]_i) dysregulation results from a constitutive [Na⁺]_i surplus relative to VMs.

Methods and Results—

Simultaneous optical mapping of voltage and [Ca²⁺]_i in CPVT hearts showed that spontaneous Ca²⁺ release preceded pacing-induced triggered activity at subendocardial PCs. On simultaneous current-clamp and Ca²⁺ imaging, early and delayed afterdepolarizations trailed spontaneous Ca²⁺ release and were more frequent in CPVT PCs than CPVT VMs. As a result of increased activity of mutant ryanodine receptor type 2 channels, sarcoplasmic reticulum Ca²⁺ load, measured by caffeine-induced Ca²⁺ transients, was lower in CPVT VMs and PCs than respective controls, and sarcoplasmic reticulum fractional release was greater in both CPVT PCs and VMs than respective controls. [Na⁺]_i was higher in both control and CPVT PCs than VMs, whereas the density of the Na⁺/Ca²⁺ exchanger current was not different between PCs and VMs. Computer simulations using a PC model predicted that the elevated [Na⁺]_i of PCs promoted delayed afterdepolarizations, which were always preceded by spontaneous Ca²⁺ release events from hyperactive ryanodine receptor type 2 channels. Increasing [Na⁺]_i monotonically increased delayed afterdepolarization frequency. Confocal imaging experiments showed that postpacing Ca²⁺ spark frequency was highest in intact CPVT PCs, but such differences were reversed on saponin-induced membrane permeabilization, indicating that differences in [Na⁺]_i played a central role.

Conclusions—

In CPVT mice, the constitutive [Na⁺]_i excess of PCs promotes triggered activity and arrhythmogenesis at lower levels of stress than VMs.

Antiarrhythmic effects of dehydroevodiamine in isolated human myocardium and cardiomyocytes

ETHNOPHARMACOLOGICAL RELEVANCE

Dehydroevodiamine alkaloid (DeHE), a bioactive component of the Chinese herbal medicine Wu-Chu-Yu (Evodiae frutus), exerted antiarrhythmic effect in guinea-pig ventricular myocytes. We further characterize the electromechanical effects of DeHE in the human atrial and ventricular tissues obtained from hearts of patients undergoing corrective cardiac surgery or heart transplantation.

MATERIALS AND METHODS

The transmembrane potentials of human myocardia were recorded with a traditional microelectrode technique while sarcolemmal Na(+) and Ca(2+) currents in single human cardiomyocytes were measured by a whole-cell patch-clamp technique. The intracellular pH (pHi) and Na(+)-H(+) exchanger (NHE) activity were determined using BCECF-fluorescence in human atria.

RESULTS

In human atria, DeHE (0.1-0.3 μM) depressed upstroke velocity, amplitude of action potential, and contractile force, both in slow and fast response action potential. Moreover, the similar depressant effects of DeHE were found in human ventricular myocardium. Both in isolated human atrial and ventricular myocytes, DeHE (0.1-1 μM) reversibly, concentration-dependently decreased the Na(+) and Ca(2+)currents. Moreover, DeHE (0.1 and 0.3 μM) suppressed delayed afterdepolarizations and aftercontractions, induced by epinephrine and high [Ca(2+)]o in atria. In human ventricular myocardium, the strophanthidin-induced triggered activities were attenuated by pretreating DeHE (0.3 μM). The resting pHi and NHE activity were also significantly increased by DeHE (0.1-0.3 μM).

CONCLUSIONS

We concluded for the first time that, in the human hearts, DeHE could antagonize triggered arrhythmias induced by cardiotonic agents through a general reduction of the Na(+) and Ca(2+) inward currents, while increase of resting pHi and NHE activity.

Modulation of the cardiac sodium/bicarbonate cotransporter by the renin angiotensin aldosterone system: pathophysiological consequences

The sodium/bicarbonate cotransporter (NBC) is one of the major alkalinizing mechanisms in the cardiomyocytes. It has been demonstrated the existence of at least two functional isoforms, one that promotes the co-influx of 1 molecule of Na⁺ per 1 molecule of HCO⁻₃ (electroneutral isoform; NBCn1) and the other one that generates the co-influx of 1 molecule of Na⁺ per 2 molecules of HCO⁻₃ (electrogenic isoform; NBCe1). Both isoforms are important to maintain intracellular pH (pH_i) and sodium concentration ([Na⁺]_i). In addition, NBCe1 generates an anionic repolarizing current that modulates the action potential duration (APD). The renin-angiotensin-aldosterone system (RAAS) is implicated in the modulation of almost all physiological cardiac functions and is also involved in the development and progression of cardiac diseases. It was reported that angiotensin II (Ang II) exhibits an opposite effect on NBC isoforms: it activates NBCn1 and inhibits NBCe1. The activation of NBCn1 leads to an increase in pH_i and [Na⁺]_i, which indirectly, due to the stimulation of reverse mode of the Na⁺/Ca²⁺ exchanger (NCX), conduces to an increase in the intracellular Ca²⁺ concentration. On the other hand, the inhibition of NBCe1 generates an APD prolongation, potentially representing a risk of arrhythmias. In the last years, the potentially altered NBC function in pathological scenarios, as cardiac hypertrophy and ischemia-reperfusion, has raised increasing interest among investigators. This review attempts to draw the attention on the relevant regulation of NBC activity by RAAS, since it modulates pH_i and [Na⁺]_i, which are involved in the development of cardiac hypertrophy, the damage produced by ischemia-reperfusion and the generation of arrhythmic events, suggesting a potential role of NBC in cardiac diseases.

Regulation of the cardiac sodium/bicarbonate cotransporter by angiotensin II: potential Contribution to structural, ionic and electrophysiological myocardial remodelling

The sodium/ bicarbonate cotransporter (NBC) is, with the Na⁺/H⁺ exchanger (NHE), an important alkalinizing mechanism that maintains cellular intracellular pH (pHi). In the heart exists at least three isoforms of NBC, one that promotes the co-influx of 1 molecule of Na⁺ per 1molecule of HCO₃^-(electroneutral isoform; nNBC) and two others that generates the co-influx of 1 molecule of Na⁺ per 2 molecules of HCO₃^- (electrogenic isoforms; eNBC). In addition, the eNBC generates an anionic repolarizing current that modulate the cardiac action potential (CAP), adding to such isoforms the relevance to modulate the electrophysiological function of the heart. Angiotensin II (Ang II) is one of the main hormones that regulate cardiac physiology. The alkalinizing mechanisms (NHE and NBC) are stimulated by Ang II, increasing pHi and intracellular Na⁺ concentration, which indirectly, due to the stimulation of the Na⁺/Ca²⁺ exchanger (NCX) operating in the reverse form, leads to an increase in the intracellular Ca²⁺ concentration. Interestingly, it has been shown that Ang II exhibits an opposite effect on NBC isoforms: it activates the nNBC and inhibits the eNBC. This inhibition generates a CAP prolongation, which could directly increase the intracellular Ca²⁺ concentration. The regulation of the intracellular Na⁺ and Ca²⁺ concentrations is crucial for the cardiac cellular physiology, but these ions are also involved in the development of cardiac hypertrophy and the damage produced by ischemia-reperfusion, suggesting a potential role of NBC in cardiac diseases.

