The effect of body temperature on myocardial protection conferred by ischaemic preconditioning or the selective adenosine A1 receptor agonist GR79236, in an anaesthetized rabbit model of myocardial ischaemia and reperfusion

Interaction between neuronal nitric oxide synthase signaling and temperature influences sarcoplasmic reticulum calcium leak: role of nitroso-redox balance

RATIONALE

Although nitric oxide (NO) signaling modulates cardiac function and excitation-contraction coupling, opposing results because of inconsistent experimental conditions, particularly with respect to temperature, confound the ability to elucidate NO signaling pathways. Here, we show that temperature significantly modulates NO effects.

OBJECTIVE

To test the hypothesis that temperature profoundly affects nitroso-redox equilibrium, thereby affecting sarcoplasmic reticulum (SR) calcium (Ca(2+)) leak.

METHODS AND RESULTS

We measured SR Ca(2+) leak in cardiomyocytes from wild-type (WT), NO/redox imbalance (neuronal nitric oxide synthase-deficient mice-1 [NOS1(-/-)]), and hyper S-nitrosoglutathione reductase-deficient (GSNOR(-/-)) mice. In WT cardiomyocytes, SR Ca(2+) leak increased because temperature decreased from 37°C to 23°C, whereas in NOS1(-/-) cells, the leak suddenly increased when the temperature surpassed 30°C. GSNOR(-/-) cardiomyocytes exhibited low leak throughout the temperature range. Exogenously added NO had a biphasic effect on NOS1(-/-) cardiomyocytes; reducing leak at 37°C but increasing it at subphysiological temperatures. Oxypurinol and Tempol diminished the leak in NOS1(-/-) cardiomyocytes. Cooling from 37°C to 23°C increased reactive oxygen species generation in WT but decreased it in NOS1(-/-) cardiomyocytes. Oxypurinol further reduced reactive oxygen species generation. At 23°C in WT cells, leak was decreased by tetrahydrobiopterin, an essential NOS cofactor. Cooling significantly increased SR Ca(2+) content in NOS1(-/-) cells but had no effect in WT or GSNOR(-/-).

CONCLUSIONS

Ca(2+) leak and temperature are normally inversely proportional, whereas NOS1 deficiency reverses this effect, increasing leak and elevating reactive oxygen species production because temperature increases. Reduced denitrosylation (GSNOR deficiency) eliminates the temperature dependence of leak. Thus, temperature regulates the balance between NO and reactive oxygen species which in turn has a major effect on SR Ca(2+).

Adenosine receptor-mediated cardioprotection: are all 4 subtypes required or redundant?

Adenosine is a purine nucleoside, which is produced primarily through the metabolism of adenosine triphosphate (ATP), therefore its levels increase during stressful situations when ATP utilization increases. Adenosine exerts potent cardioprotective effects on the ischemic/reperfused heart, reducing reversible and irreversible myocardial injury. Adenosine receptors (ARs) are G-protein-coupled receptors, and 4 subtypes exist--A(1), A(2A), A(2B), and A(3), all of which have been shown to be cardioprotective. Adenosine receptors are expressed on multiple cardiac cells, including fibroblasts, endothelial cells, smooth muscle cells, and myocytes. Activation of both A(1) and A(3) receptors prior to ischemia has been shown in multiple experimental models to reduce ischemia/reperfusion-induced cardiac injury. Additionally, activation of the A(2A) receptor at the onset of reperfusion has been shown to reduce injury. Most recently, there is evidence that the A(2B) receptor has cardioprotective effects upon its activation. However, controversy remains regarding the precise timing of activation of these receptors required to induce cardioprotection, as well as their involvement in ischemic preconditioning and postconditioning. Adenosine receptors have been suggested to reduce cell death through actions at the mitochondrial ATP-dependent potassium (K(ATP)) channel, as well as protein kinase C and mitogen-activated protein kinase (MAPK) signaling. Additionally, the ability of ARs to interact has been documented, and several recent reports suggest that these interactions play a role in AR-mediated cardioprotection. This review summarizes the current knowledge of the cardioprotective effects of each AR subtype, as well as the proposed mechanisms of AR cardioprotection. Additionally, the role of AR interactions in cardioprotection is discussed.

Dual acting antioxidant A1 adenosine receptor agonists

Herein we report the synthesis and biological evaluation of some potent and selective A(1) adenosine receptor agonists, which incorporate a functionalised linker attached to an antioxidant moiety. N(6)-(2,2,5,5-Tetramethylpyrrolidin-1-yloxyl-3-ylmethyl)adenosine (VCP28, 2e) proved to be an agonist with high affinity (K(i)=50nM) and good selectivity (A(3)/A(1) > or = 400) for the A(1) adenosine receptor. N(6)-[4-[2-[1,1,3,3-Tetramethylisoindolin-2-yloxyl-5-amido]ethyl]phenyl]adenosine (VCP102, 5a) has higher binding affinity (K(i)=7 nM), but lower selectivity (A(3)/A(1)= approximately 3). All compounds bind weakly (K(i)>1 microM) to A(2A) and A(2B) receptors. The combination of A(1) agonist activity and antioxidant activity has the potential to produce cardioprotective effects.

