Calcium entry via Na/Ca exchange during the action potential directly contributes to contraction of failing human ventricular myocytes

Prolongation of the Ca2+ transient and action potential (AP) durations are two characteristic changes in myocyte physiology in the failing human heart. The hypothesis of this study is that Ca2+ influx via reverse mode Na+/Ca2+ exchanger (NCX) or via L-type Ca2+ channels directly activates contraction in failing human myocytes while in normal myocytes this Ca2+ is transported into the sarcoplasmic reticulum (SR) to regulate SR Ca2+ stores. METHODS: Myocytes were isolated from failing human (n=6), nonfailing human (n=3) and normal feline hearts (n=9) and whole cell current and voltage clamp techniques were used to evoke and increase the duration of APs (0.5 Hz, 37 degrees C). Cyclopiazonic acid (CPA 10(-6) M), nifedipine (NIF;10(-6) M) and KB-R 7943 (KB-R; 3x10(-6) M) were used to reduce SR Ca2+ uptake, Ca2+ influx via the L-type Ca2+ current and reverse mode NCX, respectively. [Na+)i was changed by dialyzing myocytes with 0, 10 and 20 mM Na(+) pipette solutions. RESULTS: Prolongation of the AP duration caused an immediate prolongation of contraction and Ca2+ transient durations in failing myocytes. The first beat after the prolonged AP was potentiated by 21+/-5 and 27+/-5% in nonfailing human and normal feline myocytes, respectively (P<0.05), but there was no significant effect in failing human myocytes (+5+/-4% vs. steady state). CPA blunted the potentiation of the first beat after AP prolongation in normal feline and nonfailing human myocytes, mimicking the failing phenotype. NIF reduced steady state contraction in feline myocytes but the potentiation of the first beat after AP prolongation was unaltered (21+/-3% vs. base, P<0.05). KB-R reduced basal contractility and abolished the potentiation of the first beat after AP prolongation (2+/-1% vs. steady state). Increasing [Na+]i shortened AP, Ca2+ transient and contraction durations and increased steady state and post AP prolongation contractions. Dialysis with 0 Na+ eliminated these effects. CONCLUSIONS: Ca2+ enters both normal and failing cardiac myocytes during the late portion of the AP plateau via reverse mode NCX. In (normal) myocytes with good SR function, this Ca(2+) influx helps maintain and regulate SR Ca2+ load. In (failing) human myocytes with poor SR function this Ca2+ influx directly contributes to contraction. These studies suggest that the Ca2+ transient of the failing human ventricular myocytes has a higher than normal reliance on Ca2+ influx via the reverse mode of the NCX during the terminal phases of the AP.     

Enhanced Ca(2+)-activated Na(+)-Ca(2+) exchange activity in canine pacing-induced heart failure

Defective excitation-contraction coupling in heart failure is generally associated with both a reduction in sarcoplasmic reticulum (SR) Ca(2+) uptake and a greater dependence on transsarcolemmal Na(+)-Ca(2+) exchange (NCX) for Ca(2+) removal. Although a relative increase in NCX is expected when SR function is impaired, few and contradictory studies have addressed whether there is an absolute increase in NCX activity. The present study examines in detail NCX density and function in left ventricular midmyocardial myocytes isolated from normal or tachycardic pacing-induced failing canine hearts. No change of NCX current density was evident in myocytes from failing hearts when intracellular Ca(2+) ([Ca(2+)](i)) was buffered to 200 nmol/L. However, when [Ca(2+)](i) was minimally buffered with 50 micromol/L indo-1, Ca(2+) extrusion via NCX during caffeine application was doubled in failing versus normal cells. In other voltage-clamp experiments in which SR uptake was blocked with thapsigargin, both reverse-mode and forward-mode NCX currents and Ca(2+) transport were increased >2-fold in failing cells. These results suggest that, in addition to a relative increase in NCX function as a consequence of defective SR Ca(2+) uptake, there is an absolute increase in NCX function that depends on [Ca(2+)](i) in the failing heart.     

Heterogeneous transmural gene expression of calcium-handling proteins and natriuretic peptides in the failing human heart.

