Role of sodium/calcium exchange in the mechanism of myocardial stunning: protective effect of reperfusion with high sodium solution

Myocardial stunning and hibernation revisited

Unlike acute myocardial infarction with reperfusion, in which infarct size is the end point reflecting irreversible injury, myocardial stunning and hibernation result from reversible myocardial ischaemia-reperfusion injury, and contractile dysfunction is the obvious end point. Stunned myocardium is characterized by a disproportionately long-lasting, yet fully reversible, contractile dysfunction that follows brief bouts of myocardial ischaemia. Reperfusion precipitates a burst of reactive oxygen species formation and alterations in excitation-contraction coupling, which interact and cause the contractile dysfunction. Hibernating myocardium is characterized by reduced regional contractile function and blood flow, which both recover after reperfusion or revascularization. Short-term myocardial hibernation is an adaptation of contractile function to the reduced blood flow such that energy and substrate metabolism recover during the ongoing ischaemia. Chronic myocardial hibernation is characterized by severe morphological alterations and altered expression of metabolic and pro-survival proteins. Myocardial stunning is observed clinically and must be recognized but is rarely haemodynamically compromising and does not require treatment. Myocardial hibernation is clinically identified with the use of imaging techniques, and the myocardium recovers after revascularization. Several trials in the past two decades have challenged the superiority of revascularization over medical therapy for symptomatic relief and prognosis in patients with chronic coronary syndromes. A better understanding of the pathophysiology of myocardial stunning and hibernation is important for a more precise indication of revascularization and its consequences. Therefore, this Review summarizes the current knowledge of the pathophysiology of these characteristic reperfusion phenomena and highlights their clinical implications.

Dendritic Cell Amiloride-Sensitive Channels Mediate Sodium-Induced Inflammation and Hypertension

Sodium accumulates in the interstitium and promotes inflammation through poorly defined mechanisms. We describe a pathway by which sodium enters dendritic cells (DCs) through amiloride-sensitive channels including the alpha and gamma subunits of the epithelial sodium channel and the sodium hydrogen exchanger 1. This leads to calcium influx via the sodium calcium exchanger, activation of protein kinase C (PKC), phosphorylation of p47^phox, and association of p47^phox with gp91^phox. The assembled NADPH oxidase produces superoxide with subsequent formation of immunogenic isolevuglandin (IsoLG)-protein adducts. DCs activated by excess sodium produce increased interleukin-1β (IL-1β) and promote T cell production of cytokines IL-17A and interferon gamma (IFN-γ). When adoptively transferred into naive mice, these DCs prime hypertension in response to a sub-pressor dose of angiotensin II. These findings provide a mechanistic link between salt, inflammation, and hypertension involving increased oxidative stress and IsoLG production in DCs.

The evidence and rationale for the perioperative use of loop diuretics during kidney transplantation: A comprehensive review

PURPOSE

Loop diuretics (LD) attenuate ischemic injury in nephrons. They are thought to decrease delayed graft function (DGF) during kidney transplantation (KT). This review aimed to summarize the current evidence for the perioperative use of LD during KT.

METHODS

We performed an analysis of all articles that were published since the inception of Medline and Embase: 26 studies were selected for inclusion. Scope was LD use during the perioperative phase of KT only.

RESULTS

Six animal studies demonstrated mixed results in terms of renal function and survival. Of the 20 studies performed in humans, 4 were randomized clinical trials. The risk of bias was mostly unclear. Evidence supporting the role of LD to increase diuresis was mixed and to prevent DGF was weak. There was poor evidence to support LD use to improve initial and long-term graft function. No data on patient survival could be found. Overall, there was a lack of any robust clinical evidence for LD use perioperatively during KT.

IMPLICATIONS

There is poor evidence to support the perioperative use of LD during KT. Well-designed trials are needed to further explore their safety and efficacy, and we summarize several rationales. Pragmatic rationales include volume management. There is evidence to suggest that LD have a vasodilatory effect, and decrease edema, congestion and oxygen requirements. Lastly, there are several theoretical rationales to explore LD use during KT, in particular, attenuating ischemia-reperfusion injury and modulating autophagy.

Effects of Ranolazine in Patients With Chronic Angina in Patients With and Without Percutaneous Coronary Intervention for Acute Coronary Syndrome: Observations From the MERLIN-TIMI 36 Trial

BACKGROUND

Ranolazine, a piperazine derivative with anti-ischemic effects, reduces the frequency of angina and improves exercise performance in patients with chronic angina. The effects of ranolazine in patients with established ischemic heart disease and chronic angina undergoing percutaneous coronary intervention (PCI) for acute coronary syndromes (ACS) is not well described. We hypothesized that ranolazine would reduce ischemic events, regardless of revascularization.

METHODS

We examined the 1-year incidence of recurrent cardiovascular (CV) events in the subgroup of patients with prior chronic angina (n = 3565) enrolled in the randomized, double-blind, placebo-controlled Metabolic Efficiency with Ranolazine for Less Ischemia in Non-ST-Elevation ACS (MERLIN)-Thrombolysis In Myocardial Infarction (TIMI) 36 trial who did or did not have a PCI within 30 days of the index event.

RESULTS

Ranolazine reduced the risk of recurrent ischemia following admission regardless of whether patients had (hazard ratio [HR], 0.69; 95% confidence interval [CI] 0.51-0.92] or did not have PCI (HR, 0.81; 95% CI, 0.66-0.99; P interaction = 0.39). CV death, myocardial infarction, and recurrent ischemia were similarly lower with ranolazine in the PCI group (HR, 0.71; 95% CI, 0.55-0.91) vs the non-PCI group (HR, 0.91; 95% CI, 0.78-1.06; P interaction = 0.10), with a nominally significant decrease in CV death (HR, 0.39; 95% CI, 0.16-0.93) in the PCI group vs no difference in the non-PCI group (HR, 1.19; 95% CI, 0.89-1.59; P interaction = 0.02).

CONCLUSIONS

In patients with chronic angina, ranolazine reduced recurrent ischemic events, regardless of whether patients did or did not receive PCI within 30 days of a non-ST-segment ACS.

Na(+)-Ca(2+) exchanger targeting miR-132 prevents apoptosis of cardiomyocytes under hypoxic condition by suppressing Ca(2+) overload

During ischemia-reperfusion (IR) injury of the heart, Ca(2+) overload occurs, leading to cardiomyocyte dysfunction and eventual cell death by apoptosis. Since preventing Ca(2+) overload during IR injury has been reported to protect cardiomyocytes, interrupting Ca(2+) signaling cascades leading to Ca(2+) overload may exert protective effect on cardiomyocytes under hypoxic condition. One of the key regulators of the intracellular Ca(2+) level during IR injury is Na(+)-Ca(2+) exchanger 1 (NCX1), whose down-regulation during IR injury conferred protection of heart. In the present study, we examined whether down-regulation of NCX1 using exogenous microRNA ameliorates apoptosis of cardiomyocytes under hypoxic condition. Here, we identified miR-132 as a novel microRNA targeting the NCX1, whose expression increased during hypoxia. Delivery of miR-132 suppressed the increase of intracellular Ca(2+) in cardiomyocytes under hypoxia, and the expressions of apoptotic molecules, such as Bax, cytochrome C, and caspase 3, and the number of apoptotic cells were also decreased by exogenous miR-132 treatment. These results suggest the potential of miR-132 as an effective therapeutic agent against IR damage to heart by preventing Ca(2+) overload during hypoxic condition and warrant further studies to validate its anti-apoptotic effect in vivo.

