Evidence that reverse Na-Ca exchange can trigger SR calcium release

Disruption of Transverse-Tubules Eliminates the Slow Force Response to Stretch in Isolated Rat Trabeculae

Ventricular muscle has a biphasic response to stretch. There is an immediate increase in force that coincides with the stretch which is followed by a second phase that takes several minutes for force to develop to a new steady state. The initial increase in force is due to changes in myofilament properties, whereas the second, slower component of the stretch response (known as the “slow force response” or SFR) is accompanied by a steady increase in Ca²⁺ transient amplitude. Evidence shows stretch-dependent Ca²⁺ influx during the SFR occurs through some mechanism that is continuously active for several minutes following stretch. Many of the candidate ion channels are located primarily in the t-tubules, which are consequently lost in heart disease. Our aim, therefore, was to investigate the impact of t-tubule loss on the SFR in non-failing cardiac trabeculae in which expression of the different Ca²⁺ handling proteins was not altered by any disease process. For comparison, we also investigated the effect of formamide detubulation of trabeculae on β-adrenergic activation (1 μM isoproterenol), since this is another key regulator of cardiac force. Measurement of intracellular calcium ([Ca²⁺]_i) and isometric stress were made in RV trabeculae from rat hearts before, during and after formamide treatment (1.5 M for 5 min), which on washout seals the surface sarcolemmal t-tubule openings. Results showed detubulation slowed the time course of Ca²⁺ transients and twitch force, with time-to-peak, maximum rate-of-rise, and relaxation prolonged in trabeculae at optimal length (L_o). Formamide treatment also prevented development of the SFR following a step change in length from 90 to 100% L_o, and blunted the response to β-adrenergic activation (1 μM isoproterenol).

T-tubule remodeling and increased heterogeneity of calcium release during the progression to heart failure in intact rat ventricle

A highly organized transverse‐tubule (TT) system is essential to normal Ca²⁺ cycling and cardiac function. We explored the relationship between the progressive disruption of TTs and resulting Ca²⁺ cycling during the development of heart failure (HF). Confocal imaging was used to measure Ca²⁺ transients and 2‐D z‐stack images in left ventricular epicardial myocytes of intact hearts from spontaneously hypertensive rats (SHR) and Wistar‐Kyoto control rats. TT organization was measured as the organizational index (OI) derived from a fast Fourier transform of TT organization. We found little decrease in the synchrony of Ca²⁺ release with TT loss until TT remodeling was severe, suggesting a TT “reserve” characterized by a wide range of TT remodeling with little effect on synchrony of release but beyond which variability in release shows an accelerating sensitivity to TT loss. To explain this observation, we applied a computational model of spatially distributed Ca²⁺ signaling units to investigate the relationship between OI and excitation‐contraction coupling. Our model showed that release heterogeneity exhibits a nonlinear relationship on both the spatial distribution of release units and the separation between L‐type Ca²⁺ channels and ryanodine receptors. Our results demonstrate a unique relationship between the synchrony of Ca²⁺ release and TT organization in myocytes of intact rat ventricle that may contribute to both the compensated and decompensated phases of heart failure.

In silico investigation of the short QT syndrome, using human ventricle models incorporating electromechanical coupling

INTRODUCTION

Genetic forms of the Short QT Syndrome (SQTS) arise due to cardiac ion channel mutations leading to accelerated ventricular repolarization, arrhythmias and sudden cardiac death. Results from experimental and simulation studies suggest that changes to refractoriness and tissue vulnerability produce a substrate favorable to re-entry. Potential electromechanical consequences of the SQTS are less well-understood. The aim of this study was to utilize electromechanically coupled human ventricle models to explore electromechanical consequences of the SQTS.

METHODS AND RESULTS

The Rice et al. mechanical model was coupled to the ten Tusscher et al. ventricular cell model. Previously validated K(+) channel formulations for SQT variants 1 and 3 were incorporated. Functional effects of the SQTS mutations on [Ca(2+)] i transients, sarcomere length shortening and contractile force at the single cell level were evaluated with and without the consideration of stretch-activated channel current (I sac). Without I sac, at a stimulation frequency of 1Hz, the SQTS mutations produced dramatic reductions in the amplitude of [Ca(2+)] i transients, sarcomere length shortening and contractile force. When I sac was incorporated, there was a considerable attenuation of the effects of SQTS-associated action potential shortening on Ca(2+) transients, sarcomere shortening and contractile force. Single cell models were then incorporated into 3D human ventricular tissue models. The timing of maximum deformation was delayed in the SQTS setting compared to control.

