Na+/H+ exchanger inhibition at the onset of reperfusion decreases myocardial infarct size: role of reactive oxygen species

Cobalt chloride postconditioning as myoprotective therapy in cardiac ischemia-reperfusion

Since damage induced by ischemia-reperfusion (I/R) involves alterations in Ca²⁺ homeostasis and is reduced by ischemic postconditioning (IP) and that CoCl₂ can trigger changes resembling the response to a hypoxic event in normoxia and its blockade on Ca²⁺ current in heart muscle, our aim was to evaluate CoCl₂ as an IP therapeutic tool. Mechanic and energetic parameters of isolated and arterially perfused male Wistar rat heart ventricles were simultaneously analyzed in a model of I/R in which 0.23 mmol/L CoCl₂ was introduced upon reperfusion and kept or withdrawn after 20 min or introduced after 20 min of reperfusion. The presence of CoCl₂ did not affect diastolic pressure but increased post-ischemic contractile recovery, which peaked at 20 min and decreased at the end of reperfusion. This decrease was prevented when CoCl₂ was removed at 20 min of reperfusion. Total heat release increased throughout reperfusion, while economy increased between 15 and 25 min. No effect was observed when CoCl₂ was introduced at 20 min of reperfusion. In addition, both the area under the contracture curve evoked by 10 mmol/L caffeine-36 mmol/L Na⁺ and the contracture tension relaxation rate were higher with CoCl₂.Furthermore, CoCl₂ decreased the number of arrhythmias during reperfusion and the ventricular damaged area. The presence of CoCl₂ in reperfusion induces cardioprotection consistent with the improvement in cellular calcium handling. The use of CoCl₂ constitutes a potential cardioprotective tool of clinical relevance.

Chronic GPER activation prevents ischemia/reperfusion injury in ovariectomized rats

During menopause women are exposed to an increase in cardiovascular risk. G protein-coupled estrogen receptor (GPER) is known to mediate several of the protective effects of such hormones. G1 was described as a selective and synthetic agonist for GPER. The aim of the present research is to evaluate the effect of a chronic treatment with G1 in ovariectomized (OVX) rats exposed to ischemia/reperfusion (I/R). Considering the hypothesis that an impaired mitochondrial state could be involved in the alterations produced in OVX rats, other objective of this study was to investigate it in an isolated preparation. Three months old rats were assigned to undergo either bilateral ovariectomy or sham operation. The OVX rats were randomly treated during one month with either G1 or vehicle. Cardiac mitochondria from OVX rats showed a depolarized membrane potential and a decreased calcium retention capacity in comparison with Sham rats, which were prevented by chronic G1 treatment. I/R caused a higher decrease of left ventricular developed pressure and a higher increase of left ventricular end diastolic pressure in OVX compared to Sham hearts. These altered mechanical parameters were prevented by G1. The induced infarct size was significantly higher in OVX, which was reduced by G1 treatment. These results indicate that the mitochondrial state in OVX rats is impaired, accompanied by an altered mechanical response after ischemia and reperfusion injury, which was effectively prevented with chronic treatment with G1. The present study may provide further insights for the potential development of a therapy based on the GPER modulation.

Inhibition of Na+/H+ Exchanger With EMD 87580 does not Confer Greater Cardioprotection Beyond Preconditioning on Ischemia-Reperfusion Injury in Normal Dogs