Inhibition of reverse-mode sodium-calcium exchanger activity and apoptosis by levosimendan in human cardiomyocyte progenitor cell-derived cardiomyocytes after anoxia and reoxygenation

Levosimendan, a known calcium sensitizer with positive inotropic and vasodilating properties, might also be cardioprotective during ischemia-reperfusion (I/R) insult. Its effects on calcium homeostasis and apoptosis in I/R injury remain unclear. Na(+)/Ca(2+) exchanger (NCX) is a critical mediator of calcium homeostasis in cardiomyocytes, with reverse-mode NCX activity responsible for intracellular calcium overload and apoptosis of cardiomyocytes during I/R. We probed effects and underlying mechanisms of levosimendan on apoptosis and NCX activity in cultured human cardiomyocyte progenitor cells (CPC)-derived cardiomyocytes undergoing anoxia-reoxygenation (A/R), simulating I/R in vivo. Administration of levosimendan decreased apoptosis of CPC-derived cardiomyocytes induced by A/R. The increase in reverse-mode NCX activity after A/R was curtailed by levosimendan, and NCX1 was translocated away from the cell membrane. Concomitantly, endoplasmic reticulum (ER) stress response induced by A/R was attenuated in CPC-derived cardiomycytes treated with NCX-targeted siRNA or levosimendan, with no synergistic effect between treatments. Results indicated levosimendan inhibited reverse-mode NCX activity to protect CPC-derived cardiomyocytes from A/R-induced ER stress and cell death.

Regulation of ion gradients across myocardial ischemic border zones: a biophysical modelling analysis

The myocardial ischemic border zone is associated with the initiation and sustenance of arrhythmias. The profile of ionic concentrations across the border zone play a significant role in determining cellular electrophysiology and conductivity, yet their spatial-temporal evolution and regulation are not well understood. To investigate the changes in ion concentrations that regulate cellular electrophysiology, a mathematical model of ion movement in the intra and extracellular space in the presence of ionic, potential and material property heterogeneities was developed. The model simulates the spatial and temporal evolution of concentrations of potassium, sodium, chloride, calcium, hydrogen and bicarbonate ions and carbon dioxide across an ischemic border zone. Ischemia was simulated by sodium-potassium pump inhibition, potassium channel activation and respiratory and metabolic acidosis. The model predicted significant disparities in the width of the border zone for each ionic species, with intracellular sodium and extracellular potassium having discordant gradients, facilitating multiple gradients in cellular properties across the border zone. Extracellular potassium was found to have the largest border zone and this was attributed to the voltage dependence of the potassium channels. The model also predicted the efflux of [Formula: see text] from the ischemic region due to electrogenic drift and diffusion within the intra and extracellular space, respectively, which contributed to [Formula: see text] depletion in the ischemic region.

Creatine kinase overexpression improves ATP kinetics and contractile function in postischemic myocardium

Reduced myofibrillar ATP availability during prolonged myocardial ischemia may limit post-ischemic mechanical function. Because creatine kinase (CK) is the prime energy reserve reaction of the heart and because it has been difficult to augment ATP synthesis during and after ischemia, we used mice that overexpress the myofibrillar isoform of creatine kinase (CKM) in cardiac-specific, conditional fashion to test the hypothesis that CKM overexpression increases ATP delivery in ischemic-reperfused hearts and improves functional recovery. Isolated, retrograde-perfused hearts from control and CKM mice were subjected to 25 min of global, no-flow ischemia and 40 min of reperfusion while cardiac function [rate pressure product (RPP)] was monitored. A combination of (31)P-nuclear magnetic resonance experiments at 11.7T and biochemical assays was used to measure the myocardial rate of ATP synthesis via CK (CK flux) and intracellular pH (pH(i)). Baseline CK flux was severalfold higher in CKM hearts (8.1 ± 1.0 vs. 32.9 ± 3.8, mM/s, control vs. CKM; P < 0.001) with no differences in phosphocreatine concentration [PCr] and RPP. End-ischemic pH(i) was higher in CKM hearts than in control hearts (6.04 ± 0.12 vs. 6.37 ± 0.04, control vs. CKM; P < 0.05) with no differences in [PCr] and [ATP] between the two groups. Post-ischemic PCr (66.2 ± 1.3 vs. 99.1 ± 8.0, %preischemic levels; P < 0.01), CK flux (3.2 ± 0.4 vs. 14.0 ± 1.2 mM/s; P < 0.001) and functional recovery (13.7 ± 3.4 vs. 64.9 ± 13.2%preischemic RPP; P < 0.01) were significantly higher and lactate dehydrogenase release was lower in CKM than in control hearts. Thus augmenting cardiac CKM expression attenuates ischemic acidosis, reduces injury, and improves not only high-energy phosphate content and the rate of CK ATP synthesis in postischemic myocardium but also recovery of contractile function.

Triple threat: the Na+/Ca2+ exchanger in the pathophysiology of cardiac arrhythmia, ischemia and heart failure

The Na(+)/Ca(2+) exchanger (NCX) is the main Ca(2+) extrusion mechanism of the cardiac myocyte and thus is crucial for maintaining Ca(2+) homeostasis. It is involved in the regulation of several parameters of cardiac excitation contraction coupling, such as cytosolic Ca(2+) concentration, repolarization and contractility. Increased NCX activity has been identified as a mechanism promoting heart failure, cardiac ischemia and arrhythmia. Transgenic mice as well as pharmacological interventions have been used to support the idea of using NCX inhibition as a future pharmacological strategy to treat cardiovascular disease.

Recent advances in the management of chronic stable angina II. Anti-ischemic therapy, options for refractory angina, risk factor reduction, and revascularization

The objectives in treating angina are relief of pain and prevention of disease progression through risk reduction. Mechanisms, indications, clinical forms, doses, and side effects of the traditional antianginal agents - nitrates, β-blockers, and calcium channel blockers - are reviewed. A number of patients have contraindications or remain unrelieved from anginal discomfort with these drugs. Among newer alternatives, ranolazine, recently approved in the United States, indirectly prevents the intracellular calcium overload involved in cardiac ischemia and is a welcome addition to available treatments. None, however, are disease-modifying agents. Two options for refractory angina, enhanced external counterpulsation and spinal cord stimulation (SCS), are presented in detail. They are both well-studied and are effective means of treating at least some patients with this perplexing form of angina. Traditional modifiable risk factors for coronary artery disease (CAD) - smoking, hypertension, dyslipidemia, diabetes, and obesity - account for most of the population-attributable risk. Individual therapy of high-risk patients differs from population-wide efforts to prevent risk factors from appearing or reducing their severity, in order to lower the national burden of disease. Current American College of Cardiology/American Heart Association guidelines to lower risk in patients with chronic angina are reviewed. The Clinical Outcomes Utilizing Revascularization and Aggressive Drug Evaluation (COURAGE) trial showed that in patients with stable angina, optimal medical therapy alone and percutaneous coronary intervention (PCI) with medical therapy were equal in preventing myocardial infarction and death. The integration of COURAGE results into current practice is discussed. For patients who are unstable, with very high risk, with left main coronary artery lesions, in whom medical therapy fails, and in those with acute coronary syndromes, PCI is indicated. Asymptomatic patients with CAD and those with stable angina may defer intervention without additional risk to see if they will improve on optimum medical therapy. For many patients, coronary artery bypass surgery offers the best opportunity for relieving angina, reducing the need for additional revascularization procedures and improving survival. Optimal medical therapy, percutaneous coronary intervention, and surgery are not competing therapies, but are complementary and form a continuum, each filling an important evidence-based need in modern comprehensive management.