CONCLUSION

The incorporation of I sac appears to be an important consideration in modeling functional effects of SQT 1 and 3 mutations on cardiac electro-mechanical coupling. Whilst there is little evidence of profoundly impaired cardiac contractile function in SQTS patients, our 3D simulations correlate qualitatively with reported evidence for dissociation between ventricular repolarization and the end of mechanical systole.

Modeling effects of L-type ca(2+) current and na(+)-ca(2+) exchanger on ca(2+) trigger flux in rabbit myocytes with realistic T-tubule geometries

The transverse tubular system of rabbit ventricular myocytes consists of cell membrane invaginations (t-tubules) that are essential for efficient cardiac excitation-contraction coupling. In this study, we investigate how t-tubule micro-anatomy, L-type Ca²⁺ channel (LCC) clustering, and allosteric activation of Na⁺/Ca²⁺ exchanger by L-type Ca²⁺ current affects intracellular Ca²⁺ dynamics. Our model includes a realistic 3D geometry of a single t-tubule and its surrounding half-sarcomeres for rabbit ventricular myocytes. The effects of spatially distributed membrane ion-transporters (LCC, Na⁺/Ca²⁺ exchanger, sarcolemmal Ca²⁺ pump, and sarcolemmal Ca²⁺ leak), and stationary and mobile Ca²⁺ buffers (troponin C, ATP, calmodulin, and Fluo-3) are also considered. We used a coupled reaction-diffusion system to describe the spatio-temporal concentration profiles of free and buffered intracellular Ca²⁺. We obtained parameters from voltage-clamp protocols of L-type Ca²⁺ current and line-scan recordings of Ca²⁺ concentration profiles in rabbit cells, in which the sarcoplasmic reticulum is disabled. Our model results agree with experimental measurements of global Ca²⁺ transient in myocytes loaded with 50 μM Fluo-3. We found that local Ca²⁺ concentrations within the cytosol and sub-sarcolemma, as well as the local trigger fluxes of Ca²⁺ crossing the cell membrane, are sensitive to details of t-tubule micro-structure and membrane Ca²⁺ flux distribution. The model additionally predicts that local Ca²⁺ trigger fluxes are at least threefold to eightfold higher than the whole-cell Ca²⁺ trigger flux. We found also that the activation of allosteric Ca²⁺-binding sites on the Na⁺/Ca²⁺ exchanger could provide a mechanism for regulating global and local Ca²⁺ trigger fluxes in vivo. Our studies indicate that improved structural and functional models could improve our understanding of the contributions of L-type and Na⁺/Ca²⁺ exchanger fluxes to intracellular Ca²⁺ dynamics.

Integrative systems models of cardiac excitation-contraction coupling

Excitation-contraction coupling in the cardiac myocyte is mediated by a number of highly integrated mechanisms of intracellular Ca²(+) transport. The complexity and integrative nature of heart cell electrophysiology and Ca²(+) cycling has led to an evolution of computational models that have played a crucial role in shaping our understanding of heart function. An important emerging theme in systems biology is that the detailed nature of local signaling events, such as those that occur in the cardiac dyad, have important consequences at higher biological scales. Multiscale modeling techniques have revealed many mechanistic links between microscale events, such as Ca²(+) binding to a channel protein, and macroscale phenomena, such as excitation-contraction coupling gain. Here, we review experimentally based multiscale computational models of excitation-contraction coupling and the insights that have been gained through their application.

