The effects of ischemic preconditioning on resting coronary flow and reactive hyperemia: involvement of A1 adenosine receptors

Unraveling the role of adenosine in remote ischemic preconditioning-induced cardioprotection

Remote ischemic preconditioning (RIPC) induced by alternate cycles of preconditioning ischemia and reperfusion protects the heart against sustained ischemia-reperfusion-induced injury. This technique has been translated to clinical levels in patients undergoing various surgical interventions including coronary artery bypass graft surgery, abdominal aortic aneurysm repair, percutaneous coronary intervention and heart valve surgery. Adenosine is a master regulator of energy metabolism and reduces myocardial ischemia-reperfusion-induced injury. Furthermore, adenosine is a critical trigger as well as a mediator in RIPC-induced cardioprotection and scientists have demonstrated the role of adenosine by showing an increase in its levels in the systemic circulation during RIPC delivery. Furthermore, the blockade of cardioprotective effects of RIPC in the presence of specific adenosine receptor blockers and transgenic animals with targeted ablation of A1 receptors has also demonstrated its critical role in RIPC. The studies have shown that adenosine may elicit cardioprotection via activation of neurogenic pathway. The present review describes the possible role and mechanism of adenosine in mediating RIPC-induced cardioprotection.

Ischemic preconditioning changes the pattern of coronary reactive hyperemia regardless of mitochondrial ATP-sensitive K(+) channel blockade

Ischemic preconditioning increases the velocity of vasodilatation and reduces the total hyperemic flow (THF) of a subsequent coronary reactive hyperemia (CRH). The increase in the velocity of vasodilatation has been shown to depend on an up-regulation of the endothelial release of nitric oxide, while the reduction of THF is attributed to an adenosine A(1) receptor-mediated mechanism. We investigated whether the changes in CRH induced by preconditioning ischemia (PI) can still be obtained after blockade of mitochondrial ATP-sensitive K(+) channels by sodium 5-hydroxydecanoate (5-HD), and whether the blockade per se affects the pattern of CRH. In anesthetized goats, flow was recorded from the left circumflex coronary artery (LCCA). CRH was obtained with the occlusion of LCCA for 15 s. PI was obtained by 2 cycles of 2.5 min of LCCA occlusion with a 5 min interval of reperfusion between the two occlusions. CRH was studied before and after i.v. administration of 5-HD (20 mg/kg), as well as in the presence of 5-HD after PI. Following 5-HD, the pattern of CRH remained unchanged. After 5-HD and PI, velocity of vasodilatation and total hyperemic flow of CRH showed the same changes as in previous studies after PI alone. It was concluded that the blockade of mitochondrial ATP-sensitive K(+) channels, which is reported to prevent myocardial protection, does not affect CRH and does not prevent PI from increasing the velocity of vasodilatation and reducing THF. These results demonstrate that the changes induced in CRH by preconditioning are independent of the opening of the mitochondrial ATP-sensitive K(+) channels.

Pharmacologic preconditioning of estrogen by activation of the myocardial adenosine triphosphate-sensitive potassium channel in patients undergoing coronary angioplasty

OBJECTIVES

The purpose of this study was to determine whether administration of estrogen produces cardioprotective effects in patients undergoing coronary angioplasty.

BACKGROUND

We have previously demonstrated that estrogen can provide cardioprotection by activating the mitochondrial adenosine triphosphate-sensitive potassium (K(ATP)) channel, a major contributor to ischemic cardioprotection.

METHODS

Fifty patients undergoing angioplasty of a major epicardial coronary artery were randomly allocated to either ischemic preconditioning or intracoronary estrogen administration in the presence or absence of glibenclamide (glyburide).

RESULTS

The coronary collateral circulation, as quantitatively assessed by an intracoronary Doppler flow wire, was similar during balloon inflation among the groups. Patients in the preconditioned and estrogen-treated groups significantly lowered their ischemic burden, as assessed by an ST-segment shift, chest pain score and myocardial lactate extraction ratio, as compared with control subjects. The reduction in the ST-segment shift afforded by estrogen during the first inflation (-63% vs. first inflation in the preconditioned group) was similar to that afforded by preconditioning during the second inflation (-68% vs. first inflation). In contrast, the patients given glibenclamide developed significantly higher ischemic burden during the first and second inflations, as compared with those in the estrogen-treated group alone.

CONCLUSIONS

It is concluded that intracoronary administration of estrogen before balloon angioplasty rendered the myocardium relatively resistant to subsequent ischemia, and the degree of cardioprotective effect was comparable to that afforded by ischemic preconditioning. The effect of estrogen was abolished by glibenclamide, suggesting that the cardioprotective effect of estrogen may result from activation of myocardial K(ATP) channels.