OBJECTIVE: Human heart failure is associated with a disturbed intracellular calcium (Ca2+) homeostasis. In this regard, ventricular wall stress is considered to be a determinant for expression of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a). In the present study, we analyzed the transmural protein and/or mRNA levels of SERCA2a, other Ca(2+)-handling proteins, and of atrial and brain natriuretic peptides (ANP and BNP) in the human heart. METHODS: Subepicardial (epi), midmyocardial (mid), and subendocardial (endo) sections of the left ventricular free wall from end-stage failing (n = 17) and nonfailing (n = 5) human hearts were analyzed by Western blot for immunoreactive protein levels of SERCA2a, phospholamban (PLN), and calsequestrin (CS). Subepi- and subendocardial sections were analyzed by Northern blot for steady-state mRNA levels of SERCA2a, Na(+)-Ca2+ exchanger (NCX1), ANP, and BNP. RESULTS: SERCA2a protein and mRNA levels were reduced by 40 +/- 5% (P < 0.01) and 25 +/- 7% (P < 0.05) in endo compared to epi in the failing heart and by 27 +/- 14% and 16 +/- 12% (non-significant) in the nonfailing heart, respectively. PLN protein levels were reduced by 23 +/- 6% (P < 0.05) in endo compared to epi in the failing heart and by 17 +/- 25% (non-significant) in the nonfailing heart, whereas CS protein levels and NCX1 mRNA levels were similar across the left ventricular wall. Strikingly, in the failing heart, both BNP and ANP mRNA levels were upregulated predominantly in endo. CONCLUSIONS: In the failing human heart, SERCA2a and PLN, as well as natriuretic peptides but not CS and NCX1 are differentially expressed across the left ventricular wall, implicating (1) different susceptibility of subendocardium and subepicardium to factors affecting expression of these proteins and (2) differences in regulation of the distinct calcium-cycling proteins.     

Altered myocardial Ca2+ cycling after left ventricular assist device support in the failing human heart

OBJECTIVES: The objective of the present study was to determine whether improved contractility after left ventricular assist device (LVAD) support reflects altered myocyte calcium cycling and changes in calcium-handling proteins. BACKGROUND: Previous reports demonstrate that LVAD support induces sustained unloading of the heart with regression of pathologic hypertrophy and improvements in contractile performance. METHODS: In the human myocardium of subjects with heart failure (HF), with non-failing hearts (NF), and with LVAD-supported failing hearts (HF-LVAD), intracellular calcium ([Ca(2+)](i)) transients were measured in isolated myocytes at 0.5 Hz, and frequency-dependent force generation was measured in multicellular preparations (trabeculae). Abundance of sarcoplasmic reticulum Ca(2+) adenosine triphosphatase (SERCA), Na(+)/Ca(2+) exchanger (NCX), and phospholamban was assessed by Western analysis. RESULTS: Compared with NF myocytes, HF myocytes exhibited a slowed terminal decay of the Ca(2+) transient (DT(terminal), 376 +/- 18 ms vs. 270 +/- 21 ms, HF vs. NF, p < 0.0008), and HF-LVAD myocytes exhibited a DT(terminal) that was much shorter than that observed in HF myocytes (278 +/- 10 ms, HF vs. HF-LVAD, p < 0.0001). Trabeculae from HF showed a negative force-frequency relationship, compared with a positive relationship in NF, whereas a neutral relationship was observed in HF-LVAD. Although decreased SERCA abundance in HF was not altered by LVAD support, improvements in [Ca(2+)](i) transients and frequency-dependent contractile function were associated with a significant decrease in NCX abundance and activity from HF to HF-LVAD. CONCLUSIONS: Improvement in rate-dependent contractility in LVAD-supported failing human hearts is associated with a faster decay of the myocyte calcium transient. These improvements reflect decreases in NCX abundance and transport capacity without significant changes in SERCA after LVAD support. Our results suggest that reverse remodeling may involve selective, rather than global, normalization of the pathologic patterns associated with the failing heart.     

A low dose of angiotensin II increases inotropism through activation of reverse Na(+)/Ca(2+) exchange by endothelin release

