Stoichiometry of Na+-Ca2+ exchange is 3:1 in guinea-pig ventricular myocytes

Reciprocal Relationship between Ca2+ Signaling and Ca2+-Gated Ion Channels as a Potential Target for Drug Discovery

Cellular Ca²⁺ signaling functions as one of the most common second messengers of various signal transduction pathways in cells and mediates a number of physiological roles in a cell-type dependent manner. Ca²⁺ signaling also regulates more general and fundamental cellular activities, including cell proliferation and apoptosis. Among ion channels, Ca²⁺-permeable channels in the plasma membrane as well as endo- and sarcoplasmic reticulum membranes play important roles in Ca²⁺ signaling by directly contributing to the influx of Ca²⁺ from extracellular spaces or its release from storage sites, respectively. Furthermore, Ca²⁺-gated ion channels in the plasma membrane often crosstalk reciprocally with Ca²⁺ signals and are central to the regulation of cellular functions. This review focuses on the physiological and pharmacological impact of i) Ca²⁺-gated ion channels as an apparatus for the conversion of cellular Ca²⁺ signals to intercellularly propagative electrical signals and ii) the opposite feedback regulation of Ca²⁺ signaling by Ca²⁺-gated ion channel activities in excitable and non-excitable cells.

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).

Pharmacological Preconditioning Using Diazoxide Regulates Store-Operated Ca2 + Channels in Adult Rat Cardiomyocytes

Voltage-dependent Ca²⁺ channels and store-operated Ca²⁺ channels (SOCs) are the major routes of Ca²⁺ entry into mammalian cells. Previously, we reported that pharmacological preconditioning (PPC) leads to a decrease in the amplitude of L-type calcium channel current in the heart. In this study, we examined PPC-associated changes in SOC function. We measured adult cardiomyocyte membrane currents using the whole-cell patch-clamp technique, and we evaluated reactive oxygen species (ROS) production and intracellular Ca²⁺ levels in cardiomyocytes using fluorescent probes. Diazoxide (Dzx) and thapsigargin (Tg) were used to induce PPC and to deplete internal stores of Ca²⁺, respectively. Ca²⁺ store depletion generated inward currents with strong rectification, which were suppressed by the SOC blocker GSK-7975-A. These currents were completely abolished by PPC, an effect that could be countered with 5-hydroxydecanoate (5-HD; a selective mitochondrial ATP-sensitive K⁺ channel blocker), an intracellular mitochondrial energizing solution, or Ni²⁺ [a blocker of sodium-calcium exchanger (NCX)]. Buffering of ROS and intracellular Ca²⁺ also prevented PPC effects on SOC currents. Refilling of intracellular stores was largely suppressed by PPC, as determined by measuring intracellular Ca²⁺ with a fluorescent Ca²⁺ indicator. These results indicate that influx of Ca²⁺ through SOCs is inhibited by their ROS and Ca²⁺-dependent inactivation during PPC and that NCX is a likely source of PPC-inactivating Ca²⁺. We further showed that NCX associates with Orai1. Down-regulation of SOCs by PPC may play a role in cardioprotection following ischemia-reperfusion.

KB-R7943 reduces 4-aminopyridine-induced epileptiform activity in adult rats after neuronal damage induced by neonatal monosodium glutamate treatment

BACKGROUND

Neonatal monosodium glutamate (MSG) treatment triggers excitotoxicity and induces a degenerative process that affects several brain regions in a way that could lead to epileptogenesis. Na+/Ca2+ exchangers (NCX1-3) are implicated in Ca2+ brain homeostasis; normally, they extrude Ca2+ to control cell inflammation, but after damage and in epilepsy, they introduce Ca2+ by acting in the reverse mode, amplifying the damage. Changes in NCX3 expression in the hippocampus have been reported immediately after neonatal MSG treatment. In this study, the expression level of NCX1-3 in the entorhinal cortex (EC) and hippocampus (Hp); and the effects of blockade of NCXs on the seizures induced by 4-Aminopyridine (4-AP) were analysed in adult rats after neonatal MSG treatment. KB-R7943 was applied as NCXs blocker, but is more selective to NCX3 in reverse mode.

METHODS

Neonatal MSG treatment was applied to newborn male rats at postnatal days (PD) 1, 3, 5, and 7 (4 g/kg of body weight, s.c.). Western blot analysis was performed on total protein extracts from the EC and Hp to estimate the expression level of NCX1-3 proteins in relative way to the expression of β-actin, as constitutive protein. Electrographic activity of the EC and Hp were acquired before and after intracerebroventricular (i.c.v.) infusion of 4-AP (3 nmol) and KB-R7943 (62.5 pmol), alone or in combination. All experiments were performed at PD60. Behavioural alterations were also recorder.

RESULTS

Neonatal MSG treatment significantly increased the expression of NCX3 protein in both studied regions, and NCX1 protein only in the EC. The 4-AP-induced epileptiform activity was significantly higher in MSG-treated rats than in controls, and KB-R7943 co-administered with 4-AP reduced the epileptiform activity in more prominent way in MSG-treated rats than in controls.

CONCLUSIONS

The long-term effects of neonatal MSG treatment include increases on functional expression of NCXs (mainly of NCX3) in the EC and Hp, which seems to contribute to improve the control that KB-R7943 exerted on the seizures induced by 4-AP in adulthood. The results obtained here suggest that the blockade of NCXs could improve seizure control after an excitotoxic process; however, this must be better studied.

Ca(2+) -activated K(+) current is essential for maintaining excitability and gene transcription in early embryonic cardiomyocytes

AIM

Activity of early embryonic cardiomyocytes relies on spontaneous Ca(2+) oscillations that are induced by interplay between sarcoplasmic reticulum (SR) - Ca(2+) release and ion currents of the plasma membrane. In a variety of cell types, Ca(2+) -activated K(+) current (IK(Ca) ) serves as a link between Ca(2+) signals and membrane voltage. This study aimed to determine the role of IK (Ca) in developing cardiomyocytes.

METHODS

Ion currents and membrane voltage of embryonic (E9-11) mouse cardiomyocytes were measured by patch clamp; [Ca(2+) ]i signals by confocal microscopy. Transcription of specific genes was measured with RT-qPCR and Ca(2+) -dependent transcriptional activity using NFAT-luciferase assay. Myocyte structure was assessed with antibody labelling and confocal microscopy.