Regulation of sodium-calcium exchanger activity by creatine kinase under energy-compromised conditions

Na(+)/Ca(2+) exchanger (NCX) is one of the major mechanisms for removing Ca(2+) from the cytosol especially in cardiac myocytes and neurons, where their physiological activities are triggered by an influx of Ca(2+). NCX contains a large intracellular loop (NCXIL) that is responsible for regulating NCX activity. Recent evidence has shown that proteins, including kinases and phosphatases, associate with NCX1IL to form a NCX1 macromolecular complex. To search for the molecules that interact with NCX1IL and regulate NCX1 activity, we used the yeast two-hybrid method to screen a human heart cDNA library and found that the C-terminal region of sarcomeric mitochondrial creatine kinase (sMiCK) interacted with NCX1IL. Moreover, both sMiCK and the muscle-type creatine kinase (CKM) coimmunoprecipitated with NCX1 using lysates of cardiacmyocytes and HEK293T cells that transiently expressed NCX1 and various creatine kinases. Both sMiCK and CKM were able to produce a recovery in the decreased NCX1 activity that was lost under energy-compromised conditions. This regulation is mediated through a putative PKC phosphorylation site of sMiCK and CKM. The autophosphorylation and the catalytic activity of sMiCK and CKM are not required for their regulation of NCX1 activity. Our results suggest a novel mechanism for the regulation of NCX1 activity.

Donor pretreatment with hypertonic saline attenuates primary allograft dysfunction: a pilot study in a porcine model

BACKGROUND

Hypertonic saline (HTS) has been previously demonstrated to have immune modulatory and vascular protective effects. We assessed the effect of donor pretreatment with HTS on allograft preservation in a porcine model of orthotopic heart transplantation.

METHODS AND RESULTS

Orthotopic transplants were performed after 6 hours of cold static allograft storage. Donor pigs were randomly assigned to pretreatment with (n=7) or without (n=6) HTS (4.5 mL/kg of 7.5% NaCl) administered 1 hour before donor heart arrest. Administration of HTS increased serum sodium level from 138+/-2 mmol/L to 154+/-4 mmol/L, which normalized to 144+/-3 mmol/L 1 hour after infusion. Successful weaning from cardiopulmonary bypass was significantly greater in HTS-treated hearts (6/7 vs 1/6; P=0.029). Preload recruitable stroke work after transplantation was improved compared to control (88+/-21% vs 35+/-8% of baseline; P=0.0001). Similarly, end-systolic elastance was improved compared to control (85+/-17% vs 42+/-12% of baseline; P=0.0002). Posttransplantation systolic blood pressure was significantly higher in the donor HTS group (60+/-9 mm Hg vs 35+/-6 mm Hg; P=0.04). Donor HTS treatment improved coronary artery endothelial-dependent vasorelaxation compared with control (Emax: HTS, 59+/-4%; control, 47+/-3%; P=0.04). HTS also resulted in improved endothelial-independent vasorelaxation compared with control (Emax: HTS, 71+/-3%; control, 59+/-4%; P=0.03; ED-50: HTS, 0.56x10 to 6+/-0.23 mol/L; control, 2.5x10 to 6+/-1.0 mol/L; P=0.04). Sensitivity to endothelin-1-induced vasospasm was reduced with HTS pretreatment (% maximum contraction [Cmax]: HTS, 338+/-15%; control, 419+/-40%; P=0.01).

CONCLUSIONS

Donor HTS pretreatment attenuates posttransplantation cardiac allograft myocardial dysfunction, improves posttransplantation systemic hemodynamic function, and preserves posttransplantation cardiac allograft vascular function. HTS may be a novel organ donor intervention to prevent primary graft dysfunction.

Assessing manganese efflux using SEA0400 and cardiac T1-mapping manganese-enhanced MRI in a murine model

The sodium-calcium exchanger (NCX) is one of the transporters contributing to the control of intracellular calcium (Ca(2+)) concentration by normally mediating net Ca(2+) efflux. However, the reverse mode of the NCX can cause intracellular Ca(2+) concentration overload, which exacerbates the myocardial tissue injury resulting from ischemia. Although the NCX inhibitor SEA0400 has been shown to therapeutically reduce myocardial injury, no in vivo technique exists to monitor intracellular Ca(2+) fluctuations produced by this drug. Cardiac manganese-enhanced MRI (MEMRI) may indirectly assess Ca(2+) efflux by estimating changes in manganese (Mn(2+)) content in vivo, since Mn(2+) has been suggested as a surrogate marker for Ca(2+). This study used the MEMRI technique to examine the temporal features of cardiac Mn(2+) efflux by implementing a T(1)-mapping method and inhibiting the NCX with SEA0400. The change in (1)H(2)O longitudinal relaxation rate, Delta R(1), in the left ventricular free wall, was calculated at different time points following infusion of 190 nmol/g manganese chloride (MnCl(2)) in healthy adult male mice. The results showed 50% MEMRI signal attenuation at 3.4 +/- 0.6 h post-MnCl(2) infusion without drug intervention. Furthermore, treatment with 50 +/- 0.2 mg/kg of SEA0400 significantly reduced the rate of decrease in Delta R(1). At 4.9-5.9 h post-MnCl(2) infusion, the average Delta R(1) values for the two groups treated with SEA0400 were 2.46 +/- 0.29 and 1.72 +/- 0.24 s(-1) for 50 and 20 mg/kg doses, respectively, as compared to the value of 1.27 +/- 0.28 s(-1) for the control group. When this in vivo data were compared to ex vivo absolute manganese content data, the MEMRI T(1)-mapping technique was shown to effectively quantify Mn(2+) efflux rates in the myocardium. Therefore, combining an NCX inhibitor with MEMRI may be a useful technique for assessing Mn(2+) transport mechanisms and rates in vivo, which may reflect changes in Ca(2+) transport.