Alloxan reduces amplitude of ventricular myocyte shortening and intracellular Ca2+ without altering L-type Ca2+ current, sarcoplasmic reticulum Ca2+ content or myofilament sensitivity to Ca2+ in Wistar rats

Alloxan is widely used to induce diabetes mellitus in experimental animals. Recent studies have provided evidence that alloxan has direct actions on cardiac muscle contraction. The aim of this study was to further investigate the mechanisms underlying the effects of alloxan on ventricular myocyte shortening and intracellular Ca(2+) transport. Amplitude of myocyte shortening was reduced in a dose-dependent manner as the concentration of alloxan was increased in the range 10(-7)-10(-4) M. Amplitude of shortening was reduced (56.8 +/- 6.6%, n = 27) by 10(-6) M alloxan and was partially reversed during a 10 min washout. Amplitude of the Ca(2+) transient was also reduced (79.7 +/- 2.9%, n = 29) by 10(-6) M alloxan. Caffeine-evoked sarcoplasmic reticulum Ca(2+) release, fractional release of Ca(2+), assessed by comparing the amplitude of electrically evoked with that of caffeine-evoked Ca(2+) transients, and fura-2-cell length trajectory during the late stages of relaxation of myocyte twitch contraction were not significantly altered by alloxan. The amplitude of L-type Ca(2+) current was not altered by alloxan. Alterations in sarcoplasmic reticulum Ca(2+) transport, myofilament sensitivity to Ca(2+), and L-type Ca(2+) current do not appear to underlie the negative inotropic effects of alloxan.

Transformation of adult rat cardiac myocytes in primary culture

We characterized the morphological, electrical and mechanical alterations of cardiomyocytes in long-term cell culture. Morphometric parameters, sarcomere length, T-tubule density, cell capacitance, L-type calcium current (I(Ca,L)), inward rectifier potassium current (I(K1)), cytosolic calcium transients, action potential and contractile parameters of adult rat ventricular myocytes were determined on each day of 5 days in culture. We also analysed the health of the myocytes using an apoptotic/necrotic viability assay. The data show that myocytes undergo profound morphological and functional changes during culture. We observed a progressive reduction in the cell area (from 2502 +/- 70 microm(2) on day 0 to 1432 +/- 50 microm(2) on day 5), T-tubule density, systolic shortening (from 0.11 +/- 0.02 to 0.05 +/- 0.01 microm) and amplitude of calcium transients (from 1.54 +/- 0.19 to 0.67 +/- 0.19) over 5 days of culture. The negative force-frequency relationship, characteristic of rat myocardium, was maintained during the first 2 days but diminished thereafter. Cell capacitance (from 156 +/- 8 to 105 +/- 11 pF) and membrane currents were also reduced (I(Ca,L), from 3.98 +/- 0.39 to 2.12 +/- 0.37 pA pF; and I(K1), from 34.34p +/- 2.31 to 18.00 +/- 5.97 pA pF(-1)). We observed progressive depolarization of the resting membrane potential during culture (from 77.3 +/- 2.5 to 34.2 +/- 5.9 mV) and, consequently, action potential morphology was profoundly altered as well. The results of the viability assays indicate that these alterations could not be attributed to either apoptosis or necrosis but are rather an adaptation to the culture conditions over time.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Contribution of the Na+/Ca2+ exchanger to rapid Ca2+ release in cardiomyocytes

Trigger Ca(2+) is considered to be the Ca(2+) current through the L-type Ca(2+) channel (LTCC) that causes release of Ca(2+) from the sarcoplasmic reticulum. However, cell contraction also occurs in the absence of the LTCC current (I(Ca)). In this article, we investigate the contribution of the Na(+)/Ca(2+) exchanger (NCX) to the trigger Ca(2+). Experimental data from rat cardiomyocytes using confocal microscopy indicating that inhibition of reverse mode Na(+)/Ca(2+) exchange delays the Ca(2+) transient by 3-4 ms served as a basis for the mathematical model. A detailed computational model of the dyadic cleft (fuzzy space) is presented where the diffusion of both Na(+) and Ca(2+) is taken into account. Ionic channels are included at discrete locations, making it possible to study the effect of channel position and colocalization. The simulations indicate that if a Na(+) channel is present in the fuzzy space, the NCX is able to bring enough Ca(2+) into the cell to affect the timing of release. However, this critically depends on channel placement and local diffusion properties. With fuzzy space diffusion in the order of four orders of magnitude lower than in water, triggering through LTCC alone was up to 5 ms slower than with the presence of a Na(+) channel and NCX.

