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.

Long-term treatment with nipradilol, a nitric oxide-releasing beta-adrenergic blocker, enhances postischemic recovery and limits infarct size

BACKGROUND

This study examines whether the chronic administration of nipradilol, a nitric oxide-releasing beta-adrenergic blocker, decreases ischemia-reperfusion injury.

METHODS

Rats were treated with nipradilol (10 mg/kg per day orally) or a vehicle alone for 4 weeks. Isolated rat hearts were assigned to one of five groups (each n = 6): global ischemia groups treated with the vehicle or with nipradilol were subjected to 20 minutes of ischemia; ischemic preconditioning groups treated with the vehicle or with nipradilol were subjected to 3 minutes of ischemic preconditioning; and the L-arginine group treated with the vehicle received 1 mmol/L of L-arginine before global ischemia. Hemodynamic variables and coronary flow were recorded continuously. Nitrites and nitrates levels were measured 60 minutes after reperfusion, and the infarct size was determined. In another series (each n = 6), lipid peroxidation was investigated.

RESULTS

In the nipradilol group, significant preservation of the left ventricular pressure and coronary flow, as well as the level of nitrates and nitrites, was observed, compared with the global ischemia group. The infarct size was also significantly reduced in the ischemic preconditioning (23.5%+/-5.47%), L-arginine (25.6%+/-5.59%), and especially the nipradilol (10.7%+/-1.65%) groups. However, in the nipradilol plus ischemic preconditioning group, the protective effects were eliminated. Lipid peroxidation after nipradilol treatment was significantly reduced before and after global ischemia, compared with the global ischemia group.

CONCLUSIONS

The chronic administration of nipradilol improves postischemic functional recovery and infarct size, partly by preventing the formation of lipid peroxides. These cardioprotective effects were, however, abolished by ischemic preconditioning.

L-Arginine given after ischaemic preconditioning can enhance cardioprotection in isolated rat hearts

OBJECTIVE

Ischaemic or pharmacological preconditioning with L-arginine has been reported to be insufficient for optimal cardioprotection. The ability of nitric oxide (NO) to enhance ischaemic preconditioning was assessed, and the role of L-arginine-induced ischaemic preconditioning in myocardial protection was determined.

METHODS

Isolated rat hearts were prepared and divided into six groups: control hearts (control, n=6) were perfused without global ischaemia at 37 degrees C for 160 min; global ischaemia hearts (GI, n=6) were subjected to ischaemia for 20 min and reperfusion for 120 min; ischaemic preconditioned hearts (IP, n=6) received 2 min of zero-flow global ischaemia followed by 5 min reperfusion, before 20 min of global ischaemia; L-arginine hearts (ARG, n=6) received 1 mmol/l L-arginine for 5 min, before 20 min of global ischaemia; ischaemic preconditioning plus nitro-L-arginine methyl ester hearts (IP+L-NAME, n=6) received 2 min of ischaemic preconditioning and 5 min reperfusion with 3 mmol/l L-NAME in Krebs-Henseleit buffer, before 20 min of global ischaemia; and ischaemic preconditioning plus L-arginine hearts (IP+ARG, n=6) received 2 min of ischaemic preconditioning and 5 min reperfusion with 1 mmol/l L-arginine in Krebs-Henseleit buffer. Haemodynamic parameters and coronary flow were recorded continuously. Nitrites and nitrates (NOx) were measured 5 and 60 min after reperfusion, and infarct size was also determined.

RESULTS

In the IP+ARG group, significant amelioration and preservation of left ventricular peak developed pressure and coronary flow was observed compared with the GI, IP, ARG and IP+L-NAME groups. Infarct size in the IP+ARG group was reduced significantly compared with that in the GI, IP, ARG and IP+L-NAME groups. Significant preservation of NOx was observed during reperfusion in the IP+ARG group compared with the GI group.

CONCLUSIONS

Inhibition of NO synthase with L-NAME had little impact on ischaemic preconditioning, suggesting that endogenous NO is not a major mediator of ischaemic preconditioning. Nevertheless, enhancement of the effects of ischaemic preconditioning can be achieved with L-arginine, a precursor of NO, improving post-ischaemic functional recovery and infarct size in the isolated rat heart.

Coronary endothelial dysfunction after ischemia and reperfusion: a new therapeutic target?

Although cardiac ischemia is usually characterized as a disease of the myocyte, it is clear that the vasculature, and especially endothelial cells, is also a major target of this pathology. Indeed, using a rat model of ischemia/reperfusion, we were able to detect severe endothelial dysfunction (assessed as a decreased response to acetylcholine) after acute or chronic reperfusion. Given the essential role of the endothelium in the regulation of vascular tone, as well as platelet and leukocyte function, such a severe dysfunction could lead to an increased risk of vasospasm, thrombosis and accelerated atherosclerosis. This dysfunction can be prevented by free radical scavengers and by exogenous nitric oxide. Endothelial dysfunction can also be prevented by preconditioning with brief periods of intermittent ischemia, thus extending to coronary endothelial cells the concept of endogenous protection previously described at the myocyte level. Experiments performed on cultured cells showed that the endothelial protection induced by free radical scavengers or by preconditioning was due to a lesser expression of endothelial adhesion molecules such as intercellular adhesion molecule-1, leading to a lesser adhesion of neutrophils to endothelial cells. Identification of the mechanisms of this protection may lead to the development of new strategies aimed at protecting the vasculature in ischemic heart diseases.

