The effect of pH on the transient-state kinetics of Ca2+-Mg2+-ATPase of cardiac sarcoplasmic reticulum. A comparison with skeletal sarcoplasmic reticulum

Anthropogenic underwater noise: A review on physiological and molecular responses of marine biota

The detrimental effects of anthropogenic underwater noise on marine organisms have garnered significant attention among scientists. This review delves into the research concerning the repercussions of underwater noise on marine species, with specific emphasis on the physiological and molecular responses of marine biota. This review investigates the sensory mechanisms, hearing sensitivity, and reaction thresholds of diverse marine organisms, shedding light on their susceptibility to underwater noise disturbances. The physiological and molecular effects of anthropogenic underwater noise on marine biota include oxidative stress, energy homeostasis, metabolism, immune function, and respiration. Additionally, changes in the gene expression profile associated with oxidative stress, metabolism, and immunological response are among the responses reported for marine biota. These effects pose a threat to animal fitness and potentially affect their survival as individuals and populations.

RSK2 contributes to myogenic vasoconstriction of resistance arteries by activating smooth muscle myosin and the Na+/H+ exchanger

Smooth muscle contraction is triggered when Ca²⁺/calmodulin-dependent myosin light chain kinase (MLCK) phosphorylates the regulatory light chain of myosin (RLC₂₀). However, blood vessels from Mlck-deficient mouse embryos retain the ability to contract, suggesting the existence of additional regulatory mechanisms. We showed that the p90 ribosomal S6 kinase 2 (RSK2) also phosphorylated RLC₂₀ to promote smooth muscle contractility. Active, phosphorylated RSK2 was present in mouse resistance arteries under normal basal tone, and phosphorylation of RSK2 increased with myogenic vasoconstriction or agonist stimulation. Resistance arteries from Rsk2-deficient mice were dilated and showed reduced myogenic tone and RLC₂₀ phosphorylation. RSK2 phosphorylated Ser¹⁹ in RLC in vitro. In addition, RSK2 phosphorylated an activating site in the Na⁺/H⁺ exchanger (NHE-1), resulting in cytosolic alkalinization and an increase in intracellular Ca²⁺ that promotes vasoconstriction. NHE-1 activity increased upon myogenic constriction, and the increase in intracellular pH was suppressed in Rsk2-deficient mice. In pressured arteries, RSK2-dependent activation of NHE-1 was associated with increased intracellular Ca²⁺ transients, which would be expected to increase MLCK activity, thereby contributing to basal tone and myogenic responses. Accordingly, Rsk2-deficient mice had lower blood pressure than normal littermates. Thus, RSK2 mediates a procontractile signaling pathway that contributes to the regulation of basal vascular tone, myogenic vasoconstriction, and blood pressure and may be a potential therapeutic target in smooth muscle contractility disorders.

Regional acidosis locally inhibits but remotely stimulates Ca2+ waves in ventricular myocytes

Aims

Spontaneous Ca²⁺ waves in cardiomyocytes are potentially arrhythmogenic. A powerful controller of Ca²⁺ waves is the cytoplasmic H⁺ concentration ([H⁺]_i), which fluctuates spatially and temporally in conditions such as myocardial ischaemia/reperfusion. H⁺-control of Ca²⁺ waves is poorly understood. We have therefore investigated how [H⁺]_i co-ordinates their initiation and frequency.

Methods and results

Spontaneous Ca²⁺ waves were imaged (fluo-3) in rat isolated ventricular myocytes, subjected to modest Ca²⁺-overload. Whole-cell intracellular acidosis (induced by acetate-superfusion) stimulated wave frequency. Pharmacologically blocking sarcolemmal Na⁺/H⁺ exchange (NHE1) prevented this stimulation, unveiling inhibition by H⁺. Acidosis also increased Ca²⁺ wave velocity. Restricting acidosis to one end of a myocyte, using a microfluidic device, inhibited Ca²⁺ waves in the acidic zone (consistent with ryanodine receptor inhibition), but stimulated wave emergence elsewhere in the cell. This remote stimulation was absent when NHE1 was selectively inhibited in the acidic zone. Remote stimulation depended on a locally evoked, NHE1-driven rise of [Na⁺]_i that spread rapidly downstream.

Conclusion

Acidosis influences Ca²⁺ waves via inhibitory Hi+ and stimulatory Nai+ signals (the latter facilitating intracellular Ca²⁺-loading through modulation of sarcolemmal Na⁺/Ca²⁺ exchange activity). During spatial [H⁺]_i-heterogeneity, Hi+-inhibition dominates in acidic regions, while rapid Nai+ diffusion stimulates waves in downstream, non-acidic regions. Local acidosis thus simultaneously inhibits and stimulates arrhythmogenic Ca²⁺-signalling in the same myocyte. If the principle of remote H⁺-stimulation of Ca²⁺ waves also applies in multicellular myocardium, it raises the possibility of electrical disturbances being driven remotely by adjacent ischaemic areas, which are known to be intensely acidic.

Acid-base alterations in ESRD and effects of hemodialysis

Acid-base alterations in patients with kidney failure and on hemodialysis (HD) treatment contribute to (1) intradialytic hypercapnia and hypoxia, (2) hemodynamic instability and cardiac arrhythmia, (3) systemic inflammation, and (4) a number of associated electrolyte alterations including potentiating effects of hypokalemia, hypocalcemia and, chronically, soft-tissue and vascular calcification, imparting poor prognosis and mortality. This paper discusses acid-base regulation and pathogenesis of dysregulation in patients with kidney failure. Major organ and systemic effects of acid-base perturbations with a specific focus on kidney failure patients on HD are emphasized, and potential mitigating strategies proposed. The high rate of HD-related complications, specifically those that can be accounted for by rapid and steep acid-base perturbations imposed by HD treatment, attests to the pressing need for investigations to establish a better dialysis regimen.