RESULTS

E9-11 cardiomyocytes express small conductance (SK) channel subunits SK2 and SK3 and have a functional apamin-sensitive K(+) current, which is also sensitive to changes in cytosolic [Ca(2+) ]i . In spontaneously active cardiomyocytes, inhibition of IK (Ca) changed action and resting potentials, reduced SR Ca(2+) load and suppressed the amplitude and the frequency of spontaneously evoked Ca(2+) oscillations. Apamin caused dose-dependent suppression of NFAT-luciferase reporter activity, induced downregulation of a pattern of genes vital for cardiomyocyte development and triggered changes in the myocyte morphology.

CONCLUSION

The results show that apamin-sensitive IK (Ca) is required for maintaining excitability and activity of the developing cardiomyocytes as well as having a fundamental role in promoting Ca(2+) - dependent gene expression.

Function and regulation of the Na+-Ca2+ exchanger NCX3 splice variants in brain and skeletal muscle

Isoform 3 of the Na(+)-Ca(2+) exchanger (NCX3) is crucial for maintaining intracellular calcium ([Ca(2+)]i) homeostasis in excitable tissues. In this sense NCX3 plays a key role in neuronal excitotoxicity and Ca(2+) extrusion during skeletal muscle relaxation. Alternative splicing generates two variants (NCX3-AC and NCX3-B). Here, we demonstrated that NCX3 variants display a tissue-specific distribution in mice, with NCX3-B as mostly expressed in brain and NCX-AC as predominant in skeletal muscle. Using Fura-2-based Ca(2+) imaging, we measured the capacity and regulation of the two variants during Ca(2+) extrusion and uptake in different conditions. Functional studies revealed that, although both variants are activated by intracellular sodium ([Na(+)]i), NCX3-AC has a higher [Na(+)]i sensitivity, as Ca(2+) influx is observed in the presence of extracellular Na(+). This effect could be partially mimicked for NCX3-B by mutating several glutamate residues in its cytoplasmic loop. In addition, NCX3-AC displayed a higher capacity of both Ca(2+) extrusion and uptake compared with NCX3-B, together with an increased sensitivity to intracellular Ca(2+). Strikingly, substitution of Glu(580) in NCX3-B with its NCX3-AC equivalent Lys(580) recapitulated the functional properties of NCX3-AC regarding Ca(2+) sensitivity, Lys(580) presumably acting through a structure stabilization of the Ca(2+) binding site. The higher Ca(2+) uptake capacity of NCX3-AC compared with NCX3-B is in line with the necessity to restore Ca(2+) levels in the sarcoplasmic reticulum during prolonged exercise. The latter result, consistent with the high expression in the slow-twitch muscle, suggests that this variant may contribute to the Ca(2+) handling beyond that of extruding Ca(2+).

Overactive bladder mediated by accelerated Ca2+ influx mode of Na+/Ca2+ exchanger in smooth muscle

The Na(+)/Ca(2+) exchanger (NCX) is thought to be a key molecule in the regulation of cytosolic Ca(2+) dynamics. The relative importance of the two Ca(2+) transport modes of NCX activity leading to Ca(2+) efflux (forward) and influx (reverse) in smooth muscle, however, remains unclear. Unexpectedly, spontaneous contractions of urinary bladder smooth muscle (UBSM) were enhanced in transgenic mice overexpressing NCX1.3 (NCX1.3(tg/tg)). The enhanced activity was attenuated by KB-R7943 or SN-6. Whole cell outward NCX current sensitive to KB-R7943 or Ni(2+) was readily detected in UBSM cells from NCX1.3(tg/tg) but not wild-type mice. Spontaneous Ca(2+) transients in myocytes of NCX1.3(tg/tg) were larger and frequently resulted in propagating events and global elevations in cytosolic Ca(2+) concentration. Significantly, NCX1.3(tg/tg) mice exhibited a pattern of more frequent urination of smaller volumes and this phenotype was reversed by oral administration of KB-R7943. On the other hand, KB-R7943 did not improve it in KB-R7943-insensitive (G833C-)NCX1.3(tg/tg) mice. We conclude that NCX1.3 overexpression is associated with abnormal urination owing to enhanced Ca(2+) influx via reverse mode NCX leading to prolonged, propagating spontaneous Ca(2+) release events and a potentiation of spontaneous UBSM contraction. These findings suggest the possibility that NCX is a candidate molecular target for overactive bladder therapy.

Fractional Ca(2+) currents through TRP and TRPL channels in Drosophila photoreceptors

Light responses in Drosophila photoreceptors are mediated by two Ca(2+) permeable cation channels, transient receptor potential (TRP) and TRP-like (TRPL). Although Ca(2+) influx via these channels is critical for amplification, inactivation, and light adaptation, the fractional contribution of Ca(2+) to the currents (Pf) has not been measured. We describe a slow (τ ∼ 350 ms) tail current in voltage-clamped light responses and show that it is mediated by electrogenic Na(+)/Ca(2+) exchange. Assuming a 3Na:1Ca stoichiometry, we derive empirical estimates of Pf by comparing the charge integrals of the exchanger and light-induced currents. For TRPL channels, Pf was ∼17% as predicted by Goldman-Hodgkin-Katz (GHK) theory. Pf for TRP (29%) and wild-type flies (26%) was higher, but lower than the GHK prediction (45% and 42%). As predicted by GHK theory, Pf for both channels increased with extracellular [Ca(2+)], and was largely independent of voltage between -100 and -30 mV. A model incorporating intra- and extracellular geometry, ion permeation, diffusion, extrusion, and buffering suggested that the deviation from GHK predictions was largely accounted for by extracellular ionic depletion during the light-induced currents, and the time course of the Na(+)/Ca(2+) exchange current could be used to obtain estimates of cellular Ca(2+) buffering capacities.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Effects of a high-sodium diet on renal tubule Ca2+ transporter and claudin expression in Wistar-Kyoto rats

Background

Urinary Ca²⁺ excretion increases with dietary NaCl. NaCl-induced calciuria may be associated with hypertension, urinary stone formation and osteoporosis, but its mechanism and long-term effects are not fully understood. This study examined alterations in the expressions of renal Ca²⁺ transporters, channels and claudins upon salt loading to better understand the mechanism of salt-induced urinary Ca²⁺ loss.

Methods

Eight-week old Wistar-Kyoto rats were fed either 0.3% or 8% NaCl diet for 8 weeks. Renal cortical expressions of Na⁺/Ca²⁺ exchanger 1 (NCX1), Ca²⁺ pump (PCMA1b), Ca²⁺ channel (TRPV5), calbindin-D_28k, and claudins (CLDN-2, -7, -8, -16 and −19) were analyzed by quantitative PCR, western blot and/or immunohistochemistry.

