Intra- and extracellular actions of proton on the calcium current of isolated guinea pig ventricular cells

Regulation of Ca2+ signaling by acute hypoxia and acidosis in cardiomyocytes derived from human induced pluripotent stem cells

AIMS

The effects of acute (100 s) hypoxia and/or acidosis on Ca²⁺ signaling parameters of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) are explored here for the first time.

METHODS AND RESULTS

1) hiPSC-CMs express two cell populations: rapidly-inactivating I_Ca myocytes (τ_i<40 ms, in 4-5 day cultures) and slowly-inactivating I_Ca (τ_i  ≥ 40 ms, in 6-8 day cultures). 2) Hypoxia suppressed I_Ca by 10-20% in rapidly- and 40-55% in slowly-inactivating I_Ca cells. 3) Isoproterenol enhanced I_Ca in hiPSC-CMs, but either enhanced or did not alter the hypoxic suppression. 4) Hypoxia had no differential suppressive effects in the two cell-types when Ba²⁺ was the charge carrier through the calcium channels, implicating Ca²⁺-dependent inactivation in O₂ sensing. 5) Acidosis suppressed I_Ca by ∼35% and ∼25% in rapidly and slowly inactivating I_Ca cells, respectively. 6) Hypoxia and acidosis suppressive effects on Ca-transients depended on whether global or RyR2-microdomain were measured: with acidosis suppression was ∼25% in global and ∼37% in RyR2 Ca²⁺-microdomains in either cell type, whereas with hypoxia suppression was ∼20% and ∼25% respectively in global and RyR2-microdomaine in rapidly and ∼35% and ∼45% respectively in global and RyR2-microdomaine in slowly-inactivating cells.

CONCLUSIONS

Variability in I_Ca inactivation kinetics rather than cellular ancestry seems to underlie the action potential morphology differences generally attributed to mixed atrial and ventricular cell populations in hiPSC-CMs cultures. The differential hypoxic regulation of Ca²⁺-signaling in the two-cell types arises from differential Ca²⁺-dependent inactivation of the Ca²⁺-channel caused by proximity of Ca²⁺-release stores to the Ca²⁺ channels.

Regulation of Ca2+ signaling by acute hypoxia and acidosis in rat neonatal cardiomyocytes

Ischemic heart disease is an arrhythmogenic condition, accompanied by hypoxia, acidosis, and impaired Ca²⁺ signaling. Here we report on effects of acute hypoxia and acidification in rat neonatal cardiomyocytes cultures.

RESULTS

Two populations of neonatal cardiomyocyte were identified based on inactivation kinetics of L-type I_Ca: rapidly-inactivating I_Ca (τ~20ms) myocytes (prevalent in 3-4-day cultures), and slow-inactivating I_Ca (τ≥40ms) myocytes (dominant in 7-day cultures). Acute hypoxia (pO₂<5mmHg for 50-100s) suppressed I_Ca reversibly in both cell-types to different extent and with different kinetics. This disparity disappeared when Ba²⁺ was the channel charge carrier, or when the intracellular Ca²⁺ buffering capacity was increased by dialysis of high concentrations of EGTA and BAPTA, suggesting critical role for calcium-dependent inactivation. Suppressive effect of acute acidosis on I_Ca (~40%, pH6.7), on the other hand, was not cell-type dependent. Isoproterenol enhanced I_Ca in both cell-types, but protected only against suppressive effects of acidosis and not hypoxia. Hypoxia and acidosis suppressed global Ca²⁺ transients by ~20%, but suppression was larger, ~35%, at the RyR2 microdomains, using GCaMP6-FKBP targeted probe. Hypoxia and acidosis also suppressed mitochondrial Ca²⁺ uptake by 40% and 10%, respectively, using mitochondrial targeted Ca²⁺ biosensor (mito-GCaMP6).

CONCLUSION

Our studies suggest that acute hypoxia suppresses I_Ca in rapidly inactivating cell population by a mechanism involving Ca²⁺-dependent inactivation, while compromised mitochondrial Ca²⁺ uptake seems also to contribute to I_Ca suppression in slowly inactivating cell population. Proximity of cellular Ca²⁺ pools to sarcolemmal Ca²⁺ channels may contribute to the variability of inactivation kinetics of I_Ca in the two cell populations, while acidosis suppression of I_Ca appears mediated by proton-induced block of the calcium channel.

Effects of electrophysiological heterogeneity on vulnerability to re-entry in human ventricular tissue: A simulation study

In this study, we constructed a two-dimensional ventricular tissue sheet with cellular electrophysiology modified from the Ten Tusscher 2006 Model. Heterogeneity was created by dividing the tissue into endocardium, midmyocardium and epicardium, further enhanced by a central ischemic zone. Subsequently, we investigated how electrophysiological heterogeneity affects re-entry initiation and maintenance in this tissue. Furthermore, we analyzed the vulnerable window (VW) under several conditions and concluded that heterogeneity across various myocardia expands the VW further than the monolayer myocardium model does.

Effects of acidic pH on voltage-gated ion channels in rat trigeminal mesencephalic nucleus neurons

The effects of acidic pH on several voltage-dependent ion channels, such as voltage-dependent K⁺ and Ca²⁺ channels, and hyperpolarization-gated and cyclic nucleotide-activated cation (HCN) channels, were examined using a whole-cell patch clamp technique on mechanically isolated rat mesencephalic trigeminal nucleus neurons. The application of a pH 6.5 solution had no effect on the peak amplitude of voltage-dependent K⁺ currents. A pH 6.0 solution slightly, but significantly inhibited the peak amplitude of voltage-dependent K⁺ currents. The pH 6.0 also shifted both the current-voltage and conductance-voltage relationships to the depolarization range. The application of a pH 6.5 solution scarcely affected the peak amplitude of membrane currents mediated by HCN channels, which were profoundly inhibited by the general HCN channel blocker Cs⁺ (1 mM). However, the pH 6.0 solution slightly, but significantly inhibited the peak amplitude of HCN-mediated currents. Although the pH 6.0 solution showed complex modulation of the current-voltage and conductance-voltage relationships, the midpoint voltages for the activation of HCN channels were not changed by acidic pH. On the other hand, voltage-dependent Ca²⁺ channels were significantly inhibited by an acidic pH. The application of an acidic pH solution significantly shifted the current-voltage and conductance-voltage relationships to the depolarization range. The modulation of several voltage-dependent ion channels by an acidic pH might affect the excitability of mesencephalic trigeminal nucleus neurons, and thus physiological functions mediated by the mesencephalic trigeminal nucleus could be affected in acidic pH conditions.