Effect of SR load and pH regulatory mechanisms on stretch-dependent Ca(2+) entry during the slow force response

When cardiac muscle is stretched, there is an initial inotropic response that coincides with the stretch followed by a slower increase in twitch force that develops over several minutes (the "slow force response", or SFR). Unlike the initial response to stretch, the SFR is produced by an increase in Ca(2+) transient amplitude, but the cellular mechanisms that give rise to the increased transients are still debated. We have examined the relationship between the SFR, intracellular [Ca(2+)] and the inotropic state of right ventricular trabeculae from rat hearts at 37°C. The magnitude of the SFR varied with [Ca(2+)]o and stimulation frequency, so that the SFR was greatest for conditions associated with a reduced SR Ca(2+) content. The SFR was not blocked by the AT1 receptor blocker losartan, but was reduced by SN-6, an inhibitor of reverse mode Na(+)/Ca(2+)-exchange (NCX). The Na(+)/H(+)-exchange (NHE) inhibitor HOE642 had no effect in HCO3(-)-buffered solutions, but blocked 50% of the SFR in HCO3(-)-free solution. Inhibition of HCO3(-) transport by DIDS increased the SFR and made it sensitive to HOE642. The addition of cross-bridge cycle inhibitors (20mM BDM or 20μM blebbistatin) to the superfusate reduced the SFR as monitored by changes in Ca(2+). In HCO3(-)-free conditions, the SFR was associated with a slow acidification that was inhibited by BDM, and by stopping electrical stimulation. These results can be explained by stretch increasing metabolic demand and stimulating Na(+) entry via both NHE and the Na(+)/HCO3(-) transporters. This mechanism provides a novel link between inotropic state and stretch, as well as a way for the cell to compensate for increased acid load. The feedback mechanism between force and Ca(2+) transient amplitude that we describe is also limited by the degree of SR Ca(2+) load.

Influence of pH on Ca²⁺ current and its control of electrical and Ca²⁺ signaling in ventricular myocytes

Modulation of L-type Ca(2+) current (I(Ca,L)) by H(+) ions in cardiac myocytes is controversial, with widely discrepant responses reported. The pH sensitivity of I(Ca,L) was investigated (whole cell voltage clamp) while measuring intracellular Ca(2+) (Ca(2+)(i)) or pH(i) (epifluorescence microscopy) in rabbit and guinea pig ventricular myocytes. Selectively reducing extracellular or intracellular pH (pH(o) 6.5 and pH(i) 6.7) had opposite effects on I(Ca,L) gating, shifting the steady-state activation and inactivation curves to the right and left, respectively, along the voltage axis. At low pH(o), this decreased I(Ca,L), whereas at low pH(i), it increased I(Ca,L) at clamp potentials negative to 0 mV, although the current decreased at more positive potentials. When Ca(2+)(i) was buffered with BAPTA, the stimulatory effect of low pH(i) was even more marked, with essentially no inhibition. We conclude that extracellular H(+) ions inhibit whereas intracellular H(+) ions can stimulate I(Ca,L). Low pH(i) and pH(o) effects on I(Ca,L) were additive, tending to cancel when appropriately combined. They persisted after inhibition of calmodulin kinase II (with KN-93). Effects are consistent with H(+) ion screening of fixed negative charge at the sarcolemma, with additional channel block by H(+)(o) and Ca(2+)(i). Action potential duration (APD) was also strongly H(+) sensitive, being shortened by low pH(o), but lengthened by low pH(i), caused mainly by H(+)-induced changes in late Ca(2+) entry through the L-type Ca(2+) channel. Kinetic analyses of pH-sensitive channel gating, when combined with whole cell modeling, successfully predicted the APD changes, plus many of the accompanying changes in Ca(2+) signaling. We conclude that the pH(i)-versus-pH(o) control of I(Ca,L) will exert a major influence on electrical and Ca(2+)-dependent signaling during acid-base disturbances in the heart.

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.

The role of Na dysregulation in cardiac disease and how it impacts electrophysiology

Ca(2+) is well known as the central player in cardiac cell physiology, mediating Ca(2+) activation of myosin ATPase and contraction, the stimulation of Ca(2+)-activated signaling pathways and modulation of mitochondrial energy production. Abnormalities of Ca(2+) handling are a well-studied mechanism of decompensation in heart failure. Less appreciated is the role of cytosolic Na(+) (Na(i) (+)), which can dramatically influence the transfer rates and distribution of Ca(2+) among the intracellular compartments of the myocyte. Since Na(i) (+) can vary widely under different physiological and pathological conditions, and its effects depend on multiple ion gradients and membrane electrical potentials, unraveling the global influence of Na(i) (+) on cell function is complex, requiring an integrative view of cardiomyocyte physiology. Here, we discuss how abnormal Na(i) (+) regulation not only influences the cytosolic Ca(2+) transient and the cellular action potential but also alters mitochondrial Ca(2+) uptake and the balance of energy supply and demand of the cardiomyocyte, which may contribute to oxidative stress and cardiac decompensation. The implications for sudden cardiac death and the potential for novel therapeutic interventions are discussed.

Regulation of intracellular pH in cardiac muscle

Intracellular pH (pHi) in sheep cardiac Purkinje fibres is controlled by sarcolemmal Na+/H+ and Cl-/HCO3- exchange. At normal pHo (7.4), Na+/H+ exchange mediates an acid efflux whenever pHi falls and Cl-/HCO3- exchange mediates an equivalent acid influx in response to a rise in pHi. Intracellular pH is also influenced by Ca2+i, which can activate force development leading to the anaerobic production of lactic acid. This is evident after an increase in stimulation rate which reversibly reduces both pHi and extracellular surface pH (pHs). Rate-dependent pHi changes are inhibited following inhibition of glycolysis, indicating that they are caused by accumulation of lactic acid. In some cases, the efflux of lactic acid may provide a faster method for recovery of pHi from a metabolic acidosis than that provided by Na+/H+ exchange. Finally, direct pHi measurement in isolated mammalian ventricular myocytes suggests that the intrinsic intracellular buffering power (beta) of ventricular tissue may be considerably lower than previously believed. An accurate knowledge of beta is essential for calculating net membrane fluxes of acid equivalents from changes in pHi.

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.