Ischemia/reperfusion injury in kidney transplantation: mechanisms and prevention

Ischemia has been an inevitable event accompanying kidney transplantation. Ischemic changes start with brain death, which is associated with severe hemodynamic disturbances: increasing intracranial pressure results in bradycardia and decreased cardiac output; the Cushing reflex causes tachycardia and increased blood pressure; and after a short period of stabilization, systemic vascular resistance declines with hypotension leading to cardiac arrest. Free radical-mediated injury releases proinflammatory cytokines and activates innate immunity. It has been suggested that all of these changes-the early innate response and the ischemic tissue damage-play roles in the development of adaptive responses, which in turn may lead to an acute font of kidney rejection. Hypothermic kidney storage of various durations before transplantation add to ischemic tissue damage. The final stage of ischemic injury occurs during reperfusion. Reperfusion injury, the effector phase of ischemic injury, develops hours or days after the initial insult. Repair and regeneration processes occur together with cellular apoptosis, autophagy, and necrosis; the fate of the organ depends on whether cell death or regeneration prevails. The whole process has been described as the ischemia-reperfusion (I-R) injury. It has a profound influence on not only the early but also the late function of a transplanted kidney. Prevention of I-R injury should be started before organ recovery by donor pretreatment. The organ shortage has become one of the most important factors limiting extension of deceased donor kidney transplantation worldwide. It has caused increasing use of suboptimal deceased donors (high risk, extended criteria [ECD], marginal donors) and uncontrolled non-heart-beating (NHBD) donors. Kidneys from such donors are exposed to much greater ischemic damage before recovery and show reduced chances for proper early as well as long-term function. Storage of kidneys, especially those recovered from ECD (or NHBD) donors, should use machine perfusion.

Diastolic Ca2+ overload caused by Na+/Ca2+ exchanger during the first minutes of reperfusion results in continued myocardial stunning

The pathogenesis of myocardial stunning caused by brief ischemia and reperfusion remains unclear. The aim of the present study was to investigate the underlying mechanism of myocardial stunning. An isolated cell model of myocardial stunning was firstly established in isolated rat ventricular myocytes exposed to 8 min of simulated ischemia and 30 min of reperfusion, the cardiomyocyte contractile function was used to evaluate myocardial stunning. A diastolic Ca(2+) overload without significant changes in systolic Ca(2+) and the amplitude of Ca(2+) transient during the first 10 min of reperfusion played an important role in the occurrence of myocardial stunning. Decreasing Ca(2+) entry into myocardial cells with low Ca(2+) reperfusion was a very efficient way to prevent myocardial stunning. Diastolic Ca(2+) overload was closely related to the reverse mode of Na(+)/Ca(2+) exchanger (NCX) rather than L-type Ca(2+) channel. The activity of the reverse mode of NCX was found significantly higher at the initial time of reperfusion, and KB-R7943, a selective inhibitor of the reverse mode of NCX, administered at first 10 min of reperfusion rather than at the time of ischemia significantly attenuated myocardial stunning. In addition, NCX inhibition also attenuated the Ca(2+) oscillation and cardiac dysfunction when field stimulus was stopped at first 10 min of reperfusion. These data suggest that one of the important mechanisms of triggering myocardial stunning is diastolic Ca(2+) overload caused by activation of the reverse mode of NCX of cardiomyocytes during the initial period of reperfusion following brief ischemia.

Overexpression of the Na+/H+exchanger and ischemia-reperfusion injury in the myocardium

In the myocardium, the Na+/H+exchanger isoform-1 (NHE1) activity is detrimental during ischemia-reperfusion (I/R) injury, causing increased intracellular Na+(Nai+) accumulation that results in subsequent Ca2+overload. We tested the hypothesis that increased expression of NHE1 would accentuate myocardial I/R injury. Transgenic mice were created that increased the Na+/H+exchanger activity specifically in the myocardium. Intact hearts from transgenic mice at 10–15 wk of age showed no change in heart performance, resting intracellular pH (pHi) or phosphocreatine/ATP levels. Transgenic and wild-type (WT) hearts were subjected to 20 min of ischemia followed by 40 min of reperfusion. Surprisingly, the percent recovery of rate-pressure product (%RPP) after I/R improved in NHE1-overexpressing hearts (64 ± 5% vs. 41 ± 5% in WT; P < 0.05). In addition, NMR spectroscopy revealed that NHE1 overexpressor hearts contained higher ATP during early reperfusion (levels P < 0.05), and there was no difference in Na+accumulation during I/R between transgenic and WT hearts. HOE642 (cariporide), an NHE1 inhibitor, equivalently protected both WT and NHE1-overexpressing hearts. When hearts were perfused with bicarbonate-free HEPES buffer to eliminate the contribution of HCO3−transporters to pHiregulation, there was no difference in contractile recovery after reperfusion between controls and transgenics, but NHE1-overexpressing hearts showed a greater decrease in ATP during ischemia. These results indicate that the basal activity of NHE1 is not rate limiting in causing damage during I/R, therefore, increasing the level of NHE1 does not enhance injury and can have some small protective effects.

Muscarinic modulation of the sodium-calcium exchanger in heart failure

BACKGROUND

The Na-Ca exchanger (NCX) is a critical calcium efflux pathway in excitable cells, but little is known regarding its autonomic regulation.

METHODS AND RESULTS

We investigated beta-adrenergic receptor and muscarinic receptor regulation of the cardiac NCX in control and heart failure (HF) conditions in atrially paced pigs. NCX current in myocytes from control swine hearts was significantly increased by isoproterenol, and this response was reversed by concurrent muscarinic receptor stimulation with the addition of carbachol, demonstrating "accentuated antagonism." Okadaic acid eliminated the inhibitory effect of carbachol on isoproterenol-stimulated NCX current, indicating that muscarinic receptor regulation operates via protein phosphatase-induced dephosphorylation. However, in myocytes from atrially paced tachycardia-induced HF pigs, the NCX current was significantly larger at baseline but less responsive to isoproterenol compared with controls, whereas carbachol failed to inhibit isoproterenol-stimulated NCX current, and 8-Br-cGMP did not restore muscarinic responsiveness. Protein phosphatase type 1 dialysis significantly reduced NCX current in failing but not control cells, consistent with NCX hyperphosphorylation in HF. Protein phosphatase type 1 levels associated with NCX were significantly depressed in HF pigs compared with control, and total phosphatase activity associated with NCX was significantly decreased.

CONCLUSIONS

We conclude that the NCX is autonomically modulated, but HF reduces the level and activity of associated phosphatases; defective dephosphorylation then "locks" the exchanger in a highly active state.