Electrophysiological properties of computational human ventricular cell action potential models under acute ischemic conditions

Acute myocardial ischemia is one of the main causes of sudden cardiac death. The mechanisms have been investigated primarily in experimental and computational studies using different animal species, but human studies remain scarce. In this study, we assess the ability of four human ventricular action potential models (ten Tusscher and Panfilov, 2006; Grandi et al., 2010; Carro et al., 2011; O'Hara et al., 2011) to simulate key electrophysiological consequences of acute myocardial ischemia in single cell and tissue simulations. We specifically focus on evaluating the effect of extracellular potassium concentration and activation of the ATP-sensitive inward-rectifying potassium current on action potential duration, post-repolarization refractoriness, and conduction velocity, as the most critical factors in determining reentry vulnerability during ischemia. Our results show that the Grandi and O'Hara models required modifications to reproduce expected ischemic changes, specifically modifying the intracellular potassium concentration in the Grandi model and the sodium current in the O'Hara model. With these modifications, the four human ventricular cell AP models analyzed in this study reproduce the electrophysiological alterations in repolarization, refractoriness, and conduction velocity caused by acute myocardial ischemia. However, quantitative differences are observed between the models and overall, the ten Tusscher and modified O'Hara models show closest agreement to experimental data.

Altered Calcium Handling and Ventricular Arrhythmias in Acute Ischemia

Acute ischemia results in deadly cardiac arrhythmias that are a major contributor to sudden cardiac death (SCD). The electrophysiological changes involved have been extensively studied, yet the mechanisms of ventricular arrhythmias during acute ischemia remain unclear. What is known is that during acute ischemia both focal (ectopic excitation) and nonfocal (reentry) arrhythmias occur, due to an interaction of altered electrical, mechanical, and biochemical properties of the myocardium. There is particular interest in the role that alterations in intracellular calcium handling, which cause changes in intracellular calcium concentration and to the calcium transient, play in ischemia-induced arrhythmias. In this review, we briefly summarize the known contributors to ventricular arrhythmias during acute ischemia, followed by an in-depth examination of the potential contribution of altered intracellular calcium handling, which may include novel targets for antiarrhythmic therapy.

Extracellular HCO3- is sensed by mouse cerebral arteries: Regulation of tone by receptor protein tyrosine phosphatase γ

We investigate sensing and signaling mechanisms for H(+), [Formula: see text] and CO2 in basilar arteries using out-of-equilibrium solutions. Selectively varying pHo, [[Formula: see text]]o, or pCO2, we find: (a) lowering pHo attenuates vasoconstriction and vascular smooth muscle cell (VSMC) Ca(2+)-responses whereas raising pHo augments vasoconstriction independently of VSMC [Ca(2+)]i, (b) lowering [[Formula: see text]]o increases arterial agonist-sensitivity of tone development without affecting VSMC [Ca(2+)]i but c) no evidence that CO2 has direct net vasomotor effects. Receptor protein tyrosine phosphatase (RPTP)γ is transcribed in endothelial cells, and direct vasomotor effects of [Formula: see text] are absent in arteries from RPTPγ-knockout mice. At pHo 7.4, selective changes in [[Formula: see text]]o or pCO2 have little effect on pHi At pHo 7.1, decreased [[Formula: see text]]o or increased pCO2 causes intracellular acidification, which attenuates vasoconstriction. Under equilibrated conditions, anti-contractile effects of CO2/[Formula: see text] are endothelium-dependent and absent in arteries from RPTPγ-knockout mice. With CO2/[Formula: see text] present, contractile responses to agonist-stimulation are potentiated in arteries from RPTPγ-knockout compared to wild-type mice, and this difference is larger for respiratory than metabolic acidosis. In conclusion, decreased pHo and pHi inhibit vasoconstriction, whereas decreased [[Formula: see text]]o promotes vasoconstriction through RPTPγ-dependent changes in VSMC Ca(2+)-sensitivity. [Formula: see text] serves dual roles, providing substrate for pHi-regulating membrane transporters and modulating arterial responses to acid-base disturbances.

Multiscale computational analysis of the bioelectric consequences of myocardial ischaemia and infarction

Ischaemic heart disease is considered as the single most frequent cause of death, provoking more than 7 000 000 deaths every year worldwide. A high percentage of patients experience sudden cardiac death, caused in most cases by tachyarrhythmic mechanisms associated to myocardial ischaemia and infarction. These diseases are difficult to study using solely experimental means due to their complex dynamics and unstable nature. In the past decades, integrative computational simulation techniques have become a powerful tool to complement experimental and clinical research when trying to elucidate the intimate mechanisms of ischaemic electrophysiological processes and to aid the clinician in the improvement and optimization of therapeutic procedures. The purpose of this paper is to briefly review some of the multiscale computational models of myocardial ischaemia and infarction developed in the past 20 years, ranging from the cellular level to whole-heart simulations.

Intracellular ATP binding is required to activate the slowly activating K+ channel I(Ks)

Gating of ion channels by ligands is fundamental to cellular function, and ATP serves as both an energy source and a signaling molecule that modulates ion channel and transporter functions. The slowly activating K(+) channel I(Ks) in cardiac myocytes is formed by KCNQ1 and KCNE1 subunits that conduct K(+) to repolarize the action potential. Here we show that intracellular ATP activates heterologously coexpressed KCNQ1 and KCNE1 as well as I(Ks) in cardiac myocytes by directly binding to the C terminus of KCNQ1 to allow the pore to open. The channel is most sensitive to ATP near its physiological concentration, and lowering ATP concentration in cardiac myocytes results in I(Ks) reduction and action potential prolongation. Multiple mutations that suppress I(Ks) by decreasing the ATP sensitivity of the channel are associated with the long QT (interval between the Q and T waves in electrocardiogram) syndrome that predisposes afflicted individuals to cardiac arrhythmia and sudden death. A cluster of basic and aromatic residues that may form a unique ATP binding site are identified; ATP activation of the wild-type channel and the effects of the mutations on ATP sensitivity are consistent with an allosteric mechanism. These results demonstrate the activation of an ion channel by intracellular ATP binding, and ATP-dependent gating allows I(Ks) to couple myocyte energy state to its electrophysiology in physiologic and pathologic conditions.

Role of CaMKII in post acidosis arrhythmias: a simulation study using a human myocyte model

Postacidotic arrhythmias have been associated to increased sarcoplasmic reticulum (SR) Ca(2+) load and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activation. However, the molecular mechanisms underlying these arrhythmias are still unclear. To better understand this process, acidosis produced by CO2 increase from 5% to 30%, resulting in intracellular pH (pHi) change from 7.15 to 6.7, was incorporated into a myocyte model of excitation-contraction coupling and contractility, including acidotic inhibition of L-type Ca(2+) channel (I(CaL)), Na(+)-Ca(2+) exchanger, Ca(2+) release through the SR ryanodine receptor (RyR2) (I(rel)), Ca(2+) reuptake by the SR Ca(2+) ATPase2a (I(up)), Na(+)-K(+) pump, K(+) efflux through the inward rectifier K(+) channel and the transient outward K(+) flow (I(to)) together with increased activity of the Na(+)-H(+) exchanger (I(NHE)). Simulated CaMKII regulation affecting I(rel), I(up), I(CaL), I(NHE) and I(to) was introduced in the model to partially compensate the acidosis outcome. Late Na(+) current increase by CaMKII was also incorporated. Using this scheme and assuming that diastolic Ca(2+) leak through the RyR2 was modulated by the resting state of this channel and the difference between SR and dyadic cleft [Ca(2+)], postacidotic delayed after depolarizations (DADs) were triggered upon returning to normal pHi after 6 min acidosis. The model showed that DADs depend on SR Ca(2+) load and on increased Ca(2+) leak through RyR2. This postacidotic arrhythmogenic pattern relies mainly on CaMKII effect on I(CaL) and I(up), since its individual elimination produced the highest DAD reduction. The model further revealed that during the return to normal pHi, DADs are fully determined by SR Ca(2+) load at the end of acidosis. Thereafter, DADs are maintained by SR Ca(2+) reloading by Ca(2+) influx through the reverse NCX mode during the time period in which [Na(+)]i is elevated.