Na⁺ ions as spatial intracellular messengers for co-ordinating Ca²⁺ signals during pH heterogeneity in cardiomyocytes

Aims

Contraction of the heart is regulated by electrically evoked Ca²⁺ transients (CaTs). H⁺ ions, the end products of metabolism, modulate CaTs through direct interactions with Ca²⁺-handling proteins and via Na⁺-mediated coupling between acid-extruding proteins (e.g. Na⁺/H⁺ exchange, NHE1) and Na⁺/Ca²⁺ exchange. Restricted H⁺ diffusivity in cytoplasm predisposes pH-sensitive Ca²⁺ signalling to becoming non-uniform, but the involvement of readily diffusible intracellular Na⁺ ions may provide a means for combatting this.

Methods and results

CaTs were imaged in fluo3-loaded rat ventricular myocytes paced at 2 Hz. Cytoplasmic [Na⁺] ([Na⁺]_i) was imaged using SBFI. Intracellular acidification by acetate exposure raised diastolic and systolic [Ca²⁺] (also observed with acid-loading by ammonium prepulse or CO₂ exposure). The systolic [Ca²⁺] response correlated with a rise in [Na⁺]_i and sarcoplasmic reticulum Ca²⁺ load, and was blocked by the NHE1 inhibitor cariporide (CO₂/HCO₃⁻-free media). Exposure of one half of a myocyte to acetate using dual microperfusion (CO₂/HCO₃⁻-free media) raised diastolic [Ca²⁺] locally in the acidified region. Systolic [Ca²⁺] and CaT amplitude increased more uniformly along the length of the cell, but only when NHE1 was functional. Cytoplasmic Na⁺ diffusivity (D_Na) was measured in quiescent cells, with strophanthidin present to inhibit the Na⁺/K⁺ pump. With regional acetate exposure to activate a local NHE-driven Na⁺-influx, D_Na was found to be sufficiently fast (680 µm²/s) for transmitting the pH–systolic Ca²⁺ interaction over long distances.

Conclusions

Na⁺ ions are rapidly diffusible messengers that expand the spatial scale of cytoplasmic pH–CaT interactions, helping to co-ordinate global Ca²⁺ signalling during conditions of intracellular pH non-uniformity.

Ca2+ Sparks and Ca2+ waves are the subcellular events underlying Ca2+ overload during ischemia and reperfusion in perfused intact hearts

Abnormal intracellular Ca(2+) cycling plays a key role in cardiac dysfunction, particularly during the setting of ischemia/reperfusion (I/R). During ischemia, there is an increase in cytosolic and sarcoplasmic reticulum (SR) Ca(2+). At the onset of reperfusion, there is a transient and abrupt increase in cytosolic Ca(2++), which occurs timely associated with reperfusion arrhythmias. However, little is known about the subcellular dynamics of Ca(2+) increase during I/R, and a possible role of the SR as a mechanism underlying this increase has been previously overlooked. The aim of the present work is to test two main hypotheses: (1) An increase diastolic Ca(2+) sparks frequency (cspf) constitutes a mayor substrate for the ischemia-induced diastolic Ca(2+) increase; (2) an increase in cytosolic Ca(2+) pro-arrhythmogenic events (Ca(2+) waves), mediates the abrupt diastolic Ca(2+) rise at the onset of reperfusion. We used confocal microscopy on mouse intact hearts loaded with Fluo-4. Hearts were submitted to global I/R (12/30 min) to assess epicardial Ca(2+) sparks in the whole heart. Intact heart sparks were faster than in isolated myocytes whereas cspf was not different. During ischemia, cspf significantly increased relative to preischemia (2.07±0.33 vs. 1.13±0.20 sp/s/100 μm, n=29/34, 7 hearts). Reperfusion significantly changed Ca(2+) sparks kinetics, by prolonging Ca(2+) sparks rise time and decreased cspf. However, it significantly increased Ca(2+) wave frequency relative to ischemia (0.71±0.14 vs. 0.38±0.06 w/s/100 μm, n=32/33, 7 hearts). The results show for the first time the assessment of intact perfused heart Ca(2+) sparks and provides direct evidence of increased Ca(2+) sparks in ischemia that transform into Ca(2+) waves during reperfusion. These waves may constitute a main trigger for reperfusion arrhythmias.

The role of CaMKII regulation of phospholamban activity in heart disease

Phospholamban (PLN) is a phosphoprotein in cardiac sarcoplasmic reticulum (SR) that is a reversible regulator of the Ca²⁺-ATPase (SERCA2a) activity and cardiac contractility. Dephosphorylated PLN inhibits SERCA2a and PLN phosphorylation, at either Ser¹⁶ by PKA or Thr¹⁷ by Ca²⁺-calmodulin-dependent protein kinase (CaMKII), reverses this inhibition. Through this mechanism, PLN is a key modulator of SR Ca²⁺ uptake, Ca²⁺ load, contractility, and relaxation. PLN phosphorylation is also the main determinant of β1-adrenergic responses in the heart. Although phosphorylation of Thr¹⁷ by CaMKII contributes to this effect, its role is subordinate to the PKA-dependent increase in cytosolic Ca²⁺, necessary to activate CaMKII. Furthermore, the effects of PLN and its phosphorylation on cardiac function are subject to additional regulation by its interacting partners, the anti-apoptotic HAX-1 protein and Gm or the anchoring unit of protein phosphatase 1. Regulation of PLN activity by this multimeric complex becomes even more important in pathological conditions, characterized by aberrant Ca²⁺-cycling. In this scenario, CaMKII-dependent PLN phosphorylation has been associated with protective effects in both acidosis and ischemia/reperfusion. However, the beneficial effects of increasing SR Ca²⁺ uptake through PLN phosphorylation may be lost or even become deleterious, when these occur in association with alterations in SR Ca²⁺ leak. Moreover, a major characteristic in human and experimental heart failure (HF) is depressed SR Ca²⁺ uptake, associated with decreased SERCA2a levels and dephosphorylation of PLN, leading to decreased SR Ca²⁺ load and impaired contractility. Thus, the strategy of altering SERCA2a and/or PLN levels or activity to restore perturbed SR Ca²⁺ uptake is a potential therapeutic tool for HF treatment. We will review here the role of CaMKII-dependent phosphorylation of PLN at Thr¹⁷ on cardiac function under physiological and pathological conditions.