Differential effects of low glucose concentrations on seizures and epileptiform activityin vivoandin vitro

In vivo, severe hypoglycemia is frequently associated with seizures. The hippocampus is a structure prone to develop seizures and seizure-induced damage. Patients with repeated hypoglycemic episodes have frequent memory problems, suggesting impaired hippocampal function. Here we studied the effects of moderate hypoglycemia on primarily generalized flurothyl-induced seizures in vivo and, using EEG recordings, we determined involvement of the hippocampus in hypoglycemic seizures. Moderate systemic hypoglycemia had proconvulsant effects on flurothyl-induced clonic (forebrain) seizures. During hypoglycemic seizures, seizure discharges were recorded in the hippocampus. Thus, we continued the studies in combined entorhinal cortex-hippocampus slices in vitro. However, in vitro, decreases in extracellular glucose from baseline 10 mM to 2 or 1 mM did not induce any epileptiform discharges. In fact, low glucose (2 and 1 mM) attenuated preexisting low-Mg2+-induced epileptiform activity in the entorhinal cortex and hippocampal CA1 region. Osmolarity compensation in low-glucose solution using mannitol impaired slice recovery. Additionally, using paired-pulse stimuli we determined that there was no impairment of GABAA inhibition in the dentate gyrus during glucopenia. The data strongly indicate that, although forebrain susceptibility to seizures is increased during moderate in vivo hypoglycemia and the hippocampus is involved during hypoglycemic seizures, glucose depletion in vitro contributes to an arrest of epileptiform activity in the system of the entorhinal cortex-hippocampus network and there is no impairment of net GABAA inhibition during glucopenia.

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.

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.

Intracellular Ca2+ concentration and rate adaptation of the cardiac action potential

Influx of Ca(2+) ions through the cardiac plasma membrane contributes to the shaping of the action potential plateau and acts as trigger for the release of Ca(2+) ions from the sarcoplasmic reticulum and the initiation of the contractile process. The increased intracellular Ca(2+) concentration feeds back on the channels and transporters in the plasma membrane and modulates the electrical activity. This interaction and its change with rate of pacing is the topic of this review, which is subdivided in three parts. In part I a description is given of different channels and transporters that carry Ca(2+) ions, or are activated-modulated by intracellular Ca(2+) ions. In part II an analysis is given of the changes in action potential duration and shape when stimuli are applied in the relative refractory period (electrical restitution) and when rate is suddenly increased and kept at the higher level until steady-state is obtained. A description of experimental findings in each case is followed by a discussion of possible mechanisms. Part III deals with physiopathological aspects of Ca(2+) handling and discusses recent information on hypertrophy, heart failure and atrial fibrillation.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Rate-dependent changes of twitch force duration in rat cardiac trabeculae: a property of the contractile system

1. We examined the mechanisms for rate-dependent changes in twitch force duration by simultaneously measuring force and [Ca2+]i in rat cardiac trabeculae. 2. Peak force decreased when the rate of stimulation was increased from 0.2 to 0.5 Hz, whilst it increased from 1 to 2 Hz. Over the same range of frequencies, peak [Ca2+]i transients increased monotonically, whilst both force and [Ca2+]i transient duration were abbreviated. 3. Changes in peak force or peak [Ca2+]i transients were not responsible for the changes in force or [Ca2+]i transient duration. 4. The changes in twitch force and [Ca2+]i transient duration were completed roughly within one beat following an abrupt change in the rate of stimulation. 5. Rate-dependent changes resembled those observed with isoproterenol (isoprenaline) application. However, kinase inhibitors (i.e. K252-a, H-89, KN-62 and KN-93) had no effect on the rate-dependent changes of twitch force and [Ca2+]i transient kinetics, suggesting that protein kinase A (PKA), protein kinase PKG) and Ca2+-calmodulin-dependent protein kinase II (CaM/kinase II) were not responsible for these kinetic changes. 6. Despite the changes in twitch force and [Ca2+]i transient kinetics, the rate-limiting step for the rate-dependent force relaxation still resides at the level of the contractile proteins. 7. Our results suggest that rate-dependent changes in force and [Ca2+]i transients do not depend on PKA or CaM/kinase II activity but might result from intrinsic features of the contractile and/or Ca2+-handling proteins.

Role of an electrogenic Na(+)-HCO3- cotransport in determining myocardial pHi after an increase in heart rate

The contribution of electrogenic Na(+)-HCO3- cotransport to pHi regulation during changes in heart rate was explored in cat papillary muscles loaded with BCECF-AM in bicarbonate-free (HEPES) medium and in CO2/HCO3(-)-buffered medium. Stepwise increments in the frequency of contraction from 15 to 100 bpm induced a reversible increase in the pHi from 7.13 +/- 0.03 to 7.36 +/- 0.03 (P < .05, n = 5) in the presence of CO2/ HCO3- buffer. The same increase in the frequency of stimulation, however, decreased pHi from 7.10 +/- 0.02 to 6.91 +/- 0.06 (P < .05, n = 5), in the absence of bicarbonate. Moreover, in CO2/HCO3(-)-superfused muscles pretreated with SITS (0.1 mmol/L), this effect of increasing the contraction frequency was reversed, and a decrease of pHi from 7.03 +/- 0.04 to 6.88 +/- 0.06 (P < .05, n = 4) was observed when the pacing rate was increased stepwise from 15 to 100 bpm. High [K+]o-induced depolarization of cell membrane alkalinized myocardial cells in the presence of HCO3- ions, whereas acidification was observed as a consequence of hyperpolarization induced by low external [K+]o. Myocardial resting membrane potential became hyperpolarized upon exposure to HCO3(-)-buffered media. This HCO3(-)-induced hyperpolarization was not blocked by the inhibition of Na+,K(+)-ATPase activity by ouabain (0.5 mumol/L) but was prevented by SITS. The results suggested that membrane depolarization during cardiac action potential causes an increase in electrogenic Na(+)-HCO3- cotransport. Such depolarizations occurring as a consequence of increases in heart rate would thus, by means of elevated bicarbonate influxes, substantially increase the myocardial cell's ability to recover from an enhanced proton production.

Mechanism of the negative force-frequency relationship in physiologically intact rat ventricular myocardium--studies by intracellular Ca2+ monitor with indo-1 and by 31P-nuclear magnetic resonance spectroscopy

We studied the subcellular mechanisms of the negative force-frequency relationship in rat myocardium by measuring 1) intracellular Ca2+ transients by indo-1 fluorometry and 2) intracellular pH (pHi) and phosphate compounds with 31P-nuclear magnetic resonance (NMR). The data were compared with those from guinea pig hearts, which show a positive force-frequency relationship. By increasing the pacing rate from 3 Hz to 5 Hz, the peak positive first derivative of left ventricular pressure (LVdP/dt) in rat heart decreased by 10 +/- 1% (n = 6). In contrast to this negative inotropic response, simultaneously measured peak Ca2+ transients increased by 6 +/- 1%. Guinea pig heart (n = 6) showed an increase in peak positive LVdP/dt (33 +/- 1%) which was associated with an increase in peak Ca2+ transients (8 +/- 1%). Under equivalent experimental conditions in an NMR spectrometer, this increase in the pacing rate did not affect intracellular levels of phosphate compounds in either rat (n = 6) or guinea pig heart (n = 6). In contrast, pHi showed a decrease of 0.031 +/- 0.006 pH units in rat heart, while no changes were observed in guinea pig heart. These results suggest that in physiological rat myocardium, pHi is susceptible to changes in the stimulus frequency and may affect the Ca(2+)-responsiveness of contractile proteins, which results in the negative force-frequency relationship.