Modulation of ventricular transient outward K⁺ current by acidosis and its effects on excitation-contraction coupling

The contribution of transient outward current (Ito) to changes in ventricular action potential (AP) repolarization induced by acidosis is unresolved, as is the indirect effect of these changes on calcium handling. To address this issue we measured intracellular pH (pHi), Ito, L-type calcium current (ICa,L), and calcium transients (CaTs) in rabbit ventricular myocytes. Intracellular acidosis [pHi 6.75 with extracellular pH (pHo) 7.4] reduced Ito by ~50% in myocytes with both high (epicardial) and low (papillary muscle) Ito densities, with little effect on steady-state inactivation and activation. Of the two candidate α-subunits underlying Ito, human (h)Kv4.3 and hKv1.4, only hKv4.3 current was reduced by intracellular acidosis. Extracellular acidosis (pHo 6.5) shifted Ito inactivation toward less negative potentials but had negligible effect on peak current at +60 mV when initiated from -80 mV. The effects of low pHi-induced inhibition of Ito on AP repolarization were much greater in epicardial than papillary muscle myocytes and included slowing of phase 1, attenuation of the notch, and elevation of the plateau. Low pHi increased AP duration in both cell types, with the greatest lengthening occurring in epicardial myocytes. The changes in epicardial AP repolarization induced by intracellular acidosis reduced peak ICa,L, increased net calcium influx via ICa,L, and increased CaT amplitude. In summary, in contrast to low pHo, intracellular acidosis has a marked inhibitory effect on ventricular Ito, perhaps mediated by Kv4.3. By altering the trajectory of the AP repolarization, low pHi has a significant indirect effect on calcium handling, especially evident in epicardial cells.

Intracellular pH in the resistance vasculature: regulation and functional implications

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

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.

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.

Contribution of delayed intracellular pH recovery to ischemic postconditioning protection

Ischemic postconditioning (PoCo) has been proven to be a feasible approach to attenuate reperfusion injury and enhance myocardial salvage in patients with acute myocardial infarction, but its mechanisms have not been completely elucidated yet. Recent studies demonstrate that PoCo may delay the recovery of intracellular pH during initial reperfusion, and that its ability to limit infarct size critically depends on this effect. Prolongation of postischemic intracellular acidosis inhibits hypercontracture, mitochondrial permeability transition, calpain-mediated proteolysis, and gap junction-mediated spread of injury during the first minutes of reflow. This role of prolonged acidosis does not exclude the participation of other pathways in PoCo-induced cardioprotection. On the contrary, it may allow these pathways to act by preventing immediate reperfusion-induced cell death. Moreover, the existence of interactions between intracellular acidosis and endogenous protection signaling cannot be excluded and needs to be investigated. The role of prolonged acidosis in PoCo cardioprotection has important implications in the design of optimal PoCo protocols and in the translation of cardioprotective strategies to patients with on-going myocardial infarction receiving coronary reperfusion.

Mechanisms for initiation of reentry in acute regional ischemia phase 1B

BACKGROUND

During phase 1B of acute regional ischemia, the subepicardial and subendocardial layers coupled to the inexcitable midmyocardium remain viable.

OBJECTIVE

The purpose of this study was to examine how the degree of hyperkalemia in the surviving layers, the lateral width of border zone between the normal tissue and the central ischemic zone, and the degree of cellular uncoupling between the surviving layers and the midmyocardium contribute to initiation of reentry.

METHODS

Simulations were conducted on the state-of-the-art model of rabbit ventricles with realistic representation of the spatial distribution of the ischemic insult.

RESULTS

Hyperkalemia in the surviving layers led to induction of reentry by increasing refractoriness and slowing conduction in the layers. Such reentries were formed solely in the subepicardium. A minimal level of hyperkalemia was required for induction of reentry. Progress increase in hyperkalemia led to a biphasic change in vulnerability to reentry. For each level of hyperkalemia, increased cellular uncoupling between subepicardium and midmyocardium increased inducibility of reentry by restoring subepicardial tissue excitability via blocking midmyocardial electrotonic effect. In addition, increased lateral width of the border zone prevented inducibility of reentry as conduction block occurred in the central ischemic zone when the wave propagated across the border zone from the normal zone.

CONCLUSION

The degree of hyperkalemia in the surviving subepicardium, the lateral width of border zone, and cellular uncoupling between the subepicardium and midmyocardium determine dispersion of refractoriness, conduction velocity, excitability, and, therefore, inducibility of reentry during phase 1B.

Involvement of Ca(2+) channel activity in proliferation of vascular smooth muscle cells

Proliferation of vascular smooth muscle (VSM) cells is a crucial step for developing vascular diseases such as atherosclerosis, hypertension and vascular restenosis after angioplasty. Proliferation of VSM cells is regulated by many intracellular signals: second messengers (e.g. Ca(2+), phosphatydylinositol, cAMP/cGMP), protein kinases and transcription factors. Although Ca(2+) regulation of cell proliferation is very important, there is rarely any informative review paper about the topic. Increase in cytosolic intracellular Ca(2+) concentration ([Ca(2+)](i)) due to Ca(2+) entry is necessary for proliferation of VSM cells. Elevation of [Ca(2+)](i) is needed for both cell cycle progressions at G(1)/S phase and the cell division in M phase. Intracellular Ca(2+) is regulated by the balance between Ca(2+)-elevating machinery such as Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCC), Ca(2+) release from stored Ca(2+) in sarcoplasmic reticulum and Ca(2+)-lowering machinery such as Ca(2+) transport ATPases. In this review paper, we focus on the role of VDCC in the regulation of cell proliferation, especially in VSM cells. We also described significant roles of VDCC in pathophysiological conditions such as atherosclerosis, stroke and renal dysfunction.