OBJECTIVE: This work was aimed to prove that release/formation of endogenous endothelin acting in an autocrine/paracrine fashion contributes to the increase in contractility promoted by a low dose of angiotensin II. METHODS: Isolated cat papillary muscles were used for force, pH(i), [Na(+)](i) and [Ca(2+)](i) measurements and isolated cat myocytes for patch-clamp experiments. RESULTS: In papillary muscles, 1.0 nmol/l angiotensin II increased force by 23+/-2% (n=4, P<0.05), [Na(+)](i) by 2.2+/-0.2 mmol/l (n=4, P<0.05), and peak (but not diastolic) Ca(2+) from 0.674+/-0.11 to 0.768+/-0.13 micromol/l (n=4, P<0.05), without affecting pH(i). Force and [Na(+)](i) increase were abolished by inhibition of the Na(+)/H(+) exchanger (NHE) with the inhibitor HOE642, blockade of endothelin receptors with the nonselective antagonist TAK044 and by inhibition of the endothelin-converting enzyme with phosphoramidon. Force but not [Na(+)](i) increase was abolished by inhibition of reverse Na(+)/Ca(2+) exchange (NCX) with the inhibitor KB-R7943. Similar increase in force (21+/-2%, n=4, P<0.05) and in [Na(+)](i) (2.4+/-0.4 mmol/l, n=4, P<0.05) that were also suppressed by TAK044 and HOE642 were induced by exogenous 5.0 nmol/l endothelin-1. KB-R7943 reverted the endothelin-1 effect on force but not on [Na(+)](i). In isolated myocytes, exogenous endothelin-1 dose-dependently increased the NCX current and shifted the NCX reversal potential (E(NCX)) to a more negative value (DeltaE(NCX): -10+/-3 and -17+/-5 mV, with 1 and 10 nmol/l endothelin-1, respectively, n=12). The latter effect was prevented by HOE642. CONCLUSION: Taken together, the results indicate that a low dose of angiotensin II induces release of endothelin, which, in autocrine/paracrine fashion activates the Na(+)/H(+) exchanger, increases [Na(+)](i) and changes E(NCX), promoting the influx of Ca(2+) that leads to a positive inotropic effect (PIE).     

Adenoviral gene transfer of mutant phospholamban rescues contractile dysfunction in failing rabbit myocytes with relatively preserved SERCA function

In heart failure (HF) a main factor in reduced contractility is reduced SR Ca2+ content and reversed force-frequency response (FFR), ie, from positive to negative. Our arrhythmogenic rabbit HF model exhibits decreased contractility mainly due to an increase in Na/Ca exchange (NCX) activity (with only modest decrease in SR Ca2+-ATPase (SERCA) function), similar to many end-stage HF patients. Here we test whether phospholamban (PLB) inhibition using a dominant-negative mutant PLB adenovirus (K3E/R14E, AdPLB-dn, with beta-galactosidase adenovirus as control) could enhance SERCA function and restore Ca2+ transients and positive FFR in ventricular myocytes from these HF rabbits. HF myocytes infected with AdPLB-dn (versus control) had enhanced Ca2+ transient amplitude (2.0+/-0.1 versus 1.6+/-0.05 F/Fo at 0.5 Hz, P<0.05) and had a positive FFR, whereas acutely isolated HF myocytes or those infected with Adbetagal had negative FFR. Ca2+ transients declined faster in AdPLB-dn versus Adbetagal myocytes (RT50%: 317+/-29 versus 551+/-90 ms at 0.5 Hz, P<0.05) and had an increased SR Ca2+ load (3.5+/-0.3 versus 2.6+/-0.2 F/Fo at 0.5 Hz, P<0.05), indicative of increased SERCA function. Furthermore, this restoration of function was not due to changes in NCX or SERCA expression. Thus, increasing SERCA activity in failing myocytes by AdPLB-dn gene transfer reversed the contractile dysfunction (and restored positive FFR) by increasing SR Ca2+ load. This approach could enhance contractile function in failing hearts of various etiologies, even here where reduced SERCA activity is not the main dysfunction.     

Relative abundance of alpha2 Na(+) pump isoform influences Na(+)-Ca(2+) exchanger currents and Ca(2+) transients in mouse ventricular myocytes

