Jon D. Thornton

Protection against infarction afforded by preconditioning is mediated by A1 adenosine receptors in rabbit heart.

BackgroundPreconditioning (5 minutes of ischemia followed by 10 minutes of recovery) renders the heart very resistant to infarction from subsequent ischemia. This study tests whether adenosine receptors might mediate preconditioning protection. Methods and ResultsWe examined the effect on infarct size of pretreatment with either of two adenosine receptor antagonists in both control and preconditioned in situ rabbit hearts. Hearts underwent 30 minutes of regional ischemia plus 3 hours of reperfusion, and infarct size was measured with tetrazolium. Infarct size averaged 39% of the zone at risk in controls but only 8% in preconditioned hearts. Preconditioned and nonpreconditioned hearts receiving either blocker had infarcts not different in size from the controls. A 5-minute intracoronary infusion of adenosine was as effective as 5 minutes of ischemia in protecting parabiotically perfused isolated hearts against infarction from a 45-minute ischemic insult. Similarly, intracoronary infusion of N6-1-(phenyl-2R-isopropyl)adenosine, an A l-selective adenosine receptor agonist, at a dose that delayed conduction but did not dilate the coronary vessels, also limited infarct size. The protection disappeared when we reduced the coronary concentration of drug by intravenous infusion of adenosine, indicating that cardiac rather than peripheral receptors were involved in the protection. ConclusionsWe conclude that adenosine released during the preconditioning occlusion stimulates cardiac A 1 receptors, which leaves the heart protected against infarction even after the adenosine has been withdrawn. (Circulation 1991;84:350–356)

Intravenous pretreatment with A1-selective adenosine analogues protects the heart against infarction.

BackgroundRecent data from this laboratory indicate that pretreatment with adenosine can protect the heart against infarction via A1-receptors, but because of systemic hypotension, adenosine had to be given into the coronary circulation. Methods and ResultsIn this study, we tested whether that protection could be achieved by intravenous administration of the A1-selective adenosine agonists N6-(phenyl-2R-isopropyl)- adenosine (PIA) and 2-chloro-N6-cyclopentyladenosine (CCPA). Nine groups of open-chest anesthetized rabbits were subjected to 30 minutes of regional coronary ischemia and 3 hours of reperfusion. Infarct size was determined by tetrazolium staining. Control hearts receiving no treatment had 38±4% of the risk zone infarcted. Preconditioning with 5 minutes of ischemia and 10 minutes of reperfusion before ischemia limited the infarct to 8±4%. Intravenous PTA 15 minutes before 30-minute ischemia also limited infarct size to 6±2% at the highest dose. CCPA offered similar protection. When the PIA was given at reperfusion, infarct size was 46±6%, indicating that receptor activation must precede ischemia to protect. Pretreatment with CGS 21680, a selective A2-receptor agonist, caused identical hypotension but failed to limit infarct size (43±3%), indicating again that the A1-receptor is involved. When rabbits pretreated with PTA were paced at 220 beats per minute, PIA still limited infarct size (16±4%), indicating that protection was not the result of bradycardia. ConclusionsThese results indicate that stimulation of adenosine A1-receptors causes the heart to become resistant to ischemia and that this protection can be achieved with intravenous administration of A1-selective agents.

Blockade of ATP-sensitive potassium channels increases infarct size but does not prevent preconditioning in rabbit hearts.

Ischemic preconditioning renders the heart resistant to infarction by an unknown mechanism. This study tests whether preconditioning may be working through activation of ATP-sensitive potassium channels. If that were the case, then blockade of the channels should eliminate preconditionings protection, and activation of these channels should mimic it. Thirty minutes of regional coronary ischemia followed by 3 hours of reperfusion caused 38.0 +/- 3.7% of the risk zone to become infarcted in control rabbits. Preconditioning with 5-minute ischemia followed by a 10-minute reperfusion before the 30-minute insult caused only 8.8 +/- 2.1% infarction, which was a reduction of 29.2% in infarct size by preconditioning (p < 0.01 versus control value). Pretreatment with the potassium channel blocker glibenclamide at three different concentrations significantly elevated infarct size in the nonpreconditioned hearts at all doses. Preconditioning, however, continued to limit infarct size by an amount not different from that seen in the control group at all doses of glibenclamide. Pinacidil, a potassium channel agonist, given before a 30-minute ischemic insult resulted in infarct sizes no different from that seen in nonpreconditioned control rabbits. We conclude that ATP-sensitive potassium channels are not involved in preconditioning in the rabbit heart; however, blocking those channels does exacerbate ischemia.

Streptozotocin-induced non-insulin-dependent diabetes protects the heart from infarction.