Cardioprotective effects of novel tetrahydroisoquinoline analogs of nitrobenzylmercaptopurine riboside in an isolated perfused rat heart model of acute myocardial infarction

We have investigated the cardioprotective effects of novel tetrahydroisoquinoline nitrobenzylmercaptopurine riboside (NBMPR) analog nucleoside transport (NT) inhibitors, compounds 2 and 4, in isolated perfused rat hearts. Langendorff-perfused heart preparations were subjected to 10 min of treatment with compound 2, compound 4, or vehicle (control) followed by 30 min of global ischemia and 120 min of reperfusion. For determination of infarct size, reperfusion time was 180 min. At 1 microM, compounds 2 and 4 provided excellent cardioprotection, with left ventricular developed pressure (LVDP) recovery and end-diastolic pressure (EDP) increase of 82.9 +/- 4.0% (P<0.001) and 14.1 +/- 2.0 mmHg (P<0.03) for compound 2-treated hearts and 79.2 +/- 5.9% (P<0.002) and 7.5 +/- 2.7 mmHg (P<0.01) for compound 4-treated hearts compared with 41.6 +/- 5.2% and 42.5 +/- 6.5 mmHg for control hearts. LVDP recovery and EDP increase were 64.1 +/- 4.2% and 29.1 +/- 2.5 mmHg for hearts treated with 1 microM NBMPR. Compound 4 was the best cardioprotective agent, affording significant cardioprotection, even at 0.1 microM, with LVDP recovery and EDP increase of 76.0 +/- 4.9% (P<0.003) and 14.1 +/- 1.0 mmHg (P<0.03). At 1 microM, compound 4 and NBMPR reduced infarct size, with infarct area-to-total risk area ratios of 29.13 +/- 3.17 (P<0.001) for compound 4 and 37.5 +/- 3.42 (P<0.01) for NBMPR vs. 51.08 +/- 5.06% for control hearts. Infarct size was more effectively reduced by compound 4 than by NBMPR (P<0.02). These new tetrahydroisoquinoline NBMPR analogs are not only potent cardioprotective agents but are, also, more effective than NBMPR in this model.

Amelioration of Ischemia/Reperfusion-Induced Myocardial Infarction by the 2-Alkynyladenosine Derivative 2-Octynyladenosine (YT-146)

The present study was aimed at determining whether the novel adenosine A2-agonist YT-146 may have cardioprotective effects against ischemia-reperfusion injury. Anesthetized open-chest dogs underwent 90-min occlusion of the left anterior descending artery and subsequent 300-min reperfusion. The animals were randomly assigned to receive vehicle, 3, or 10 microg/kg YT-146 or ischemic preconditioning (4 episodes of 5 min occlusion followed by 5 min of reperfusion). Blood pressure, heart rate, and regional myocardial blood flow throughout the experiment were measured, as was the myocardial infarct size after reperfusion. The infarct size of the vehicle-treated dog was 56.2% +/- 2.7% (n = 5), whereas that of 3 or 10 microg/kg YT-146-treated dog was smaller (ie, 29.5% +/- 8.7% or 20.2% +/- 7.0%, respectively; n = 5). The infarct size of the dog treated with 10 microg/kg YT-146 was reduced to a degree similar to that of the ischemic preconditioning (19.2% +/- 6.3%, n = 5). YT-146 at both doses elicited a dose-dependent increase in acute hyperemic coronary flow immediately after reperfusion. The cardioprotective effect may be attributed to the limitation of the infarct size, probably via A2-receptor-mediated coronary artery dilatation during the early period of reperfusion.

Does a selective adenosine A(1) receptor agonist protect against exercise induced ischaemia in patients with coronary artery disease?

BACKGROUND

The "warm up" effect in angina may represent ischaemic preconditioning, which is mediated by adenosine A(1) receptors in most models.

OBJECTIVE

To investigate the effect of a selective A(1) agonist, GR79236 (GlaxoSmithKline), on exercise induced angina and ischaemic left ventricular dysfunction in patients with coronary artery disease.

DESIGN

A double blind crossover study.

PATIENTS

25 patients with multivessel coronary artery disease.

INTERVENTIONS

On mornings one week apart, patients received intravenous GR79236 10 microgram/kg or placebo, and then carried out two supine bicycle exercise tests separated by 30 minutes. Equilibrium radionuclide angiography was done before and during exercise.

RESULTS

The onset of chest pain or 1 mm ST depression was delayed and occurred at a higher rate-pressure product during the second exercise test following either placebo or GR79236. Compared with placebo, GR79236 did not affect these indices during equivalent tests. GR79236 reduced resting global ejection fraction from (mean (SD)) 63 (7)% to 61 (5)% (p < 0.05) by a selective reduction in the regional ejection fraction of "ischaemic" left ventricular sectors (those where the ejection fraction fell during the first exercise test following placebo). Ischaemic sectors showed increased function during the second test following placebo (72 (21)% v 66 (20)%; p = 0.0001), or during the first test following GR79236 (69 (21)% v 66 (20)%; p = 0.0001). Sequential exercise further increased the function of ischaemic sectors even after drug administration.

CONCLUSIONS

GR79236 failed to mimic the warm up effect, and warm up occurred even in the presence of this agent. This suggests that ischaemic preconditioning is not an important component of this type of protection. The complex actions of the drug on regional left ventricular function at rest and during exercise suggest several competing A(1) mediated actions.

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.