Mechanism of hydrogen peroxide and hydroxyl free radical-induced intracellular acidification in cultured rat cardiac myoblasts

After a transient ischemic attack of the cardiac vascular system, reactive oxygen-derived free radicals, including the superoxide (O2-.) and hydroxyl (.OH) radicals can be easily produced during reperfusion. These free radicals have been suggested to be responsible for reperfusion-induced cardiac stunning and reperfusion-induced arrhythmia. Hydrogen peroxide (H2O2) is often used as an experimental source of oxygen-derived free radicals. Using freshly dissociated single rat cardiac myocytes and the rat cardiac myoblast cell line, H9c2, we have shown, for the first time, that an intriguing pHiota acidification (approximately 0.24 pH unit) is induced by the addition of 100 micromol/L H2O2 and that this dose is without effect on the intracellular free Ca2+ levels or viability of the cells. Using H9c2 as a model cardiac cell, we have shown that it is the intracellular production of .OH, and not O2-. or H2O2, that results in this acidification. We have excluded any involvement of (1) the three known cardiac pHi regulators (the Na+-H+ exchanger, the Cl--HCO3 exchanger, and the Na+-HCO3 co-transporter), (2) a rise in intracellular Ca2+ levels, and (3) inhibition of oxidative phosphorylation. However, we have found that H2O2-induced acidosis is due to inhibition of the glycolytic pathway, with hydrolysis of intracellular ATP and the resultant intracellular acidification. In cardiac muscle and in skinned cardiac muscle fiber, it has been shown that a small intracellular acidification may severely inhibit contractility. Therefore, the sustained pHi decrease caused by hydroxyl radicals may contribute, in some part, to the well-documented impairment of cardiac mechanical function (ie, reperfusion cardiac stunning) seen during reperfusion ischemia.

Regional differences in rest decay and recoveries of contraction and the calcium transient in rabbit ventricular muscle

The rates of rest decay (for rest periods of between 0.5 min and 10 min) and recovery from the rested state (following 10 min of rest) of cell shortening and the amplitude of the intracellular calcium transient were compared in epicardial and endocardial ventricular myocytes isolated from rabbit hearts. The object of these experiments was to determine whether reported transmural differences in action potential duration, myosin type expression and metabolic enzyme content are able to influence the control of contraction. Cells isolated from these two regions of the ventricular wall displayed almost identical twitch shortening and calcium transient characteristics during steady-state electrical stimulation at 0.5 Hz. Despite this, rest decay of cell shortening was faster and recovery from the rested state slower in endocardial cells than in epicardial cells. Neither of these differences could be explained in terms of changes of calcium transient amplitude or time course. We tried to mimic the effect of prolonged rest by application of caffeine to empty the sarcoplasmic reticulum of calcium. The regional differences in recovery of contraction from the rested state were not reproduced in the recovery of contraction after caffeine application, suggesting that the effect is produced by something other than refilling of the sarcoplasmic reticulum. It is suggested that changes in factors that affect myofilament calcium sensitivity produce the regional differences in rest decay and post-rest recovery of contraction.

The metabolic consequences of an increase in the frequency of stimulation in isolated ferret hearts

1. The metabolic consequences of an increase in the frequency of stimulation were examined in isolated ferret hearts. Intracellular pH (pHi) and the intracellular concentrations of phosphocreatine ([PCr]i), inorganic phosphate ([Pi]i) and ATP were measured by 31P nuclear magnetic resonance (NMR) spectroscopy. 2. Increasing the stimulus rate from 0.1-0.7 to 2 Hz caused an increase in [Pi]i and a decrease recovery of both [PCr]i and [Pi]i during continued stimulation. There was no change in [ATP]i during stimulation at 2 Hz. Increasing the stimulus rate caused an intracellular acidosis of around 0.1 pH units. 3. Increasing the stimulus rate generally caused an initial increase in developed pressure, followed by a decrease over 1-2 min to a steady level slightly lower than developed pressure at the low (control) stimulus rate. The increase in stimulus rate caused a 4- to 6-fold increase in time-averaged muscle activity. 4. Both oxygen uptake and production of lactate increased on 2 Hz stimulation. Lactate production accounted for less than 5% of ATP production at low or high stimulus rates, suggesting that significant anoxia was not occurring during stimulation. The observed lactate production was, however, sufficient to explain most of the intracellular acidosis observed when the stimulus rate was raised. When glycolysis was prevented by removal of glucose and depletion of glycogen stores, 2 Hz stimulation was accompanied by an intracellular alkalosis rather than an acidosis, suggesting that lactate production by glycolysis was the cause of the intracellular acidosis. 5. Reducing the rate of glycolysis increased the size of changes in [PCr]i and [Pi]i evoked by stimulation at 2 Hz. Furthermore, there was now no partial reversal of the changes in [PCr]i and [Pi]i during 2 Hz stimulation. 6. When oxidative phosphorylation was inhibited by replacing O2 with N2, increasing the rate of stimulation from 0.1-0.7 to 1-2 Hz caused an initial increase followed by a large fall in developed pressure, which declined to a level well below that at the control stimulus rate. The increase in stimulus rate was accompanied by a large fall in [PCr]i, an increase in [Pi]i, and an intracellular acidosis of 0.1-0.3 pH units. The fall in developed pressure was consistent with the known effects of the rise in [Pi]i and the fall in pHi on the contractile apparatus.

Different effects of alpha- and beta-adrenergic stimulation on cytosolic pH and myofilament responsiveness to Ca2+ in cardiac myocytes

alpha-Adrenergic stimulation (alpha-AS) and beta-adrenergic stimulation (beta-AS) of the myocardium are associated respectively with an increase and a decrease in myofilament responsiveness to Ca2+. We hypothesized that changes in cytosolic pH (pH(i)) may modulate these opposite actions of alpha-AS and beta-AS. The effects of alpha-AS (50 microM phenylephrine and 1 microM nadolol) and beta-AS (0.05 microM isoproterenol) on contraction and either cytosolic Ca2+ (Cai) or pH(i) were assessed in adult rat ventricular myocytes bathed in bicarbonate buffer (pH 7.36 +/- 0.05). In cells loaded with the ester derivative (AM form) of indo-1, the 410/490-nm ratio of emitted fluorescence indexed Cai. Myofilament responsiveness to Ca2+ was assessed by the relaxation phase of the length-indo-1 fluorescence relation during a twitch. alpha-AS and beta-AS shifted this relation in opposite directions, indicating that alpha-AS increased and beta-AS decreased myofilament responsiveness to Ca2+. In addition, the positive inotropic action of alpha-AS was associated with an increased Cai transient amplitude in 50% of the myocytes (n = 12), whereas beta-AS always increased Cai (n = 5). In cells loaded with the fluorescent pH(i) probe SNARF-1 AM, the emitted 590/640-nm fluorescence is a measure of pH(i). The effect of alpha-AS on the extent of cell shortening during the twitch (ES) was expressed as the percentage of resting cell length. Both ES and pH(i) were assessed in myocytes bathed in 1.5 mM [Ca2+] and stimulated at 0.5 Hz (control ES, 7.4 +/- 1.5%; control pH(i), 7.11 +/- 0.05; n = 10). alpha-AS enhanced both ES (delta ES, 1.8 +/- 0.6%; p less than 0.05) and pH(i) (delta pH(i), 0.06 +/- 0.01; p less than 0.005), and there was a significant correlation between delta ES and delta pH(i) (r = 0.76, p less than 0.05). A similar effect of alpha-AS on pH(i) was observed in the absence of electrical stimulation (n = 8). The alpha-AS-induced enhancement of ES and pH(i) was abolished by 10 microM ethylisopropylamiloride, a Na(+)-H+ exchange inhibitor (n = 7). In additional experiments, myocytes were preincubated either with 0.2 microM 4 beta-phorbol 12-myristate 13-acetate (n = 8) or with 5 nM staurosporine (n = 8), which have been shown to downregulate and inhibit Ca(2+)-activated phospholipid-dependent protein kinase C, respectively. In either group, alpha-AS had no effect on pH(i) and decreased ES to approximately 60% of control.(ABSTRACT TRUNCATED AT 400 WORDS)