Transmural heterogeneity of calcium activity and mechanical function in the canine left ventricle

Although electrical heterogeneity within the ventricular myocardium has been the focus of numerous studies, little attention has been directed to the mechanical correlates. This study examines unloaded cell shortening, Ca2+ transients, and inward L-type Ca2+ current ( ICa,L) characteristics of epicardial, endocardial, and midmyocardial cells isolated from the canine left ventricle. Unloaded cell shortening was recorded using a video edge detector, Ca2+ transients were measured in cells loaded with 15 μM fluo-3 AM and voltage and current-clamp recordings were obtained using patch-clamp techniques. Time to peak and latency to onset of contraction were shortest in epicardial and longest in endocardial cells; midmyocardial cells displayed an intermediate time to peak. When contraction was elicited using uniform voltage-clamp square waves, epicardial versus endocardial distinctions persisted and midmyocardial cells displayed a time to peak comparable to that of epicardium. The current-voltage relationship for ICa,L and fluorescence-voltage relationship were similar in the three cell types when quantitated using square pulses. However, peak ICa,L and total charge were significantly larger when an epicardial versus endocardial action potential waveform was used to elicit the current under voltage-clamp conditions. Sarcoplasmic reticulum Ca2+ content, assessed by rapid application of caffeine, was largest in epicardial cells and contributed to a faster time to peak. Our data point to important differences in calcium homeostasis and mechanical function among the three ventricular cell types. These differences serve to synchronize contraction across the ventricular wall. Although these distinctions are conferred in part by differences in electrical characteristics of the three cell types, intrinsic differences in excitation-contraction coupling are evident.

Ca2+ sparks in rabbit ventricular myocytes evoked by action potentials: involvement of clusters of L-type Ca2+ channels

It is not clear how many L-type Ca2+ channels (LCCs) are required to ensure that a Ca2+ spark is triggered during a normal mammalian action potential (AP). We investigated this in rabbit ventricular myocytes by examining both the properties of sparks evoked by APs and the activity of LCCs. We measured Ca2+ sparks evoked by repeated APs with pipettes containing 2 mmol/L EGTA and single LCC activity in cell-attached patches depolarized to +50 mV using pipettes containing 110 mmol/L Ba2+. With 2 mmol/L Ca2+ in the external solution, we observed sparks at the beginning of every evoked AP at numerous locations. Each spark was observed repeatedly at a fixed location and began during a limited interval after the AP peak. These sparks occurred with a probability of approximately unity. However, the chance that an LCC does not open during the interval when a spark is triggered is quite high ( approximately 0.13). Therefore, because single channels open with a probability significantly lower than 1, more than one LCC must be available to ensure that sparks are triggered with a probability of approximately unity. We conclude that it is likely that a cluster of LCCs is involved in gating a cluster of ryanodine receptors at the beginning of an AP.

Recombinant expression of the plasma membrane Na(+)/Ca(2+) exchanger affects local and global Ca(2+) homeostasis in Chinese hamster ovary cells

The cardiac type Na(+)/Ca(2+) exchanger (NCX1) has been transiently expressed in Chinese hamster ovary cells, which do not contain an endogenous exchanger, together with aequorin chimeras that are targeted to different intracellular compartments to investigate intracellular Ca(2+) homeostasis. The expression of NCX decreased the endoplasmic reticulum Ca(2+) concentration, [Ca(2+)](er), in resting cells, showing that the exchanger was operative under these conditions. It induced a greater reduction in the height of the mitochondrial and cytosolic Ca(2+) transients in agonist-stimulated cells than would have been expected from the [Ca(2+)](er) decrease. It also had a major effect on the sub-plasma membrane Ca(2+) concentration, [Ca(2+)](pm): after a transient [Ca(2+)](pm) rise induced by the activation of capacitative Ca(2+) influx, [Ca(2+)](pm) settled to a value about 3-fold higher than in controls. The sustained [Ca(2+)](pm) increase after the transient was due to the operation of the exchanger, either directly by operating in the Ca(2+) entry mode, or indirectly by removing the Ca(2+) inhibition on the capacitative Ca(2+) influx channels.