Results

Fractional excretion of Ca²⁺ increased 6.0 fold with high-salt diet. Renal cortical claudin-2 protein decreased by approximately 20% with decreased immunological staining on tissue sections. Claudin-16 and −19 expressions were not altered. Renal cortical TRPV5, calbindin-D_28k and NCX1 expressions increased 1.6, 1.5 and 1.2 fold, respectively.

Conclusions

Chronic high-salt diet decreased claudin-2 protein and increased renal TRPV5, calbindin-D_28k, and NCX1. Salt loading is known to reduce the proximal tubular reabsorption of both Na⁺ and Ca²⁺. The reduction in claudin-2 protein expression may be partly responsible for the reduced Ca²⁺ reabsorption in this segment. The concerted upregulation of more distal Ca²⁺-transporting molecules may be a physiological response to curtail the loss of Ca²⁺, although the magnitude of compensation does not seem adequate to bring the urinary Ca²⁺ excretion down to that of the normal-diet group.

Alteration in NCX-3 immunoreactivity within the gerbil hippocampus following spontaneous seizures

Although NCX-3 is highly expressed in the brain, the distribution of NCX-3 in the epileptic hippocampus is still controversial. Therefore, to assess the distribution and pattern of NCX-3 expression in epileptic hippocampus, we performed a comparative analysis of NCX-3 immunoreactivities in the hippocampus of seizure-resistant (SR) and seizure-sensitive (SS) gerbils. In SR gerbils, NCX-3 immunoreactivity was higher than pre-seizure SS gerbils, particularly in the pavalbumin (PV)-positive interneurons. Three h post-ictal, NCX-3 immunoreactivity in the SS gerbil hippocampus was markedly elevated to the level of SR gerbils. Six h post-ictal, the expression of NCX-3 was reduced to the level of pre-seizure SS gerbils. Therefore, the results of the present study suggest that down-regulation of NCX-3 expression in the SS gerbil hippocampus may be involved in the hyperexcitability of SS gerbils due to an imbalance of intracellular Na(+)/Ca(2+) homeostasis and Ca(2+) concentration.

Effects of estradiol and IGF-1 on the sodium calcium exchanger in rat cultured cortical neurons

The Na(+)/Ca(2+) exchanger (NCX) is an important bidirectional transporter of calcium in neurons and has been shown to be involved in neuroprotection. Calcium can activate a number of cascades that can result in apoptosis and cell death, and NCX is a key factor in regulating the cytoplasmic concentration of this ion. 17-β-estradiol and insulin-like growth factor 1 (IGF-1) are known neuroprotective hormones with interacting mechanisms and effects on intracellular calcium; however, their relationship with the NCX has not been explored. In this article, the effects of these two hormones on neuronal NCX were tested using the whole-cell patch clamp technique on rat primary culture neurons. Both 17-β-estradiol and IGF-1 produced an increase in the NCX-mediated inward current and a decrease in the NCX-mediated outward current. However, the IGF-1 effect was lower than that of 17-β-estradiol, and the effect of both agents together was greater than the sum of each agent alone. Neither of the agents affected the pattern of regulation by extracellular or intrapipette calcium. Inhibitors of the IGF-1 and 17-β-estradiol receptors and inhibitors of the main signaling pathways failed to change the observed effects, indicating that these actions were not mediated by the classical receptors of these hormones. These effects on the NCX could be a mechanism explaining the neuroprotective actions of 17-β-estradiol and IGF-1, and these findings could help researchers to understand the role of the NCX in neuroprotection.

A transmural gradient in the cardiac Na/K pump generates a transmural gradient in Na/Ca exchange

We previously demonstrated a transmural gradient in Na/K pump current (I (P)) and [Na(+)]( i ), with the highest maximum I (P) and lowest [Na(+)]( i ) in epicardium. The present study examines the relationship between the transmural gradient in I (P) and Na/Ca exchange (NCX). Myocytes were isolated from canine left ventricle. Whole-cell patch clamp was used to measure current generated by NCX (I (NCX)) and inward background calcium current (I (ibCa)), defined as inward current through Ca(2+) channels less outward current through Ca(2+)-ATPase. When resting myocytes from endocardium (Endo), midmyocardium (Mid) or epicardium (Epi) were studied in the same conditions, I (NCX) was the same and I (ibCa) was zero. Moreover, Western blots were consistent with NCX protein being uniform across the wall. However, the gradient in [Na(+)]( i ), with I (ibCa) = 0, should create a gradient in [Ca(2+)]( i ). To test this hypothesis, we measured resting [Ca(2+)]( i ) using two methods, based on either transport or the Ca(2+)-sensitive dye Fura2. Both methods demonstrated a significant transmural gradient in resting [Ca(2+)]( i ), with Endo > Mid > Epi. This gradient was eliminated by exposing Epi to sufficient ouabain to partially inhibit Na/K pumps, thus increasing [Na(+)]( i ) to values similar to those in Endo. These data support the existence of a transmural gradient for Ca(2+) removal by NCX. This gradient is not due to differences in expression of NCX; rather, it is generated by a transmural gradient in [Na(+)]( i ), which is due to a transmural gradient in plasma membrane expression of the Na/K pump.

Nonanticoagulant heparin reduces myocyte Na+ and Ca2+ loading during simulated ischemia and decreases reperfusion injury

Heparin desulfated at the 2-O and 3-O positions (ODSH) decreases canine myocardial reperfusion injury. We hypothesized that this occurs from effects on ion channels rather than solely from anti-inflammatory activities, as previously proposed. We studied closed-chest pigs with balloon left anterior descending coronary artery occlusion (75-min) and reperfusion (3-h). ODSH effects on [Na(+)](i) (Na Green) and [Ca(2+)](i) (Fluo-3) were measured by flow cytometry in rabbit ventricular myocytes after 45-min of simulated ischemia [metabolic inhibition with 2 mM cyanide, 0 glucose, 37 degrees C, pacing at 0.5 Hz; i.e., pacing-metabolic inhibition (PMI)]. Na(+)/Ca(2+) exchange (NCX) activity and Na(+) channel function were assessed by voltage clamping. ODSH (15 mg/kg) 5 min before reperfusion significantly decreased myocardial necrosis, but neutrophil influx into reperfused myocardium was not consistently reduced. ODSH (100 microg/ml) reduced [Na(+)](i) and [Ca(2+)](i) during PMI. The NCX inhibitor KB-R7943 (10 microM) or the late Na(+) current (I(Na-L)) inhibitor ranolazine (10 microM) reduced [Ca(2+)](i) during PMI and prevented effects of ODSH on Ca(2+) loading. ODSH also reduced the increase in Na(+) loading in paced myocytes caused by 10 nM sea anemone toxin II, a selective activator of I(Na-L). ODSH directly stimulated NCX and reduced I(Na-L). These results suggest that in the intact heart ODSH reduces Na(+) influx during early reperfusion, when I(Na-L) is activated by a burst of reactive oxygen production. This reduces Na(+) overload and thus Ca(2+) influx via NCX. Stimulation of Ca(2+) extrusion via NCX later after reperfusion may also reduce myocyte Ca(2+) loading and decrease infarct size.