Myocardial force interval relationships: influence of external sodium and calcium, muscle length, muscle diameter and stimulation frequency

Several inotropic interventions were studied in thin papillary muscles under dynamic conditions. The effects on mechanical restitution and postextrasystolic potentiation were analysed. The decay of postextrasystolic potentiation was taken as a measure of recirculation fraction of activator calcium. The mechanical restitution curve had a plateau phase on its rising phase which was abolished in low extracellular sodium but pronounced in increased extracellular calcium. The recirculation fraction (RF) in control was 0.35 +/- 0.03; lowering the extracellular sodium by 20% increased the RF to 0.46 +/- 0.04 (n = 10). A reduction of sodium by 40% increased the RF to 0.57 +/- 0.04, whereas increasing extracellular calcium to 4 mM gave an RF of 0.48 +/- 0.05 (n = 10 in all cases). There was no significant effect on RF of changing basic stimulation frequency or muscle preparation length. These findings support RF as a good index of myocardial contractility. Furthermore, at muscle diameters above 0.65 mm the RF was found to be reduced, suggesting this diameter as critical for muscle function. Also, postextrasystolic potentiation in relation to preceding steady state contraction was markedly increased at these diameters. In conclusion, this study shows that RF is independent of stimulation frequency and muscle length, and that it is increased when calcium extrusion by the sodium/calcium exchange is reduced. Furthermore, RF is critically dependent upon the diameter of the preparation and mechanical restitution is changed by altered extracellular sodium concentration.

The relationship between contraction and intracellular sodium in rat and guinea-pig ventricular myocytes

1. The contraction, measured optically, and the intracellular Na+ activity (aNai), measured with the Na(+)-sensitive fluorescent dye SBFI, have been recorded simultaneously in rat and guinea-pig ventricular myocytes. 2. In rat and guinea-pig ventricular myocytes at rest, aNai was 7.8 +/- 0.3 mM (n = 4) and 5.1 +/- 0.3 mM (n = 16), respectively. 3. When both rat and guinea-pig ventricular myocytes were stimulated at 1 Hz after a rest there was usually a gradual increase in twitch shortening (referred to as a 'staircase') over several minutes accompanied by an increase in aNai over a similar time course. Twitch shortening increased by 21 +/- 3% (n = 6) and 20 +/- 4% (n = 16) (of steady-state twitch shortening during 1 Hz stimulation) per millimolar rise in aNai in rat and guinea-pig ventricular myocytes, respectively. 4. When rat and guinea-pig ventricular myocytes were exposed to strophanthidin to block the Na(+)-K+ pump, there were increases in twitch shortening and aNai over similar time courses. Twitch shortening increased by 24 +/- 4% (n = 5) and 20 +/- 3% (n = 10) (of control twitch shortening) per millimolar rise in aNai in rat and guinea-pig ventricular myocytes respectively. 5. The inotropic effect of cardiac glycosides, such as strophanthidin, is widely regarded to be principally the result of the rise in aNai. The similarity of the relation between twitch shortening and aNai during the staircase and on application of strophanthidin suggests that the progressive increase in the strength of contraction during the staircase was also linked to the rise in aNai. 6. In guinea-pig, but not rat, ventricular myocytes there was hysteresis in the relation between twitch shortening and aNai on application and wash-off of strophanthidin. This indicates that strophanthidin has another inotropic action in guinea-pig ventricular myocytes. 7. A computer model of excitation-contraction coupling has been developed to simulate the staircase and the action of cardiac glycoside and to account for the relation between contraction and intracellular Na+.

Aging: stimulation rate on cardiac intracellular Na+ activity and developed tension

Previous reports suggested that Na,K-ATPase activity and Na(+)-pump capacity decrease with senescence in left atrial myocardium of F344 rats. Current experiments were designed to determine if this reduction in the Na(+)-pump affects free intracellular Na+ levels. Mean intracellular Na+ ion activity (aiNa) was measured with Na-selective microelectrodes in left atrial muscle isolated from hearts of 4-, 14- and 25-month-old F344 rats. Preparations were stimulated randomly at frequencies between 0 and 12 h. There were no age-associated differences in aiNa measured at any frequency or in the decay of Na+ activity following discontinuation of electrical stimulation. These data indicate that the aging-related decline in Na,K-ATPase does not result in elevated aiNa even at extremely high stimulation frequencies, thus suggesting that other routes of Na+ influx and efflux are also altered in atrial muscle.

Endothelin and increased contractility in adult rat ventricular myocytes. Role of intracellular alkalosis induced by activation of the protein kinase C-dependent Na(+)-H+ exchanger

Endothelin, a 21-amino acid vasoactive peptide, is among the most potent positively inotropic agents yet described in mammalian heart. Having demonstrated that endothelin's inotropic effect is due, in part, to an apparent sensitization of cardiac myofilaments to intracellular calcium, we determined whether this could be due to a rise in intracellular pH (pHi). In isolated adult rat ventricular cells loaded with the H(+)-selective fluorescent probe BCECF, 100 pM endothelin increased contractile amplitude to 190 +/- 26% of baseline and pHi by 0.08 +/- 0.02 (n = 8), whereas 1 nM endothelin increased pHi by 0.13 +/- 0.03 with little further increase in contractility. Amiloride (10(-4)M) prevented the increase in pHi in response to endothelin and reduced the inotropic response by 45%, although the inotropic effect could be readily restored by subsequent NH4Cl-induced alkalinization. Similarly, inhibitors of protein kinase C (H-7 and sphingosine) diminished or abolished the rise in pHi after endothelin superfusion while causing a decline in its inotropic effect comparable with that observed with amiloride. Pretreatment with pertussis toxin, which we have demonstrated results in complete ADP-ribosylation of the alpha-subunits of Go and Gi GTP-binding proteins and abolition of endothelin's positive inotropic effect, only partially reduced the intracellular alkalinization induced by the peptide, suggesting a complex signal transduction mechanism. Thus, the positive inotropic action of endothelin is due in part to stimulation of the sarcolemmal Na(+)-H+ exchanger by a protein kinase C-mediated pathway, resulting in a rise in pHi and sensitization of cardiac myofilaments to intracellular Ca2+.