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.

H⁺-activated Na⁺ influx in the ventricular myocyte couples Ca²⁺-signalling to intracellular pH

Acid extrusion on Na(+)-coupled pH-regulatory proteins (pH-transporters), Na(+)/H(+) exchange (NHE1) and Na(+)-HCO3(-) co-transport (NBC), drives Na(+) influx into the ventricular myocyte. This H(+)-activated Na(+)-influx is acutely up-regulated at pHi<7.2, greatly exceeding Na(+)-efflux on the Na(+)/K(+) ATPase. It is spatially heterogeneous, due to the co-localisation of NHE1 protein (the dominant pH-transporter) with gap-junctions at intercalated discs. Overall Na(+)-influx via NBC is considerably lower, but much is co-localised with L-type Ca(2+)-channels in transverse-tubules. Through a functional coupling with Na(+)/Ca(2+) exchange (NCX), H(+)-activated Na(+)-influx increases sarcoplasmic-reticular Ca(2+)-loading and release during intracellular acidosis. This raises Ca(2+)-transient amplitude, rescuing it from direct H(+)-inhibition. Functional coupling is biochemically regulated and linked to membrane receptors, through effects on NHE1 and NBC. It requires adequate cytoplasmic Na(+)-mobility, as NHE1 and NCX are spatially separated (up to 60μm). The relevant functional NCX activity must be close to dyads, as it exerts no effect on bulk diastolic Ca(2+). H(+)-activated Na(+)-influx is up-regulated during ischaemia-reperfusion and some forms of maladaptive hypertrophy and heart failure. It is thus an attractive system for therapeutic manipulation. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

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.

The relative influences of phosphometabolites and pH on action potential morphology during myocardial reperfusion: a simulation study

Myocardial ischemia-reperfusion (IR) injury represents a constellation of pathological processes that occur when ischemic myocardium experiences a restoration of perfusion. Reentrant arrhythmias, which represent a particularly lethal manifestation of IR injury, can result when ischemic tissue exhibits decreased excitability and/or changes of action potential duration (APD), conditions that precipitate unidirectional conduction block. Many of the cellular components that are involved with IR injury are modulated by pH and/or phosphometabolites such as ATP and phosphocreatine (PCr), all of which can be manipulated in vivo and potentially in the clinical setting. Using a mathematical model of the cardiomyocyte that we previously developed to study ischemia and reperfusion, we performed a series of simulations with the aim of determining whether pH- or phosphometabolite-related processes play a more significant role in generating changes in excitability and action potential morphology that are associated with the development of reentry. In our simulations, persistent shortening of APD, action potential amplitude (APA), and depolarization of the resting membrane potential were more severe when ATP and PCr availability were suppressed during reperfusion than when extracellular pH recovery was inhibited. Reduced phosphometabolite availability and pH recovery affected multiple ion channels and exchangers. Some of these effects were the result of direct modulation by phosphometabolites and/or acidosis, while others resulted from elevated sodium and calcium loads during reperfusion. In addition, increasing ATP and PCr availability during reperfusion was more beneficial in terms of increasing APD and APA than was increasing the amount of pH recovery. Together, these results suggest that therapies directed at increasing ATP and/or PCr availability during reperfusion may be more beneficial than perturbing pH recovery with regard to mitigating action potential changes that increase the likelihood of reentrant arrhythmias.

Calcium-mediated cell death during myocardial reperfusion

Reperfusion may induce additional cell death in patients with acute myocardial infarction receiving primary angioplasty or thrombolysis. Altered intracellular Ca(2+) handling was initially considered an essential mechanism of reperfusion-induced cardiomyocyte death. However, more recent studies have demonstrated the importance of Ca(2+)-independent mechanisms that converge on mitochondrial permeability transition (MPT) and are shared by cardiomyocytes and other cell types. This article analyses the importance of Ca(2+)-dependent cell death in light of these new observations. Altered Ca(2+) handling includes increased cytosolic Ca(2+) levels, leading to activation of calpain-mediated proteolysis and sarcoplasmic reticulum-driven oscillations; this can induce hypercontracture, but also MPT due to the privileged Ca(2+) transfer between sarcoplasmic reticulum and mitochondria through cytosolic Ca(2+) microdomains. In the opposite direction, permeability transition can worsen altered Ca(2+) handling and favour hypercontracture. Ca(2+) appears to play an important role in cell death during the initial minutes of reperfusion, particularly after brief periods of ischaemia. Developing effective and safe treatments to prevent Ca(2+)-mediated cardiomyocyte death in patients with transient ischaemia, by targeting Ca(2+) influx, intracellular Ca(2+) handling, or Ca(2+)-induced cell death effectors, is an unmet challenge with important therapeutic implications and large potential clinical impact.