The relative role of refractoriness and source-sink relationship in reentry generation during simulated acute ischemia

During acute myocardial ischemia, reentrant episodes may lead to ventricular fibrillation (VF), giving rise to potentially mortal arrhythmias. VF has been traditionally related to dispersion of refractoriness and more recently to the source-sink relationship. Our goal is to theoretically investigate the relative role of dispersion of refractoriness and source-sink mismatch in vulnerability to reentry in the specific situation of regional myocardial acute ischemia. The electrical activity of a regionally ischemic tissue was simulated using a modified version of the Luo-Rudy dynamic model. Ischemic conditions were varied to simulate the time-course of acute ischemia. Our results showed that dispersion of refractoriness increased with the severity of ischemia. However, no correlation between dispersion of refractoriness and the width of the vulnerable window was found. Additionally, in approximately 50% of the reentries, unidirectional block (UDB) took place in cells completely recovered from refractoriness. We examined patterns of activation after premature stimulation and they were intimately related to the source-sink relationship, quantified by the safety factor (SF). Moreover, the isoline where the SF dropped below unity matched the area where propagation failed. It was concluded that the mismatch of the source-sink relationship, rather than solely refractoriness, was the ultimate cause of the UDB leading to reentry. The SF represents a very powerful tool to study the mechanisms responsible for reentry.

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.

Increased intracellular Ca2+ and SR Ca2+ load contribute to arrhythmias after acidosis in rat heart. Role of Ca2+/calmodulin-dependent protein kinase II

Returning to normal pH after acidosis, similar to reperfusion after ischemia, is prone to arrhythmias. The type and mechanisms of these arrhythmias have never been explored and were the aim of the present work. Langendorff-perfused rat/mice hearts and rat-isolated myocytes were subjected to respiratory acidosis and then returned to normal pH. Monophasic action potentials and left ventricular developed pressure were recorded. The removal of acidosis provoked ectopic beats that were blunted by 1 muM of the CaMKII inhibitor KN-93, 1 muM thapsigargin, to inhibit sarcoplasmic reticulum (SR) Ca(2+) uptake, and 30 nM ryanodine or 45 muM dantrolene, to inhibit SR Ca(2+) release and were not observed in a transgenic mouse model with inhibition of CaMKII targeted to the SR. Acidosis increased the phosphorylation of Thr(17) site of phospholamban (PT-PLN) and SR Ca(2+) load. Both effects were precluded by KN-93. The return to normal pH was associated with an increase in SR Ca(2+) leak, when compared with that of control or with acidosis at the same SR Ca(2+) content. Ca(2+) leak occurred without changes in the phosphorylation of ryanodine receptors type 2 (RyR2) and was blunted by KN-93. Experiments in planar lipid bilayers confirmed the reversible inhibitory effect of acidosis on RyR2. Ectopic activity was triggered by membrane depolarizations (delayed afterdepolarizations), primarily occurring in epicardium and were prevented by KN-93. The results reveal that arrhythmias after acidosis are dependent on CaMKII activation and are associated with an increase in SR Ca(2+) load, which appears to be mainly due to the increase in PT-PLN.

Practical application of CellML 1.1: The integration of new mechanisms into a human ventricular myocyte model

CellML 1.1 was released as a formal specification in February 2006 with the first release of a complete implementation of the CellML API following in December. The combination of these two developments paves the way for a powerful new paradigm in mathematically modeling cardiac cellular electrophysiology. In this article we explore the practical application of this paradigm using the example of integrating new mechanisms into a well known model of human ventricular myocyte electrophysiology. Through practical application of the CellML 1.1 paradigm we demonstrate the advantages inherent in such an approach and contrast them to more traditional methods of model description, exchange, and publication. This work has also provided the impetus for some recent developments in regard to CellML metadata specifications. The development of the tools and techniques used in this work has helped define some guidelines that should prove useful in future developments in this field. By following these guidelines model authors can increase the usability of their work by other scientists. This work presents the first attempt to utilize annotated CellML models to present not only the underlying mathematical models but also specify the numerical simulations and graphical outputs in an interchangeable, machine readable format. By doing so, all simulations are able to be duplicated by anyone with access to a capable tool. Similarly, identical graphical representations of the numerical simulation results can be produced.

Effect of metabolic inhibition on couplon behavior in rabbit ventricular myocytes

We investigated the effect of combined inhibition of oxidative and glycolytic metabolism on L-type Ca(2+) channels (LCCs) and Ca(2+) spikes in isolated patch-clamped rabbit ventricular myocytes. Metabolic inhibition (MI) reduced LCC open probability, increased null probability, increased first latency, and decreased open time but left conductance unchanged. These results explain the reduction in macroscopic Ca(2+) current observed during MI. MI also produced a gradual reduction in action potential duration at 90% repolarization (APD(90)), a clear decline in spike probability, and an increase in spike latency and variance. These effects are consistent with the changes we observed in LCC activity. MI had no effect on the amplitude or time to peak of Ca(2+) spikes until APD(90) reached 10% of control, suggesting preserved sarcoplasmic reticulum Ca(2+) stores and ryanodine receptor (RyR) conductance in those couplons that remained functioning. Ca(2+) spikes disappeared completely when APD(90) reached <2% of control, although in two cells, spikes were reactivated in a highly synchronized fashion by very short action potentials. This reactivation is probably due to the increased driving force for Ca(2+) entry through a reduced number of LCCs that remain open during early repolarization. The enlarged single channel flux produced by rapid repolarization is apparently sufficient to trigger RyRs whose Ca(2+) sensitivity is likely reduced by MI. We suggest that loss of coupling fidelity during MI is explained by loss of LCC activity (possibly mediated by Ca(2+)-calmodulin kinase II activity). In addition, the results are consistent with loss of RyR activity, which can be mitigated under conditions likely to enlarge the trigger.

Simulated ischemia enhances L-type calcium current in pacemaker cells isolated from the rabbit sinoatrial node

Ischemic-like conditions (a glucose-free, pH 6.6 Tyrode solution bubbled with 100% N2) enhance L-type Ca current ( ICa,L) in single pacemaker cells (PCs) isolated from the rabbit sinoatrial node (SAN). In contrast, studies of ventricular myocytes have shown that acidic extracellular pH, as employed in our “ischemic” Tyrode, reduces ICa,L. Therefore, our goal was to explain why ICa,Lis increased by “ischemia” in SAN PCs. The major findings were the following: 1) blockade of Ca-induced Ca release with ryanodine, exposure of PCs to BAPTA-AM, or replacement of extracellular Ca2+with Ba2+failed to prevent the ischemia-induced enhancement of ICa,L; 2) inhibition of protein kinase A with H-89, or calcium/calmodulin-dependent protein kinase II with KN-93, reduced ICa,Lbut did not prevent its augmentation by ischemia; 3) ischemic Tyrode or pH 6.6 Tyrode shifted the steady-state inactivation curve in the positive direction, thereby reducing inactivation; 4) ischemic Tyrode increased the maximum conductance but did not affect the activation curve; 5) in rabbit atrial myocytes isolated and studied with exactly the same techniques used for SAN PCs, ischemic Tyrode reduced the maximum conductance and shifted the activation curve in the positive direction; pH 6.6 Tyrode also shifted the steady-state inactivation curve in the positive direction. We conclude that the acidic pH of ischemic Tyrode enhances ICa,Lin SAN PCs, because it increases the maximum conductance and reduces inactivation. Furthermore, the opposite results obtained with rabbit atrial myocytes cannot be explained by differences in cell isolation or patch-clamp techniques.