Postischemic accumulation of intracellular Na⁺ promotes calcium overload and contributes to cellular necrosis. Cardioprotection afforded by pharmacologic blockade of the sodium-hydrogen exchanger subtype 1 (NHE1) is thought to be more remarkable than that obtained by ischemic conditioning (IC). The window of protection provided by IC pretreatment is maintained even when performed up to 48 hours before ischemia. In addition, the perception exists that combined NHE1 inhibition plus IC produces greater than additive protection against ischemic injury. The current study compared the efficacy of NHE1 blockade by N-[2-methyl-4,5-bis(methylsulfonyl)-benzoyl]-guanidine (EMD 87580 5 mg/kg) combined with first- or second-window IC on ischemic tolerance in dogs subject to 90-minute acute ischemia and 180-minute reperfusion. Infarct size (tetrazolium staining), vascular responses, and myocardial perfusion (microspheres) were assessed. EMD 87580 given before ischemia or before reperfusion did not reduce infarct size (compared to vehicle-treated group). Significant protection against tissue necrosis was obtained by both first- and second-window IC, but additive cardioprotection (ie, greater than that afforded by IC) was not observed by treatment with EMD 87580. Vascular reactivity in the infarct-related artery was not preserved after ischemia-reperfusion in any of the experimental groups. Likewise, either the pharmacologic or the nonpharmacologic interventions did not modify myocardial perfusion. These data demonstrate that EMD 87580 did not protect against ischemia-reperfusion injury regardless of the time of drug administration. Combined EMD 87580 and IC did not antagonize protection that was achieved by either first- or second-window IC alone; no additive protection beyond preconditioning was obtained. Further study is necessary to assess the value of NHE1 blockers as protective agents against myocardial injury.

The electrogenic cardiac sodium bicarbonate co-transporter (NBCe1) contributes to the reperfusion injury

BACKGROUND

Although the participation of the electrogenic sodium/bicarbonate cotransporter (NBCe1) in the recovery from an intracellular acid load is recognized, its role in ischemia-reperfusion is still unclear.

METHODS AND RESULTS

Our objective was to assess the role of NBCe1 in reperfusion injury. We use selective functional antibodies against extracellular loop 3 (a-L3) and loop 4 (a-L4) of NBCe1. a-L3 inhibits and a-L4 stimulates NBCe1 activity. Isolated rat hearts were submitted to 40 min of coronary occlusion and 1 h of reperfusion. a-L3, a-L4 or S0859--selective Na(+)-HCO3(-) co-transport inhibitor--were administered during the initial 10 min of reperfusion. The infarct size (IS) was measured by triphenyltetrazolium chloride staining technique. Postischemic systolic and diastolic functions were also assessed. a-L3 and S0859 treatments decreased significantly (P < .05) the IS (16 ± 3% for a-L3 vs. 32 ± 5% in hearts treated with control nonimmune serum and 19 ± 3% for S0859 vs. 39 ± 2% in untreated hearts). Myocardial function during reperfusion improved after a-L3 treatment, but it was not modified by S0859. The infusion of a-L4 did not modify neither the IS nor myocardial function.

CONCLUSIONS

The NBCe1 hyperactivity during reperfusion leads to Na(+) and Ca(2+) loading, conducing to Ca(2+) overload and myocardial damage. Consistently, we have shown herein that the selective NBCe1 blockade with a-L3 exerted cardioprotection. This beneficial action strongly suggests that NBCe1 could be a potential target for the treatment of coronary disease.

Mitochondrial NHE1: a newly identified target to prevent heart disease

Mitochondrial damage has been associated with early steps of cardiac dysfunction in heart subjected to ischemic stress, oxidative stress and hypertrophy. A common feature for the mitochondrial deterioration is the loss of the mitochondrial membrane potential (ΔΨ m) with the concomitant irreversible opening of the mitochondrial permeability transition pore (MPTP) which follows the mitochondrial Ca²⁺ overload, and the subsequent mitochondrial swelling. We have recently characterized the expression of the Na⁺/H⁺ exchanger 1 (mNHE1) in mitochondrial membranes. This surprising observation provided a unique target for the prevention of the Ca²⁺-induced MPTP opening, based on the inhibition of the NHE1 m. In this line, inhibition of NHE1 m activity and/or reduction of NHE1 m expression decreased the Ca²⁺-induced mitochondrial swelling and the release of reactive oxygen species (ROS) in isolated cardiac mitochondria and preserved the ΔΨ m in isolated cardiomyocytes. Mitochondrial NHE1 thus represents a novel target to prevent cardiac disease, opening new avenues for future research.