NHE inhibition does not improve Na(+) or Ca(2+) overload during reperfusion: using modeling to illuminate the mechanisms underlying a therapeutic failure

Reperfusion injury results from pathologies of cardiac myocyte physiology that develop when previously ischemic myocardium experiences a restoration of normal perfusion. Events in the development of reperfusion injury begin with the restoration of a proton gradient upon reperfusion, which then allows the sodium-proton exchanger (NHE) to increase flux, removing protons from the intracellular space while importing sodium. The resulting sodium overload drives increased reverse-mode sodium-calcium exchanger (NCX) activity, creating a secondary calcium overload that has pathologic consequences. One of the attempts to reduce reperfusion-related damage, NHE inhibition, has shown little clinical benefit, and only when NHE inhibitors are given prior to reperfusion. In an effort to further understand why NHE inhibitors have been largely unsuccessful, we employed a new mathematical cardiomyocyte model that we developed for the study of ischemia and reperfusion. Using this model, we simulated 20 minutes of ischemia and 10 minutes of reperfusion, while also simulating NHE inhibition by reducing NHE flux in our model by varying amounts and at different time points. In our simulations, when NHE inhibition is applied at the onset of reperfusion, increasing the degree of inhibition increases the peak sodium and calcium concentrations, as well as reducing intracellular pH recovery. When inhibition was instituted at earlier time points, some modest improvements were seen, largely due to reduced sodium concentrations prior to reperfusion. Analysis of all sodium flux pathways suggests that the sodium-potassium pump (NaK) plays the largest role in exacerbated sodium overload during reperfusion, and that reduced NaK flux is largely the result of impaired pH recovery. While NHE inhibition does indeed reduce sodium influx through that exchanger, the resulting prolongation of intracellular acidosis paradoxically increases sodium overload, largely mediated by impaired NaK function.

Myocardial ischemia, commonly observed when arteries supplying the heart become occluded, results when cardiac tissue receives inadequate blood perfusion. In order to minimize the amount of cardiac damage, ischemic tissue must be reperfused. However, reperfusion can result in deleterious effects that leave the heart muscle sicker than if the ischemia had been allowed to continue. Examples of these reperfusion injuries include lethal arrhythmias and an increased region of cell death. Some of the early events that result in reperfusion injury include changes in pH and an overload of sodium inside the cell. During reperfusion, the sodium-proton exchanger (NHE) removes protons from the cell in an effort to restore normal pH, in turn importing sodium ions. Many strategies have been attempted to prevent reperfusion injury, including inhibition of the NHE, with little clinical effect. Using a mathematical model that we developed to study ischemia and reperfusion in cardiac cells, we found that NHE inhibition produces more severe sodium overload, largely due to adverse consequences of the delayed pH recovery produced by NHE inhibition. These results suggest that NHE inhibition alone may not be a viable strategy, and that therapies which prolong intracellular acidosis may be problematic.

Acidosis and ischemia increase cellular Ca2+ transient alternans and repolarization alternans susceptibility in the intact rat heart

Cardiac cellular Ca(2+) transient (CaT) alternans and electrocardiographic T-wave alternans (TWA) often develop in myocardial ischemia, but the mechanisms for this relationship have not been elucidated. Acidosis is a major component of ischemia, but there is no direct evidence linking acidosis-induced cellular CaT alternans to ischemia-induced CaT alternans and TWA in whole heart. We used laser-scanning confocal microscopy to measure intracellular Ca(2+) (Ca(i)(2+)) cycling in individual myocytes of fluo-4 AM-loaded rat hearts and simultaneously recorded pseudo-ECGs to investigate changes in CaTs and late-phase repolarization, respectively, during baseline and rapid pacing under control and either globally acidic or globally ischemic conditions. Acidosis (hypercapnia; pH 6.6) increased diastolic Ca(i)(2+) levels, prolonged CaT duration, and shifted to slower heart rates both the development of pacing-induced acidosis-induced CaT alternans (both concordant and discordant) and of repolarization alternans (RPA, a measure of TWA in rat ECGs). The magnitudes of these shifts were equivalent for both CaT alternans and RPA, suggesting a close association between them. Nearly identical results were found in low-flow global ischemia. Additionally, ischemic preconditioning reduced the increased propensity for CaT alternans and RPA development and was mimicked by preconditioning by acidosis alone. Our results demonstrate that global acidosis or ischemia modifies Ca(i)(2+) cycling in myocytes such that the diastolic Ca(i)(2+) rises and the cellular CaT duration is prolonged, causing spatially concordant as well as spatially discordant cellular CaT alternans to develop at slower heart rates than in controls. Since RPA also developed at slower heart rates, our results suggest that acidosis is a major contributor to CaT alternans, which underlies the proarrhythmic state induced by myocardial ischemia and therefore may play a role in its modulation and prevention.

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 effect of cAMP-dependent protein kinase phosphorylation on the external Ca2+ binding sites of cardiac sarcoplasmic reticulum

Canine cardiac sarcoplasmic reticulum (SR) is known to be phosphorylated by adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase on a 22,000-dalton protein, Phosphorylation is associated with an increase in both the initial rate of Ca2+ uptake and the Ca(2+)-ATPase activity which is partially due to an increase in the affinity of the Ca(2+)-Mg(2+)-ATPase (E) of sarcoplasmic reticulum for calcium. In this study, the effect of cAMP-dependent protein kinase phosphorylation on the binding of calcium to the SR and on the dissociation of calcium from the SR was examined. The rate of dissociation of the E x Ca2 was measured directly and was not found to be significantly altered by cAMP-dependent protein kinase phosphorylation. Since the affinity of the enzyme for Ca2+ is equal to the ratio of the on and off rates of calcium, these results demonstrate that the observed change in affinity must be due to an increase in the rate of calcium binding to the Ca(2+)-Mg(2+)-ATPase of SR. In addition, an increase in the degree of positive cooperativity between the two calcium binding sites was associated with protein kinase phosphorylation.

Ca2+/calmodulin-dependent protein kinase: a key component in the contractile recovery from acidosis

Intracellular acidosis exerts substantial effects on the contractile performance of the heart. Soon after the onset of acidosis, contractility diminishes, largely due to a decrease in myofilament Ca(2+) responsiveness. This decrease in contractility is followed by a progressive recovery that occurs despite the persistent acidosis. This recovery is the result of different mechanisms that converge to increase diastolic Ca(2+) levels and Ca(2+) transient amplitude. Recent experimental evidence indicates that activation of the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is an essential step in the sequence of events that increases the Ca(2+) transient amplitude and produces contractile recovery. CaMKII may act as an amplifier, providing compensatory pathways to offset the inhibitory effects of acidosis on many of the Ca(2+) handling proteins. CaMKII-induced phosphorylation of the SERCA2a regulatory protein phospholamban (PLN) has the potential to promote an increase in sarcoplasmic reticulum (SR) Ca(2+) uptake and SR Ca(2+) load, and is a likely candidate to mediate the mechanical recovery from acidosis. In addition, CaMKII-dependent phosphorylation of proteins other than PLN may also contribute to this recovery.