BACKGROUND The vulnerability of the myocardium of a diabetic animal to an ischemic insult is controversial. To address this issue, streptozotocin-induced non-insulin-dependent diabetes (NIDD) was induced in rats, and the effects of regional myocardial ischemia were assessed by measuring infarct size. METHODS AND RESULTS Open-chest rats with NIDD and age-matched control rats underwent 30 or 45 minutes of regional ischemia and 2-hour reperfusion. Infarct size was measured by tetrazolium. Control rats had 32.0 +/- 3.3% infarction of the risk zone after a 30-minute coronary occlusion, whereas NIDD rats had significantly smaller infarcts (11.5 +/- 3.1% of the risk area, P < .005). When ischemic time was extended to 45 minutes, infarct size in control animals averaged 57.9 +/- 6.2%, whereas only 37.3 +/- 5.6% of ischemic myocardium was infarcted in NIDD rats (P < .05). In a subset NIDD group, rats experienced a period of ischemic preconditioning (three cycles of 5-minute ischemia/5-minute reperfusion) before 45-minute ischemia. Infarct size in these rats averaged only 6.9 +/- 3.0% (P < .01 vs nonpreconditioned NIDD rats with 45-minute coronary occlusions). Collateral flow was measured in NIDD rat hearts with radioactive microspheres. Collateral flow was < 1% of normal myocardial blood flow. CONCLUSIONS We conclude that NIDD protects the heart from infarction and that this protection is not related to the development of coronary collaterals. Furthermore, preconditioning can further protect the NIDD heart.

Catecholamines can induce adenosine receptor-mediated protection of the myocardium but do not participate in ischemic preconditioning in the rabbit.

The role of catecholamines in ischemic preconditioning is unclear. Accordingly, the effects of tyramine-induced norepinephrine release and alpha 1-receptor blockade were examined. Ischemic preconditioning with a 5-minute coronary occlusion 10 minutes before a 30-minute ischemic interval resulted in only 7.7 +/- 3.1% infarction of the risk area, significantly less than that in control rabbits with isolated 30-minute coronary occlusions (34.4 +/- 3.2%, P < .01). Intravenous infusion of tyramine 10 minutes before 30 minutes of ischemia also protected the heart from infarction to an extent similar to that seen with ischemic preconditioning (6.9 +/- 2.4% infarction). This protection observed with tyramine infusion was eliminated by alpha 1-receptor blockade with BE 2254 (36.8 +/- 2.6% infarction) but was unaffected by beta-blockade with propranolol (10.5 +/- 2.4% infarction). Furthermore, the protection was unaffected when the tyramine-induced hypertension was attenuated by allowing blood to flow into a volume reservoir (3.9 +/- 0.8% infarction, P < .01 vs control value). The nonselective adenosine-receptor blocker PD 115,199 also eliminated tyramine-induced protection (40.2 +/- 5.6% infarction), indicating that adenosine is involved in adrenergic-mediated protection. BE 2254 could not block ischemic preconditioning (3.9 +/- 1.1% infarction, P < .01 vs control value). Therefore, catecholamine release before prolonged ischemia can protect the heart from infarction via the alpha 1-receptor, but adenosine receptor stimulation is also involved. alpha-Adrenergic stimulation does not appear to be critical to the protection observed after ischemic preconditioning.

Hyperbaric oxygen limits infarct size in ischemic rabbit myocardium in vivo.

BackgroundWe explored the ability of increased oxygen pressure to modify necrosis in an open-chest rabbit model of myocardial ischemia and reperfusion. Methods and ResultsA branch of the left coronary artery was occluded for 30 minutes followed by 3 hours of reperfusion. Infarction was measured by triphenyl tetrazolium staining and expressed as a percentage of the ischemic zone. Untreated rabbits were ventilated with 100% oxygen at 1 atm absolute. Treatment animals were exposed to hyperbaric oxygen at 2.5 atm absolute. The l.O-atm control hearts developed 41.5±4.6% infarction of the ischemic zone. Animals exposed to hyperbaric oxygen during ischemia only, reperfhsion only, or ischemia and reperfusion had significantly smaller infarcts with respect to control animals (16.2±2.9%, 14.5±3.7%,. and 9.8±2.7%, respectively; P

Adenosine is an endogenous protectant against stunning during repetitive ischemic episodes in the heart.

.01), indicating that they had been protected by the procedure. When hyperbaric oxygen was begun 30 minutes after the onset of reperfusion, no protection was seen (35.8±3.8%).Concuvions. We conclude that hyperbaric oxygen limits infarct size in the reperfused rabbit heart and that the effect can be achieved when hyperbaric oxygen is begun at reperfusion.

