Do NHE inhibition and ischemic preconditioning convey cardioprotection through a common mechanism?

Remote ischemic preconditioning of cardiomyocytes inhibits the mitochondrial permeability transition pore independently of reduced calcium-loading or sarcKATP channel activation

Ischemic preconditioning (IPC) inhibits Ca²⁺‐loading during ischemia which contributes to cardioprotection by inhibiting mechanical injury due to hypercontracture and biochemical injury through mitochondrial permeability transition (MPT) pores during reperfusion. However, whether remote‐IPC reduced Ca²⁺‐loading during ischemia and its subsequent involvement in inhibiting MPT pore formation during reperfusion has not been directly shown. We have developed a cellular model of remote IPC to look at the impact of remote conditioning on Ca²⁺‐regulation and MPT pore opening during simulated ischemia and reperfusion, using fluorescence microscopy. Ventricular cardiomyocytes were isolated from control rat hearts, hearts preconditioned with three cycles of ischemia/reperfusion or naïve myocytes remotely conditioned with effluent collected from preconditioned hearts. Both conventional‐IPC and remote‐IPC reduced the loss of Ca²⁺‐homeostasis and contractile function following reenergization of metabolically inhibited cells and protected myocytes against ischemia/reperfusion injury. However, only conventional‐IPC reduced the Ca²⁺‐loading during metabolic inhibition and this was independent of any change in sarcK_ATP channel activity but was associated with a reduction in Na⁺‐loading, reflecting a decrease in Na/H exchanger activity. Remote‐IPC delayed opening of the MPT pores in response to ROS, which was dependent on PKCε and NOS‐signaling. These data show that remote‐IPC inhibits MPT pore opening to a similar degree as conventional IPC, however, the contribution of MPT pore inhibition to protection against reperfusion injury is independent of Ca²⁺‐loading in remote IPC. We suggest that inhibition of the MPT pore and not Ca²⁺‐loading is the common link in cardioprotection between conventional and remote IPC.

Remote ischemic preconditioning (IPC) provides a similar level of protection against ischemia–reperfusion injury to that of conventional‐IPC. This study shows that unlike conventional‐IPC, this was independent of any reduction in Na or Ca²⁺‐loading during the simulated ischemic event but results from a direct PKCε‐dependent inhibition of the mitochondrial permeability transition pore.

Exercise alters the regulation of myocardial Na(+)/H(+) exchanger-1 activity

The myocardial Na(+)/H(+) exchanger-1 (NHE1) plays a major role in regulation of intracellular pH, and its upregulation has been implicated in increased ischemia-reperfusion injury and other pathologies. Hydrogen peroxide (H2O2) increases NHE1 activity acutely via ERK1/2 signaling. Chronic strenuous exercise upregulates NHE1 in skeletal muscle, but we hypothesize this will not occur in the heart, because exercise creates a cardioprotective phenotype. NHE1 activity and its regulation by H2O2 were examined at physiological pH using isolated cardiomyocytes from female Sprague-Dawley rats exercised on a treadmill for 5 wk (E; n = 11). Compared with sedentary (S; n = 15), E displayed increases (P < 0.05) in heart-to-body weight ratio (6.8%) and plantaris mitochondria content (89%). NHE1 activity (acid efflux rate following an acid load) was 209% greater in E (0.65 ± 0.12 vs. 2.01 ± 0.29 fmol/min). The difference was attributed primarily to greater cell volume (22.2 ± 0.6 vs. 34.3 ± 1.1 pl) and intracellular pH-buffering capacity (33.94 ± 1.59 vs. 65.82 ± 5.20 mM/pH unit) of E myocytes. H2O2 stimulation (100 μM) raised NHE1 activity significantly less in E (45%) than S (167%); however, activity remained 185% greater in E. ERK1/2 inhibition abrogated the increases. H2O2-stimulated ERK1/2 phosphorylation levels normalized to total ERK1/2 were similar between groups. Content of NHE1 and activities of H2O2 scavengers were also similar. We observed that intracellular pH-buffering capacity differences between groups became progressively less with declining pH, which may be an exercise-induced cardioprotective adaptation to lower NHE1 activity during certain pathological situations. We conclude that strenuous endurance exercise increases myocardial NHE1 activity at physiological pH, which would likely enhance cardiac performance under physiological conditions.