Involvement of Na+/Ca2+ exchanger in migration and contraction of rat cultured tendon fibroblasts

In response to injury and inflammation of tendons, tendon fibroblasts are activated, migrate to the wound, and eventually induce contraction of the extracellular matrices to repair the tissue. Under such conditions, Ca(2+) signalling is involved in motility and contractility of tendon fibroblasts. Using cultured tendon fibroblasts isolated from rat Achilles tendons, we investigated functional expression of Na(+)/Ca(2+) exchangers (NCX). The fluorometric study showed that the intracellular Ca(2+) concentration ([Ca(2+)](i)) was increased by reducing extracellular Na(+) concentration ([Na(+)](o)) in tendon fibroblasts. Selective NCX inhibitors, KB-R7943 and SEA0400, both attenuated [Na(+)](o)-dependent [Ca(2+)](i) elevation and the resting [Ca(2+)](i) in tendon fibroblasts. RT-PCR, Western blots and sequence analyses revealed that NCX1.3 and NCX1.7 were expressed in cultured tendon fibroblasts. NCX2 mRNA was undetected. NCX3 expression was negligibly low. Immunofluorescence microscopy indicated that NCX1 protein localized in the plasma membrane especially at the microspikes of tendon fibroblasts. In the wound-healing scratch assay, the cells migrated toward the space created by a scratch and almost completely filled the space within 48 h. This phenomenon was significantly suppressed by KB-R7943 and SEA0400. Furthermore, the NCX inhibitors abrogated the tendon fibroblast-mediated collagen-matrix contractions. Two types of siRNAs for NCX1 also suppressed the migration and contraction of tendon fibroblasts. We conclude that NCX is expressed and mediates Ca(2+) influx in cultured tendon fibroblasts. Since the pharmacological inhibitors and siRNA for NCX1 suppressed motility and contractility of tendon fibroblasts, NCX may play an important role in the function of tendon fibroblasts in the wound healing.

Regulation of cardiac Na+-Ca2+ exchanger activity by protein kinase phosphorylation--still a paradox?

The cardiac Na+-Ca2+ exchanger (NCX) is an important regulator of intracellular ion homeostasis and cardiac function. Gaining insight into modulation of the NCX is therefore important in order to understand ion handling in the heart under physiological and pathological conditions. Typically, the functional contribution of the NCX is often regarded as "secondary" to the changes in luminal Na+ and Ca2+. Whilst it is well accepted that the NCX can be regulated by various factors, including the concentrations of transported ions, direct receptor-mediated modulation of the cardiac NCX is more controversial. Evidence from several different laboratories supports the notion that the cardiac NCX is a direct target of neurotransmitters and hormones and their downstream signalling pathways; however, the issue remains unresolved due to conflicting data showing a lack of direct modulation. The present review summarizes overall findings regarding the modulation of the cardiac NCX, in particular on molecular mechanisms of direct phosphorylation of NCX by beta-adrenergic/adenylate cyclase/protein kinase A and (for comparative purposes) on endothelin-1/protein kinase C signalling pathways. It also aims to consider whether it is currently possible to reconcile discrepancies between studies in the interpretation of the regulation of the cardiac NCX by agents stimulating the beta-adrenoceptor/PKA pathway.

Acidic preconditioning inhibits Na+/H+ and Na+/Ca2+ exchanger interaction via PKCepsilon in guinea-pig ventricular myocytes

An interaction between the Na(+)/Ca(2+) exchanger (NCX) and the Na(+)/H(+) exchanger (NHE) induces reperfusion injury. We investigated the effect of brief repetitive acidosis as acidic preconditioning on NCX and NHE interaction during recovery from acidosis. NCX current with the reversal potential was measured in guinea-pig ventricular myocytes using the whole-cell voltage clamp. The cells were exposed to 5 min of acidosis preceded by two episodes of brief acidosis as acidic preconditioning. Acidosis inhibited NCX current and upon recovery shifted its reversal potential in the negative direction. The shift was prevented by cariporide, but was augmented by a high concentration of phorbol 13-myristate acetate (PMA). Acidic preconditioning prevented the shift, but not in the presence of a selective PKCepsilon inhibitor. A low concentration of PMA, which activates PKCepsilon selectively, prevented the shift, but together with PKCepsilon inhibitor (epsilonV1-2) restored the shift during recovery. 5-Hydroxydecanoate inhibited the effects of acidic preconditioning and those of both low and high concentrations of PMA. The negative shift of NCX reversal potential during recovery from acidosis may be due to [Na(+)](i) accumulation by the NHE. Acidic preconditioning prevented the shift most likely by activating PKCepsilon, which in turn inhibited the NHE. The NHE-NCX interaction may be one of the important end-effectors of preconditioning.

Acute inhibitory effect of dronedarone, a noniodinated benzofuran analogue of amiodarone, on Na+/Ca2+ exchange current in guinea pig cardiac ventricular myocytes

Using the whole-cell voltage-clamp method, we examined an acute effect of dronedarone, a noniodinated benzofuran analogue of amiodarone, on Na+/Ca2+ exchange current (INCX) in guinea pig cardiac ventricular cells. The INCX was recorded by ramp pulses with a holding potential of -60 mV using a pipette solution containing 226 nM free Ca2+ (20 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid and 10 mM Ca2+) and 20 mM Na+. The external solution contained 140 mM Na+, 1 mM Ca2+, and blockers of other currents and pumps such as Cs+, nifedipine, ryanodine, and ouabain. A selective potent NCX inhibitor, KB-R7943 (100 microM), was used to completely inhibit INCX. Dronedarone inhibited INCX in a concentration-dependent manner. The IC50 values for the outward and inward INCX inhibition were about 33 and 28 microM, respectively, with the Hill coefficient of 1 for both. The inhibitory effect of dronedarone at 50 microM on INCX did not change in the presence of trypsin in the pipette solution. Therefore, dronedarone is classified as a trypsin-insensitive NCX inhibitor and distinct from amiodarone which is a trypsin sensitive. We conclude that dronedarone inhibits INCX but the potency is tenfold less than that of amiodarone. Dronedarone may modestly inhibit INCX in a therapeutic concentration range.