Cardiac-Specific Ablation of the Na+-Ca2+Exchanger Confers Protection Against Ischemia/Reperfusion Injury

During ischemia and reperfusion, with an increase in intracellular Na+ and a depolarized membrane potential, Ca2+ may enter the myocyte in exchange for intracellular Na+ via reverse-mode Na+-Ca2+ exchange (NCX). To test the role of Ca2+ entry via NCX during ischemia and reperfusion, we studied mice with cardiac-specific ablation of NCX (NCX-KO) and demonstrated that reverse-mode Ca2+ influx is absent in the NCX-KO myocytes. Langendorff perfused hearts were subjected to 20 minutes of global ischemia followed by 2 hours of reperfusion, during which time we monitored high-energy phosphates using 31P-NMR and left-ventricular developed pressure. In another group of hearts, we monitored intracellular Na+ using 23Na-NMR. Consistent with Ca2+ entry via NCX during ischemia, we found that hearts lacking NCX exhibited less of a decline in ATP during ischemia, delayed ischemic contracture, and reduced maximum contracture. Furthermore, on reperfusion following ischemia, NCX-KO hearts had much less necrosis, better recovery of left-ventricular developed pressure, improved phosphocreatine recovery, and reduced Na+ overload. The improved recovery of function following ischemia in NCX-KO hearts was not attributable to the reduced preischemic contractility in NCX-KO hearts, because when the preischemic workload was matched by treatment with isoproterenol, NCX-KO hearts still exhibited improved postischemic function compared with wild-type hearts. Thus, NCX-KO hearts were significantly protected against ischemia-reperfusion injury, suggesting that Ca2+ entry via reverse-mode NCX is a major cause of ischemia/reperfusion injury.

cDNA cloning and expression of the cardiac Na+/Ca2+ exchanger from Mozambique tilapia (Oreochromis mossambicus) reveal a teleost membrane transporter with mammalian temperature dependence

The complete cDNA sequence of the tilapia cardiac Na(+)/Ca2+ exchanger (NCX-TL1.0) was determined. The 3.1-kb transcript encodes a protein 957 amino acids in length, with a predicted signal peptide cleaved at residue 31 and two potential N-glycosylation sites in the extracellular N terminus. Hydropathy analysis and sequence comparison predicted a mature protein with nine transmembrane-spanning segments, consistent with the structural topologies of other known mammalian and teleost NCX isoforms. Overall sequence comparison shows high identity to both trout NCX-TR1.0 ( approximately 81%) and mammalian NCX1.1 ( approximately 73%), and phylogenetic analyses confirmed its identity as a member of the NCX1 gene family, expressing exons A, C, D, and F in the alternative splice site. Sequence identity is even higher in the alpha-repeats, the exchanger inhibitory peptide (XIP) site, and Ca(2+)-binding domains, which is reflected in the functional and regulatory properties of tilapia NCX-TL1.0. When NCX-TL1.0 was expressed in Xenopus oocytes and the currents were measured in giant excised patches, they displayed both positive regulation by Ca2+ and Na(+)-dependent inactivation in a manner similar to trout NCX-TR1.0. However, tilapia NCX-TL1.0 exhibited a relatively high sensitivity to temperature compared with trout NCX-TR1.0. Whereas trout NCX-TR1.0 currents displayed activation energies of approximately 7 kJ/mol, tilapia NCX-TL1.0 currents showed mammal-like temperature dependence, with peak and steady-state current activation energies of 53 +/- 9 and 67 +/- 21 kJ/mol, respectively. Using comparative sequence analysis, we highlighted 10 residue positions in the N-terminal domain of the NCX that, in combination, may confer exchanger temperature dependence through subtle changes in protein flexibility. Tilapia NCX-TL1.0 represents the first non-mammalian NCX to exhibit a mammalian temperature dependence phenotype and will prove to be a useful model in defining the interplay between molecular flexibility and stability in NCX function.

Genetic manipulation of cardiac Na+/Ca2+ exchange expression

The Na+/Ca2+ exchanger (NCX) is the primary Ca2+ extrusion mechanism in cardiomyocytes. To further investigate the role of NCX in excitation-contraction coupling and Ca2+ homeostasis, we created murine models with altered expression levels of NCX. Homozygous overexpression of NCX resulted in mild cardiac hypertrophy. Decline of the Ca2+ transient and relaxation of contraction were increased and the reverse mode of NCX was augmented. Overexpression also led to a higher susceptibility to ischemia-reperfusion injury and to a greater ability of NCX to trigger Ca2+-induced Ca2+ release. Furthermore, an increase in peak L-type Ca2+ current was observed suggesting a direct influence of NCX on L-type Ca2+ current. Whereas global knockout of NCX led to prenatal death, a recently generated cardiac-specific NCX knockout mouse was viable with surprisingly normal contractile properties. Expression levels of other Ca2+-handling proteins were not altered. Ca2+ influx in these animals is limited by a decrease of peak L-type Ca2+ current. An alternative Ca2+ efflux mechanism, presumably the plasma membrane Ca2+-ATPase, is sufficient to maintain Ca2+-homeostasis in the NCX knockout mice.

Inhibitory profile of SEA0400 [2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline] assessed on the cardiac Na+-Ca2+ exchanger, NCX1.1

SEA0400 (2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline) has recently been described as a potent and selective inhibitor of Na(+)-Ca(2+) exchange in cardiac, neuronal, and renal preparations. The inhibitory effects of SEA0400 were investigated on the cloned cardiac Na(+)-Ca(2+) exchanger, NCX1.1, expressed in Xenopus laevis oocytes to gain insight into its inhibitory mechanism. Na(+)-Ca(2+) exchange currents were measured using the giant excised patch technique using conditions to evaluate both inward and outward currents. SEA0400 inhibited outward Na(+)-Ca(2+) exchange currents with high affinity (IC(50) = 78 +/- 15 and 23 +/- 4 nM for peak and steady-state currents, respectively). Considerably less inhibitory potency (i.e., micromolar) was observed for inward currents. The inhibitory profile was reexamined after proteolytic treatment of excised patches with alpha-chymotrypsin, a procedure that eliminates ionic regulatory mechanisms. After this treatment, an IC(50) value of 1.2 +/- 0.6 microM was estimated for outward currents, whereas inward currents became almost insensitive to SEA0400. The inhibitory effects of SEA0400 on outward exchange currents were evident at both high and low concentrations of regulatory Ca(2+), although distinct features were noted. SEA0400 accelerated the inactivation rate of outward currents. Based on paired pulse experiments, SEA0400 altered the recovery of exchangers from the Na(+)(i)-dependent inactive state, particularly at higher regulatory Ca(2+)(i) concentrations. Finally, the inhibitory potency of SEA0400 was strongly dependent on the intracellular Na(+) concentration. Our data confirm that SEA0400 is the most potent inhibitor of the cardiac Na(+)-Ca(2+) exchanger described to date and provide a reasonable explanation for its apparent transport mode selectivity.

Stunning and myocardial contractile autoregulation studied on the isolated isovolumic blood-perfused dog heart

AIM

To study, for the first time, the effects of stunning on homeometric and heterometric autoregulation.