Na+/H+ exchange inhibitor cariporide attenuates skeletal muscle infarction when administered before ischemia or reperfusion

Administration of Na(+)/H(+) exchange isoform-1 (NHE-1) inhibitors before ischemia has been shown to attenuate myocardial infarction in several animal models of ischemia-reperfusion injury. However, controversy still exists as to the efficacy of NHE-1 inhibitors in protection of myocardial infarction when administered at the onset of reperfusion. Furthermore, the efficacy of NHE-1 inhibition in protection of skeletal muscle from infarction (necrosis) has not been studied. This information has potential clinical applications in prevention or salvage of skeletal muscle from ischemia-reperfusion injury in elective and trauma reconstructive surgery. The objective of this research project is to test our hypothesis that the NHE-1 inhibitor cariporide is effective in protection of skeletal muscle from infarction when administered at the onset of sustained ischemia or reperfusion and to study the mechanism of action of cariporide. In our studies, we observed that intravenous administration of cariporide 10 min before ischemia (1 or 3 mg/kg) or reperfusion (3 mg/kg) significantly reduced infarction in pig latissimus dorsi muscle flaps compared with the control, when these muscle flaps were subjected to 4 h of ischemia and 48 h of reperfusion (P < 0.05; n = 5 pigs/group). Both preischemic and postischemic cariporide treatment (3 mg/kg) induced a significant decrease in muscle myeloperoxidase activity and mitochondrial-free Ca(2+) content and a significant increase in muscle ATP content within 2 h of reperfusion (P < 0.05; n = 4 pigs/group). Preischemic and postischemic cariporide treatment (3 mg/kg) also significantly inhibited muscle NHE-1 protein expression within 2 h of reperfusion after 4 h of ischemia, compared with the control (P < 0.05; n = 3 pigs/group). These observations support our hypothesis that cariporide attenuates skeletal muscle infarction when administered at the onset of ischemia or reperfusion, and the mechanism involves attenuation of neutrophil accumulation and mitochondrial-free Ca(2+) overload and preservation of ATP synthesis in the early stage of reperfusion.

Acetaminophen is cardioprotective against H2O2-induced injury in vivo

Here we report our ongoing investigation of the cardiovascular effects of acetaminophen, with emphasis on oxidation-induced canine myocardial dysfunction. The objective of the current study was to investigate whether acetaminophen could attenuate exogenous H(2)O(2)-mediated myocardial dysfunction in vivo. Respiratory, metabolic, and hemodynamic indices such as left ventricular function (LVDP and +/-dP/dt(max)), and percent ectopy were measured in anesthetized, open-chest dogs during intravenous administration of 0.88 mM, 2.2 mM, 6.6 mM H(2)O(2). Following 6.6 mM H(2)O(2), tissue from the left ventricle was harvested for electron microscopy. Left ventricular function did not vary significantly between vehicle and acetaminophen groups under baseline conditions. Acetaminophen-treated dogs regained a significantly greater fraction of baseline function after high concentrations of H(2)O(2) than vehicle-treated dogs. Moreover, the incidence of H(2)O(2)-induced ventricular arrhythmias was significantly reduced in the acetaminophen-treated group. Percent ectopy following 6.6 mM concentrations of H(2)O(2) was 1 +/- 0.3 vs. 0.3 +/- 0.1 (P < 0.05) for vehicle- and acetaminophen-treated dogs, respectively. Additionally, electron micrograph images of left ventricular tissue confirmed preservation of tissue ultrastructure in acetaminophen-treated hearts when compared to vehicle. We conclude that, in the canine myocardium, acetaminophen is both functionally cardioprotective and antiarrhythmic against H(2)O(2)-induced oxidative injury.

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.

Effects of KR-33028, a novel Na+/H+exchanger-1 inhibitor, on ischemia and reperfusion-induced myocardial infarction in rats and dogs

The present study was performed to evaluate the cardioprotective effects of KR-33028, a novel Na+/H+ exchanger subtype 1 (NHE-1) inhibitor, in rat and dog models of coronary artery occlusion and reperfusion. In anesthetized rats subjected to a 45-min coronary occlusion and a 90-min reperfusion, KR-33028 at 5 min before occlusion (i.v. bolus) dose-dependently reduced myocardial infarct size from 58.0% to 46.6%, 40.3%, 39.7%, 33.1%, and 27.8% for 0.03, 0.1, 0.3, 1.0, and 3.0 mg/kg respectively (P < 0.05). In anesthetized beagle dogs that underwent a 1.0-h occlusion followed by a 3.0-h reperfusion, KR-33028 (3 mg/kg, i.v. bolus) markedly decreased infarct size from 45.6% in vehicle-treated group to 16.4% (P < 0.05), and reduced the reperfusion-induced release in creatine kinase myocardial band isoenzyme (MB), lactate dehydrogenase, troponin-I, glutamic oxaloacetic transaminase, and glutamic pyruvic transaminase. In separate experiments to assess the effects of timing of treatment, KR-33028 (1 mg/kg, i.v. bolus) given 10 min before or at reperfusion in rat models also significantly reduced the myocardial infarct size (46.3% and 44.1% respectively) compared with vehicle-treated group. In all studies, KR-33028 caused no significant changes in any hemodynamic profiles. In an isolated rat heart model of hypothermic cardioplegia, KR-33028 (30 mum), which was added to the heart preservation solution (histidin-tryptophan-ketoglutarate) during hypothermic cardioplegic arrest, significantly improved the recovery of left ventricular developed pressure, heart rate and dP/dt(max) after reperfusion. Taken together, these results indicate that KR-33028 significantly reduced the myocardial infarction induced by ischemia and reperfusion in rats and dogs, without affecting hemodynamic profiles.

