Ischemic Preconditioning at a Distance
Reduction of Myocardial Infarct Size by Partial Reduction of Blood Supply Combined With Rapid Stimulation of the Gastrocnemius Muscle in the Rabbit
Background Limitation of myocardial infarct size by an earlier brief complete occlusion of a coronary artery is defined as ischemic preconditioning. However, myocardial protection also can be achieved by partial reduction of coronary flow, rapid cardiac pacing, or brief ischemia-reperfusion of a remote region of the heart. Our study assesses the effect on myocardial infarct size of preconditioning at a distance induced by partial reduction of blood flow to a hind limb with or without increase of demand by electrical stimulation of a skeletal muscle.
Methods and Results Anesthetized rabbits were randomized to 30 minutes of waiting period (controls), 55% to 65% reduction of femoral artery blood flow (stenosis), electrical stimulation of the gastrocnemius muscle at a rate of one per second (stimulation), or stenosis+stimulation. Thereafter, rabbits underwent 30 minutes of coronary artery occlusion and 4 hours of reperfusion. Each group included 8 rabbits. Risk zones were comparable among groups. However, the ratio of infarct size to risk zone was smaller in the stenosis+stimulation group (0.09±0.02) compared with the control (0.26±0.03), stenosis (0.36±0.05), and stimulation (0.30±0.05) groups (P=.0006). ANCOVA performed on the fraction of infarction (infarct size/left ventricular weight) and the fraction of risk zone revealed a significant group effect (P=.0004).
Conclusions Remote ischemia of a skeletal muscle induced by muscle stimulation combined with restriction of blood flow preconditioned the myocardium. The combination of muscle stimulation with reduction of femoral arterial blood flow but not muscle stimulation without blood flow restriction or of flow restriction without muscle stimulation reduced myocardial infarct size considerably.
The original experimental models of ischemic myocardial preconditioning involved brief complete obstruction of the infarct-related coronary artery before a long episode of ischemia.1 2 However, myocardial protection simulating preconditioning can be achieved by partial coronary artery stenosis,3 4 5 6 by rapid cardiac pacing,7 8 9 10 11 or by stimulation of the left stellate cardiac nerve combined with restriction of the increase in coronary flow by a pneumatic occluder, thus interfering with the supply-demand balance.12 Moreover, it has been demonstrated that myocardial protection can be conferred by an earlier brief ischemia of a remote myocardial region (“preconditioning at a distance”)13 or even by brief ischemia of a remote organ, such as the kidney14 or mesentery.15 However, it is unclear whether ischemia of a remote organ induced by imbalance between myocardial oxygen demand and supply can protect the myocardium from ischemia-reperfusion injury. The purpose of this study was to assess the effect on myocardial infarct size of preconditioning at a distance induced by partial reduction of blood flow to a hind limb with or without increase of oxygen demand by rapid electrical stimulation of the gastrocnemius muscle.
Male New Zealand White rabbits (2.0 to 3.0 kg) were anesthetized with a mixture of ketamine (≈130 mg/kg IM) and xylazine (≈65 mg/kg IM). Repeated injections of ketamine and xylazine were given during the protocol as required to maintain a deep level of anesthesia. The rabbits were intubated and mechanically ventilated with room air enriched with 100% oxygen delivered at a rate of 1.0 L/min. The left jugular vein and carotid artery were cannulated with fluid-filled catheters. The right femoral artery was exposed below the right inguinal ligament. An ultrasonic flow probe for continuous measurement of mean blood flow and an occluder balloon cuff were placed around the femoral artery. In the rabbits randomized to flow reduction, the occluder balloon was inflated with glycerol. The femoral flow was kept constant by an occlusive pressure maintained with an angioplasty inflator. The right gastrocnemius muscle was exposed through proximal and distal skin incisions, and pacing electrodes were attached by a 4-0 silk suture to this muscle. The chest was opened through the left fourth intercostal space. The pericardium was incised and the heart exposed. Near the base of the heart, a large anterolateral branch of the left circumflex artery or the artery itself was encircled with a 4-0 silk suture. The ends of the suture were threaded through a piece of tubing, forming a snare that could be tightened to occlude the artery. A 21-gauge butterfly catheter was inserted into the left atrial appendage and fixed by a metal clip for injection of radioactive microspheres to measure regional myocardial blood flow and blue dye at the end of the protocol to delineate the ischemic zone at risk. Systemic arterial pressure and heart rate were recorded throughout the protocol. Blood flow in the right femoral artery was monitored until coronary artery occlusion.