METHODS AND RESULTS

Ischaemia (15 min)/reperfusion (30 min) was induced in the isovolumic blood-perfused dog heart preparation. Heart rate elevations (n = 9) from 60 to 200 beats min-1, in steps of 20 beats min-1, promoted the same inotropic stimulation in control (C) and stunning (S), indicating that ischaemia/reperfusion does not affect the changes in calcium kinetics elicited by the Bowditch effect. Sudden ventricular dilation (VD) (n = 10) evoked an instantaneous increase in developed pressure (Delta1DP) followed by a continuous slow performance increase (Delta2DP) in C and S. Delta1DP (C: 35 +/- 2.2 mmHg; S: 27 +/- 2.1 mmHg; P = 0.002) and Delta2DP (C: 20 +/- 1.6 mmHg; S: 14 +/- 1.3 mmHg; P = 0.002) decreased proportionally, while Delta2/Delta1DP (C: 0.57 +/- 0.13; S: 0.58 +/- 0.14) and slow response time course (T/2) were unchanged (C: 55 +/- 6.6 s; S: 57 +/- 7.7 s) after ischaemia/reperfusion. The reduction of Delta1DP can be understood as a decline of the myofilaments calcium responsiveness, the main pathophysiological effect of stunning. The reason for the weakening of Delta2DP, due to intracellular calcium gain, was not determined but it was supposed that its complete manifestation could be restricted by cyclic adenosine monophosphate (cAMP) myocardial content reduction. As reported by others, Delta2DP depends on myocardial cAMP, and it has been shown that myocardial cAMP is decreased after ischaemia/reperfusion.

CONCLUSIONS

Contractile depression due to stunning has no effect on the inotropic stimulation generated by the Bowditch phenomenon. Immediate and time-dependent enhancements of contraction evoked by sudden VD are proportionally reduced and the slow response time course is unaffected in the stunned myocardium.

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.

Effects of brief ischemia and reperfusion on the myocardium and the role of nitric oxide

Brief myocardial ischemia/reperfusion has complex effects on the myocardium. In the short term the myocardium may be stunned with temporarily reduced contractile function, though this may also be accompanied by the modification and de novo synthesis of proteins that protect the heart against subsequent early or delayed insults. Repeated episodes of non-lethal ischemia, which are common in the clinical setting, combine all of these phenomena and may ultimately result in chronic contractile dysfunction. Nitric oxide is intimately linked to many of these alterations in cellular function and defense. This article examines data predominantly from in vivo large animal studies that relate to these ischemia-induced changes, the evidence for the proposed mechanisms behind both myocardial stunning and preconditioning while concentrating on the role of nitric oxide in these conditions.

Role of intracellular Na(+) kinetics in preconditioned rat heart

To elucidate the role of intracellular Na(+) kinetics in the mechanism for ischemic preconditioning (IPC), we measured intracellular Na(+) concentration ([Na(+)](i)) using (23)Na-magnetic resonance spectroscopy in isolated rat hearts. IPC significantly delayed the initial [Na(+)](i) increase (d[Na(+)](i)/dt) compared with non-IPC control, resulting in attenuation of Na(+) accumulation (Delta[Na(+)](i)) during 27 minutes of ischemia with better functional recovery. [Na(+)](i) in IPC, but not in control, recovered to preischemic level during a 6-minute reperfusion. The Na(+)-H(+) exchange inhibitor further suppressed d[Na(+)](i)/dt in both control and IPC hearts with concomitant improvement of functional recovery, suggesting little contribution to the mechanism of IPC. The mitochondrial ATP-sensitive K(+) (mito K(ATP)) channel activator diazoxide (30 micromol/L) completely mimicked both [Na(+)](i) kinetics and functional recovery in IPC without any additive effects to IPC. The mito K(ATP) channel blocker 5-hydroxydecanoic acid (100 micromol/L) lost protective effect as well as the attenuation of d[Na(+)](i)/dt and [Na(+)](i) recovery induced by diazoxide. However, 5-hydroxydecanoic acid also lost IPC-induced protection, but incompletely abolished the alteration of d[Na(+)](i)/dt and the [Na(+)](i) recovery. The Na(+)/K(+)-ATPase inhibitor ouabain (200 micromol/L) did not change d[Na(+)](i)/dt in non-IPC hearts, but it abolished the IPC- or diazoxide-induced reduction of d[Na(+)](i)/dt and the [Na(+)](i) recovery, whereas IPC followed by ouabain treatment showed partial functional recovery with smaller Delta[Na(+)](i) than other ouabain groups. In conclusion, alteration of Na(+) kinetics by preserving Na(+) efflux via Na(+)/K(+)-ATPase mediated by mito K(ATP) channel activation mainly contributes to functional protection in IPC hearts. The contribution of mito K(ATP) channel-independent pathway relating to Na(+) kinetics including reduced Na(+) influx is limited in functional protection of IPC.

Enhanced gene expression of Na(+)/Ca(2+) exchanger attenuates ischemic and hypoxic contractile dysfunction

Enhanced gene expression of the Na(+)/Ca(2+) exchanger in failing hearts may be a compensatory mechanism to promote influx and efflux of Ca(2+), despite impairment of the sarcoplasmic reticulum (SR). To explore this, we monitored intracellular calcium (Ca(i)(2+)) and cardiac function in mouse hearts engineered to overexpress the Na(+)/Ca(2+) exchanger and subjected to ischemia and hypoxia, conditions known to impair SR Ca(i)(2+) transport and contractility. Although baseline Ca(i)(2+) and function were similar between transgenic and wild-type hearts, significant differences were observed during ischemia and hypoxia. During early ischemia, Ca(i)(2+) was preserved in transgenic hearts but significantly altered in wild-type hearts. Transgenic hearts maintained 40% of pressure-generating capacity during early ischemia, whereas wild-type hearts maintained only 25% (P < 0.01). During hypoxia, neither peak nor diastolic Ca(i)(2+) decreased in transgenic hearts. In contrast, both peak and diastolic Ca(i)(2+) decreased significantly in wild-type hearts. The decline of Ca(i)(2+) was abbreviated in hypoxic transgenic hearts but prolonged in wild-type hearts. Peak systolic pressure decreased by nearly 10% in hypoxic transgenic hearts and >25% in wild-type hearts (P < 0.001). These data demonstrate that enhanced gene expression of the Na(+)/Ca(2+) exchanger preserves Ca(i)(2+) homeostasis during ischemia and hypoxia, thereby preserving cardiac function in the acutely failing heart.

A Na+/Ca2+ exchanger inhibitor, KB-R7943, caused negative inotropic responses and negative followed by positive chronotropic responses in isolated, blood-perfused dog heart preparations

Effects of a Na+/Ca2+ exchanger inhibitor, KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl] isothiourea methanesulfonate), on the sinoatrial nodal pacemaker activity, atrial contractility and ventricular contractility were investigated in the isolated and blood-perfused right atrium and left ventricle of the dog. KB-R7943 (0.03- 3 micromol) induced negative inotropic effects and negative followed by positive chronotropic effects in the right atrium and negative inotropic effects in the left ventricle. Neither atropine nor hexamethonium affected the cardiac responses to KB-R7943. Propranolol attenuated the positive chronotropic response to KB-R7943 but imipramine did not. Tetrodotoxin potentiated the positive chronotropic response to KB-R7943 in 6 of 11 isolated atria. When NaCl infusion increased atrial contractile force and atrial rate, KB-R7943-induced negative inotropic and chronotropic responses were attenuated in a dose-dependent manner. CaCl2 infusion potentiated the negative chronotropic response to KB-R7943 but did not affect the inotropic response significantly. On the other hand, ouabain (17 nmol) attenuated the negative inotropic response, but not chronotropic response, to KB-R7943. These results suggest that KB-R7943-induced cardiac effects relate to the Na+ activity, probably mediated through the Na+/Ca2+ exchanger, and the Na+/Ca2+ exchanger modifies the pacemaker activity and myocardial contractility in the dog heart.