Effect of sodium/hydrogen exchange inhibition on myocardial infarct size after coronary artery thrombosis and thrombolysis

This study examines the cardioprotective effects of Na+/H+ exchange inhibition with BIIB-722 or ischemic preconditioning after occlusive thrombus formation and subsequent thrombolysis for reperfusion. Coronary artery thrombosis was induced by vessel wall electrolytic injury. Thrombotic occlusion was maintained for 60 or 90 min in 4 different groups: (1) control; (2) Na+/H+ exchange inhibitor, BIIB-722 (3 mg/kg) before occlusion; (3) BIIB-722 (0.75 mg/kg) before reperfusion; (4) ischemic preconditioning (4 x 5 min). Thrombolysis with intracoronary recombinant tissue plasminogen activator produced reperfusion in 6.3 +/- 1.4 min (average for 68 dogs). After restoration of blood flow, vessel patency was maintained for 4 h with the glycoprotein IIb/IIIa receptor antagonist, BIBU 52ZW. BIIB-722, administered before (26.9 +/- 3.6%) or after (22.0 +/- 2.3%) 60-min ischemia or preconditioning (18.4 +/- 2.8%), produced comparable and significant reductions in infarct size (percent of area at risk) compared to controls (47.2 +/- 2.0%). After 90 min of ischemia, BIIB-722 administered before occlusion (37.3 +/- 1.1%) and ischemic preconditioning (35.0 +/- 4.8%) provided significant cardioprotection compared to control (45.9 +/- 1.8%). BIIB-722 was not cardioprotective when administered during occlusion (48.0 +/- 2.4%). The results indicate that Na+/H+ exchange inhibition and preconditioning provide a comparable degree of cardioprotection against 60 min of regional ischemia. However, when the regional ischemic period is extended to 90 min, the degree of cardioprotection is markedly reduced. Further studies incorporating clinically relevant events such as thrombosis and thrombolysis are required before one can conclude that Na+/H+ exchange inhibition is effective against more prolonged myocardial ischemia.

Effects of KR-32570, a new sodium hydrogen exchanger inhibitor, on myocardial infarction and arrhythmias induced by ischemia and reperfusion

The present study was performed to evaluate the cardioprotective effects of [5-(2-methoxy-5-chloro-5-phenyl)furan-2-ylcarbonyl]guanidine (KR-32570) in rat and dog models of coronary artery occlusion and reperfusion. In addition, we sought to clarify the efficacy of KR-32570 on reperfusion-induced fatal ventricular arrhythmia. In anesthetized rats subjected to 45-min coronary occlusion and 90-min reperfusion, KR-32570 (i.v. bolus) dose-dependently reduced myocardial infarct size from 58.0% to 50.7%, 35.3%, 33.5% and 27.0% for 0.03, 0.1, 0.3 and 1.0 mg/kg, respectively (P<0.05). In anesthetized beagle dogs that underwent 1.2-h occlusion followed by 3.0-h reperfusion, KR-32570 (3 mg/kg, i.v. bolus) markedly decreased infarct size from 28.9% in vehicle-treated group to 8.0% (P<0.05), and reduced the reperfusion-induced release in creatine kinase isoenzyme MB, lactate dehydrogenase, Troponin-I and glutamic-oxaloacetic transaminase. KR-32570 dose-dependently decreased the incidence of premature ventricular contraction, ventricular tachycardia or ventricular fibrillation induced by ischemia and reperfusion in rats. Similar results were obtained in dogs with reperfusion-induced arrhythmia. In separate experiments to assess the effects of timing of treatment, KR-32570 given 10 min before or at reperfusion in rat models also significantly reduced the myocardial infarct size (40.9% and 46.1%, respectively) compared with vehicle-treated group. In all studies, KR-32570 caused no significant changes in any hemodynamic profiles. Taken together, these results indicate that KR-32570 significantly reduced the myocardial infarction and incidence of arrhythmias induced by ischemia and reperfusion in rats and dogs, without affecting hemodynamic profiles. Thus, it could be potentially useful in the prevention and treatment of myocardial injuries and lethal ventricular arrhythmias.