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

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

Effects of cytosolic NADH/NAD(+) levels on sarcoplasmic reticulum Ca(2+) release in permeabilized rat ventricular myocytes

In the heart ischaemic conditions induce metabolic changes known to have profound effects on Ca(2+) signalling during excitation-contraction coupling. Ischaemia also affects the redox state of the cell. However, the role of cytosolic redox couples, such as the NADH/NAD(+) redox system, for the regulation of Ca(2+) homeostasis has remained elusive. We studied the effects of NADH and NAD(+) on sarcoplasmic reticulum (SR) Ca(2+) release in permeabilized rat ventricular myocytes as well as on Ca(2+) uptake by SR microsomes and ryanodine receptor (RyR) single channel activity. Exposure of permeabilized myocytes to NADH (2 mm; [Ca(2+)](cyt)= 100nm) decreased the frequency and the amplitude of spontaneous Ca(2+) sparks by 62% and 24%, respectively. This inhibitory effect was reversed by NAD(+) (2 mm) and did not depend on mitochondrial function. The inhibition of Ca(2+) sparks by NADH was associated with a 52% decrease in SR Ca(2+) load. Some of the effects observed with NADH may involve the generation of superoxide anion (O(2)(-).) as they were attenuated to just a transient decrease of Ca(2+) spark frequency by superoxide dismutase (SOD). O(2)(-). generated in situ from the xanthine/xanthine oxidase reaction caused a slowly developing decrease of Ca(2+) spark frequency and SR Ca(2+) load by 44% and 32%, respectively. Furthermore, in studies with cardiac SR microsomes NADH slowed the rate of ATP-dependent Ca(2+) uptake by 39%. This effect also appeared to depend on O(2)(-). formation. Single channel recordings from RyRs incorporated into lipid bilayers revealed that NADH (2 mm) inhibited the activity of RyR channels by 84%. However, NADH inhibition of RyR activity was O(2)(-).-independent. In summary, an increase of the cytoplasmic NADH/NAD(+) ratio depresses SR Ca(2+) release in ventricular cardiomyocytes. The effect appears to be mediated by direct NADH inhibition of RyR channel activity and by indirect NADH inhibition (O(2)(-). mediated) of SR Ca(2+)-ATPase activity with a subsequent decrease in SR Ca(2+) content.

Pyruvate modulates cardiac sarcoplasmic reticulum Ca2+ release in rats via mitochondria-dependent and -independent mechanisms

The glycolytic product pyruvate has beneficial effects on cardiac contractile function. The postulated cellular mechanisms underlying the positive inotropic effect of pyruvate, however, are contradictory or have remained elusive. Therefore, we studied the effects of pyruvate on cardiac Ca2+ regulation, intracellular pH (pHi) and flavoprotein oxidation using fluorescence confocal microscopy in intact and permeabilized rat ventricular myocytes and single channel recordings from rat cardiac ryanodine receptors (RyRs) incorporated into planar lipid bilayers. In intact cells extracellular pyruvate (10 mM) elevated diastolic [Ca2+]i, which was due, at least in part, to a concomitant acidification of the cytosol. Furthermore, pyruvate increased the amplitude and slowed the kinetics of the electrically evoked [Ca2+]i transient, and augmented sarcoplasmic reticulum (SR) Ca2+ content. Recording of flavoprotein (FAD) fluorescence indicated that pyruvate caused a reduction of mitochondrial redox potential, which is proportional to an increase of the rate of ATP synthesis. Inhibitors of mitochondrial monocarboxylate transport (alpha-cyano-4-hydroxycinnamate, 0.5 mM), adenine nucleotide translocation (atractyloside, 0.3 mM) and the electron transport chain (cyanide, 4 mM) abolished or attenuated the pyruvate-mediated increase of the amplitude of the [Ca2+]i transient, but did not change the effect of pyruvate on diastolic [Ca2+]i. Results from experiments with permeabilized myocytes indicated a direct correlation between ATP/ADP ratio and SR Ca2+ content. Furthermore, pyruvate (4 mM) reduced the frequency of spontaneous Ca2+ sparks by approximately 50%. Single RyR channel recordings revealed a approximately 60% reduction of the open probability of the channel by pyruvate (1 mM), but no change in conductance. This effect of pyruvate on RyR channel activity was neither Ca2+ nor ATP dependent. Taken together, these findings suggest that, in cardiac tissue, pyruvate has a dual effect on SR Ca2+ release consisting of a direct inhibition of RyR channel activity and elevation of SR Ca2+ content. The latter effect was most probably mediated by an enhanced SR Ca2+ uptake due to an augmentation of mitochondria-dependent ATP synthesis.

Sarcoplasmic reticulum function and muscle contractile character following fatiguing exercise in humans

1. This study examined the alterations in calcium release, calcium uptake and calcium ATPase activity of skeletal muscle sarcoplasmic reticulum in response to a bout of intense dynamic knee extensor exercise, and the relationship between these changes and alterations in muscle contractile characteristics in the human quadriceps. 2. In biopsy samples taken from the vastus lateralis, sarcoplasmic reticulum calcium release and calcium uptake were significantly depressed (P < 0.01 and 0.05, respectively) immediately following the exercise with no alteration in the sarcoplasmic reticulum Ca2+-ATPase activity. 3. A 33 % reduction in the maximum voluntary isometric torque was found following the exercise, with reduced torques from electrically evoked isometric contractions at low frequencies of stimulation (10 and 20 Hz) but not at higher frequencies (50 and 100 Hz). 4. The depressed calcium release was correlated (P < 0.05) with a decreased ratio of torques generated at 20:50 Hz, indicating an involvement in low frequency fatigue; however, no correlations between the muscle relaxation times or rates of change of torque and calcium uptake were observed.