Comparison of Na(+)/Ca(2+) exchanger current and of its response to isoproterenol between acutely isolated and short-term cultured adult ventricular myocytes

The Na(+)/Ca(2+) exchanger protein is present in the cell membrane of many tissue types and plays key roles in Ca(2+) homeostasis, excitation-contraction coupling, and generation of electrical activity in the heart. The use of adult ventricular myocyte cell culture is important to molecular biological approaches to study the roles and modulation of the cardiac Na(+)/Ca(2+) exchanger. Therefore, we characterised the functional expression of the exchanger in adult guinea-pig ventricular myocytes maintained in short-term culture (for 4 days) and compared the response of ionic current (I(NaCa)) carried by the exchanger from acutely isolated and Day 4 cells to beta-adrenoceptor activation with isoproterenol (ISO). Functional activity of the exchanger was assessed by measuring I(NaCa) using whole cell patch clamp, under selective recording conditions. I(NaCa) amplitude measured at both +60 and -100mV declined significantly by Day 1 of cell culture, showing a further small decline by Day 4. However, cell surface area (assessed by measuring membrane capacitance) also declined over this time-frame. I(NaCa) normalised to membrane capacitance (I(NaCa) density) did not differ significantly between acutely isolated and cells cultured for 4 days. However, although ISO (1 microM) increased I(NaCa) in acutely isolated myocytes, it exerted no significant effect on I(NaCa) from Day 4 cells. This was not due to an inherent inability of these cells to respond to ISO, as L-type calcium current amplitude from Day 4 cells was increased by ISO to a similar extent as that from acutely isolated cells. Our data suggest that the functional expression of the Na/Ca exchanger is well maintained during short-term culture of adult ventricular myocytes. The lack of response to ISO of I(NaCa) from Day 4 cells suggests: (a) that, despite a well-maintained I(NaCa) density, cultured adult myocytes may not necessarily be suitable for studies of exchanger modulation by some agonists and (b) that there may exist subtle differences between beta-adrenergic regulation of the exchanger protein and of L-type Ca channels.

Glutamate-induced calcium increase in myotubes depends on up-regulation of a sodium-dependent transporter

We report a study on the regulation by 2-chloro adenosine (2CA) of a glutamate (Glu) transporter in myogenic C2C12 cells. Long-term 2CA exposition significantly increased the V(max) of the Glu transporter. Moreover, 2CA-treated cells responded to Glu challenge by a rapid and transient increase in their intracellular calcium level. The above reported effects were totally abolished by treating C2C12 cells with the Na(+)-dependent Glu transporter inhibitors DL-threo-b-hydroxyaspartic acid and L-trans-pyrrolidine-2,4-dicarboxylic acid. We propose that the possible link between the Glu uptake increase and the Glu induction of calcium rise could be the depolarizing currents carried by Na(+) coupled with transporter activity.

Na(+)/Ca(2+) exchanger in GABAergic presynaptic boutons of rat central neurons

Rat Meynert neurons were acutely isolated using a dissociation technique that maintains functional GABAergic presynaptic boutons. Miniature inhibitory postsynaptic currents (mIPSCs) were recorded under voltage-clamp conditions using whole cell patch-clamp recordings. Using the frequency of mIPSCs as a measure of presynaptic terminal excitability, the existence of a Na(+)/Ca(2+) exchanger (NCX) in these GABAergic nerve terminals was clearly demonstrated. Both the frequency and the amplitude of mIPSCs were unaffected by replacement of extracellular Na(2+). However, in this Na(+)-free external solution, ouabain could now induce a transient increase of mIPSCs frequency, which was not inhibited by adding Cd(2+) or cyclopiazonic acid but was inhibited by removing external Ca(2+). This indicates that this transient potentiation was dependent on external Ca(2+), but that this Ca(2+) influx was not via voltage-dependent Ca(2+) channels. KB-R7943, an inhibitor of NCX, at a concentration of 3 x 10(-6) M, reduced this transient increase of mIPSCs frequency without affecting mIPSCs amplitude and the response to exogenous GABA. These results demonstrate the existence of NCX in these GABAergic nerve terminals. In zero external Na(+), ouabain causes an accumulation of intraterminal Na(+) and a resultant influx of Ca(2+) through the reversed mode operation of NCX. However, under more physiological conditions, NCX may also operate in a forward mode and serve to maintain low intracellular [Ca(2+)] in nerve terminals.