In the mouse, genetic reduction in the Na(+), K(+)-ATPase alpha1 or alpha2 isoforms results in different functional phenotypes: heterozygous alpha2 isolated hearts are hypercontractile, whereas heterozygous alpha1 hearts are hypocontractile. We examined Na(+)/Ca(2+) exchange (NCX) currents in voltage clamped myocytes (pipette [Na(+)]=15 mM) induced by abrupt removal of extracellular Na(+). In wild-type (WT) myocytes, peak exchanger currents were 0.59+/-0.04 pA/pF (mean+/-S.E.M., n=10). In alpha1(+/-) myocytes (alpha2 isoform increased by 54%), NCX current was reduced to 0.33+/-0.05 (n=9, P<0.001) indicating a lower subsarcolemmal [Na(+)]. In alpha2(+/-) myocytes (alpha2 isoform reduced by 54%), the NCX current was increased to 0.89+/-0.11 (n=8, P=0.03). The peak sarcolemmal Na(+) pump currents activated by abrupt increase in [K(+)](o) to 4 mM in voltage clamped myocytes in which the Na(+) pump had been completely inhibited for 5 min by exposure to 0 [K(+)](o) were similar in alpha1(+/-) (0.86+/-0.12, n=10) and alpha2(+/-) myocytes (0.94+/-0.08 pA/pF, n=16), and were slightly but insignificantly reduced relative to WT (1.03+/-0.05, n=24). The fluo-3 [Ca(2+)](i) transient (F/F(o)) in WT myocytes paced at 0.5 Hz was 2.18+/-0.09, n=34, was increased in alpha2(+/-) myocytes (F/F(o)=2.56+/-0.14, n=24, P=0.02), and was decreased in alpha1(+/-) myocytes (F/F(o)=1.93+/-0.08, n=28, P<0.05). Thus the alpha2 isoform rather than the alpha1 appears to influence Na(+)/Ca(2+) exchanger currents [Ca(2+)](i) transients, and contractility. This finding is consistent with the proposal that alpha2 isoform of the Na pump preferentially alters [Na(+)] in a subsarcolemmal micro-domain adjacent to Na(+)/Ca(2+) exchanger molecules and SR Ca(2+) release sites.     

Cellular basis of abnormal calcium transients of failing human ventricular myocytes

Depressed contractility is a central feature of the failing human heart and has been attributed to altered [Ca2+]i. This study examined the respective roles of the L-type Ca2+ current (ICa), SR Ca2+ uptake, storage and release, Ca2+ transport via the Na+-Ca2+ exchanger (NCX), and Ca2+ buffering in the altered Ca2+ transients of failing human ventricular myocytes. Electrophysiological techniques were used to measure and control V(m) and measure I(m), respectively, and Fluo-3 was used to measure [Ca2+]i in myocytes from nonfailing (NF) and failing (F) human hearts. Ca2+ transients from F myocytes were significantly smaller and decayed more slowly than those from NF hearts. Ca2+ uptake rates by the SR and the amount of Ca2+ stored in the SR were significantly reduced in F myocytes. There were no significant changes in the rate of Ca2+ removal from F myocytes by the NCX, in the density of NCX current as a function of [Ca2+]i, ICa density, or cellular Ca2+ buffering. However, Ca2+ influx during the late portions of the action potential seems able to elevate [Ca2+]i in F but not in NF myocytes. A reduction in the rate of net Ca2+ uptake by the SR slows the decay of the Ca2+ transient and reduces SR Ca2+ stores. This leads to reduced SR Ca2+ release, which induces additional Ca2+ influx during the plateau phase of the action potential, further slowing the decay of the Ca2+ transient. These changes can explain the defective Ca2+ transients of the failing human ventricular myocyte.     

Evidence for calcineurin-mediated regulation of SERCA 2a activity in human myocardium