Intracellular sodium accumulation during ischemia as the substrate for reperfusion injury

To elucidate the role of intracellular Na+ kinetics during ischemia and reperfusion in postischemic contractile dysfunction, intracellular Na+ concentration ([Na+]i) was measured in isolated perfused rat hearts using 23Na nuclear magnetic resonance spectroscopy. The extension of the ischemic period from 9 minutes to 15, 21, and 27 minutes (at 37 degrees C) increased [Na+]i at the end of ischemia from 270.0+/-10.4% of preischemic level (mean+/-SE, n=5) to 348.4+/-12.0% (n=5), 491.0+/-34.0% (n=7), and 505.3+/-12.1% (n=5), respectively, whereas the recovery of developed pressure worsened with the prolongation of the ischemic period (95.1+/-4.2%, 84.3+/-1. 2%, 52.8+/-13.7%, and 16.9+/-6.4% of preischemic level). The kinetics of [Na+]i recovery during reperfusion was analyzed by the fitting of a monoexponential function. When the hearts were reperfused with low-[Ca]o (0.15 mmol/L) solution, the time constants of the recovery (tau) after 15-minute (8.07+/-0.85 minutes, n=5) and 21-minute ischemia (6.44+/-0.90, n=5) were significantly extended, with better functional recovery (98.5+/-1.4% for 15-minute [P<0.05]; 98.0+/-1.0% for 21-minute [P<0.05]) compared with standard reperfusion ([Ca]o=2.0 mmol/L, tau=3.58+/-0.28 minutes for 15-minute [P<0.0001]; tau=3.02+/-0.20 for 21-minute [P<0.0001]). A selective inhibitor of Na+/Ca2+ exchanger also decelerated the [Na+]i recovery, which suggests that the recovery reflects the Na+/Ca2+ exchange activity. In contrast, high-[Ca]o reperfusion (5 mmol/L) accelerated the [Na+]i recovery after 9-minute ischemia (tau=2.48+/-0.11 minute, n=5 [P<0.0001]) and 15-minute ischemia (tau=2.10+/-0.07, n=6 [P<0. 05]), but functional recovery deteriorated only in the hearts with 15-minute ischemia (29.8+/-9.4% [P<0.05]). [Na+]i recovery after 27-minute ischemia was incomplete and decelerated by low-[Ca]o reperfusion, with limited improvement of functional recovery (42. 5+/-7.9%, n=5 [P<0.05]). These results indicate that intracellular Na+ accumulation during ischemia is the substrate for reperfusion injury and that the [Na+]i kinetics during reperfusion, which is coupled with Ca2+ influx, also determines the degree of injury.

Molecular and cellular mechanisms of myocardial stunning

The past two decades have witnessed an explosive growth of knowledge regarding postischemic myocardial dysfunction or myocardial "stunning." The purpose of this review is to summarize current information regarding the pathophysiology and pathogenesis of this phenomenon. Myocardial stunning should not be regarded as a single entity but rather as a "syndrome" that has been observed in a wide variety of experimental settings, which include the following: 1) stunning after a single, completely reversible episode of regional ischemia in vivo; 2) stunning after multiple, completely reversible episodes of regional ischemia in vivo; 3) stunning after a partly reversible episode of regional ischemia in vivo (subendocardial infarction); 4) stunning after global ischemia in vitro; 5) stunning after global ischemia in vivo; and 6) stunning after exercise-induced ischemia (high-flow ischemia). Whether these settings share a common mechanism is unknown. Although the pathogenesis of myocardial stunning has not been definitively established, the two major hypotheses are that it is caused by the generation of oxygen-derived free radicals (oxyradical hypothesis) and by a transient calcium overload (calcium hypothesis) on reperfusion. The final lesion responsible for the contractile depression appears to be a decreased responsiveness of contractile filaments to calcium. Recent evidence suggests that calcium overload may activate calpains, resulting in selective proteolysis of myofibrils; the time required for resynthesis of damaged proteins would explain in part the delayed recovery of function in stunned myocardium. The oxyradical and calcium hypotheses are not mutually exclusive and are likely to represent different facets of the same pathophysiological cascade. For example, increased free radical formation could cause cellular calcium overload, which would damage the contractile apparatus of the myocytes. Free radical generation could also directly alter contractile filaments in a manner that renders them less responsive to calcium (e.g., oxidation of critical thiol groups). However, it remains unknown whether oxyradicals play a role in all forms of stunning and whether the calcium hypothesis is applicable to stunning in vivo. Nevertheless, it is clear that the lesion responsible for myocardial stunning occurs, at least in part, after reperfusion so that this contractile dysfunction can be viewed, in part, as a form of "reperfusion injury." An important implication of the phenomenon of myocardial stunning is that so-called chronic hibernation may in fact be the result of repetitive episodes of stunning, which have a cumulative effect and cause protracted postischemic dysfunction. A better understanding of myocardial stunning will expand our knowledge of the pathophysiology of myocardial ischemia and provide a rationale for developing new therapeutic strategies designed to prevent postischemic dysfunction in patients.

Intracellular calcium dynamics in mouse model of myocardial stunning

Intracellular calcium (Cai2+) and left ventricular (LV) function were determined in the coronary-perfused mouse heart to study Cai2+-related mechanisms of injury from myocardial ischemia and reperfusion. Specifics for loading of the photoprotein aequorin into isovolumically contracting mouse hearts under constant-flow conditions are provided. The method allows detection of changes in Cai2+ on a beat-to-beat basis in a model of myocardial stunning and permits correlation of interventions that regulate Ca2+ exchange with functional alterations. Twenty-three coronary-perfused mouse hearts were subjected to 15 min of ischemia followed by 20 min of reperfusion. In 13 hearts, the perfusate included the calmodulin antagonist W7 (10 microM) to inhibit Ca(2+)-calmodulin-regulated mechanisms. Peak Cai2+ was 0.77 +/- 0.03 microM in the control group and was unaffected by W7 at baseline. Ischemia was characterized by a rapid decline in LV function, followed by ischemic contracture, accompanied by a gradual rise in Cai2+. Reperfusion was characterized by an initial burst of Cai2+ and a gradual recovery to nearly normal systolic Cai2+ while LV pressure recovered to 55% after 20 min of reperfusion (stunned myocardium). These results in the mouse heart confirm that stunning does not result from deficiency of Cai2+ but rather from a decreased myofilament responsiveness to Cai2+ due to changes in the myofilaments themselves. In hearts perfused with W7, the rise in Cai2+ during ischemia was significantly attenuated, as was the magnitude of mean Cai2+ during early reflow. Ischemic contracture was abolished or delayed. Hearts perfused with W7 showed significantly improved recovery of LV pressure, rate of contraction, and rate of relaxation. Diastolic Cai2+ was increased in control hearts during stunning but returned to baseline in hearts perfused with W7. Simultaneous assessment of Cai2+ and LV function demonstrates that calmodulin-regulated mechanisms may contribute to the pathogenesis of myocardial stunning in the mouse heart.