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.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Mechanism of Na+/Ca2+ exchanger activation by hydrogen peroxide in guinea-pig ventricular myocytes

Using the whole-cell voltage clamp, we examined the mechanism of activation of the Na(+)/Ca(2+) exchanger (NCX) by hydrogen peroxide (H(2)O(2)) in isolated guinea-pig cardiac ventricular myocytes. Exposure to H(2)O(2) increased the NCX current. The effect was inhibited by cariporide, an inhibitor of the Na(+)/H(+) exchanger (NHE), suggesting that there are NHE-dependent and -independent pathways in the effect of H(2)O(2) on NCX. In addition, both pathways were blocked by edaravone, a hydroxyl radical (*OH) scavenger; pertussis toxin, a Galpha(i/o) protein inhibitor; and U0126, an inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK). On the other hand, wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, inhibited only the NHE-dependent pathway, while PP2, a Src family protein tyrosine kinase inhibitor, inhibited only the NHE-independent pathway. Taken together, our data suggest that H(2)O(2) increases the NCX current via two signal transduction pathways. The common pathway is the conversion of H(2)O(2) to *OH, which activates Galpha(i/o) protein and a mitogen-activated protein (MAP) kinase signaling pathway. Then, one pathway activates NHE with a PI3K-dependent mechanism and indirectly increases the NCX current. Another pathway involves activation of a Src family tyrosine kinase.

Na:Ca Stoichiometry and Cytosolic Ca-Dependent Activation of NCX in Intact Cardiomyocytes

We are studying both Na:Ca exchange stoichiometry and cytosolic [Ca] ([Ca]i)-dependent regulation of Na-Ca exchange (NCX) in intact rabbit ventricular myocytes. Analysis of NCX fluxes in subcellular systems strongly supports a dominant 3Na:1Ca exchange, and our measurements in intact cells confirm this. However, in intact native cells, local ion gradients and other factors complicate the process of inferring stoichiometry. From a functional viewpoint, NCX stoichiometry is near 3:1 but is affected by ion accumulation/depletion as well as non-NCX fluxes. We and others have viewed [Ca]i-dependent NCX regulation as a static process (dependent on instantaneous local [Ca]i). However, evidence from subcellular and expression systems shows the process to be dynamic, and our observations confirm this to be the case in intact cardiac cells as well.

Tachyphylaxis to the inhibitory effect of L-type channel blockers on ACh-induced [Ca2+]i oscillations in porcine tracheal myocytes

Discrepancies about the role of L-type voltage-gated calcium channels (VGCC) in acetylcholine (ACh)-induced [Ca(2+)](i) oscillations in tracheal smooth muscle cells (TSMCs) have been seen in recent reports. We demonstrate here that ACh-induced [Ca(2+)](i) oscillations in TMCS were reversibly inhibited by three VGCC blockers, nicardipine, nifedipine and verapamil. Prolonged (several minutes) application of VGCC blockers, led to tachyphylaxis; that is, [Ca(2+)](i) oscillations resumed, but at a lower frequency. Brief (15-30 s) removal of VGCC blockers re-sensitized [Ca(2+)](i) oscillations to inhibition by the agents. Calcium oscillations tolerant to VGCC blockers were abolished by KB-R7943, an inhibitor of the reverse mode of Na(+)/Ca(2+) exchanger (NCX). KB-R7943 alone also abolished ACh-induced [Ca(2+)](i) oscillations. Enhancement of the reverse mode of NCX via removing extracellular Na(+) reversed inhibition of ACh-induced [Ca(2+)](i) oscillations by VGCC blockers. Inhibition of non-selective cation channels using Gd(3+) slightly reduced the frequency of ACh-induced [Ca(2+)](i) oscillations, but did not prevent the occurrence of tachyphylaxis. Altogether, these results suggest that VGCC and the reverse mode of NCX are two primary Ca(2+) entry pathways for maintaining ACh-induced [Ca(2+)](i) oscillations in TSMCs. The two pathways complement each other, and may account for tachyphylaxis of ACh-induced [Ca(2+)](i) oscillations to VGCC blockers.

Comprehensive Analyses of Arrhythmogenic Substrates and Vulnerability to Ventricular Tachycardia in Left Ventricular Hypertrophy in Salt-Sensitive Hypertensive Rats

BACKGROUND

The contribution of conduction disturbances to susceptibility to ventricular tachycardia (VT) has not been directly examined in the process of left ventricular hypertrophy (LVH).

METHODS AND RESULTS

Dahl salt-sensitive (S) and -resistant rats were raised on a high-salt diet. After echocardiography, a fractionation study was conducted using wavelet transform analysis, which reflects conduction disturbances, recording of monophasic action potential (MAP), measurement of conduction velocity, and programmed extrastimuli for the induction of VT, as well as patch-clamp analysis at 10, 13 and 16 weeks (W) (n=7, each). Systolic blood pressure, wall thickness and MAP duration significantly increased at 10W in S rats, whereas conduction velocity decreased and MAP duration and the power of wavelet transform increased at 13W and 16W. Paired extrastimuli induced polymorphic VT only at 13W and 16W in S rats. VT frequency correlated inversely with conduction velocity and positively with MAP duration. Power of wavelet transform correlated negatively with conduction velocity and positively with VT vulnerability. The patch-clamp study revealed that the action potential duration significantly increased in S rats with aging, and correlated with MAP duration.

CONCLUSIONS

Latently progressing slow and inhomogeneous conduction, as well as a repolarization abnormality, may contribute to VT vulnerability in hypertensive LVH.

Topics on the Na+/Ca2+ exchanger: role of vascular NCX1 in salt-dependent hypertension

Excess salt intake is a major risk factor for hypertension. However, the molecular mechanisms underlying salt-dependent hypertension remain obscure. Our recent studies using selective Na(+)/Ca(2+) exchange inhibitors and genetically engineered mice provide compelling evidence that salt-dependent hypertension is triggered by Ca(2+) entry through Na(+)/Ca(2+) exchanger type 1 (NCX1) in arterial smooth muscle. Endogenous cardiac glycosides, which may contribute to salt-dependent hypertension, seem to be necessary for NCX1-mediated hypertension. Intriguingly, recent studies by Dostanic-Larson et al. using knock-in mice with modified cardiac glycoside binding affinity of Na(+),K(+)-ATPases demonstrate that this binding site plays an important physiological role in blood pressure control. Thus, when cardiac glycosides inhibit Na(+),K(+)-ATPase in arterial smooth muscle cells, the elevation of local Na(+) on the submembrane area is believed to facilitate Ca(2+) entry through NCX1, resulting in vasoconstriction. This proposed pathway may have enabled us to explain how to link dietary salt to hypertension.