Protective effects of N-(2-mercaptopropionyl)-glycine against ischemia-reperfusion injury in hypertrophied hearts

The beneficial effects of N-(2-mercaptopropionyl)-glycine (MPG) against ischemia-reperfusion injury in normotensive animals have been previously studied. Our objective was to test the action of MPG during ischemia and reperfusion in hearts from spontaneously hypertensive rats (SHR). Isolated hearts from SHR and age-matched normotensive rats Wistar Kyoto (WKY) were subjected to 50-min global ischemia (GI) and 2-hour reperfusion (R). In other hearts MPG 2mM was administered during 10 min before GI and the first 10 min of R. Infarct size (IS) was assessed by TTC staining technique and expressed as percentage of risk area. Postischemic recovery of myocardial function was assessed. Reduced glutathione (GSH), thiobarbituric acid reactive substances (TBARS) and SOD cytosolic activity - as estimators of oxidative stress and MnSOD cytosolic activity - as an index of (mPTP) opening were determined. In isolated mitochondria H(2)O(2)-induced mPTP opening was also measured. The treatment with MPG decreased infarct size, preserved GSH levels and decreased SOD and MnSOD cytosolic activities, TBARS concentration, and H(2)O(2) induced-mPTP opening in both rat strains. Our results show that in both hypertrophied and normal hearts an attenuation of mPTP opening via reduction of oxidative stress appears to be the predominant mechanism involved in the cardioprotection against reperfusion injury MPG-mediated.

Disruption of chronic cariporide treatment abrogates myocardial ion homeostasis during acute ischemia reperfusion

Cariporide, an Na/H exchanger inhibitor, is a drug with cardioprotective properties. However, chronic treatment with cariporide may modify the protein phenotype of the cardiomyocytes. Disruption of the equilibrium between a cariporide-modified phenotype and the supply of cariporide could be deleterious. The aim of this study was to test the effects of this equilibrium rupture (EqR) on cardiac function at baseline and acute ischemia reperfusion. Rats were chronically treated with cariporide (2.5 mg·kg·d) or with placebo for 21 days, after which isolated Langendorff-mode heart perfusion experiments utilized cariporide-free buffer. During this type of perfusion, the drug is rapidly cleared from the cellular environment. After 30 minutes of stabilization, the hearts were subjected to global zero-flow ischemia (25 minutes) followed by reperfusion (45 minutes). Measures of mechanical function, oxygen consumption, lactate plus pyruvate, CO2 and proton release into the coronary effluent were determined. The gene and protein expression of proton extruders was also evaluated. Chronic cariporide administration followed by EqR reduced the expression of the Na/H exchanger, increased the expression of the HCO3 or Na exchanger, decreased monocarboxylate/H carrier expression, reduced the lactate plus pyruvate release but did not change the glucose oxidation rate and mechanical function compared with baseline conditions. The resulting low glycolytic rate was associated with a stronger contracture during ischemia. During reperfusion, the early release of acidic forms was higher and redirected toward the use of the Na/H and HCO3 /Na exchangers to the detriment of the safe monocarboxylate/H carrier. Both phenomena were assumed to increase the Na uptake and activate the Na/Ca exchanger, resulting in Na and Ca overload and further cellular damage. This explains the impaired recovery of the contractile function observed in the EqR group during reperfusion. In conclusion, although cariporide is usually cardioprotective, a disruption of its chronic treatment followed by an ischemia/reperfusion event can become deleterious.

Silencing of cardiac mitochondrial NHE1 prevents mitochondrial permeability transition pore opening