Compromised SERCA 2a activity is a key malfunction leading to the Ca(2+) cycling alterations in failing human myocardium. SERCA 2a activity is regulated by the Ca(2+)/calmodulin-dependent protein kinase (CaM-kinase) but alterations of the CaM-kinase pathway regarding SERCA 2a in heart failure are unresolved. Therefore we investigated the CaM-kinase and phosphatase calcineurin mediated regulation of SERCA 2a in failing and non-failing human myocardium. We studied human myocardial preparations from explanted hearts from non-failing organ donors (NF, n=8) and from patients with terminal heart failure undergoing cardiac transplantation (dilated cardiomyopathy, DCM, n=8). SERCA 2a activity was determined using a NADH-coupled enzyme assay [expressed in nmol ATP/(mg protein x min)] and by(45)Ca(2+) uptake. Protein expression of SERCA 2a, phospholamban, calsequestrin and calcineurin was assessed by Western blotting (expressed as densitometric units/microg protein); phosphorylation of cardiac proteins was detected with specific phospho-antibodies for phospholamban at threonine-17 (PT17) or by incorporation of [gamma -(32)P] (expressed as pmol(32)P/mg). Maximal(45)Ca(2+) uptake (in pmol/mg/min) (NF: 3402+/-174; DCM: 2488+/-189) and maximal SERCA 2a activity were reduced in DCM compared to NF (V(max): NF: 125+/-9; DCM: 98+/-5). The V(max) reduction could be mimicked by calcineurin in vitro in NF (NF(control): 72.1+/-3.7; NF(+calcineurin): 49.8+/-2.9) and restored in DCM by CaM-kinase in vitro (DCM(control): 98+/-5; DCM(+CaM-kinase): 120+/-6). Protein expression of SERCA 2a, phospholamban and calsequestrin remained similar, but calcineurin expression was significantly increased in failing human hearts (NF: 11.6+/-1.5 v DCM: 17.1+/-1.6). Although the capacity of endogenous CaM-kinase to phosphorylate PT17 was significantly higher in DCM (DCM(control): 128+/-36; DCM(+endogenous CaM-kinase): 205+/-20) compared to NF myocardium (NF(control): 273+/-37; NF(+endogenous CaM-kinase): 254+/-31), net phosphorylation at threonine-17 phospholamban was significantly lower in DCM (DCM 130+/-11 v NF 170+/-11). A calcineurin-dependent dephosphorylation of phospholamban could be mimicked in vitro by incubation of NF preparations with calcineurin (NF(control) 80.7+/-4.4 v NF(+calcineurin) 30.7+/-4.1, P<0.05). In human myocardium, the V(max) of SERCA 2a and the phosphorylation of phospholamban is modulated by CaM-kinase and calcineurin, at least in vitro. In failing human myocardium, despite increased CaM-kinase activity, calcineurin dephosphorylation leads to decreased net phosphorylation of threonine-17 phospholamban in vivo. Increased calcineurin activity contributes to the impaired V(max) of SERCA 2a in failing human myocardium and the disorder in Ca(2+)-handling in heart failure.     

Impaired contractile performance of cultured rabbit ventricular myocytes after adenoviral gene transfer of Na(+)-Ca(2+) exchanger

Na(+)-Ca(2+) exchanger (NCX) gene expression is increased in the failing human heart. We investigated the hypothesis that upregulation of NCX can induce depressed contractile performance. Overexpression of NCX was achieved in isolated rabbit ventricular myocytes through adenoviral gene transfer (Ad-NCX). After 48 hours, immunoblots revealed a virus dose-dependent increase in NCX protein. Adenoviral beta-galactosidase transfection served as a control. The fractional shortening (FS) of electrically stimulated myocytes was analyzed. At 60 min(-1), FS was depressed by 15.6% in the Ad-NCX group (n=143) versus the control group (n=163, P:<0.05). Analysis of the shortening-frequency relationship showed a steady increase in FS in the control myocytes (n=26) from 0.027+/-0.002 at 30 min(-1) to 0. 037+/-0.002 at 120 min(-1) (P:<0.05 versus 30 min(-1)) and to 0. 040+/-0.002 at 180 min(-1) (P:<0.05 versus 30 min(-1)). Frequency potentiation of shortening was blunted in NCX-transfected myocytes (n=27). The FS was 0.024+/-0.002 at 30 min(-1), 0.029+/-0.002 at 120 min(-1) (P:<0.05 versus 30 min(-1), P:<0.05 versus control), and 0. 026+/-0.002 at 180 min(-1) (NS versus 30 min(-1), P:<0.05 versus control). Caffeine contractures, which indicate sarcoplasmic reticulum Ca(2+) load, were significantly reduced at 120 min(-1) in NCX-transfected cells. An analysis of postrest behavior showed a decay of FS with longer rest intervals in control cells. Rest decay was significantly higher in the Ad-NCX group; after 120 seconds of rest, FS was 78+/-4% in control and 65+/-3% in the Ad-NCX group (P:<0.05) relative to steady-state FS before rest (100%). In conclusion, the overexpression of NCX in rabbit cardiomyocytes results in the depression of contractile function. This supports the hypothesis that upregulation of NCX can result in systolic myocardial failure.     

Altered hetero- and homeometric autoregulation in the terminally failing human heart