SEA0400: A Novel Sodium-Calcium Exchange Inhibitor with Cardioprotective Properties1

The cardiac sodium-calcium exchanger (NCX) plays an important role in calcium homeostasis. It is the primary mechanism for removing calcium ions that enter myocytes through L-type calcium channels on a beat-to-beat basis. Its direction of transport is determined by the membrane potential and the ionic concentrations of Na+ and Ca2+, with the forward (or Ca2+-efflux) mode of transport being the dominant mode under physiological conditions. In contrast, the Ca2+-influx mode (or reverse mode) of NCX becomes important in certain pathophysiological conditions, such as myocardial ischemia and reperfusion. Recent discovery of compounds that inhibit the Ca2+-influx mode (or reverse mode) of NCX has generated intense research interest in the pharmacology of NCX. Among the newer NCX inhibitors described to date, 2-[4-[(2,5-difluorophenyl)methoxy]-phenoxy]-5-ethoxyaniline (SEA0400) appears particularly promising in attenuating cardiac, renal, and cerebral ischemia/reperfusion injuries in various experimental models. Moreover, the mixed results that have emerged from clinical trials evaluating the efficacy and safety of inhibitors of the sodium-hydrogen exchanger (an upstream target in relation to the sodium-calcium exchanger) in myocardial protection stimulated interest in evaluating NCX as an alternative therapeutic target. This article reviews the pharmacological profile of SEA0400, as presented in the published literature, and discusses the therapeutic potential of this compound in attenuating myocardial ischemia/reperfusion injury.

Extracellular lactate as a dynamic vasoactive signal in the rat retinal microvasculature

We tested the hypothesis that extracellular lactate regulates the function of pericyte-containing retinal microvessels. Although abluminally positioned pericytes appear to adjust capillary perfusion by contracting and relaxing, knowledge of the molecular signals that regulate the contractility of these mural cells is limited. Here, we focused on lactate because this metabolic product is in the retinal extracellular space under both physiological and pathophysiological conditions. In microvessels freshly isolated from the adult rat retina, we used perforated-patch pipettes to monitor ionic currents, fura-2 to measure calcium levels, and time-lapse photography to visualize changes in mural cell contractility and lumen diameter. During lactate exposure, pericyte calcium rose; these cells contracted, and lumens constricted. This contractile response appears to involve a cascade of events resulting in the inhibition of Na+/Ca2+exchangers (NCXs), the decreased of which function causes pericyte calcium to increase and contraction to be triggered. On the basis of our observation that gap junction uncouplers minimized the lactate-induced rise in pericyte calcium, we propose that the NCXs inhibited by lactate are predominately located in the endothelium. Indicative of the importance of endothelial/pericyte gap junctions, uncouplers of these junctions switched the pericyte response to lactate from contraction to relaxation. In addition, we observed that hypoxia, which closes microvascular gap junctions, also switched lactate’s effect from vasocontraction to vasorelaxation. Thus the response of pericyte-containing retinal microvessels to extracellular lactate is metabolically modulated. The ability of lactate to serve as a vasoconstrictor when energy supplies are ample and a vasodilator under hypoxic conditions may be an efficient mechanism to link capillary function with local metabolic need.

Topics on the Na+/Ca2+ Exchanger: Pharmacological Characterization of Na+/Ca2+ Exchanger Inhibitors

Using the whole-cell voltage clamp, we examined acute effects of various agents on Na(+)/Ca(2+) exchange current (I(NCX)) in guinea-pig cardiac ventricular cells and transfected cells. Among the antiarrhythmic drugs, amiodarone, bepridil, dronedarone, cibenzoline, azimilide, and aprindine inhibited I(NCX) in a concentration-dependent manner. We also investigated the effects on NCX of 2,3-buanedione monoxim (BDM) and selective NCX inhibitors such as KB-R7943, SEA0400, and SN-6. The presence of trypsin in the pipette solution attenuated the inhibitory effects on NCX of amiodarone, bepridil, and BDM, suggesting that these drugs inhibit NCX from the cytosolic side. In contrast, the trypsin-insensitive NCX inhibitors were aprindine, azimilide, dronedarone, cibenzoline, KB-R7943, SEA0400, and SN-6. KB-R7943, SEA0400, and SN-6 suppressed the uni-directional outward I(NCX) more potently than the uni-directional inward I(NCX). The mechanism of this mode-dependency is unknown, but is suggested to be related to intracellular Na(+) concentration.

Electrophysiological demonstration of Na+/Ca2+ exchange in bovine articular chondrocytes

Altered fluxes of Ca2+ across the chondrocyte membrane have been proposed as one pathway by which mechanical load can modulate cartilage turnover. In many cells, Na+/Ca2+ exchange (NCX) plays a key role in Ca2+ homeostasis, and recent studies have suggested it is operative in articular chondrocytes. In this study, an electrophysiological characterisation of NCX in articular bovine chondrocytes has been performed, using the whole-cell patch clamp technique, and the effects of inhibitors and the transmembrane electrochemical gradients of Na+ and Ca2+ on NCX function have been assessed. A Ni2+-sensitive current (I(NCX)) which exhibited outward rectification, was elicited by a voltage ramp protocol. The current was also attenuated by the NCX inhibitors benzamil and KBR7943, without significant differences between the effect of these two compounds upon outward and inward currents. The Ni2+-sensitive current was modulated by changes in extracellular and pipette Na+ and Ca2+ in a manner characteristic of I(NCX). Measured values for the reversal potential differed significantly from those predicted for an exchanger stoichiometry of 3Na+ : 1Ca2+, implying that accumulation of intracellular Ca2+ (from influx or release from stores) or more than one transport mode is occurring. These results demonstrate the operation of NCX in articular chondrocytes and suggest that changes in its turnover rate, as might occur in response to mechanical load, may modify cell composition and thereby dictate cartilage turnover.

Rapid turnover of the "functional" Na(+)-Ca2+ exchanger in cardiac myocytes revealed by an antisense oligodeoxynucleotide approach

Antisense oligodeoxynucleotides (AS-ODNs) were used in combination with transient functional expression of the cardiac Na(+)-Ca2+ exchanger (NCX1) to correlate suppression of the Na(+)-Ca2+ exchange function with down-regulation of NCX1 protein expression. In a de-novo expression system (Sf9 cells), a decrease in both, NCX1 mRNA and protein after AS-ODN application was paralleled by diminished NCX1 activity, a typical hallmark of a true "antisense effect". Although AS-ODN uptake was also efficient in rat neonatal cardiac myocytes, in whole-cell extracts of these cells treated with AS-ODNs, the amount of NCX1 protein determined in a quantitative binding assay remained almost unchanged, despite a prompt loss of NCX1 function. Immunocytochemical staining of myocytes revealed that most of the immunoreactivity was not localized in the plasma membrane, but in intracellular compartments and was barely affected by AS-ODN treatment. These results indicate that the "functional half-life" of the NCX1 protein in the plasma membrane of neonatal cardiac myocytes is surprisingly short, much shorter than reported half-lifes of about 30 h for other membrane proteins.