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.

Modification of intracellular calcium concentration in cardiomyocytes by inhibition of sarcolemmal Na+/H+exchanger

Although the Na+/H+exchanger (NHE) is considered to be involved in regulation of intracellular Ca2+concentration ([Ca2+]i) through the Na+/Ca2+exchanger, the exact mechanisms of its participation in Ca2+handling by cardiomyocytes are not fully understood. Isolated rat cardiomyocytes were treated with or without agents that are known to modify Ca2+movements in cardiomyocytes and exposed to an NHE inhibitor, 5-( N-methyl- N-isobutyl)amiloride (MIA). [Ca2+]iin cardiomyocytes was measured spectrofluorometrically with fura 2-AM in the absence or presence of KCl, a depolarizing agent. MIA increased basal [Ca2+]iand augmented the KCl-induced increase in [Ca2+]iin a concentration-dependent manner. The MIA-induced increase in basal [Ca2+]iwas unaffected by extracellular Ca2+, antagonists of the sarcolemmal (SL) L-type Ca2+channel, and inhibitors of the SL Na+/Ca2+exchanger, SL Ca2+pump ATPase and mitochondrial Ca2+uptake. However, the MIA-induced increase in basal [Ca2+]iwas attenuated by inhibitors of SL Na+-K+-ATPase and sarcoplasmic reticulum (SR) Ca2+transport. On the other hand, the MIA-mediated augmentation of the KCl response was dependent on extracellular Ca2+concentration and attenuated by agents that inhibit SL L-type Ca2+channels, the SL Na+/Ca2+exchanger, SL Na+-K+-ATPase, and SR Ca2+release channels and the SR Ca2+pump. However, the effect of MIA on the KCl-induced increase in [Ca2+]iremained unaffected by treatment with inhibitors of SL Ca2+pump ATPase and mitochondrial Ca2+uptake. MIA and a decrease in extracellular pH lowered intracellular pH and increased basal [Ca2+]i, whereas a decrease in extracellular pH, in contrast to MIA, depressed the KCl-induced increase in [Ca2+]iin cardiomyocytes. These results suggest that NHE may be involved in regulation of [Ca2+]iand that MIA-induced increases in basal [Ca2+]i, as well as augmentation of the KCl-induced increase in [Ca2+]i, in cardiomyocytes are regulated differentially.

Involvement of non-selective Ca2+ channels in the contraction induced by alkalinization of rat anococcygeus muscle cells

Intracellular pH is a modulator of cellular functions such as smooth muscle contraction. Changes in cytosolic Ca(2+) concentration ([Ca(2+)](c)) associated with contraction are brought about by Ca(2+) influx and release from the sarcoplasmic reticulum, and alterations in the intracellular pH can affect both processes. In this work, therefore, we have investigated the Ca(2+) influx pathway that contributes to the contraction induced by the alkalinizing agent NH(4)Cl in the rat anococcygeus smooth muscle. For this purpose, we measured the isometric tension in muscle preparations, and [Ca(2+)](c) was measured on isolated cells loaded with 5 micromol/l FURA2/AM by using the ratio 340/380 nm. NH(4)Cl (10 mmol/l) induced a larger increase in [Ca(2+)](c) (100%) when compared with the [Ca(2+)](c) increase induced by 0.1 micromol/l phenylephrine (57.0+/-12.3% n=4). Incubation of the muscle preparations for 1 min in Ca(2+)-free medium reduced the contractions induced by 10 mmol/l NH(4)Cl to 11.5+/-5.1% (n=5), when compared with the contractions induced in 2.5 mmol/l Ca(2+) solution (100%). After 3 min in Ca(2+) free medium, contractions stimulated with NH(4)Cl were almost abolished (0.6+/-0.4%, n=5). In the same way, incubation with 10 micromol/l 1-[beta-[3[(4-methoxyphenyl)propoxyl]-4-methoxy-phenetyl]-1H-imidazole hydrochloride (SKF96365), a non-selective Ca(2+) channels, reduced the contractions stimulated with NH(4)Cl to 47.6+/-6.7% (n=7). On the other hand, 1 micromol/l verapamil, a voltage-operated Ca(2+) channel blocker and 0.05 micromol/l calphostin C, a protein kinase-C inhibitor, did not alter the contractions induced by NH(4)Cl. On isolated cells, [Ca(2+)](c) was reduced to 72.2+/-1.7% (n=4) by 10 micromol/l SKF96365. Taken together, our results suggest that NH(4)Cl induces contraction of rat anococcygeus smooth muscle cells, as well as [Ca(2+)](c) increase due to Ca(2+) influx through non-selective Ca(2+) channels.

Modeling cardiac ischemia

Myocardial ischemia is one of the main causes of sudden cardiac death, with 80% of victims suffering from coronary heart disease. In acute myocardial ischemia, the obstruction of coronary flow leads to the interruption of oxygen flow, glucose, and washout in the affected tissue. Cellular metabolism is impaired and severe electrophysiological changes in ionic currents and concentrations ensue, which favor the development of lethal cardiac arrhythmias such as ventricular fibrillation. Due to the burden imposed by ischemia in our societies, a large body of research has attempted to unravel the mechanisms of initiation, sustenance, and termination of cardiac arrhythmias in acute ischemia, but the rapidity and complexity of ischemia-induced changes as well as the limitations in current experimental techniques have hampered evaluation of ischemia-induced alterations in cardiac electrical activity and understanding of the underlying mechanisms. Over the last decade, computer simulations have demonstrated the ability to provide insight, with high spatiotemporal resolution, into ischemic abnormalities in cardiac electrophysiological behavior from the ionic channel to the whole organ. This article aims to review and summarize the results of these studies and to emphasize the role of computer simulations in improving the understanding of ischemia-related arrhythmias and how to efficiently terminate them.

Store-operated Ca2+-channels are sensitive to changes in extracellular pH

The sensitivity of store-operated Ca(2+)-entry to changes in the extra- and intracellular pH (pH(o) and pH(i), respectively) was investigated in SH-SY5Y human neuroblastoma cells. The intracellular Ca(2+)-stores were depleted either with 1 mM carbachol (CCH) or with 2 microM thapsigargin (TG). Extracellular acidification suppressed both the CCH- and TG-mediated Ca(2+)-entry while external alkalinization augmented both the CCH- and the TG-induced Ca(2+)-influx. Mn(2+)-quenching experiments revealed that the rates of Ca(2+)-entry at the thapsigargin- or carbachol-induced plateau were both accelerated at pH(o) 8.2 and slowed down at pH(o) 6.8 with respect to the control at pH(o) 7.4. Alteration of pH(o) between 6.8 and 8.2 did not have any significant prompt effect on pH(i) and changes in pH(i) left the CCH-induced Ca(2+)-entry unaffected. These findings demonstrate that physiologically relevant changes in pH(o) affect the store-operated Ca(2+)-entry in SH-SY5Y cells and suggest that endogenous pH(o) shifts may regulate cell activity in situ via modulating the store-operated Ca(2+)-entry.