Effect of intracellular pH on spontaneous Ca2+ sparks in rat ventricular myocytes

1. A fall of intracellular pH (pHi) typically depresses cardiac contractility. Among the many mechanisms underlying this depression, an inhibitory effect of acidosis upon the sarcoplasmic reticulum (SR) Ca2+ release channel has been predicted, but not so far demonstrated in the intact cardiac myocyte. In the present work, pHi was manipulated experimentally while confocal imaging was used to record spontaneous 'Ca2+ sparks' (local SR Ca2+ release events) in rat isolated myocytes loaded with the fluorescent Ca2+ indicator fluo-3. In other experiments, whole cell (global) pHi or [Ca2+]i was measured by microfluorimetry (using, respectively, intracellular carboxy SNARF-1 and indo-1). 2. Reducing pHi (i) increased whole cell intracellular [Ca2+] transients induced either electrically or by addition of caffeine, whereas (ii) it decreased spontaneous Ca2+ spark frequency. Conversely, raising pHi increased spontaneous Ca2+ spark frequency. 3. Blocking sarcolemmal Ca2+ influx with 10 mM Ni2+, or reducing external pH by 1.0 unit, had no effect on the pHi-dependent changes in spontaneous Ca2+ spark frequency. 4. Decreasing pHi over the range 7.78-7.20, decreased Ca2+ spark frequency exponentially as a function of pHi, with frequency declining by approximately 33 % for a 0.2 unit fall in pHi. In contrast, over the same pHi range, Ca2+ spark amplitude was unaffected. Intracellular acidosis produced a slight slowing of Ca2+ spark relaxation. 5. The results indicate that, in the intact myocyte, a reduced pHi decreases the probability of opening of the SR Ca2+ release channel. This phenomenon may contribute to the negative inotropic effects of acidosis.

Effect of acidosis on Ca2+ uptake and release by sarcoplasmic reticulum of intact rat ventricular myocytes

The effect of acidosis on Ca2+ uptake and release by the sarcoplasmic reticulum (SR) of rat ventricular myocytes has been investigated. Intracellular Ca2+ concentration ([Ca2+]i) was monitored using fura 2; the L-type Ca2+ current (ICa) was monitored using the perforated patch-clamp technique. Acidosis was produced either by superfusing the cells with an acid solution (intracellular and extracellular acidosis) or by NH4Cl withdrawal (intracellular acidosis). Both types of acidosis increased the amplitude, and slowed the declining phase, of the Ca2+ transient. Application of caffeine produced a rise of [Ca2+]i, which declined in the continued presence of caffeine; the declining phase was slowed by the acid solution but was unaffected by NH4Cl withdrawal. Acidosis decreased the fraction of the caffeine-induced release that was released by electrical stimulation but had no effect on ICa. It is concluded that acidosis inhibits SR Ca2+ uptake and Ca2+-induced Ca2+ release in intact myocytes but that these effects are compensated by an increase in SR Ca2+ content secondary to a rise in cytoplasmic [Ca2+].

Sarco/endoplasmic reticulum Ca2+-ATPase isoforms: diverse responses to acidosis

The effects of acidic pH on the kinetics of Ca2+-ATPase isoforms from intracellular membranes of skeletal muscle, cardiac muscle, cerebellum and blood platelets were studied. At neutral pH, all four Ca2+-ATPase isoforms exhibited similar Ca2+-concentration requirements for half-maximal rates of Ca2+ uptake and ATP hydrolysis. A decrease in the pH from 7.0 to 6.0 promoted a decrease in both the apparent affinity for Ca2+ [increasing half-maximal activation (K0.5)] and the maximal velocity (Vmax) of Ca2+ uptake. With skeletal muscle vesicles these effect were 5 to 10 times smaller than those observed with all the other isoforms. Acidification of the medium from pH 7.0 to 6.5 caused the release of Ca2+ from loaded vesicles and a decrease in the amount of Ca2+ retained by the vesicles at the steady state. With the vesicles derived from skeletal muscle these effects were smaller than for vesicles derived from other tissues. The rate of passive Ca2+ efflux from skeletal and cardiac muscle vesicles, loaded with Ca2+ and diluted in a medium containing none of the ligands of Ca2+-ATPase, was the same at pH 7.0 and 6.0. In contrast, the rate of Ca2+ efflux from cerebellar and platelet vesicles increased 2-fold after acidification of the medium. The effects of DMSO, Mg2+ with Pi and arsenate on the rate of Ca2+ efflux varied among the different preparations tested. The differences became more pronounced when the pH of the medium was decreased from 7.0 to 6.0. It is proposed that the kinetic differences among the Ca2+-ATPase isoforms may reflect different adaptations to cellular acidosis, such as that which occurs during ischaemia.

Phosphorylation and regulation of the Ca(2+)-pumping ATPase in cardiac sarcoplasmic reticulum by calcium/calmodulin-dependent protein kinase

In cardiac muscle, a membrane-associated Ca2+/calmodulin-dependent protein kinase (CaM kinase) phosphorylates the Ca(2+)-pumping ATPase in addition to its previously characterized substrates, phospholamban and Ca(2+)-release channel (ryanodine receptor). The phosphorylated amino acid in the Ca(2+)-ATPase has been identified as serine. Posphorylation of the Ca(2+)-ATPase is rapid and is reversible by a membrane-associated protein phosphatase, Ca(2+)-ATPase purified from cardiac SR underwent phosphorylation by exogenous CaM kinase, and the phosphorylated enzyme displayed twofold greater catalytic activity without alteration in its Ca(2+)-sensitivity. The phosphorylation of the Ca(2+)-ATPase was found to be isoform-specific in that the cardiac and slow-twitch skeletal muscle isoform (SERCA 2), but not the fast-twitch skeletal muscle isoform (SERCA 1), underwent phosphorylation by CaM kinase. Studies using SERCA 1 and SERCA 2 isoforms and their mutants expressed in a heterelogous cell system have resulted in i) confirmation of the isoform specificity of Ca(2+)-ATPase phosphorylation by CaM kinase, ii) identification of Ser38 as the site in SERCA 2 phosphorylated by CaM kinase, and iii) demonstration of phosphorylation-induced increase in Vmax of Ca2+ transport by the SERCA 2 enzyme. These observations suggest that in cardiac and slow-twitch skeletal muscle direct phosphorylation of the SR Ca(2+)-ATPase by the membrane-bound CaM kinase may serve to stimulate Ca2+ sequestration and therefore, the speed of muscle relaxation.