The effect of acidosis on the interval-force relation and mechanical restitution in ferret papillary muscle

1. The effect of a respiratory acidosis on the interval-force relation and on mechanical restitution was investigated in ferret papillary muscles. 2. Acidosis (pH 6.85) decreased developed force over a range of stimulation frequencies (1.0.06 Hz); the percentage decrease was greatest at the lowest stimulation frequencies. Qualitatively similar effects of acidosis on developed force were observed in the presence of the sarcoplasmic reticulum (SR) inhibitor ryanodine. 3. Mechanical restitution curves were constructed by interpolating extra-systoles at different test intervals following a train of steady-state beats. Mechanical restitution in ferret papillary muscle was triphasic: an initial, rapid, exponential increase in force with test intervals to 2 s, a further increase with test intervals between 60 and 90 s and then a slow decline, with a plateau at about 30 min (0.33 Hz, 30 degrees C). 4. Acidosis slowed the initial phase of mechanical restitution. The degree of slowing depended on the steady-state stimulation frequency, being greatest at low frequencies. 5. Inhibition of the SR abolished the initial phase of mechanical restitution, suggesting that this phase depends on Ca2+ release from the SR. 6. The strength of the first contraction after the extra-systole varied inversely with the size of the extra-systole under all conditions studied. 7. It is concluded that acidosis may inhibit the SR by altering the time required for Ca2+ recycling between contractions. This effect may alter Ca2+ release from the SR during acidosis, and may underlie the mechanical alternans (the alternation of small and large contractions) that can occur during acidosis.

The pH dependence of the cardiac sarcolemmal Ca2(+)-transporting ATPase: evidence that the Ca2+ translocator bears a doubly negative charge

The pH dependence of the Ca2(+)-transporting ATPase of bovine cardiac sarcolemma was determined in a membrane vesicle preparation. The maximal velocity (Vmax) at saturating external Ca2+ showed a sigmoidal pH dependence with maximal values in the 6.0-6.5 range, a half-maximal value at 7.2 and minimal (less than or equal to 15%) values at pH greater than or equal to 8.0. The apparent affinity for Ca2+ (1/Km) varied over 10(4)-fold for 6.0 less than or equal to pH less than or equal to 8.5, increasing with increasing pH. Plots of log(1/Km) vs. pH were biphasic. In the acid range (6.0 less than or equal to pH less than or equal to 7.2), a slope of 2.6 was observed for the calmodulin-activated form of the pump. For 7.2 less than or equal to pH less than or equal to 8.5, a slope of 0.5 was observed. At pH 7.4, the Km is approx. 48 +/- 19 nM. The Ca2+ pump of cardiac sarcoplasmic reticulum in the same preparation had a Km of 304 +/- 115 nM and showed a similar pH dependence except that the slope in the acid range was 1.7. When calmodulin was removed from the sarcolemmal pump, its Km was raised to approx. 1.0 microM, the slope in the acid range was reduced to 1.7 and the Vmax was markedly reduced. The results are explicable in terms of a model in which each of the two Ca2+ binding sites on the pump contains two buried COO- groups responsible for high affinity. The Km effect is explained by 2 H+ vs. 1 Ca2+ competition for occupation of each of the two cytoplasmically-oriented translocators (4 H+ vs. 2 Ca2+). The Vmax effect is explained by counter-transport of H+. The findings are considered in terms of the published amino acid sequence of the cardiac sarcolemmal pump and recent site-directed mutagenesis vs. function studies identifying the Ca2+ binding site in the skeletal sarcoplasmic reticulum pump. The kinetic data are also applied to pump behavior under conditions of ischemia and acidosis.

Dependence of intracellular free calcium and tension on membrane potential and intracellular pH in single crayfish muscle fibres

The dependence of intracellular free calcium ([Ca2+]i) and tension on membrane potential and intracellular pH (pHi) was studied in single isolated fibres of the crayfish claw-opener muscle using ion-selective microelectrodes. Tension (T) was quantified as a percentage of the maximum force, or as force per cross-sectional area (N/cm2). In resting fibres, pHi had a mean value of 7.06. Contractions evoked by an increase extracellular potassium [( K+]0) produced a fall in pHi of 0.01-0.05 units. The lowest measured levels of resting [Ca2+]i corresponded to a pCai (= -log [Ca2+]i) of 6.8. Intracellular Ca2+ transients recorded during K(+)-induced contractions did not reveal any distinct threshold for force development. Both the resting [Ca2+]i and resting tension were decreased by an intracellular alkalosis and increased by an acidosis. The sensitivity of resting tension to a change in pHi (quantified as -dT/dpHi) showed a progressive increase during a fall in pHi within the range examined (pHi 6.2-7.5). The pHi/[Ca2+]i and pHi/tension relationships were monotonic throughout the multiphasic pHi change caused by NH4Cl. A fall of 0.5-0.6 units in pHi did not produce a detectable shift in the pCai/tension relationship at low levels of force development. The results indicate that resting [Ca2+]i is slightly higher than the level required for contractile activation. They also show that the dependence of tension on pHi in crayfish muscle fibres is attributable to a direct H+ and Ca2+ interaction at the level of Ca2+ sequestration and/or transport. Finally, the results suggest that in situ, the effect of pH on the Ca2+ sensitivity of the myofibrillar system is not as large as could be expected on the basis of previous work on skinned crustacean muscle fibres.

Effects of changes of pH on the contractile function of cardiac muscle

It has been known for over 100 years that acidosis decreases the contractility of cardiac muscle. However, the mechanisms underlying this decrease are complicated because acidosis affects every step in the excitation-contraction coupling pathway, including both the delivery of Ca2+ to the myofilaments and the response of the myofilaments to Ca2+. Acidosis has diverse effects on Ca2+ delivery. Actions that may diminish Ca2+ delivery include 1) inhibition of the Ca2+ current, 2) reduction of Ca2+ release from the sarcoplasmic reticulum, and 3) shortening of the action potential, when such shortening occurs. Conversely, Ca2+ delivery may be increased by the prolongation of the action potential that is sometimes observed and by the rise of diastolic Ca2+ that occurs during acidosis. This rise, which will increase the uptake and subsequent release of Ca2+ by the sarcoplasmic reticulum, may be due to 1) stimulation of Na+ entry via Na(+)-Ca2+ exchange; 2) direct inhibition of Na(+)-Ca2+ exchange; 3) mitochondrial release of Ca2+; and 4) displacement of Ca2+ from cytoplasmic buffer sites by H+. Acidosis inhibits myofibrillar responsiveness to Ca2+ by decreasing the sensitivity of the contractile proteins to Ca2+, probably by decreasing the binding of Ca2+ to troponin C, and by decreasing maximum force, possibly by a direct action on the cross bridges. Thus the final amount of force developed by heart muscle during acidosis is the complex sum of these changes.

pH dependence of intrinsic H+ buffering power in the sheep cardiac Purkinje fibre