Identification of an endogenous inhibitor of the cardiac Na+/Ca2+ exchanger, phospholemman

Rapid and precise control of Na(+)/Ca(2+) exchanger (NCX1) activity is essential in the maintenance of beat-to-beat Ca(2+) homeostasis in cardiac myocytes. Here, we show that phospholemman (PLM), a 15-kDa integral sarcolemmal phosphoprotein, is a novel endogenous protein inhibitor of cardiac NCX1. Using a heterologous expression system that is devoid of both endogenous PLM and NCX1, we first demonstrated by confocal immunofluorescence studies that both exogenous PLM and NCX1 co-localized at the plasma membrane. Reciprocal co-immunoprecipitation studies revealed specific protein-protein interaction between PLM and NCX1. The functional consequences of direct association of PLM with NCX1 was the inhibition of NCX1 activity, as demonstrated by whole-cell patch clamp studies to measure NCX1 current density and radiotracer flux assays to assess Na(+)-dependent (45)Ca(2+) uptake. Inhibition of NCX1 by PLM was specific, because a single mutation of serine 68 to alanine in PLM resulted in a complete loss of inhibition of NCX1 current, although association of the PLM mutant with NCX1 was unaltered. In native adult cardiac myocytes, PLM co-immunoprecipitated with NCX1. We conclude that PLM, a member of the FXYD family of small ion transport regulators known to modulate Na(+)-K(+)-ATPase, also regulates Na(+)/Ca(2+) exchange in the heart.

Calcium extrusion protein expression in the hippocampal formation of chronic epileptic rats after kainate-induced status epilepticus

PURPOSE

The plasma membrane Ca2+ -adenosine triphosphatase (ATPase) (PMCA) and (potassium-dependent) sodium-calcium exchange [NC(K)X] represent two main calcium-extrusion mechanisms that are important for the restoration of [Ca2+]i levels after electrical activity. We investigated whether the expression of these calcium-extrusion proteins is altered in the course of epileptogenesis.

METHODS

Hippocampal-parahippocampal protein expression of NCX1, 2, and 3, PMCA1-4, and NCKX2 at an early and late stage after kainate-induced status epilepticus (SE) was compared with that in control rats by using immunocytochemistry.

RESULTS

Several alterations were found in chronic epileptic rats: (a) NCX1 expression was permanently decreased in the inner molecular layer (IML) of the dentate gyrus (DG) and entorhinal cortex layer III (ECIII), related to neuronal loss in hilus and ECIII, respectively; (b) PMCA and NCKX2 expression was transiently upregulated in the IML, and decreased in several areas where cell loss had occurred, (c) NCX3 expression, which in control rats is abundant in presynaptic terminals of mossy fibers (MF), was extensively and permanently decreased in stratum lucidum and hilar region. In addition, newly formed MF sprouts that project to the DG iml did not noticeably express NCX3; (d) NCX2 and NCKX2 were (transiently) upregulated in astrocytes of epileptic rats throughout the hippocampal formation, including ECIII.

CONCLUSIONS

These region-specific changes in calcium-extrusion proteins reflect a change in calcium regulation. Whether these regional-specific changes of calcium-extrusion proteins are associated with an abnormal calcium homeostasis must be determined. Because some alterations of calcium-extrusion protein expression are already present at an early stage of epileptogenesis, they could be involved in this process.

New insights into the molecular and cellular workings of the cardiac Na+/Ca2+exchanger

The cardiac Na+/Ca2+exchanger (NCX1) is almost certainly the major Ca2+extrusion mechanism in cardiac myocytes, although the driving force for Ca2+extrusion is quite small. To explain multiple recent results, it is useful to think of the exchanger as a slow Ca2+buffer that can reverse its function multiple times during the excitation-contraction cycle (ECC). An article by the group of John Reeves brings new insights to this function by analyzing the role of regulatory domains of NCX1 that mediate its activation by a rise of cytoplasmic Ca2+. It was demonstrated that the gating reactions are operative just in the physiological range of Ca2+changes, a few fold above resting Ca2+level, and that they prevent the exchanger from damping out the influence of mechanisms that transiently increase Ca2+levels. Furthermore, exchangers with deleted regulatory domains are shown to reduce resting Ca2+to lower levels than achieved by wild-type exchangers. A study by the group of Kenneth Philipson demonstrated that the NCX1 regulatory domain can bind and respond to Ca2+changes on the time scale of the ECC in rat myocytes. At the same time, studies of transgenic mice and NCX1 knockout mice generated by the Philipson group revealed that large changes of NCX1 activity have rather modest effects on ECC. Simple simulations predict these results very well: murine cardiac ECC is very sensitive to small changes of the Na+gradient, very sensitive to changes of the sarcoplasmic reticulum Ca2+pump activity, and very insensitive to changes of NCX1 activity. It is speculated that the NCX1 gating reactions not only regulate coupled 3Na+:1Ca2+exchange but also control the exchanger’s Na+leak function that generates background Na+influx and depolarizing current in cardiac myocytes.

Forefront of Na+/Ca2+ exchanger studies: stoichiometry of cardiac Na+/Ca2+ exchanger; 3:1 or 4:1?

The stoichiometry of the Na+/Ca2+ exchanger (NCX) had been generally accepted as 3 Na+:1 Ca2+. However, recently a challenging stoichiometry of 4:1 was proposed. Therefore, using guinea pig ventricular cells, we re-examined the stoichiometry by measuring the reversal potential of the NCX current and intracellular Ca2+ concentrations under the whole-cell voltage clamp. We confirmed that the stoichiometry of NCX is 3:1 not 4:1. In addition, we explored the possible reasons for obtaining erroneous results of a 4:1 stoichiometry.