Effects of acidosis on phosphorylation of phospholamban and troponin I in rat cardiac muscle

Acidosis inhibits Ca2+ transport by the sarcoplasmic reticulum of cardiac muscle and decreases the Ca2+ sensitivity of the contractile proteins, although the mechanisms underlying these changes are unclear. We have investigated the hypothesis that changes in the phosphorylation of the regulatory proteins phospholamban and troponin I might play a role in the acidosis-induced changes in the function of the sarcoplasmic reticulum and the myofilaments, respectively. Langendorff-perfused rat hearts were labeled with 32P and then perfused with either control (pH 7.4) or acid (pH 6.8) physiological salt solution, in both the absence and presence of isoproterenol. The incorporation of 32P into phospholamban and troponin I was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of sarcoplasmic reticulum and myofibrillar proteins, followed by autoradiography and liquid scintillation counting. The data show that acidosis has no effect on the phosphorylation of phospholamban in the absence of isoproterenol but that, in the presence of isoproterenol, acidosis increased the phosphorylation of phospholamban. However, acidosis increased the phosphorylation of troponin I, in both the absence and the presence of isoproterenol. Acidosis did not alter the adenosine 3',5'-cyclic monophosphate content of the hearts but did inhibit type 1 phosphatase. These data show that acidosis can alter the phosphorylation of these two proteins and suggest that these changes underlie, in part the changes observed in cardiac muscle during acidosis.

Effects of prolonged low frequency stimulation on skeletal muscle sarcoplasmic reticulum

The role of prolonged electrical stimulation on sarcoplasmic reticulum (SR) Ca2+ sequestration measured in vitro and muscle energy status in fast white and red skeletal muscle was investigated. Fatigue was induced by 90 min intermittent 10-Hz stimulation of rat gastrocnemius muscle, which led to reductions (p < 0.05) in ATP, creatine phosphate, and glycogen of 16, 55, and 49%, respectively, compared with non-stimulated muscle. Stimulation also resulted in increases (p < 0.05) in muscle lactate, creatine, Pi, total ADP, total AMP, IMP, and inosine. Calculated free ADP (ADPf) and free AMP (AMPf) were elevated 3- and 15-fold, respectively. No differences were found in the metabolic response between tissues obtained from the white (WG) and red (RG) regions of the gastrocnemius. No significant reductions is SR Ca2+ ATPase activity were observed in homogenate (HOM) or a crude SR fraction (CM) from WG or RG muscle following exercise. Maximum Ca2+ uptake in HOM and CM preparations was similar in control (C) and stimulated (St) muscles. However, Ca2+ uptake at 400 nM free Ca2+ was significantly reduced in CM from RG (0.108 +/- 0.04 to 0.076 +/- 0.02 mumol.mg-1 protein.min-1 in RG - C and RG - St, respectively). Collectively, these data suggest that reductions in muscle energy status are dissociated from changes in SR Ca2+ ATPase activity in vitro but are related to Ca2+ uptake at physiological free [Ca2+ bd in fractionated SR from highly oxidative muscle. Dissociation of SR Ca2+ ATPase activity from Ca2+ uptake may reflect differences in the mechanisms evaluated by these techniques.

Failure of short term stimulation to reduce sarcoplasmic reticulum Ca(2+)-ATPase function in homogenates of rat gastrocnemius

To examine the effect of short term intense activity on sarcoplasmic reticulum (SR) Ca2+ sequestering function, the gastrocnemius (G) muscles of 11 anaesthetized male rats (weight, 411 +/- 8 g, X +/- SE) were activated using supramaximal, intermittent stimulation (one train of 0.2 msec impulses per sec of 100 msec at 100 Hz). Homogenates were obtained from stimulated white (WG-S) and red (RG-S) tissues, assayed for Ca2+ uptake and maximal Ca2+ ATPase activity and compared to contralateral controls (WG-C, RG-C). Calcium uptake (nmoles/mg protein/min) determined using Indo-1 and at [Ca2+]i concentrations between 300-400 nM was unaffected (p > 0.05) by activity in both WG (6.14 + 0.43 vs 5.37 + 0.43) and RG (3.21 + 0.18 vs 3.07 + 0.20). Similarly, no effect (p > 0.05) of contractile activity was found for maximal Ca2+ ATPase activity (mumole/mg protein/min) determined spectrophotometrically in RG (0.276 + 0.03 vs 0.278 + 0.02). In WG, Ca2+ ATPase activity was 15% higher in WG-S compared to WG-C (0.412 + 0.03 vs 0.385 + 0.04). Repetitive stimulation resulted in a reduction in tetanic tension of 74% (p < 0.05) by 2 min in the G muscle. By the end of the stimulation period, ATP concentration was reduced (p < 0.05) by 57% in the WG and by 47% in the RG.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium exchange hypothesis of skeletal muscle fatigue: a brief review

Skeletal muscle fatigue is often associated with diminished athletic performance and work productivity as well as increased susceptibility to injury. The exact cause of muscle fatigue probably involves a number of factors which influence force production in a manner dependent on muscle fiber type and activation pattern. However, a growing body of evidence implicates alterations in intracellular Ca2+ exchange as a major role in the fatigue process. These changes are thought to occur secondary to reductions in the rates of Ca2+ uptake and release by the sarcoplasmic reticulum (SR). This hypothesis is based on the finding that peak myoplasmic Ca2+ concentration ([Ca2+]i) is reduced as force declines during fatigue. In addition, direct measurements of Ca2+ uptake and release show that fatiguing activity causes intrinsic alterations in the functional properties of the SR. We also propose that fatigue-induced alterations in Ca2+ exchange may be beneficial, reducing the rate of energy utilization by the muscle fiber and preventing irreversible damage to the cell.