Inhibition of Na(+)/H(+) exchanger 1 (NHE1) reduces cardiac ischemia-reperfusion (I/R) injury and also cardiac hypertrophy and failure. Although the mechanisms underlying these NHE1-mediated effects suggest delay of mitochondrial permeability transition pore (MPTP) opening, and reduction of mitochondrial-derived superoxide production, the possibility of NHE1 blockade targeting mitochondria has been incompletely explored. A short-hairpin RNA sequence mediating specific knock down of NHE1 expression was incorporated into a lentiviral vector (shRNA-NHE1) and transduced in the rat myocardium. NHE1 expression of mitochondrial lysates revealed that shRNA-NHE1 transductions reduced mitochondrial NHE1 (mNHE1) by ∼60%, supporting the expression of NHE1 in mitochondria membranes. Electron microscopy studies corroborate the presence of NHE1 in heart mitochondria. Immunostaining of rat cardiomyocytes also suggests colocalization of NHE1 with the mitochondrial marker cytochrome c oxidase. To examine the functional role of mNHE1, mitochondrial suspensions were exposed to increasing concentrations of CaCl(2) to induce MPTP opening and consequently mitochondrial swelling. shRNA-NHE1 transduction reduced CaCl(2)-induced mitochondrial swelling by 64 ± 4%. Whereas the NHE1 inhibitor HOE-642 (10 μM) decreased mitochondrial Ca(2+)-induced swelling in rats transduced with nonsilencing RNAi (37 ± 6%), no additional HOE-642 effects were detected in mitochondria from rats transduced with shRNA-NHE1. We have characterized the expression and function of NHE1 in rat heart mitochondria. Because mitochondria from rats injected with shRNA-NHE1 present a high threshold for MPTP formation, the beneficial effects of NHE1 inhibition in I/R resulting from mitochondrial targeting should be considered.

Activation of microglia depends on Na+/H+ exchange-mediated H+ homeostasis

H(+) extrusion is important for sustained NADPH oxidase activation after "respiratory" burst in macrophage/microglia activation. In this study, we investigated the role of Na(+)/H(+) exchanger isoform 1 (NHE-1) in activation of microglia after lipopolysaccharide (LPS) or oxygen and glucose deprivation and reoxygenation (OGD/REOX) exposure. NHE-1 functioned in maintaining basal pH(i) of immortalized M4T.4 microglia or mouse primary microglia. Pharmacological inhibition of NHE-1 activity with the potent inhibitor cariporide [HOE 642 (4-isopropyl-3-methylsulfonyl-benzoyl-guanidine-methanesulfonate)] abolished pH(i) regulation in microglia under basal conditions. Activation of microglia either by LPS, phorbol myristate acetate, or OGD/REOX accelerated pH(i) regulation and caused pH(i) elevation, which was accompanied with an increase in [Na(+)](i) and [Ca(2+)](i) as well as production of superoxide anion and cytokines. Interestingly, inhibition of NHE-1 not only abolished pH(i) regulation but also reduced production of superoxide anion as well as expression of cytokines and inducible nitric oxide synthase. Together, these results reveal that there was a concurrent activation of NHE-1 in microglia in response to proinflammatory stimuli. The study suggests that NHE-1 functions to maintain microglial pH(i) homeostasis allowing for sustained NADPH oxidase function and "respiratory" burst.

Na+/H+ exchanger-1 inhibitors decrease myocardial superoxide production via direct mitochondrial action

The possibility of a direct mitochondrial action of Na(+)/H(+) exchanger-1 (NHE-1) inhibitors decreasing reactive oxygen species (ROS) production was assessed in cat myocardium. Angiotensin II and endothelin-1 induced an NADPH oxidase (NOX)-dependent increase in anion superoxide (O(2)(-)) production detected by chemiluminescence. Three different NHE-1 inhibitors [cariporide, BIIB-723, and EMD-87580] with no ROS scavenger activity prevented this increase. The mitochondria appeared to be the source of the NOX-dependent ROS released by the "ROS-induced ROS release mechanism" that was blunted by the mitochondrial ATP-sensitive potassium channel blockers 5-hydroxydecanoate and glibenclamide, inhibition of complex I of the electron transport chain with rotenone, and inhibition of the permeability transition pore (MPTP) by cyclosporin A. Cariporide also prevented O(2)(-) production induced by the opening of mK(ATP) with diazoxide. Ca(2+)-induced swelling was evaluated in isolated mitochondria as an indicator of MPTP formation. Cariporide decreased mitochondrial swelling to the same extent as cyclosporin A and bongkrekic acid, confirming its direct mitochondrial action. Increased O(2)(-) production, as expected, stimulated ERK1/2 and p90 ribosomal S6 kinase phosphorylation. This was also prevented by cariporide, giving additional support to the existence of a direct mitochondrial action of NHE-1 inhibitors in preventing ROS release. In conclusion, we report a mitochondrial action of NHE-1 inhibitors that should lead us to revisit or reinterpret previous landmark observations about their beneficial effect in several cardiac diseases, such as ischemia-reperfusion injury and cardiac hypertrophy and failure. Further studies are needed to clarify the precise mechanism and site of action of these drugs in blunting MPTP formation and ROS release.