Disruption of a single copy of the SERCA2 gene results in altered Ca2+ homeostasis and cardiomyocyte function

A mouse model carrying a null mutation in one copy of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase isoform 2 (SERCA2) gene, in which SERCA2 protein levels are reduced by approximately 35%, was used to investigate the effects of decreased SERCA2 level on intracellular Ca(2+) homeostasis and contractile properties in isolated cardiomyocytes. When compared with wild-type controls, SR Ca(2+) stores and Ca(2+) release in myocytes of SERCA2 heterozygous mice were decreased by approximately 40-60% and approximately 30-40%, respectively, and the rate of myocyte shortening and relengthening were each decreased by approximately 40%. However, the rate of Ca(2+) transient decline (tau) was not altered significantly, suggesting that compensation was occurring in the removal of Ca(2+) from the cytosol. Phospholamban, which inhibits SERCA2, was decreased by approximately 40% in heterozygous hearts, and basal phosphorylation of Ser-16 and Thr-17, which relieves the inhibition, was increased approximately 2- and 2.1-fold. These results indicate that reduced expression and increased phosphorylation of phospholamban provides compensation for decreased SERCA2 protein levels in heterozygous heart. Furthermore, both expression and current density of the sarcolemmal Na(+)-Ca(2+) exchanger were up-regulated. These results demonstrate that a decrease in SERCA2 levels can directly modify intracellular Ca(2+) homeostasis and myocyte contractility. However, the resulting deficit is partially compensated by alterations in phospholamban/SERCA2 interactions and by up-regulation of the Na(+)-Ca(2+) exchanger.

Sodium-calcium exchange: a molecular perspective

Plasma membrane Na(+)-Ca2+ exchange is an essential component of Ca2+ signaling pathways in several tissues. Activity is especially high in the heart where the exchanger is an important regulator of contractility. An expanding exchanger superfamily includes three mammalian Na(+)-Ca2+ exchanger genes and a number of alternative splicing products. New information indicates that the exchanger protein has nine transmembrane segments. The exchanger, which transports Na+ and Ca2+, is also regulated by these substrates. Some molecular information is available on regulation by Na+ and Ca2+ and by PIP2 and phosphorylation. Altered expression of the exchanger in pathophysiological states may contribute to various cardiac phenotypes. Use of transgenic approaches is beginning to improve our knowledge of exchanger function.

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.

Effects of diabetes and hypertension on myocardial Na+-Ca2+ exchange

Abnormalities in cardiac function have been extensively documented in experimental and clinical diabetes. These aberrations are well known to be exaggerated when hypertension and diabetes co-exist. The objective of the present study was to examine whether alterations in the activity of the myocardial Na+-Ca2+ exchanger (NCX) can account for the deleterious effects of diabetes and (or) hypertension on the heart. To this aim, the following experimental groups were studied: (i) control; (ii) diabetic; (iii) hypertensive; and (iv) hypertensive-diabetic. Wistar rats served as the control group (C) while Wistar rats injected with streptozotocin (STZ, 55 mg/kg) served as the diabetic (D) group. Spontaneously hypertensive (SH) rats were used as the hypertensive group (H) while SH rats injected with STZ served as the hypertensive-diabetic (HD) group. Sarcolemma was isolated from the ventricles of the C, D, H, and HD groups and NCX activity was examined using rapid quenching techniques to study initial rates over a [Ca2+]o range of 10-160 µM. The Vmax of NCX was lower in the D group when compared with the C group (D, 2.96 ± 0.26 vs. C, 4.0 ± 0.46 nmol·mgprot-1·s-1, P < 0.05), however combined diabetes and hypertension (HD) did not affect the Vmax of NCX activity (HD, 3.84 ± 0.88 vs. H, 3.59 ± 0.24 nmol·mgprot-1·s-1, P > 0.05). However, analysis of the Km values for Ca2+ indicated that both the D and HD groups exhibited a significantly lower Km when compared with their respective control groups (D, 42 ± 4 vs. C, 56 ± 4 µM,P < 0.05; HD, 33 ± 7 vs. H, 51 ± 8 µM, P < 0.05). Immunoblotting using polyclonal antibodies (against canine cardiac NCX) exhibited the typical banding of 160, 120, and 70 kDa. The 120 kDa band is believed to represent the native exchanger with its post-translational modifications. Examination of the blots revealed a lower intensity of the 120 kDa band in the D group when compared with the C group, however, no significant difference in the HD group was observed. We speculate that the lower Vmax in the D group may be due to a reduced concentration of exchanger protein in the membrane. The absence of this defect in the HD group may be a result of compensatory mechanisms to the overall hemodynamic overload, however, this remains to be determined. The increased affinity for Ca2+ in both the D and HD groups (determined by the lower Km values) is an interesting finding and may be due to changes in sarcolemmal lipid bilayer composition secondary to diabetes-induced hyperlipidemia.Key words: diabetes, hypertension, cardiac, Na+-Ca2+ exchange, contractility.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Role of Na-Ca exchange in the action potential changes caused by drive in cardiac myocytes exposed to different Ca2+ loads