Pharmacological modulation, preclinical studies, and new clinical features of myocardial ischemic preconditioning

The term "ischemic preconditioning (PC)" was first applied to canine myocardium subjected to brief episodes of ischemia and reperfusion that tolerated a more prolonged episode of ischemia better than myocardium not previously exposed to ischemia. Protective effect of myocardial ischemic PC was demonstrated in several animal species, resulting in the strongest endogenous form of protection against myocardial injury, jeopardized myocardium, infarct size, and arrhythmias other than early reperfusion. New onset angina before acute myocardial infarction, episodes of myocardial ischemia during coronary angioplasty or bypass surgery, and the "warm-up" phenomenon may represent clinical counterparts of the PC phenomenon in humans. Here, we have attempted to summarize pharmacological modulation, preclinical studies, and new clinical features of ischemic PC. To date, the pathophysiological basis of the "chemical PC" is still not well established, and "putting PC in a bottle" for clinical applications still remains a new pharmacological venture.

Preconditioning during simulated MIDCABG attenuates blood flow defects and neutrophil accumulation

BACKGROUND

Ischemic preconditioning (IP) may be cardioprotective in minimally invasive direct coronary artery bypass where cardioplegia is not used. This study tested the hypothesis that IP of the area at risk (AAR) would attenuate postischemic injury from transient coronary artery occlusion.

METHODS

In 19 anesthetized dogs, the left anterior descending coronary artery was occluded for 30 minutes (simulating coronary occlusion during anastomosis) followed by 3 hours of reperfusion. In 10 dogs, occlusion was preceded by 5 minutes of occlusion and 5 minutes of reperfusion (IP), whereas 9 other dogs had no IP (control, C).

RESULTS

Thirty minutes of left anterior descending occlusion caused comparable dyskinesis (systolic shortening, sonomicrometry) in the AAR in C (baseline, 29% +/- 3% to 3% +/- 2%) and in IP (baseline, 29% +/- 2% to -0.3% +/- 2%). After 3 hours of reperfusion, systolic shortening was significantly depressed in C (20% +/- 4%), and was not significantly improved by IP (24% +/- 3%, p = 0.8 versus C). Postischemic diastolic stiffness in the AAR was not altered by IP versus C (0.60 +/- 0.12 versus 0.41 +/- 0.13). Plasma creatine kinase activity was similar between C and IP at the end of reperfusion (20 +/- 11 versus 16 +/- 5 U/g). Postischemic AAR blood flow (in milliliters per minute per gram of tissue) at 180 minutes of reperfusion decreased by 56% versus baseline in C (from 1.04 +/- 0.4 to 0.46 +/- 0.12; p < 0.05) compared with no change in IP (from 0.74 +/- 0.23 to 0.60 +/- 0.10), but there was no significant group difference at this time. Myeloperoxidase activity as an index of neutrophil accumulation in AAR was decreased in IP versus C (0.4 +/- 0.09 versus 0.7 +/- 0.04 U/microg tissue).

CONCLUSIONS

Ischemic preconditioning does not decrease postischemic wall motion and only modestly increases postischemic blood flow abnormalities in the AAR, but does significantly inhibit neutrophil accumulation.

Preconditioning by transient myocardial ischemia confers protection against ischemia-induced ventricular arrhythmias in variant angina

BACKGROUND

In experimental models, ischemic preconditioning of the heart protects against ischemic damage and ventricular arrhythmias during subsequent coronary occlusion. In this study, we investigated whether protection against ischemic suffering and ischemia-induced arrhythmias may occur after spontaneous transmural ischemia in humans.

METHODS AND RESULTS

We performed 24-hour Holter monitoring in 10 patients with variant angina who developed complex ventricular arrhythmias (CVAs, more than five premature ventricular beats per minute or repetitive ventricular arrhythmias) during episodes of ST-segment elevation. A total of 150 episodes of ST-segment elevation were detected on Holter monitoring, 21 (14%) of which showed CVAs. Episodes separated from the previous one by a time interval of < or = 30 minutes or by a time interval of > 30 minutes did not differ in either magnitude or duration of ST-segment elevation, but CVAs occurred more frequently in the second group (3% versus 29%, P < .0001). The time interval from the preceding ischemic episode was longer for the episodes with compared with those without CVAs (197 +/- 275 versus 57 +/- 87 minutes, P < .001), but these two groups of episodes also had similar severities and durations of ST-segment elevation. Finally, when we analyzed 13 clusters of two to six ischemic episodes, CVAs were found much more frequently in the first (92%) than in the last (23%, P = .009) episode of the clusters, while ST-segment elevations were similar (2.1 +/- 1.6 versus 2.2 +/- 1.1 mm) and ischemia durations shorter in the first than in the last episode (3.9 +/- 3.6 versus 6.1 +/- 1.7 minutes, P = .03).

CONCLUSIONS

Our data indicate that preconditioning by transient ischemia induces a significant protection against ischemia-induced CVAs in patients with variant angina. This beneficial effect was not related to a reduction in either severity or duration of ischemia, suggesting that arrhythmic protection was a direct consequence of preconditioning rather than an epiphenomenon of ischemic protection.