OBJECTIVE AND METHODS: To further investigate length-dependent force generation in human heart, nonfailing (donor hearts, NF) and terminally failing (heart transplants, dilated cardiomyopathy, DCM) left ventricular myocardium was studied under various preload (4-40 mN/mm2) or length conditions. In addition, morphological studies (van Giesson Trichrome staining, electron microscopy) were performed. RESULTS: In NF, a biphasic increase in force of contraction (FOC) was observed after elevating the preload (4-40 mN/mm2): there was an immediate fast increase (FOCf,), followed by a slow increase over several minutes (FOCs), which was paralleled by an increase in the systolic fura-2 transient. In DCM, FOCf, FOCs and the systolic fura-2 transient were blunted and diastolic tension was increased at increasing muscle length. Only in NF, a stretched induced increase in diastolic fura-2 ratio was observed. In DCM, no obvious interstitial fibrosis and no difference in basement membrane structure and attachment were observed. CONCLUSIONS: Since FOCf has been attributed to the Frank-Starling mechanism, whereas FOCs represents a length-dependent increase in the intracellular Ca2+-transient, the impaired length-dependent force generation in failing myocardium results from a dysregulation of both myofibrillar Ca2+-sensitivity as well as the intracellular Ca2+-homeostasis. Interstitial fibrosis may have only minor impact on force generation in human end-stage heart failure.     

Partial inhibition of sodium/calcium exchange restores cellular calcium handling in canine heart failure

Sodium/calcium (Na+/Ca2+) exchange (NCX) overexpression is common to human heart failure and heart failure in many animal models, but its specific contribution to the cellular Ca2+ ([Ca2+]i) handling deficit is unclear. Here, we investigate the effects of exchange inhibitory peptide (XIP) on Ca2+ handling in myocytes isolated from canine tachycardic pacing-induced failing hearts. Whole-cell patch-clamped left ventricular myocytes from failing hearts (F) showed a 52% decrease in steady-state sarcoplasmic reticulum (SR) Ca2+ load and a 44% reduction in the amplitude of the [Ca2+]i transient, as compared with myocytes from normal hearts (N). Intracellular application of XIP (30 micromol/L) normalized the [Ca2+]i transient amplitude in F (3.86-fold increase), concomitant with a similar increase in SR Ca2+ load. The degree of NCX inhibition at this concentration of XIP was 27% and was selective for NCX: L-type Ca2+ currents and plasmalemmal Ca2+ pumps were not affected. XIP also indirectly improved the rate of [Ca2+]i removal at steady-state, secondary to Ca2+-dependent activation of SR Ca2+ uptake. The findings indicate that in the failing heart cell, NCX inhibition can improve SR Ca2+ load by shifting the balance of Ca2+ fluxes away from trans-sarcolemmal efflux toward SR accumulation. Hence, inhibition of the Ca2+ efflux mode of the exchanger could potentially be an effective therapeutic strategy for improving contractility in congestive heart failure.     

Sinoatrial nodal cell ryanodine receptor and Na(+)-Ca(2+) exchanger: molecular partners in pacemaker regulation

The rate of spontaneous diastolic depolarization (DD) of sinoatrial nodal cells (SANCs) that triggers recurrent action potentials (APs) is a fundamental aspect of the heart's pacemaker. Here, in experiments on isolated SANCs, using confocal microscopy combined with a patch clamp technique, we show that ryanodine receptor Ca(2+) release during the DD produces a localized subsarcolemmal Ca(2+) increase that spreads in a wavelike manner by Ca(2+)-induced Ca(2+) release and produces an inward current via the Na(+)-Ca(2+) exchanger (NCX). Ryanodine, a blocker of the sarcoplasmic reticulum Ca(2+) release channel, in a dose-dependent manner reduces the SANC beating rate with an IC(50) of 2.6 micromol/L and abolishes the local Ca(2+) transients that precede the AP upstroke. In voltage-clamped cells in which the DD was simulated by voltage ramp, 3 micromol/L ryanodine decreased an inward current during the voltage ramp by 1.6+/-0.3 pA/pF (SEM, n=4) leaving the peak of L-type Ca(2+) current unchanged. Likewise, acute blockade of the NCX (via rapid substitution of bath Na(+) by Li(+)) abolished SANC beating and reduced the inward current to a similar extent (1.7+/-0.4 pA/pF, n=4), as did ryanodine. Thus, in addition to activation/inactivation of multiple ion channels, Ca(2+) activation of the NCX, because of localized sarcoplasmic reticulum Ca(2+) release, is a critical element in a chain of molecular interactions that permits the heartbeat to occur and determines its beating rate.     