[The mechanism of contractile dysfunction in heart failure, focussing on SERCA2a function]

Cytosolic Ca(2+) is a key regulator of excitation-contraction coupling in myocardium. Myocardial contractile dysfunction in heart failure is characterized by a decrease in contraction and prolonged relaxation. These alterations are mainly due to changes in 1) intracellular Ca(2+) transients (CaT), 2) Ca(2+) sensitivity of the contractile elements, and/or 3) contractile proteins. It is useful to investigate the relationship between CaT and contraction for understanding of the mechanism of contractile dysfunction in heart failure. There are many reports regarding the alterations in CaT, Ca(2+) sensitivity, and contractile proteins in heart failure. Changes in the activity of the sarcoplasmic Ca(2+) pump protein, SERCA2a, may be involved in the altered contractility in heart failure. We generated cardiac-restricted overexpression of SERCA2a transgenic mice (TG) and non-transgenic littermates (NTG). To investigate the role of SERCA2a activity for ischemic heart, we used acidosis as a model of acute contractile dysfunction. During acidosis and recovery from acidosis, the peaks of CaT and tension in TG were significantly larger than those in NTG. These results suggest that an increase in the activity of SERCA2a could be beneficial to preserve contractility during acidosis and recovery. Thus, a disturbance of the intracellular Ca(2+) homeostasis is one of the key factors for the contractile dysfunction in heart failure.

Protons and calcium alter gating of the hyperpolarization-activated cation current (I(h)) in rod photoreceptors

We investigated the effects of protons and calcium ions on the voltage-dependent gating of the hyperpolarization-activated, nonselective cation channel current, I(h), in rod photoreceptors. I(h) is a cesium-sensitive current responsible for the peak-plateau sag during the rod response to bright light. The voltage dependence of I(h) activation shifted about 5 mV per pH unit, with external acidification producing positive shifts and alkalinization producing negative shifts. Increasing external [Ca(2+)] from 3 to 20 mM resulted in a large (approximately 17 mV) positive shift in I(h) activation. External [Ca(2+)] (20 mM) blocked pH-induced shifts in activation. Cytoplasmic acidification produced by 25 mM sodium acetate led to a negative shift in inactivation (-9 mV) and internal alkalinization produced with 20 mM ammonium chloride resulted in a positive shift (+6 mV). Surface charge binding and screening theory (Gouy-Chapman-Stern) accounted for the observed shifts in I(h) activation, with the best fit achieved when protons and calcium ions were assumed to bind to distinct sites on the membrane. Since light induces changes in the retinal ionic environment, these results permit us to gauge the degree to which rod light responses could be modified via alterations in I(h) activation.

CaMKII-dependent reactivation of SR Ca(2+) uptake and contractile recovery during intracellular acidosis

In hearts, intracellular acidosis disturbs contractile performance by decreasing myofibrillar Ca(2+) response, but contraction recovers at prolonged acidosis. We examined the mechanism and physiological implication of the contractile recovery during acidosis in rat ventricular myocytes. During the initial 4 min of acidosis, the twitch cell shortening decreased from 2.3 +/- 0.3% of diastolic length to 0.2 +/- 0.1% (means +/- SE, P < 0.05, n = 14), but in nine of these cells, contractile function spontaneously recovered to 1.5 +/- 0.3% at 10 min (P < 0.05 vs. that at 4 min). During the depression phase, both the diastolic intracellular Ca(2+) concentration ([Ca(2+)](i)) and Ca(2+) transient (CaT) amplitude increased, and the twitch [Ca(2+)](i) decline prolonged significantly (P < 0.05). In the cells that recovered, a further increase in CaT amplitude and a reacceleration of twitch [Ca(2+)](i) decline were observed. The increase in diastolic [Ca(2+)](i) was less extensive than the increase in the cells that did not recover (n = 5). Blockade of sarcoplasmic reticulum (SR) function by ryanodine (10 microM) and thapsigargin (1 microM) or a selective inhibitor of Ca(2+)-calmodulin kinase II, 2-[N- (2-hydroxyethyl)-N-(4-methoxybenzenesulfonyl)] amino-N-(4-chlorocinnamyl)-N-methyl benzylamine (1 microM) completely abolished the reacceleration of twitch [Ca(2+)](i) decline and almost eliminated the contractile recovery. We concluded that during prolonged acidosis, Ca(2+)-calmodulin kinase II-dependent reactivation of SR Ca(2+) uptake could increase SR Ca(2+) content and CaT amplitude. This recovery can compensate for the decreased myofibrillar Ca(2+) response, but may also cause Ca(2+) overload after returning to physiological pH(i).

Electrophysiological response of rat ventricular myocytes to acidosis

The effects of acidosis on the action potential, resting potential, L-type Ca(2+) (I(Ca)), inward rectifier potassium (I(K1)), delayed rectifier potassium (I(K)), steady-state (I(SS)), and inwardly rectifying chloride (I(Cl,ir)) currents of rat subepicardial (Epi) and subendocardial (Endo) ventricular myocytes were investigated using the patch-clamp technique. Action potential duration was shorter in Epi than in Endo cells. Acidosis (extracellular pH decreased from 7.4 to 6.5) depolarized the resting membrane potential and prolonged the time for 50% repolarization of the action potential in Epi and Endo cells, although the prolongation was larger in Endo cells. At control pH, I(Ca), I(K1), and I(SS) were not significantly different in Epi and Endo cells, but I(K) was larger in Epi cells. Acidosis did not alter I(Ca), I(K1), or I(K) but decreased I(SS); this decrease was larger in Endo cells. It is suggested that the acidosis-induced decrease in I(SS) underlies the prolongation of the action potential. I(Cl,ir) at control pH was Cd(2+) sensitive but 4,4'-disothiocyanato-stilbene-2,2'-disulfonic acid resistant. Acidosis increased I(Cl,ir); it is suggested that the acidosis-induced increase in I(Cl,ir) underlies the depolarization of the resting membrane potential.