1. Intrinsic, intracellular H+ buffering power (beta) was estimated in the isolated sheep cardiac Purkinje fibre at various values of intracellular pH (pHi) in the range 6.2-7.5 and for various values of extracellular pH (pHo) in the range 6.5-8.5. Buffering power was calculated from the fall of pHi (recorded with an intracellular pH-selective microelectrode) induced by addition and removal of extracellular, permeant weak acids and bases (NH4Cl, trimethylamine chloride, sodium propionate). Experiments were performed under conditions nominally free of CO2-HCO3. 2. beta was estimated firstly following acid loads induced by NH4Cl removal (10-20 mM) under conditions where Na(+)-H+ exchange was operational (i.e. in Na(+)-containing Tyrode solution). At constant pHi, the value of beta appeared to double (from a control level of 39.7 mM) as pHo was increased from 7.5 to 8.5. Notably, raising pHo in this range greatly accelerated pHi recovery from an intracellular acid load, indicating stimulation of acid extrusion. It is likely that this stimulation results in an overestimation of beta because it blunts the intracellular acid load. The apparent elevation of beta at high pHo may therefore be an artifact. 3. Estimates of beta were compared (NH4Cl removal) before and after inhibiting Na(+)-H+ exchange in Na(+)-free solution or with amiloride (1 mM). The acid load was larger and in many (but not all) cases the apparent value of beta decreased after inhibition of acid extrusion. This indicates that, if Na(+)-H+ exchange is operational, it can result in an overestimate of beta. In amiloride, beta was 26.6 +/- 1.4 mM (n = 8) at a mean pHi of 6.84 +/- 0.03. 4. Small stepwise reductions of external NH4Cl (from 40 to 0 mM), in the presence of Na(+)-free solution plus 5 mM-BaCl2 at constant pHo, resulted in small stepwise reductions of pHi (approximately 0.1 units). When these were used to calculate beta, we observed that beta increased roughly linearly as pHi became more acid. For a pHi of 7.2, beta approximately 20 mM. 5. An almost identical relationship between beta and pHi was found when using the method of sodium propionate addition (10-50 mM): amiloride (1 mM) was present and pHi was manipulated to various test levels by changing pHo. This confirms that beta varies inversely with pHi and also that it is independent of pHo. We conclude that the apparent variation of beta with pHo observed earlier was indeed an artifact.(ABSTRACT TRUNCATED AT 400 WORDS)

Decreased sensitivity of contraction to changes of intracellular pH in papillary muscle from diabetic rat hearts

1. The relationship between intracellular pH (pHi) and contractile activity was investigated in papillary muscles isolated from right ventricle of normal and streptozotocin (STZ)-induced diabetic rats. pHi changes induced by 20 mM-NH4Cl were recorded with H(+)-sensitive microelectrodes. 2. An increase in pHi of approximately 0.20 pH units on exposure to NH4Cl led to an increase of the maximum developed tension, which was 707.8 +/- 57.5% (mean +/- S.E. of mean, n = 10) of control in normal muscles and 271 +/- 16.3% (n = 10) in diabetic muscles. On the other hand, acidosis induced by NH4Cl withdrawal was associated with a fall in developed tension to 48.2 +/- 6.7% of control in diabetic muscles, as compared to 79.2 +/- 8% in normal muscles. 3. The decrease in tension associated with acidosis was rapidly followed (in approximately 2 min) by a transient redevelopment of force, which peaked at 80.2 +/- 8.6% of control in the diabetic muscles as compared to 153.5 +/- 11.7% in normal papillary muscles. The peak of this secondary positive inotropy coincided in both groups of muscles with the maximum decrease of pHi, i.e. -0.40 +/- 0.02 and -0.28 +/- 0.04 pH units in diabetic and normal muscles, respectively. 4. Caffeine (10 mM), which had a marked positive inotropic effect in both groups of muscles, abolished the transient recovery of tension occurring after NH4Cl withdrawal. Ryanodine (2 microM) which had a marked negative inotropic effect on both normal and diabetic papillary muscles, also suppressed the transient recovery of tension. 5. The presence of amiloride (1 mM) during acidosis induced by NH4Cl withdrawal abolished the observed differences in developed tension, in particular the transient recovery of tension, between normal and diabetic muscles, as it abolished the differences in the amplitude of pHi decrease and in the time course of pHi recovery. 6. The presence of 2',4'-dichlorobenzamil amiloride (40 microM) significantly and similarly delayed and reduced the amplitude of transient recovery of tension in both normal and diabetic papillary muscles. 7. We conclude that STZ-induced diabetes induces a decrease in pHi sensitivity of contractile force. This may be the consequence of a change in sarcoplasmic reticulum (SR) composition and function, and may also indirectly result from changes in Na(+)-H+ exchange activity, particularly during intracellular acidosis.

Sodium-hydrogen exchange and its role in controlling contractility during acidosis in cardiac muscle

Intracellular pH (pHi) and Na (aina) were recorded in isolated sheep cardiac Purkinje fibres using ion-selective microelectrodes while simultaneously recording twitch tension. A fall of pHi stimulated acid-extrusion via sarcolemmal Na-H exchange but the extrusion was inhibited by reducing extracellular pH (pHo), indicating an inhibitory effect of external H ions upon the exchanger. Intracellular acidosis can reduce contraction by directly reducing myofibrillar Ca2+ sensitivity. The activation of Na-H exchange at low pHi can offset this direct inhibitory effect of H+ ions since exchange-activation elevates aina which then indirectly elevates Ca2+i (via Na-Ca exchange) thus tending to restore tension. This protection of contraction during intracellular acidosis can be removed if extracellular pH is also allowed to fall since, under these conditions, Na-H exchange is inhibited.

The effect of lactate on intracellular pH and force recovery of fatigued sartorius muscles of the frog, Rana pipiens

1. The effects of pHo (extracellular pH) and lactic acid on pHi (intracellular pH) and tetanic force were examined in frog sartorius muscle. Ion-selective microelectrodes were used to measure pHi. Tetanic force was elicited by field stimulation. Experiments were performed in HEPES-buffered solution equilibrated with 100% O2. 2. Mean pHi values (+/- S.E.M.) of unfatigued frog sartorius muscles were 7.14 +/- 0.02 and 7.05 +/- 0.09 at pHo 7.2 and 6.4, respectively. 3. A stimulation at a rate of one 100 ms tetanic contraction per second for 3 min reduced pHi to 6.21 +/- 0.09 and 6.20 +/- 0.04 at pHo 7.2 and 6.4, respectively. Meanwhile at pHo 7.2, the tetanic force (defined as the maximum force developed during a tetanus) decreased by 82.9 +/- 2.6%, the maximum rate of relaxation decreased by 92.9 +/- 0.9%, and the rate constant of the relaxation decreased by 88.5 +/- 1.6%. At pHo 6.4, the decrease in tetanic force, maximum rate of relaxation and rate constant were 90.6 +/- 1.8%, 93.8 +/- 0.5 and 87.5 +/- 2.7%, respectively. 4. The maximum rates of recovery of pHi following fatigue were 0.068 +/- 0.05 and 0.025 +/- 0.05 pH units min-1 at pHo 7.2 and 6.4, respectively. Recovery of normal tetanic force and relaxation rate was also slower at acidic pHo than at neutral pHo. 5. In the presence of 40 mmol l-1 L-lactic acid at pHo 7.2, the maximum rate of pHi recovery following fatigue was only 0.027 +/- 0.03 pH units min-1 at pHo 7.2. The presence of lactic acid also reduced the recovery of the relaxation phase, but not the recovery of tetanic force. 6. It is suggested that pHi recovery is not a limiting factor for tetanic force recovery and that the extracellular H+ inhibits tetanic force recovery by acting at a site located on the outer surface of the sarcolemma. The recovery of the relaxation phase is believed to be pHi dependent.