Effects of KR-32570, a new Na+/H+ exchanger inhibitor, on functional and metabolic impairments produced by global ischemia and reperfusion in the perfused rat heart

The present study was performed to evaluate the cardioprotective effects of [5-(2-methoxy-5-chloro-5-phenyl)furan-2-ylcarbonyl]guanidine (KR-32570) on ischemia/reperfusion-induced mechanical and metabolic dysfunction in isolated rat hearts. In addition, the effects of KR-32570 on the Na(+)/H(+)-exchanger (NHE) and lipid peroxidation were also evaluated. KR-32570 strongly inhibited the recovery from acidosis induced by an NH(4)Cl prepulse in PS120 fibroblast cells expressing the human NHE-1 isoform (IC(50): 0.05 and 1.16 microM for KR-32570 and cariporide, respectively). In isolated perfused rat hearts subjected to 30-min ischemia/30-min reperfusion, KR-32570 (1-10 microM) significantly and concentration dependently improved cardiac contractile function and severe contracture in conjunction with causing a marked reduction in lactate dehydrogenase release. Additionally, it (1-10 microM) significantly increased the content of ATP, creatine phosphate and glycogen as well as decreased the tissue lactate content in heart homogenates following ischemia and reperfusion. KR-32570 (1-10 microM) significantly decreased the concentration of 8-iso-prostaglandin F(2 alpha), a reliable marker for oxidant stress, in perfusates from rat hearts subjected to ischemia and reperfusion. In separate experiments, KR-32570 significantly lowered the concentration of malondialdehyde in rat liver homogenate and inhibited Cu(2+)-induced peroxidation of low-density lipoprotein. Taken together, these results suggest that KR-32570 possesses potent cardioprotective effects in perfused rat hearts, and its effects may be mediated by inhibition of NHE-1, preservation of high-energy phosphates, and inhibition of lipid peroxidation.

Enhanced cardioprotection of the rat heart during hypothermic storage with combined Na+-H+ exchange inhibition and ATP-dependent potassium channel activation

BACKGROUND

We investigated the ability of mitochondrial adenosine triphosphate-dependent potassium-channel activation to augment the protection of Na(+)-H(+) exchanger inhibition in isolated working rat hearts after 6 hours of hypothermic storage in an extracellular-based cardioplegic solution.

METHODS

We treated hearts with the potassium-channel openers diazoxide (100 micromol/liter) or BMS-180448 (10 micromol/liter) or with the Na(+)-H(+) exchanger inhibitor cariporide (10 micromol/liter). Cariporide also was administered in combination with either diazoxide or BMS-180448 in 2 other treatment groups. All hearts were arrested and stored at 2 to 3 degrees C. After storage, we reperfused hearts for 10 minutes before performing work for a further 15 minutes, and then we measured and assessed cardiac function using a 2-way analysis of variance model.

RESULTS

Neither diazoxide nor BMS-180448 significantly improved recovery of cardiac output. Cariporide therapy significantly improved cardiac output compared with control. However, we obtained the greatest recovery of cardiac output when we combined cariporide with either diazoxide or BMS-180448.

CONCLUSIONS

Cariporide is more cardioprotective than the potassium-channel openers diazoxide and BMS-180448 after prolonged hypothermic storage. Co-administration of diazoxide or BMS-180448 with cariporide results in additive cardioprotection, with significantly improved cardiac function when compared with either treatment given alone. Such a combination could be used to improve the functional recovery of hearts stored for cardiac transplantation.

The cardioprotective effects of Na+/H+ exchange inhibition and mitochondrial KATP channel activation are additive in the isolated rat heart

The mechanisms of recovery of the isolated rat heart were studied after 30 min of global ischemia. Functional recovery was assessed by the percentage recovery of developed pressure after 30 min reperfusion and by the magnitude of the contracture on reperfusion. After a control ischemia, developed pressure recovered to only 12+/-2% of pre-ischemic control and the reperfusion contracture was very large (81+/-6 mmHg). Activation of the mitochondrial KATP channel with 100 microM diazoxide present throughout ischemia and reperfusion improved recovery of developed pressure to 36+/-3% and reduced the reperfusion contracture (53+/-4 mmHg). Inhibition of the sodium/hydrogen exchanger with 10 microM cariporide caused a larger recovery of developed pressure to 72+/-4% and further reduced the reperfusion contracture (11+/-3 mmHg). The combination of both drugs increased recovery of developed pressure to 96+/-4% and the reperfusion contracture remained small (11+/-5 mmHg). The effectiveness of the timing of exposure to these drugs was explored. When both diazoxide and cariporide were applied 2 min before the end of ischaemia and remained present during reperfusion the recovery of developed pressure was 81+/-4% and the reperfusion contracture was small (12+/-3 mmHg); neither was significantly different to the recovery when both drugs were present throughout ischemia and reperfusion. We conclude that mitochondrial damage, blocked by diazoxide, and the coupled exchanger pathway, blocked by cariporide, are two of the principal damage pathways and functional recovery appears to be complete when both are blocked. The combination of these drugs is also highly effective when given 2 min before the end of ischemia.