Na+/Ca2+ exchanger overexpression impairs frequency- and ouabain-dependent cell shortening in adult rat cardiomyocytes

The Na(+)/Ca(2+) exchanger (NCX) may influence cardiac function depending on its predominant mode of action, forward mode or reverse mode, during the contraction-relaxation cycle. The intracellular Na(+) concentration ([Na(+)](i)) and the duration of the action potential as well as the level of NCX protein expression regulate the mode of action of NCX. [Na(+)](i) and NCX expression have been reported to be increased in human heart failure. Nevertheless, the consequences of altered NCX expression in heart failure are still a matter of discussion. We aimed to characterize the influence of NCX expression on intracellular Ca(2+) transport in rat cardiomyocytes by adenoviral-mediated gene transfer. A five- to ninefold (dose dependent) overexpression of NCX protein was achieved after 48 h by somatic gene transfer (Ad.NCX.GFP) versus control (Ad.GFP). NCX activity, determined by Na(+) gradient-dependent (45)Ca(2+)-uptake, was significantly increased. The protein expressions of sarco(endo)plasmic reticulum Ca(2+)-ATPase, phospholamban, and calsequestrin were unaffected by NCX overexpression. Fractional shortening (FS) of isolated cardiomyocytes was significantly increased at low stimulation rates in Ad.NCX.GFP. After a step-wise enhancing frequency of stimulation to 3.0 Hz, FS remained unaffected in Ad.GFP cells but declined in Ad.NCX.GFP cells. The positive inotropic effect of the cardiac glycoside ouabain was less effective in Ad.NCX.GFP cells, whereas the positive inotropic effect of beta-adrenergic stimulation remained unchanged. In conclusion, NCX overexpression results in a reduced cell shortening at higher stimulation frequencies as well as after inhibition of sarcolemmal Na(+)-K(+)-ATPase, i.e., in conditions with enhanced [Na(+)](i). At low stimulation rates, increased NCX expression enhances both intracellular systolic Ca(2+) and contraction amplitude.

Multiple transport modes of the cardiac Na+/Ca2+ exchanger

The cardiac Na+/Ca2+ exchanger (NCX1; ref. 2) is a bi-directional Ca2+ transporter that contributes to the electrical activity of the heart. When, and if, Ca2+ is exported or imported depends on the Na+/Ca2+ exchange ratio. Whereas a ratio of 3:1 (Na+:Ca2+) has been indicated by Ca2+ flux equilibrium studies, a ratio closer to 4:1 has been indicated by exchange current reversal potentials. Here we show, using an ion-selective electrode technique to quantify ion fluxes in giant patches, that ion flux ratios are approximately 3.2 for maximal transport in either direction. With Na+ and Ca2+ on both sides of the membrane, net current and Ca2+ flux can reverse at different membrane potentials, and inward current can be generated in the absence of cytoplasmic Ca2+, but not Na+. We propose that NCX1 can transport not only 1 Ca2+ or 3 Na+ ions, but also 1 Ca2+ with 1 Na+ ion at a low rate. Therefore, in addition to the major 3:1 transport mode, import of 1 Na+ with 1 Ca2+ defines a Na+-conducting mode that exports 1 Ca2+, and an electroneutral Ca2+ influx mode that exports 3 Na+. The two minor transport modes can potentially determine resting free Ca2+ and background inward current in heart.

Resting membrane potential regulates Na(+)-Ca2+ exchange-mediated Ca2+ overload during hypoxia-reoxygenation in rat ventricular myocytes

In the heart, reperfusion following an ischaemic episode can result in a marked increase in [Ca2+]i and cause myocyte dysfunction and death. Although the Na(+)-Ca2+ exchanger has been implicated in this response, the ionic mechanisms that are responsible have not been identified. In this study, the hypothesis that the diastolic membrane potential can influence Na(+)-Ca2+ exchange and Ca2+ homeostasis during chemically induced hypoxia-reoxygenation has been tested using right ventricular myocytes isolated from adult rat hearts. Superfusion with selected [K+]o of 0.5, 2.5, 5, 7, 10 and 15 mM yielded the following resting membrane potentials: -27.6+/-1.63 mV, -102.2+/-1.89, -86.5+/-1.03, -80.1+/-1.25, -73.6+/-1.02 and -66.4+/-1.03, respectively. In a second set of experiments myocytes were subjected to chemically induced hypoxia-reoxygenation at these different [K+]o, while [Ca2+]i was monitored using fura-2. These results demonstrated that after chemically induced hypoxia-reoxygenation had caused a marked increase in [Ca2+]i, hyperpolarization of myocytes with 2.5 mM [K+]o significantly reduced [Ca2+]i (7.5+/-0.32 vs. 16.9+/-0.55%); while depolarization (with either 0.5 or 15 mM [K+]o) significantly increased [Ca2+]i (31.8+/-3.21 and 20.8+/-0.36 vs. 16.9+/-0.55%, respectively). As expected, at depolarized membrane potentials myocyte hypercontracture and death increased in parallel with Ca2+ overload. The involvement of the Na(+)-Ca2+ exchanger in Ca2+ homeostasis was evaluated using the Na(+)-Ca2+ exchanger inhibitor KB-R7943. During reoxygenation KB-R7943 (5 microM) almost completely prevented the increase in [Ca2+]i both in control conditions (in 5 mM [K+]o: 2.2+/-0.40 vs. 10.8+/-0.14%) and in depolarized myocytes (in 15 mM [K+]o: -2.1+/-0.51 vs. 11.3+/-0.05%). These findings demonstrate that the resting membrane potential of ventricular myocytes is a critical determinant of [Ca2+]i during hypoxia-reoxygenation. This appears to be due mainly to an effect of diastolic membrane potential on the Na(+)-Ca2+ exchanger, since at depolarized potentials this exchanger mechanism operates in the reverse mode, causing a significant Ca2+ influx.

Cardiac submembrane [Na+] transients sensed by Na+-Ca2+ exchange current

Na+ influx via INa during cardiac action potentials can raise bulk [Na+]i by 10 to 15 micromol/L. However, larger rises in submembrane [Na+] ([Na+]sm) local to Na+-Ca2+ exchangers (NCX) could enhance Ca2+ influx via NCX (and Ca2+-induced Ca2+ release). We tested whether INa could increase [Na+]sm, using NCX current (INCX) as a biosensor in rabbit ventricular myocytes (with [Ca2+]i buffered, [Na+]i=10 mmol/L, and other currents blocked). We measured INCX as early as 5 ms after INa. Prior INa activation did not affect INCX at physiological membrane potentials (Em=-100 to +50 mV), but for Em >+50 mV (where INCX is especially sensitive to [Na+]i), INCX shifted outward. At 5 ms and +100 mV, INa shifted INCX outward by 0.23 A/F (corresponding to Delta[Na+]sm=0.24 mmol/L). The effect of INa dissipated with a time constant of approximately 15 ms. Thus, the impact of INa on NCX is almost undetectable at physiological Em and short lived. This suggests that INa effects on excitation-contraction coupling (via outward INCX) are minimal and limited to early during the action potential. However, local Delta[Na+]sm during INa may be 60 times higher than bulk Delta[Na+]i.