Substrate dependence of energy metabolism in isolated guinea-pig cardiac muscle: a microcalorimetric study

1. The effects of glucose, pyruvate and lactate on basal metabolism and on contraction-related energy expenditure of thin trabeculae isolated from guinea-pig heart were studied using a microcalorimetric technique. 2. Resting heat rates of cardiac ventricular muscle measured in the presence of substrate-free solution (56 +/- 20 mW (g dry weight)-1), 10 mM-lactate (54 +/- 12 mW (g dry weight)-1) and 10 mM-glucose (63 +/- 24 mW (g dry weight)-1) did not differ significantly. Increasing the external glucose concentration (up to 100 mM) and/or adding insulin (up to 80 units l-1) had virtually no effect on the measured resting heat rate. 3. With 10 mM-pyruvate as substrate resting heat rate was substantially larger (106 +/- 40 mW (g dry weight)-1) than with glucose, lactate or substrate-free solution. The concentrations of pyruvate producing a half-maximal increase in resting heat rate as compared to substrate-free solution ranged between 0.4 and 1.2 mM. 4. In order to test whether the development of an anoxic core contributed to the substrate dependence of resting heat production the critical PO2 (i.e. the PO2 that produced a just-noticeable decrease in heat rate) was determined in cylindrical preparations of various diameters. It was found that none of the preparations had an anoxic core at rest in a solution equilibrated with 100% oxygen. 5. From the dependence of the critical PO2 on the diameter of the preparation the diffusion coefficient of oxygen through cardiac muscle was calculated using a modification of Hill's equation (Hill, 1928). The O2 diffusion coefficient was found to be 1.09 X 10(-5) cm2 s-1. 6. Contraction-related heat production was also found to be dependent on the substrate used. In the presence of 10 mM-pyruvate it was about 60% larger than in the presence of 10 mM-glucose, 10 mM-lactate or with substrate-free solution. 7. Isometric force of contraction showed the same substrate dependence as contraction-related heat production and increased with a similar time course during repetitive stimulation. 8. The possible mechanisms underlying the substrate dependence of myocardial energy metabolism are discussed. It is suggested that the increased energy expenditure observed in the presence of pyruvate may be related to a decrease in intracellular phosphate and/or to an increase in intracellular pH.

Mechanism of rate-dependent pH changes in the sheep cardiac Purkinje fibre

1. The mechanism of the rate-dependent decrease in intracellular pH (pHi) and its recovery were studied in isolated sheep cardiac Purkinje fibres. Intracellular Na+ activity (aiNa) and pHi were measured using ion-selective microelectrodes. Twitches were elicited by field stimulation or by depolarizing pulses applied using a two-microelectrode voltage clamp. 2. A 3 Hz train of short (50 ms) depolarizing voltage-clamp pulses induced a reversible fall in pHi which was accompanied by a reversible increase in aiNa. A train of longer (200 ms) pulses also produced a fall in pHi which was now paralleled by a decrease in aiNa. These observations indicate that the rate-dependent acidosis is not dependent upon a rise in aiNa. 3. Neither the fall in pHi nor the increase in aiNa seen upon an increase in action potential frequency was inhibited by amiloride (1 mmol l-1) which indicates that Na+-H+ exchange is not involved in the generation of the acidosis. Furthermore, the rate-dependent acidosis was not abolished in Na+-free solution (Li+ or N-methyl glucamine substituted) indicating that other Na+-requiring processes (such as Na+-Ca2+ exchange) are not a necessary requirement. Rate-dependent pHi changes were also unaffected by the stilbene compound DIDS indicating no participation by Cl--HCO-3 exchange. 4. The rate-dependent acidosis was inhibited by the organic calcium antagonist D600 (20 mumol l-1) which also inhibited twitch tension. This suggests that the acidosis is related to the activation by Ca2+ of developed tension. D600 also inhibited the rate-dependent rise in aiNa (field stimulation). 5. The rate-dependent acidosis was not inhibited by cyanide (2 mmol l-1) but it was blocked by iodoacetate (0.5 mmol l-1) and by 2-deoxyglucose (DOG) (10 mmol l-1, applied in glucose-free solution). These results suggest that the acidosis is generated metabolically via stimulation of glycolysis, following an increase in contraction. Contributions from aerobic metabolism are likely to be small. 6. Twitch tension was inhibited by ryanodine (10 mumol l-1) but the drug had little inhibitory effect on the rate-dependent acidosis. A tonic component of tension was observed, however, in the presence of ryanodine. The lack of effect of ryanodine upon the rate-induced acidosis is discussed. 7. The half-time of pHi recovery from the frequency-dependent acidosis was consistently shorter than that from an intracellular acid load induced by adding and then removing external NH4Cl (10 mmol l-1).(ABSTRACT TRUNCATED AT 400 WORDS)

Role of aiNa in positive force-frequency staircase in guinea pig papillary muscle

In the ventricular papillary muscle of guinea pig heart, membrane potential, intracellular sodium activity (aiNa), and twitch force were measured simultaneously and continuously for many hours at stimulation rates of 0, 0.5, 1, 2, 3, 4, 5, and 6 Hz to investigate the relation of aiNa to twitch force and membrane potential both in the steady state and during the changes in these variables. After an increase in stimulation rate, both aiNa and twitch force increased progressively, reaching steady-state levels. The relation between twitch force and aiNa in the steady state was generally sigmoidal over the range of 0.5-5 Hz and steep in the 1- to 4-Hz range. After either increase or decrease in stimulation rate, the time course of change in aiNa was exponential and similar to that of change in twitch force. Moreover, the force-aiNa relation observed after increase in stimulation rate from 0.5 to 3 Hz resembled that observed after decrease in the rate from 3 to 0.5 Hz, indicating an absence of hysteresis in the relation. The results suggest that an increase in aiNa is an important factor involved in the force staircase. As stimulation rate was increased from 0.5 to higher rates (5 or 6 Hz) and then decreased back to 0.5 Hz, a hysteresis phenomenon was observed in the relation between twitch force and aiNa. This suggests that some secondary factor may alter the relation between twitch force and aiNa. As stimulation rate increased and aiNa rose, the steady-state diastolic membrane potential hyperpolarized. This result is consistent with the view that an increase in aiNa enhances the electrogenic Na+-K+ pump and hyperpolarizes the cell membrane.