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

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

Cytochalasin D reduces Ca2+ currents via cofilin-activated depolymerization of F-actin in guinea-pig cardiomyocytes

1. L-type Ca2+ channel currents (I(Ca)) were measured in guinea-pig ventricular myocytes (22 degrees C, 300 ms steps from -45 to +10 mV). Pulsing at 0.5 Hz reduced I(Ca) within 5 min to 92 +/- 3% (mean +/- S.E.M., n = 14) and within 10 min to 83 +/- 4 % ('run-down' with reference to I(Ca) after a 5 min equilibration period). 2. Bath-applied cytochalasin D (cytD, 10 microM) reduced I(Ca) to 75 +/- 4% within 5 min and to 61 +/- 4% within 10 min ('cytD reduction of I(Ca)') by reduction of maximal Ca2+ conductance (suggested by fits of time course and of current-potential (I-V) curves). 3. Preincubation with phalloidin (bath applied, 100 microM, 5 h) prevented the cytD reduction of I(Ca). Since phalloidin specifically blocks F-actin depolymerization, cytD reduction of I(Ca) is linked to depolymerization of F-actin. 4. CytD did not attenuate the beta-adrenergic stimulation of I(Ca) (30 nM isoproterenol), suggesting that A kinase anchoring proteins are unlikely to mediate the cytD reduction of I(Ca). The cytD reduction of I(Ca) was abolished by extra-/intracellular acidosis (pH(o) 6.9), by cell dialysis of 5 mM BAPTA, or by serine/threonine protein phosphatase inhibitors. 5. Actin-depolymerizing factor (ADF)/cofilin are proteins that bind to actin, mediate a pH-sensitive depolymerization of F-actin, and are activated by dephosphorylation. Western blots from hearts perfused with solutions containing zero or 10 microM cytD indicated that cytD reduces the ratio of phosphorylated to total ADF/cofilin content by 50%. 6. The data support the concept that cytD mediates dephosphorylation and activation of ADF/cofilin, leading to depolymerization of F-actin with a subsequent reduction of I(Ca).

Influence of calcitonin gene-related peptide release on pH-induced mechanical depression in rat atria

Rat atria is richly innervated by sensory nerve fibers that release CGRP when stimulated either by capsaicin or acid pH. We studied the physiological relevance of acid pH-induced CGRP release on changes in atrial contractility and relaxation produced by lowering the pH. Isolated atria electrically paced at 2.77 Hz were exposed to a 10-minute period of metabolic acidosis (pH=6.73+/-0.01, n=28) after: 1) CGRP release induced by capsaicin 0.5 microM; 2) blockage of CGRP release with ruthenium red (RR) 5 microM; 3) no pretreatment; and 4) CGRP receptor blockage with CGRP(8-37) 1 microM. Contractility and relaxation were significantly less depressed by acid pH when CGRP release was prevented by RR or CGRP receptor activation was blocked by CGRP(8-37). The results suggest that CGRP release and the activation of CGRP receptors may be physiologically involved in contributing to the depression of contractility and relaxation induced by acid pH in rat atria.

Modulation by extracellular pH of low- and high-voltage-activated calcium currents of rat thalamic relay neurons

The effects of changes in the extracellular pH (pH(o)) on low-voltage- (LVA) and high-voltage- (HVA) activated calcium currents of acutely isolated relay neurons of the ventrobasal thalamic complex (VB) were examined using the whole cell patch-clamp technique. Modest extracellular alkalinization (pH 7.3 to 7.7) reversibly enlarged LVA calcium currents by 18.6 +/- 3.2% (mean +/- SE, n = 6), whereas extracellular acidification (pH 7.3 to 6.9) decreased the current by 24.8 +/- 3.1% (n = 9). Normalized current amplitudes (I/I(7.3)) fitted as a function of pH(o) revealed an apparent pK(a) of 6.9. Both, half-maximal activation voltage and steady-state inactivation were significantly shifted to more negative voltages by 2-4 mV on extracellular alkalinization and to more positive voltages by 2-3 mV on extracellular acidification, respectively. Recovery from inactivation of LVA calcium currents was not significantly affected by changes in pH(o). In contrast, HVA calcium currents were less sensitive to changes in pH(o). Although extracellular alkalinization increased maximal HVA current by 6.0 +/- 2.0% (n = 7) and extracellular acidification decreased it by 11.9 +/- 0.02% (n = 11), both activation and steady-state inactivation were only marginally affected by the moderate changes in pH(o) used in the present study. The results show that calcium currents of thalamic relay neurons exhibit different pH(o) sensitivity. Therefore activity-related extracellular pH transients might selectively modulate certain aspects of the electrogenic behavior of thalamic relay neurons.

The effect of acidosis on systolic Ca2+ and sarcoplasmic reticulum calcium content in isolated rat ventricular myocytes

We have investigated the mechanisms responsible for the changes of systolic Ca2+ that occur in voltage-clamped rat ventricular myocytes during acidosis produced by application of the weak acid butyrate (30 mM). Intracellular pH regulation was inhibited with dimethylamiloride (bicarbonate-free solution). The application of butyrate produced an intracellular acidification of 0.33 pH units. This was accompanied by a decrease in systolic Ca2+ to about 50% of control. However, within 2 min, systolic Ca2+ returned to control levels. The decrease in systolic Ca2+ was accompanied by a decrease in the Na+-Ca2+ exchange current observed on repolarisation so that the calculated Ca2+ efflux on Na+-Ca2+ exchange was less than the entry on the L-type Ca2+ current. The magnitude of the Na+-Ca2+ exchange current recovered along with systolic Ca2+ until it equalled the Ca2+ entry on the L-type Ca2+ current. From the measurement of Ca2+ fluxes, it was calculated that, during acidosis, the cell gains 121.6+/-16.2 micromol l(-1) of Ca2+. This is equal to the measured increase of sarcoplasmic reticulum (SR) calcium content obtained by applying caffeine (20 mM) and integrating the resulting Na+-Ca2+ exchange current. We conclude that the recovery of the amplitude of the systolic Ca2+ transient is due to decreased SR calcium release, resulting in reduced Ca2+ efflux from the cell leading to increased SR calcium content.

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.

Mechanism of effect of extracellular pH on L-type Ca(2+) channel currents in human mesenteric arterial cells

Extracellular pH (pH(o)) influences vasoconstriction partly by modulating Ca(2+) influx through voltage-gated Ca(2+) channels in the vasculature. The mechanism of this effect of pH(o) is, however, controversial. Using the whole cell voltage-clamp technique, we examined the influence of pH(o) on L-type Ca(2+) channel currents in isolated human mesenteric arterial myocytes. Acidification to pH 6.2 and alkalinization to 8.2 from 7.2 decreased by approximately 50% and increased by 25-30%, respectively, the peak amplitude of Ca(2+) and Ba(2+) currents (1.5 and 10 mM), with an apparent pK(a) of 6.8. Activation and inactivation of Ca(2+) and Ba(2+) currents were shifted toward positive membrane voltages during acidification and in the opposite direction during alkalinization. The relationship between the current amplitude and shifts in the gating parameters in solutions of different pH(o) conformed closely to that predicted by the Gouy-Chapman model, in which the divalent cation concentration at the outer surface of the membrane varies with the extent to which protons neutralize the membrane surface potential.