LOSS OF MYOCARDIAL PROTECTION FROM ISCHEMIC PRECONDITIONING FOLLOWING CHRONIC EXPOSURE TO R(-)-N6-(2-PHENYLISOPROPYL)ADENOSINE IS RELATED TO DEFECT AT THE ADENOSINE A1 RECEPTOR

To investigate whether adenosine receptor blockade alters the response to serial coronary occlusions in the rabbit heart, we measured changes in segment length with ultrasonic dimension gauges placed in the field of a coronary branch. The coronary branch was subjected to four 5-minute occlusions, each separated by 10 minutes of reperfusion. In control hearts, the percentage of segment shortening fell from 15.7% to 9.3% after release of the first occlusion with no further deterioration after each of the subsequent three occlusions. In hearts in which adenosine receptors were blocked with PD 115,199, a potent blocker of both A1 and A2 receptors, segment shortening again recovered to about two thirds of the preischemic value during the reperfusion period after the first occlusion. Each subsequent occlusion, however, resulted in a further deterioration of function to the extent that shortening was less than 20% of the preischemic value during reperfusion after the fourth occlusion. Therefore the first occlusion of a series of four occlusions, while mildly stunning the heart, also preconditions the myocardium against further stunning by subsequent occlusions. Because that protection was when adenosine receptors were blocked, adenosine must play an important role in the mediation of this protective effect.

Intravenous co-infusion of adenosine and norepinephrine preconditions the heart without adverse hemodynamic effects☆☆☆★

Exogenously administered adenosine agonist will protect myocardium against infarction during ischemia. However, long-term exposure to adenosine agonists is associated with loss of this protection. To determine why this protection is lost, isolated, perfused rabbit hearts were studied after administration of R(-)-N6-(2-phenylisopropyl)adenosine (PIA), 0.25 mg/h IP, for 3-4 days to intact animals. All hearts experienced 30 min of regional ischemia and 120 min of reperfusion. Control groups 1 and 2 were untreated. In group 1 this ischemia/reperfusion was the only intervention, whereas group 2 hearts were preconditioned with a cycle of 5 min global ischemia/10 min reperfusion preceding the 30 min regional ischemia. Groups 3-5 had been chronically exposed to PIA. Group 3 hearts had 1 preconditioning ischemia/reperfusion cycle before the prolonged ischemia. Group 4 received a 5 min infusion of 0.1 μmol/L phenylephrine in lieu of global ischemia, whereas group 5 was instead treated with 1 μmol/L carbachol. Infarct size averaged 32% of the risk zone in group 1, whereas ischemic preconditioning limited infarction to 8.2 in group 2. Prolonged exposure of group 3 hearts to PIA resulted in the inability of preconditioning with 5 min global ischemia to protect (28.7 ± 4.4% infarction). However, protection was restored by either phenylephrine, an agonist of α1-adrenergic receptors which couple to Gq and stimulate PKC, or carbachol, an agonist of M2-muscarinic receptors which couple instead to Gi as do adenosine A1 receptors (5.2 ± 1.7% and 9.2 ± 2.1% infarction, resp.). Therefore, cross tolerance to ischemic preconditioning develops after chronic PIA infusion. Since both the Gi and the PKC components of the preconditioning pathway were shown to be intact, tolerance must have been related to downregulation or desensitization of the A1 adenosine receptor.

Ischemic Preconditioning of Myocardium: Effect of Adenosine

OBJECTIVE A simple intervention is needed that could protect the heart against infarction during limited-access coronary artery bypass grafting. Adenosine and norepinephrine can precondition the heart with resulting protection, but adverse hemodynamic effects prevent clinical application. Because heart rate, blood pressure, and contractility effects of these two drugs are diametrically opposite, a mixture might be beneficial. METHODS A superficial branch of the left coronary artery of rabbits was surrounded with a suture. Infarction was produced in all hearts by a 30-minute coronary artery occlusion. Infarct size after reperfusion was measured and is presented as a percentage of the risk zone. The effect of 5-minute intravenous co-infusion of adenosine (20 mg/kg) and norepinephrine (0.1 mg/kg) 15 minutes before ischemia was examined. In addition, the protective effect of three sequential intravenous bolus injections of adenosine at either 0.2 or 0.4 mg/kg was evaluated. RESULTS Thirty minutes of regional ischemia caused infarction of 40% +/- 4% of the risk zone. The combination of adenosine and norepinephrine caused no change in blood pressure but rather protected the heart, with infarction of only 9% +/- 2% of the risk zone (p = 0.0001 vs control). Adenosine-norepinephrine co-infusion still protected the heart when the interval between infusion and ischemia was extended to 60 minutes, but it did not protect with a 120-minute interval. Intravenous bolus injections of adenosine resulted in cardiac slowing and marked hypotension. Boluses of 0.2 mg/kg resulted in a minimal, but significant, reduction in infarct size, whereas the higher dose provided no protection. CONCLUSION Adenosine-norepinephrine co-infusion provides a feasible and safe parenteral method for preconditioning the heart.