Differential roles of p38-MAPK and JNKs in mediating early protection or apoptosis in the hyperthermic perfused amphibian heart

In the present study the activation of p38 mitogen-activated protein kinase(p38-MAPK) and c-Jun N-terminal kinases (JNKs) by hyperthermia was investigated in the isolated perfused Rana ridibunda heart. Hyperthermia (42°C) was found to profoundly stimulate p38-MAPK phosphorylation within 0.5 h, with maximal values being attained at 1 h[4.503(±0.577)-fold relative to control, P&lt;0.01]. JNKs were also activated under these conditions in a sustained manner for at least 4 h[2.641(±0.217)-fold relative to control, P&lt;0.01]. Regarding their substrates, heat shock protein 27 (Hsp27) was maximally phosphorylated at 1 h [2.261(±0.327)-fold relative to control, P&lt;0.01] and c-Jun at a later phase [3 h: 5.367(±0.081)-fold relative to control, P&lt;0.001]. Hyperthermia-induced p38-MAPK activation was found to be dependent on the Na+/H+ exchanger 1 (NHE1) and was also suppressed by catalase (Cat) and superoxide dismutase (SOD), implicating the generation of reactive oxygen species (ROS). ROS were also implicated in the activation of JNKs by hyperthermia, with the Na+/K+-ATPase acting as a mediator of this effect at an early stage and the NHE1 getting involved at a later time point. Finally, JNKs were found to be the principal mediators of the apoptosis induced under hyperthermic conditions, as their inhibition abolished poly(ADP-ribose)polymerase (PARP) cleavage after 4 h at 42°C. Overall, to our knowledge,this study highlights for the first time the variable mediators implicated in the transduction of the hyperthermic signal in the isolated perfused heart of an ectotherm and deciphers a potential salutary effect of p38-MAPK as well as the fundamental role of JNKs in the induced apoptosis.

Age dependency of the cariporide-mediated cardio-protection after simulated ischemia in isolated human atrial heart muscles

Experimental and clinical investigations suggest that blockade of Na(+)/H(+) exchange (NHE) with cariporide provides functional protection during ischemia and reperfusion in mature hearts. The benefit on aged human myocardium is unknown. Therefore, the impact of cardiac aging on cardio-protection by cariporide after prolonged ischemia was studied in isolated myocardium of adult (<or=55 years), old (56-69 years), and very old (>or=70 years) patients with coronary artery disease. Isolated atrial trabeculae were subjected to 30 min of simulated ischemia with and without cariporide, and early post-ischemic contractile recovery was determined. During the reoxygenation period, trabeculae of adults, but not those of old or very old patients, improved after treatment with cariporide. After 90 min of reoxygenation, cariporide-treated adult trabeculae developed 41+/-5% of their pre-ischemic force (non-treated control group, 27+/-5%; P<0.05), and old trabeculae recovered to 41+/-7% (control, 25+/-6%), whereas very old trabeculae recovered to only 26+/-2% (control, 28+/-6%). Trabeculae of all patients <70 years with CCS stage I-II angina pectoris recovered well (45+/-6%; control, 22+/-5%; P<0.01), which was in contrast to patients with CCS stage III (34+/-4%; control, 31+/-5%). Subsequent immunoblot analyses indicated no concomitant alterations in the myocardial NHE1 protein level depending on age. In very old myocardium, higher levels of active p38MAPK in atrial trabeculae after ischemia pointed at an increased cellular stress, which was even more pronounced after post-ischemic reperfusion. In summary, cariporide is protective against ischemia-reperfusion injury in mature human hearts but has no benefit on the post-ischemic functional recovery of the aging myocardium.