Action potential and contractility changes in [Na(+)](i) overloaded cardiac myocytes: a simulation study

Sodium overload of cardiac cells can accompany various pathologies and induce fatal cardiac arrhythmias. We investigate effects of elevated intracellular sodium on the cardiac action potential (AP) and on intracellular calcium using the Luo-Rudy model of a mammalian ventricular myocyte. The results are: 1) During rapid pacing, AP duration (APD) shortens in two phases, a rapid phase without Na(+) accumulation and a slower phase that depends on [Na(+)](i). 2) The rapid APD shortening is due to incomplete deactivation (accumulation) of I(Ks). 3) The slow phase is due to increased repolarizing currents I(NaK) and reverse-mode I(NaCa), secondary to elevated [Na(+)](i). 4) Na(+)-overload slows the rate of AP depolarization, allowing time for greater I(Ca(L)) activation; it also enhances reverse-mode I(NaCa). The resulting increased Ca(2+) influx triggers a greater [Ca(2+)](i) transient. 5) Reverse-mode I(NaCa) alone can trigger Ca(2+) release in a voltage and [Na(+)](i)-dependent manner. 6) During I(NaK) block, Na(+) and Ca(2+) accumulate and APD shortens due to enhanced reverse-mode I(NaCa); contribution of I(K(Na)) to APD shortening is negligible. By slowing AP depolarization (hence velocity) and shortening APD, Na(+)-overload acts to enhance inducibility of reentrant arrhythmias. Shortened APD with elevated [Ca(2+)](i) (secondary to Na(+)-overload) also predisposes the myocardium to arrhythmogenic delayed afterdepolarizations.

Intracellular acidosis modulates the stretch-induced changes in E-C coupling of the rat atrium

By inducing a small reduction of the intracellular pH (0.18 units) with 20 mmol L-1 propionate we demonstrated that acidification changed the responses of isolated rat atria to stretch. Stretch (increase of the intra-atrial pressure) in normal pH increased the Ca2+ transients' amplitude (Indo-1 fluorescence) from 0.26 +/- 0.09 in 1 mmHg to 0.36 +/- 0.13 in 4 mmHg (P < 0.05, n=6), without affecting the diastolic [Ca2+]i level (n.s. n=6). The changes in Ca2+ balance during stretch were accompanied by a biphasic increase in the contraction force. Five minutes of continuous stretch increased the action potential duration (APD90%, P < 0.01, n=13) and decreased the APD15% (P < 0.001, n=13). During acidosis, the stretch-induced increase of the Ca2+ transient amplitude (0.4 +/- 0. 13 vs. 0.3 +/- 0.08, P < 0.05, n=6) was accompanied by the increase of the diastolic [Ca2+]i (1.16 +/- 0.07, P < 0.05, n=6) compared with non-acidotic control (1.06 +/- 0.06, n=6). Acidic intracellular pH also inhibited the stretch-induced changes in the action potentials (n=10) and slowed down the development of the contractile force during stretch. The results showed that acidosis modulates the mechanotransduction. It does this by interfering with the intracellular Ca2+ balance, inhibiting the Ca2+ extrusion mechanisms and reducing the Ca2+-buffering power of the cells. The physiological and pathological processes associated with stretch are therefore modulated by intracellular pH owing to its concerted effects on intracellular Ca2+ handling caused by a competitive inhibition of various Ca2+-binding molecules.

Acidosis antagonizes intracellular calcium response to kappa-opioid receptor stimulation in the rat heart

To study the effects of kappa-opioid receptor stimulation on intracellular Ca2+ concentration ([Ca2+]i) homeostasis during extracellular acidosis, we determined the effects of kappa-opioid receptor stimulation on [Ca2+]i responses during extracellular acidosis in isolated single rat ventricular myocytes, by a spectrofluorometric method. U-50488H (10-30 microM), a selective kappa-opioid receptor agonist, dose dependently decreased the electrically induced [Ca2+]i transient, which results from the influx of Ca2+ and the subsequent mobilization of Ca2+ from the sarcoplasmic reticulum (SR). U-50488H (30 microM) also increased the resting [Ca2+]i and inhibited the [Ca2+]i transient induced by caffeine, which mobilizes Ca2+ from the SR, indicating that the effects of the kappa-opioid receptor agonist involved mobilization of Ca2+ from its intracellular pool into the cytoplasm. The Ca2+ responses to 30 microM U-50488H were abolished by 5 microM nor-binaltorphimine, a selective kappa-opioid receptor antagonist, indicating that the event was mediated by the kappa-opioid receptor. The effects of the agonist on [Ca2+]i and the electrically induced [Ca2+]i transient were significantly attenuated when the extracellular pH (pHe) was lowered to 6.8, which itself reduced intracellular pH (pHi) and increased [Ca2+]i. The inhibitory effects of U-50488H were restored during extracellular acidosis in the presence of 10 microM ethylisopropyl amiloride, a potent Na+/H+ exchange blocker, or 0.2 mM Ni2+, a putative Na+/Ca2+ exchange blocker. The observations indicate that acidosis may antagonize the effects of kappa-opioid receptor stimulation via Na+/H+ and Na+/Ca2+ exchanges. When glucose at 50 mM, known to activate the Na+/H+ exchange, was added, both the resting [Ca2+]i and pHi increased. Interestingly, the effects of U-50488H on [Ca2+]i and the electrically induced [Ca2+]i transient during superfusion with glucose were significantly attenuated; this mimicked the responses during extracellular acidosis. When a high-Ca2+ (3 mM) solution was superfused, the resting [Ca2+]i increased; the increase was abolished by 0.2 mM Ni2+, but the pHi remained unchanged. Like the responses to superfusion with high-concentration glucose and extracellular acidosis, the responses of the [Ca2+]i and electrically induced [Ca2+]i transients to 30 microM U-50488H were also significantly attenuated. Results from the present study demonstrated for the first time that extracellular acidosis antagonizes the effects of kappa-opioid receptor stimulation on the mobilization of Ca2+ from SR. Activation of both Na+/H+ and Na+/Ca2+ exchanges, leading to an elevation of [Ca2+]i, may be responsible for the antagonistic action of extracellular acidosis against kappa-opioid receptor stimulation.

Na+/H+ exchangers. Molecular diversity and relevance to heart

During the last several years, significant advances have been made in our understanding of the molecular, cellular, and physiological diversity of mammalian Na+/H+ exchangers. This transporter forms a multigene family of at least six members (NHE1-NHE6) that share approximately 20-60% amino acid identity. NHE1 is the most predominant isoform expressed in heart and it contributes significantly to myocardial pHi homeostasis, which is important for maintaining contractility. However, hyperactivation of NHE1 during episodes of cardiac ischemia and reperfusion disrupts the intracellular ion balance, leading to cardiac dysfunction and damage in several animal models, but which can be prevented by pharmacological antagonists of NHE1. Molecular studies have indicated that the predicted transmembrane segments M4 and M9 contain several residues involved in drug sensitivity. Molecular dissection of the drug binding region should facilitate the rational design of more potent and isoform-specific drugs that may provide therapeutic benefit in the prevention of cardiac ischemia and reperfusion injuries.