A novel antagonist, No. 7943, of the Na+/Ca2+ exchange current in guinea-pig cardiac ventricular cells

1. The effects of No. 7943 on the Na+/Ca2+ exchange current and on other membrane currents were investigated in single cardiac ventricular cells of guinea-pig with the whole-cell voltage-clamp technique. 2. No. 7943 at 0.1-10 microM suppressed the outward Na+/Ca2+ exchange current in a concentration-dependent manner. The suppression was reversible and the IC50 value was approximately 0.32 microM. 3. No. 7943 at 5-50 microM suppressed also the inward Na+/Ca2+ exchange current in a concentration-dependent manner but with a higher IC50 value of approximately 17 microM. 4. In a concentration-response curve, No. 7943 raised the K(m)Ca2+ value, but did not affect the Imax value, indicating that No. 7943 is a competitive antagonist with external Ca2+ for the outward Na+/ Ca2+ exchange current. 5. The voltage-gated Na+ current, Ca2+ current and the inward rectifier K+ current were also inhibited by No. 7943 with IC50S of approximately 14, 8 and 7 microM, respectively. 6. In contrast to No. 7943, 3', 4'-dichlorobenzamil (DCB) at 3-30 microM suppressed the inward Na+/Ca2+ exchange current with IC50 of 17 microM, but did not affect the outward exchange current at these concentrations. 7. We conclude that No. 7943 inhibits the outward Na+/Ca2+ exchange current more potently than any other currents as a competitive inhibitor with external Ca2+. This effect is in contrast to DCB which preferentially inhibits the inward rather than the outward Na+/Ca2+ exchange current.

Biphasic changes in relaxation following reperfusion after myocardial ischemia

The present study provides evidences of left ventricular diastolic alterations following reperfusion in a model of global ischemia. Isolated perfused rabbit and rat hearts, were subjected to ischemia for 15 and 20 min respectively, followed by 30 min of reperfusion. In rabbit heart at the end of the reperfusion period, isovolumic left ventricular developed pressure (LVDP) and +dP/dtmax stabilized at 55 +/- 3% and 60 +/- 2% of preischemic values respectively and, in rat heart LVDP = 61 +/- 8% and +dP/dtmax = 57 +/- 9% of preischemic values. Stunned heart was then obtained from both species. Left ventricular end diastolic pressure (LVEDP) values stabilized at the end of reperfusion period at values higher than preischemic conditions in both species (38.9 +/- 4.4 mmHg and 30.3 +/- 3.1 mmHg in rabbit and rat respectively). The time constant of relaxation (T) increased early in reperfusion in both species, but then decreased and stabilized at the end of reperfusion period at values lower than preischemic values. The ratio between both maximal velocities (+P/-P), also showed a transitory impairment in relaxation, followed by normalization and stabilization at values lower than preischemic values. This biphasic pattern in relaxation was detected in both species. The changes in relaxation were dissociated from the diastolic compliance and could be the result of a transitory calcium overload and/or sarcoplasmic reticulum dysfunction. The faster myocardial relaxation at the end of reperfusion period is consistent with the decreased myofilament sensitivity, which characterizes the stunned myocardium.

Long-term nifedipine treatment reduces calcium overload in isolated reperfused hearts of diabetic rats

1. Streptozotocin-induced diabetic rats showed poor post-ischemic recovery in isolated working rat hearts. 2. Diabetic rats showed myocardial Na+ accumulation after ischemia, and Ca2+ level and water content elevation after reperfusion. 3. A 6-wk nifedipine treatment improved post-ischemic recovery of cardiac parameters and prevented myocardial Na+ accumulation after ischemia and myocardial Ca2+ level and water content elevation after reperfusion of diabetic rats. 4. Results suggest that nifedipine treatment improves cardiac dysfunction in the reperfused ischemic hearts of diabetic rats through normalization of the Na+-Ca2+ imbalance and water content.

Influence of jet direction on pulmonary vein flow patterns in severe mitral regurgitation

Pulmonary vein flow patterns measured with transesophageal echocardiography have been used recently to assess the severity of mitral valve regurgitation. This study was designed to determine whether regurgitant jet direction selectively influences the pattern of flow in right and left pulmonary veins. Thirty-seven patients undergoing mitral valve repair or replacement for severe valvular regurgitation were studied intraoperatively with biplane transesophageal echocardiography. Regurgitant jets were classified by color flow mapping as central or wall, with the latter further classified as septal, lateral, anterior, or posterior in the two orthogonal scan planes. Pulmonary vein flow patterns were measured with pulsed wave Doppler ultrasonography and categorized as showing normal, blunted, or reversed systolic flow. Right and left pulmonary vein flow patterns were identical in the majority of patients studied (78%). Eight patients had discordant flow patterns. In seven of eight patients, the more abnormal pattern was seen in the right pulmonary vein, despite the fact that the regurgitant jets were directed centrally in four of these seven patients. Since discordant pulmonary vein flow patterns occurred in 5 of 15 patients (33%) with central jets, but in only 3 of 22 patients (14%) with eccentric wall jets, it is unlikely that mitral regurgitation jet direction per se causes predictable and selective unilateral alteration in pulmonary vein flow patterns.

Alterations of excitation-contraction coupling in stunned myocardium and in failing myocardium

Although both myocardial stunning and chronic heart failure are characterized by contractile dysfunction, there are profound differences in their underlying mechanisms. Changes in cardiac contractile force can be effected by modulation of intracellular [Ca2+] or by alteration of the contractile protein response to intracellular Ca2+. New evidence suggests that the principal lesion in the stunned myocardium resides at the level of the contractile proteins, which may be injured by proteases activated early during reperfusion. In contrast, failing myocardium is known to display abnormal intracellular Ca2+ handling, indicative of dysfunction of the sarcoplasmic reticulum. Alterations of gene expression and isoform switching of the myofilaments also occur in failing myocardium, consistent with an observed shift of the kinetics of crossbridge cycling. In conclusion, changes in both intracellular Ca2+ homeostasis and myofilament function occur in failing myocardium, while stunned myocardium primarily reflects an uncoupling between Ca2+ and contractile force.

Myocardial stunning--are calcium antagonists useful?

Considerable data support the point of view that calcium antagonists, whether given before the onset of ischemia or exactly at the time of reperfusion, ameliorate stunning. Benefit after the onset of reperfusion is much more controversial. It is proposed that the mechanisms whereby calcium antagonists act vary between these situations. When given before or at the onset of ischemia, then an antiischemic effect is likely. According to the hypothesis that the severity of ischemic damage determines the severity of reperfusion damage, the calcium antagonists indirectly lessen reperfusion damage. When given exactly at the time of reperfusion, the proposal is that the calcium antagonists are specifically limiting the entry of calcium ions via the calcium channel and thereby diminishing pathogenic cytosolic calcium oscillations. The reported benefit of calcium antagonists when given postreperfusion to the heart in situ, in the presence of established stunning, is of unknown mechanism and controversial significance. The hypothesis of a two-stage model of stunning with calcium as a pathogen is in accord with most of the available evidence.