Redox signaling triggers protection during the reperfusion rather than the ischemic phase of preconditioning

In ischemic preconditioning (IPC) brief ischemia/reperfusion renders the heart resistant to infarction from any subsequent ischemic insult. Protection results from binding of surface receptors by ligands released during the preconditioning ischemia. The downstream pathway involves redox signaling as IPC will not protect in the presence of a free radical scavenger. To determine when in the IPC protocol the redox signaling occurs, seven groups of isolated rabbit hearts were studied. All hearts underwent 30 min of coronary branch occlusion and 2 h of reperfusion. IPC groups were subjected to 5 min of regional ischemia followed by 10 min of reperfusion prior to the 30-min coronary occlusion. The Control group had only the 30-min occlusion and 2-h reperfusion. In the second group IPC preceded the index coronary occlusion. The third group was also preconditioned, but the free radical scavenger N-2-mercaptopropionyl glycine (MPG 300 microM) was infused during the 10-min reperfusion and therefore was present in the myocardium in the distribution of the snared coronary artery during the entire reperfusion phase and also during the subsequent 30-min ischemia. In another preconditioned group MPG was added to the perfusate before the preconditioning ischemia and therefore was present in the tissue only during the preconditioning ischemia and then was washed out during reperfusion. In the fifth group MPG was added to the perfusate for only the last 5 min of the preconditioning reperfusion and therefore was present in the tissue during the last minutes of the reperfusion phase and the 30 min of ischemia. In an additional group of IPC hearts MPG was infused for only the initial 5 min of the preconditioning reperfusion and then allowed to wash out so that the scavenger was present for only the first half of the reperfusion phase. Infarct and risk zone sizes were measured by triphenyltetrazolium staining and fluorescent microspheres, resp. IPC reduced infarct size from 31.3 +/- 2.7% of the ischemic zone in control hearts to only 8.4 +/- 1.9%. MPG completely blocked IPC's protection in the third (39.4 +/- 2.8%) and sixth (36.1 +/- 7.7%) groups but did not affect its protection in groups 4 (8.1 +/- 1.5%) or 5 (7.8 +/- 1.1%). When deoxygenated buffer was used during IPC's reperfusion phase in the seventh group of hearts, protection was lost and infarct size was increased over that seen in control hearts (74.5 +/- 9.0%). Hence redox signaling occurs during the reperfusion phase of IPC, and the critical component in that reperfusion phase appears to be molecular oxygen.

Sodium/hydrogen exchange inhibition with cariporide reduces leukocyte adhesion via P-selectin suppression during inflammation

BACKGROUND AND PURPOSE

The Na(+)/H(+) exchange (NHE) inhibitor cariporide is known to ameliorate ischaemia/reperfusion (I/R) injury by reduction of cytosolic Ca(2+) overload. Leukocyte activation and infiltration also mediates I/R injury but whether cariporide reduces I/R injury by affecting leukocyte activation is unknown. We studied the effect of cariporide on thrombin and I/R induced leukocyte activation and infiltration models and examined P-selectin expression as a potential mechanism for any identified effects.

EXPERIMENTAL APPROACH

An in vivo rat mesenteric microcirculation microscopy model was used with stimulation by thrombin (0.5 micro ml(-1)) superfusion or ischaemia (by haemorrhagic shock for 60 min) and reperfusion (90 min).

KEY RESULTS

Treatment with cariporide (10 mg kg(-1) i.v.) significantly reduced leukocyte rolling, adhesion and extravasation after thrombin exposure. Similarly, cariporide reduced leukocyte rolling (54+/-6.2 to 2.4+/-1.0 cells min(-1), P<0.01), adherence (6.3+/-1.9 to 1.2+/-0.4 cells 100 microm(-1), P<0.01) and extravasation (9.1+/-2.1 to 2.4+/-1.1 cells per 20 x 100 microm perivascular space, P<0.05), following haemorrhagic shock induced systemic ischaemia and reperfusion. The cell adhesion molecule P-selectin showed a profound decrease in endothelial expression following cariporide administration in both thrombin and I/R stimulated groups (35.4+/-3.2 vs 14.2+/-4.1% P-selectin positive cells per tissue section, P<0.01).