The role of Na-Ca exchange in the membrane potential changes caused by repetitive activity ("drive") was studied in guinea pig single ventricular myocytes exposed to different [Ca2+]o. The following results were obtained. (i) In 5.4 mM [Ca2+]o, the action potentials (APs) gradually shortened during drive, and the outward current during a train of depolarizing voltage clamp steps gradually increased. (ii) The APs shortened more and were followed by a decaying voltage tail during drive in the presence of 5 mM caffeine; the outward current became larger and there was an inward tail current on repolarization during a train of depolarizing steps. (iii) These effects outlasted drive so that immediately after a train of APs, currents were already bigger and, after a train of steps, APs were already shorter. (iv) In 0.54 mM [Ca2+]o, the above effects were much smaller. (v) In high [Ca2+]o APs were shorter and outward currents larger than in low [Ca2+]o. (vi) In 10.8 mM [Ca2+]o, both outward and inward currents during long steps were exaggerated by prior drive, even with steps (+80 and +120 mV) at which there was no apparent inward current identifiable as ICa. (vii) In 0.54 mM [Ca2+]o, the time-dependent outward current was small and prior drive slightly increased it. (viii) During long steps, caffeine markedly increased outward and inward tail currents, and these effects were greatly decreased by low [Ca2+]o. (ix) After drive in the presence of caffeine, Ni2+ decreased the outward and inward tail currents. It is concluded that in the presence of high [Ca2+]o drive activates outward and inward Na-Ca exchange currents. During drive, the outward current participates in the plateau shortening and the inward tail current in the voltage tail after the action potential.Key words: ventricular myocytes, repetitive activity, outward and inward Na-Ca exchange currents, caffeine, nickel.

Overexpression of the Na(+)-Ca2+ exchanger leads to enhanced inotropic responsiveness to Na(+)-channel agonist without sarcoplasmic reticulum protein changes in transgenic mice

The present study investigated the influence of over-expression of the cardiac Na(+)-Ca2+ exchanger on myocardial force of contraction and alterations in sarcoplasmic reticulum (SR) Ca(2+)-handling proteins in a transgenic mouse model. Inotropic effects of Na+ channel agonist BDF 9148 and isoprenaline were determined in isolated electrically driven atria. Protein levels of key SR Ca(2+)-handling proteins were determined by Western blot analysis. Transgenic animals had no myocardial hypertrophy or failure. The positive inotropic effect of BDF 9148 was significantly more pronounced in myocardium for transgenic animals, whereas the inotropic response to isoprenaline was similar in both groups. Strong immunoreactivity of the transgene Na(+)-Ca2+ exchanger was detected in myocardium of transgenic animals. Protein levels of SR Ca(2+)-ATPase, phospholamban, and calsequestrin were unchanged. In conclusion, transgenic overexpression of the Na(+)-Ca2+ exchanger is accompanied by increased force development following Na+ channel agonist administration, even though Ca2+ proteins of the SR are unchanged.