Na(+)-Ca(2+) exchange current and submembrane [Ca(2+)] during the cardiac action potential

Na(+)-Ca(2+) exchange (NCX) is crucial in the regulation of [Ca(2+)](i) and cardiac contractility, but key details of its dynamic function during the heartbeat are not known. In the present study, we assess how NCX current (I(NCX)) varies during a rabbit ventricular action potential (AP). First, we measured the steady-state voltage and [Ca(2+)](i) dependence of I(NCX) under conditions when [Ca(2+)](i) was heavily buffered. We then used this relationship to infer the submembrane [Ca(2+)](i) ([Ca(2+)](sm)) sensed by NCX during a normal AP and [Ca(2+)](i) transient (when the AP was interrupted to produce an I(NCX) tail current). The [Ca(2+)](i) dependence of I(NCX) at -90 mV allowed us to convert the peak inward I(NCX) tail currents to [Ca(2+)](sm). Peak [Ca(2+)](sm) measured via this technique was >3.2 micromol/L within < 32 ms of the AP upstroke (versus peak [Ca(2+)](i) of 1.1 micromol/L at 81 ms measured with the global Ca(2+) indicator indo-1). The voltage and [Ca(2+)](sm) dependence of I(NCX) allowed us to infer I(NCX) during the normal AP and Ca(2+) transient. The early rise in [Ca(2+)](sm) causes I(NCX) to be inward for the majority of the AP. Thus, little Ca(2+) influx via NCX is expected under physiological conditions, but this can differ among species and in pathophysiological conditions.     

Isoproterenol does not enhance Ca-dependent Na/Ca exchange current in intact rabbit ventricular myocytes

Cardiac Na/Ca exchange (NCX, NCX1.1) is critical in cardiac myocyte Ca regulation, and its altered function contributes to inotropic state, systolic dysfunction in heart failure and arrhythmogenesis. Regulation of NCX is multifaceted, but protein kinase A (PKA) effects on NCX function are controversial. Here, we use three different and complementary approaches to compare NCX function +/-1 microM isoproterenol (ISO) in intact rabbit cardiac myocytes (in paired comparisons). First, in field-stimulated intact cells we inferred the cytosolic [Ca] ([Ca](i)) dependence of NCX function from the decay rate of caffeine-induced [Ca](i) transients. Second, we measured caffeine-induced [Ca](i) and inward I(NCX) simultaneously (perforated patch voltage clamp), to measure directly the [Ca](i) dependence of NCX rate. Third, using whole cell ruptured patch with [Ca](i) heavily buffered to 100 nM, [Na](i)=10 mM, and I(Ca), SR Ca release and Na/K pump all blocked, we recorded I(NCX) ramps at 37 degrees C. We find that NCX function is not altered by PKA activation under any of these three protocols, where intracellular conditions ranged from near-physiological to highly controlled. This does not rule out NCX modulation by PKA under all conditions, or in species other than rabbit. However, such effects are likely to be either minor (vs. other PKA actions on myocyte Ca handling) or indirect, such as secondary effects dependent on altered local [Ca](i) and [Na](i).     

培哚普利对心力衰竭大鼠心肌细胞钙瞬变及其调控蛋白的影响

目的研究培哚普利在心力衰竭(心衰)治疗中对心肌细胞的收缩特性、钙瞬变及其调控蛋白的影响。方法通过缩窄雄性Wistar大鼠的腹主动脉,制成压力负荷增高性心衰模型。随机分成培哚普利治疗组(培哚普利组,3mg·kg-1·d-1)、心衰对照组和假手术对照组(假手术组)。12周后分离左室心肌细胞,测定单个心肌细胞在电刺激时的缩短分数、钙瞬变以及钠-钙交换蛋白(NCX1)、内质网Ca2+-ATPase(SERCA2)、受磷蛋白等钙调控蛋白的转录与翻译水平。结果心肌细胞的缩短分数(FS)和[Ca2+]imax在心衰对照组均明显低于假手术组[FS(%):7.51±1.15与13.21±1.49,[Ca2+]imax(nmol/L):330.85±50.05与498.16±14.07;均为P<0.01]。给予培哚普利治疗后,培哚普利组的FS和[Ca2+]imax分别为(10.89±1.18)%和(488.72±22.27)nmol/L,明显高于心衰对照组(均为P<0.01)。在心衰对照组,NCX1和受磷蛋白的mRNA水平显著高于假手术组,而SERCA2的mRNA却明显低于假手术组。在培哚普利组,其NCX1和SERCA2的mRNA水平则介于心衰对照组和假手术组之间,差异有统计学意义(均为P<0.05)。在心衰对照组及培哚普利组,NCX1的蛋白表达量分别是假手术组的(1.141±0.047)倍(P<0.01)和(1.074±0.081)倍(P=0.018),SERCA2的表达量则是假手术组的(0.803±0.100)倍(P<0.01)和(0.893±0.084)倍(P=0.003)。结论培哚普利抗心衰治疗能够减轻心肌细胞的钙瞬变及其调控蛋白的异常变化,使心衰中单个心肌细胞的收缩特性得到保护。     