CONCLUSIONS AND IMPLICATIONS

The NHE inhibitor cariporide is known to limit reperfusion injury by controlling Ca(2+) overload but these data are novel evidence for a vasculoprotective effect of NHE inhibition at all levels of leukocyte activation, an effect which is likely to be mediated at least in part by a reduction of P-selectin expression.

Mitochondrial reactive oxygen species activate the slow force response to stretch in feline myocardium

When the length of the myocardium is increased, a biphasic response to stretch occurs involving an initial rapid increase in force followed by a delayed slow increase called the slow force response (SFR). Confirming previous findings involving angiotensin II in the SFR, it was blunted by AT1 receptor blockade (losartan). The SFR was accompanied by an increase in reactive oxygen species (ROS) of approximately 30% and in intracellular Na(+) concentration ([Na(+)](i)) of approximately 2.5 mmol l(-1) over basal detected by H(2)DCFDA and SBFI fluorescence, respectively. Abolition of ROS by 2-mercapto-propionyl-glycine (MPG) and EUK8 suppressed the increase in [Na(+)](i) and the SFR, which were also blunted by Na(+)/H(+) exchanger (NHE-1) inhibition (HOE642). NADPH oxidase inhibition (apocynin or DPI) or blockade of the ATP-sensitive mitochondrial potassium channels (5HD or glybenclamide) suppressed both the SFR and the increase in [Na(+)](i) after stretch, suggesting that endogenous angiotensin II activated NADPH oxidase leading to ROS release by the ATP-sensitive mitochondrial potassium channels, which promoted NHE-1 activation. Supporting the notion of ROS-mediated NHE-1 activation, stretch increased the ERK1/2 and p90rsk kinases phosphorylation, effect that was cancelled by losartan. In agreement, the SFR was cancelled by inhibiting the ERK1/2 signalling pathway with PD98059. Angiotensin II at a dose that mimics the SFR (1 nmol l(-1)) induced an increase in .O(2)(-) production of approximately 30-40% detected by lucigenin in cardiac slices, an effect that was blunted by losartan, MPG, apocynin, 5HD and glybenclamide. Taken together the data suggest a pivotal role of mitochondrial ROS in the genesis of the SFR to stretch.

Phosphodiesterase 5A Inhibition Induces Na + /H + Exchanger Blockade and Protection Against Myocardial Infarction

Acute phosphodiesterase 5A inhibition by sildenafil or EMD360527/5 promoted profound inhibition of the cardiac Na(+)/H(+) exchanger (NHE-1), detected by the almost null intracellular pH recovery from an acute acid load (ammonium prepulse) in isolated papillary muscles from Wistar rats. Inhibition of phosphoglycerate kinase-1 (KT5823) restored normal NHE-1 activity, suggesting a causal link between phosphoglycerate kinase-1 increase and NHE-1 inhibition. We then tested whether the beneficial effects of NHE-1 inhibitors against the deleterious postmyocardial infarction (MI) remodeling can be detected after sildenafil-mediated NHE-1 inhibition. MI was induced by left anterior descending coronary artery ligation in Wistar rats, which were randomized to placebo or sildenafil (100 mg kg(-1) day(-1)) for 6 weeks. Sildenafil significantly increased left ventricular phosphoglycerate kinase-1 activity in the post-MI group without affecting its expression. MI increased heart weight/body weight ratio, left ventricular myocyte cross-sectional area, interstitial fibrosis, and brain natriuretic peptide and NHE-1 expression. Sildenafil blunted these effects. Neither a significant change in infarct size nor a change in arterial or left ventricular systolic pressure was detected after sildenafil. MI decreased fractional shortening and the ratio of the maximum rate of rise of LVP divided by the pressure at the moment such maximum occurs, effects that were prevented by sildenafil. Intracellular pH recovery after an acid load was faster in papillary muscles from post-MI hearts (versus sham), whereas sildenafil significantly inhibited NHE-1 activity in both post-MI and sildenafil-treated sham groups. We conclude that increased phosphoglycerate kinase-1 activity after acute phosphodiesterase 5A inhibition blunts NHE-1 activity and protects the heart against post-MI remodeling and dysfunction.