Lusitropic changes induced by acid base alterations in cat papillary muscles

The present work investigates the effects of acid-base alterations upon myocardial relaxation. Experiments were performed in cat papillary muscles contracting isometrically at constant frequency (0.2 Hz) and temperature (29 degrees C). To induce intracellular alkalosis at constant pH0, 20 mM NH4Cl were added to the perfusate. Alkalosis at variable pH0 was induced by switching from the control solution (5% CO2-95% O2, pH0 7.40) to a solution identical to the control one, equilibrated with 3% CO2-97% O2. Acidosis was induced by switching the control perfusate to a solution equilibrated with 12% CO2-88% O2 in which pH0 was either allowed to change or kept constant by manipulation of bicarbonate concentration. Alkalosis produced a negative lusitropic effect either when pH0 was kept constant or when it was allowed to increase. For an increase in myocardial contractility of 30%, half relaxation tme (T50) and time to peak tension (TTP) were prolonged 9.4 +/- 5% and 5.4 +/- 2% respectively at constant pH0 and 6.8 +/- 0.8 and 4.7 +/- 1% respectively at variable pHo. It is suggested that this negative lusitropic effect of alkalosis can be attributed to an increase in myofilament sensitivity to calcium. Either at constant or at variable pHo acidosis decreased myocardial contractility by approximately 50%. This decrease in contractility was accompanied by a positive lusitropic action only when pHo was allowed to decrease, or when acidosis at constant pHo was evoked in the presence of EIPA, a specific inhibitor of the Na+/H+ exchanger.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in myoplasmic pH and calcium concentration during exposure to lactate in isolated rat ventricular myocytes

1. We investigated the mechanisms involved in the rise of myoplasmic calcium concentration ([Ca2+]i) when isolated rat ventricular myocytes were exposed to lactate. The intracellular pH (pHi) and [Ca2+]i were measured using the fluorescent indicators 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) and fura-2, respectively. Cell shortening was used as a measure of contractile performance. 2. Exposure to 20 mM lactate at the normal extracellular pH (pHo 7.4) for 10 min caused the pHi to fall rapidly by 0.24 pH units and cell shortening was reduced. Thereafter, pHi partially recovered by 0.16 pH units, which was paralleled by a recovery of shortening. 3. Exposure to lactate at a reduced extracellular pH (pHo 6.4) induced a very large acidosis of 0.70 pH units and cell shortening was abolished. During maintained exposure to lactate the pHi remained constant and cell shortening did not recover. 4. Application of Na(+)-H+ exchanger inhibitors, amiloride or ethylisopropyl-amiloride (EIPA), abolished the recovery of pHi and shortening during maintained exposure to lactate at pHo 7.4 and caused an additional acidosis during maintained application of lactate at pHo 6.4. 5. Application of lactate at both the normal and reduced pHo resulted in a rapid, followed by a slower, rise in [Ca2+]i. The diastolic and systolic [Ca2+]i and the amplitude of the systolic rise in the [Ca2+]i (the Ca2+ transient) all increased in both the rapid and the slow phase. 6. When lactate was applied at pHo 7.4, in the presence of EIPA, the initial rise of [Ca2+]i still occurred but the slower increase was abolished. This suggests an involvement of the Na(+)-H+ exchanger in the slower rise of [Ca2+]i. 7. In conclusion, the Na(+)-H+ exchanger is an important regulator of pHi during a lactate-induced intracellular acidosis. The rise of [Ca2+]i involves at least two mechanisms: (i) a rapid component which may represent reduced myoplasmic Ca2+ buffering, impaired Ca2+ removal by the sarcoplasmic reticulum or a direct inhibitory effect of protons on the Na(+)-Ca2+ exchanger; (ii) a slower component linked to stimulation of Na(+)-H+ exchanger which causes an increased [Na+]i and stimulates the Na(+)-Ca2+ exchanger, resulting in an enhanced Ca2+ influx.

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.

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.

The alteration of rabbit skeletal sarcoplasmic reticulum function by N-acylethanolamine, a lipid associated with myocardial infarction

The ability of N-acylethanolamines (pharmacologically active lipid metabolites which accumulate in canine myocardium during experimentally induced infarctions) to alter Ca2+ fluxes in a biological membrane system was studied using sarcoplasmic reticulum vesicles prepared from rabbit skeletal muscle. The effects of two N-acylethanolamines, the N-oleyl and N-lauryl derivatives, were compared to those of the lipophilic drugs, dibucaine and propranolol. The rate and extent of Ca2+ sequestration, Ca2+-Mg2+-ATPase activity and retention time of Ca2+ by the vesicles were all stimulated at low concentrations of the four compounds studied and inhibited at higher concentrations. The stoichiometry between Ca2+-pumping rates and ATPase activity was partially "uncoupled" indicating that both the calcium pump and the membrane permeability were affected by the drugs. However, although all four compounds exhibited the same qualitative behavior, the effects of the two N-acylethanolamines were more pronounced than dibucaine and propranolol and occurred at much lower concentrations. These results suggest that the N-acylethanolamines may have important physiological effects in the myocardium and, at least at lower concentrations, stimulate myocardial contractility by increasing the rate of calcium flux across the sarcoplasmic reticulum.