2-deoxyglucose enhances epileptic tolerance evoked by transient incomplete brain ischemia in mice

The aim of the study was to assess the influence of chronic treatment with a non-metabolisable glucose analogue, 2-deoxyglucose (2-DG) at a 150 mg/kg dose on long-term epileptic tolerance (ET) evoked by 30 min bilateral carotid artery clamping (BCCA) in mice. The effects of protein synthesis inhibition with cycloheximide (CHX), given in three daily doses of 2.5 mg/kg starting either 1 day before (peri-insult regimen) or 1 day after the priming insult (post-insult regimen), on ET development was also studied. Seizures were induced 14 days after BCCA with 3.5 mg/kg of bicuculline; this dose (CD97) evokes convulsions in 97% of normal untreated mice. BCCA resulted in decreased mortality, prolonged latency to the onset of generalised convulsions and decreased overall seizure score. CHX given in the post-insult regimen did not influence, while the peri-insult regimen abolished, all signs of BCCA-evoked ET. 2-DG treatment of sham-operated animals resulted in a moderate but significant decrease in mortality rate and a tendency toward a lower seizure score. BCCA combined with 2-DG treatment resulted in a marked decrease in mortality rate, as well as reduction in all indicators of seizure susceptibility. CHX abolished the antiepileptic effects of BCCA alone, as well as BCCA combined with 2-DG, while it did not influence the 2-DG-related decrease in mortality. We conclude that the development of BCCA-induced epileptic tolerance, as well as unmasking antiepileptic effects of 2-DG by BCCA, is dependent on protein synthesis.

Adenosine preconditions against endothelin-induced constriction of coronary arterioles

Myocardial hypoperfusion is accompanied by concomitant increases in adenosine and endothelin-1 (ET-1) production, but the vasodilatory effect of adenosine prevails over that of ET-1. Therefore, we hypothesized that adenosine-induced or ischemic preconditioning reduces the vasoconstrictive effect of ET-1. Coronary arteriolar diameter in vivo was measured using fluorescence microangiography in anesthetized open-thorax dogs. ET-1 (5 ng. kg(-1). min(-1) administered intracoronary, n = 10) induced progressive constriction over 45 min [25 +/- 6% (SE)]. The constriction was blocked by preconditioning with adenosine (25 microgram. kg(-1). min(-1) administered intracoronary) for 20 min and 10 min of washout (n = 10) or attenuated by ischemic preconditioning (four 5-min periods of ischemia, 9 +/- 5% at 45 min). To investigate the receptor involved in this process, coronary arterioles (50-150 micrometer) were isolated and pressurized at 60 mmHg in vitro. The ET-1 dose-response curve (1 pM-5 nM) was rightward shifted after preconditioning with adenosine (1 microM) for 20 min and 10 min of washout (n = 11). Blockade of A(2) receptors [8-(3-chlorostyryl)caffeine, 1 microM, n = 9] but not A(1) receptors (8-cyclopentyl-1,3-dipropylxanthine, 100 nM, n = 7) prevented this shift. These results suggest that adenosine confers a vascular preconditioning effect, mediated via the A(2) receptor, against endothelin-induced constriction. This effect may offer a new protective function of adenosine in preventing excessive coronary constriction.

New insights into nitric oxide and coronary circulation

Since its discovery over 20 years ago as an intercellular messenger, nitric oxide (NO), has been extensively studied with regard to its involvement in the control of the circulation and, more recently, in the prevention of atherosclerosis. The importance of NO in coronary blood flow control has also been recognized. NO-independent vasodilation causes increased shear stress within the blood vessel which, in turn, stimulates endothelial NO synthase activation, NO release and prolongation of vasodilation. Reactive hyperemia, myogenic vasodilation and vasodilator effects of acetylcholine and bradykinin are all mediated by NO. Ischemic preconditioning, which protects the myocardium from cellular damage and arrhythmias, is itself linked with NO and both the first and second windows of protection may be due to NO release. Exercise increases NO synthesis via increases in shear stress and pulse pressure and so it is likely that NO is an important blood flow regulatory mechanism in exercise. This phenomenon may account for the beneficial effects of exercise seen in atherosclerotic individuals. Whilst NO plays a protective role in preventing atherosclerosis via superoxide anion scavenging, risk factors such as hypercholesterolemia reduce NO release leading the way for endothelial dysfunction and atherosclerotic lesions. Exercise reverses this process by stimulating NO synthesis and release. Other factors impacting on the activity of NO include estrogens, endothelins, adrenomedullin and adenosine, the last appearing to be a compensatory pathway for coronary control in the presence of NO inhibition. These studies reinforce the pivotal role played by the substance in the control of coronary circulation.