Increased SR Ca2+ cycling contributes to improved contractile performance in SERCA2a-overexpressing transgenic rats

OBJECTIVE: Heart failure is associated with reduced function of sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2a) but increased function of sarcolemmal Na+/Ca2+ exchanger (NCX), leading to decreased SR Ca2+ content and loss of frequency-potentiation of contractile force. We reported that SERCA2a-overexpression in transgenic rat hearts (TG) results in improved contractility. However, it was not clear whether TG have improved contractility due to frequency-dependent improved SR Ca2+ handling. METHODS: Therefore, we characterized TG (n=35) vs. wild-type (WT) control rats (n=39) under physiological conditions (37 degrees C, stimulation rate <8 Hz). Twitch force, intracellular Ca2+ transients ([Ca2+]i), and SR Ca2+ content were measured in isolated muscles. The contribution of transsarcolemmal Ca2+ influx (I(Ca)) through L-type Ca2+ channels (LTCC) and reverse mode NCX (I(Na/Ca)) to Ca2+ cycling were studied in isolated myocytes. RESULTS: With increasing frequency, force increased in TG muscles by 168+/-35% (8 Hz; P<0.05) and SR Ca2+ content increased by maximally 118+/-31% (4 Hz; P<0.05). In WT, there was a flat force-frequency response without changes in SR Ca2+ content. Relaxation parameters of force and [Ca2+]i decay were accelerated at each frequency in TG vs. WT by approximately 10%. At prolonged rest intervals (<240 s), force and SR Ca2+ content increased significantly more in TG. Consequently, absolute SR Ca2+ content measured in myocytes was increased approximately 2-fold in TG. Transsarcolemmal Ca2+ fluxes estimated by I(Ca) (at 0 mV -10.2+/-1.1 vs. -16.9+/-1.3 pA/pF) and I(Na/Ca) (0.17+/-0.02 vs. 0.46+/-0.05 pA/pF) were decreased in TG vs. WT (P<0.05), whereas NCX and LTCC protein expression was only slightly reduced (P=n.s.). CONCLUSION: In summary, SERCA2a-overexpression improved contractility in a frequency-dependent way due to increased SR Ca2+ loading whereas transsarcolemmal Ca2+ fluxes were decreased.     

Endothelin-1 stimulates the Na+/Ca2+ exchanger reverse mode through intracellular Na+ (Na+i)-dependent and Na+i-independent pathways

This study aimed to explore the signaling pathways involved in the positive inotropic effect (PIE) of low doses of endothelin-1 (ET-1). Cat papillary muscles were used for force and intracellular Na(+) concentration (Na(+)(i)) measurements, and isolated cat ventricular myocytes for patch-clamp experiments. ET-1 (5 nmol/L) induced a PIE and an associated increase in Na(+)(i) that were abolished by Na(+)/H(+) exchanger (NHE) inhibition with HOE642. Reverse-mode Na(+)/Ca(2+) exchanger (NCX) blockade with KB-R7943 reversed the ET-1-induced PIE. These results suggest that the ET-1-induced PIE is totally attributable to the NHE-mediated Na(+)(i) increase. However, an additional direct stimulating effect of ET-1 on NCX after the necessary increase in Na(+)(i) could occur. Thus, the ET-1-induced increase in Na(+)(i) and contractility was compared with that induced by partial inhibition of the Na(+)/K(+) ATPase by lowering extracellular K(+) (K(+)(o)). For a given Na(+)(i), ET-1 induced a greater PIE than low K(+)(o). In the presence of HOE642 and after increasing contractility and Na(+)(i) by low K(+)(o), ET-1 induced an additional PIE that was reversed by KB-R7943 or the protein kinase C (PKC) inhibitor chelerythrine. ET-1 increased the NCX current and negatively shifted the NCX reversal potential (E(NCX)). HOE642 attenuated the increase in NCX outward current and abolished the E(NCX) shift. These results indicate that whereas the NHE-mediated ET-1-induced increase in Na(+)(i) seems to be mandatory to drive NCX in reverse and enhance contractility, Na(+)(i)-independent and PKC-dependent NCX stimulation appears to additionally contribute to the PIE. However, it is important to stress that the latter can only occur after the primary participation of the former.