After a 10-minute stabilization period, baseline heart rate, blood pressure, and right femoral flow rate were measured and rabbits were randomized to one of four treatment groups (Fig 1⇓): (1) control: 30-minute waiting period without inflation of the vascular occluder or electrical stimulation of the gastrocnemius muscle; (2) stenosis without muscle stimulation: reduction of the femoral blood flow by 55% to 65% for 30 minutes without electrical stimulation of the gastrocnemius muscle; (3) muscle stimulation without stenosis: electrical stimulation of the gastrocnemius muscle, at a rate of one per second, with a stimulus of 9 V and 20-ms duration for 30 minutes without reduction of femoral blood flow; and (4) muscle stimulation with stenosis: reduction of the femoral blood flow by 55% to 65% followed by electrical stimulation of the gastrocnemius muscle at a rate of one per second with a stimulus of 9 V and 20-ms duration for 30 minutes.
After 30 minutes, the coronary artery was ligated for 30 minutes by tightening of the snare. Thereafter, the vascular occluder was released and muscle stimulation stopped. Measurement of regional myocardial blood flow was obtained at 25 minutes of ischemia. After 30 minutes of coronary artery occlusion, the snare was released and the heart reperfused for 4 hours. Thirty minutes after reperfusion, regional myocardial blood flow measurement was repeated to verify reperfusion.
Heart rate and systemic pressure were measured at baseline, 2 minutes before coronary artery occlusion (28 minutes of treatment), 25 minutes of coronary artery occlusion, 30 and 60 minutes after reperfusion, and every 60 minutes thereafter. Heart rate and arterial blood pressure were recorded at a paper speed of 25 mm/s. Three to five beats were averaged at each time point.
At the end of the protocol, after 4 hours of reperfusion, the coronary artery was reoccluded, and 4 mL of a 50% solution of Unisperse blue dye (CIBA-Geigy) was injected through the left atrial catheter to delineate the ischemic region at risk of necrosis. The deeply anesthetized rabbit was then killed by an injection of potassium chloride into the left atrium, and the heart was excised.
Analysis of Myocardium at Risk of Infarction and Infarct Size
The heart was sliced transversely into six to eight sections ≈2 mm thick and photographed with magnification to identify the ischemic risk zones (uncolored by the blue dye) and the nonischemic zones (colored by blue dye). The slices were then incubated for 10 minutes in a 1% solution of buffered triphenyltetrazolium chloride (TTC) preheated to 37°C, immersed in 10% phosphate-buffered formalin, and rephotographed. The second photograph showed the necrotic zone (unstained by TTC) and the noninfarcted region (stained red by TTC). The photographic slides were later projected with a magnification of ×3 in a random order and traced without knowledge of the treatment group. The areas of ischemic and normally perfused zones and the areas of necrotic and nonnecrotic zones in each slice were determined by computerized planimetry (Summagraphics). These areas were multiplied by the weight of each slice, and the results were summed to obtain the weight of the ischemic zone (IZ) at risk and the weight of the necrotic zone (NZ). Infarct size was defined as the ratio of the weight of the necrotic zone to the ischemic zone (NZ/IZ).
Measurement of Regional Myocardial Blood Flow
Regional myocardial blood flow was measured by use of 11-μm radioactive microspheres (New England Nuclear), ≈5×105 microspheres per injection, labeled with 141Ce, 96Nb, or 103Ru. Microspheres were injected into the left atrium via the left atrial catheter, and a reference blood sample was obtained from the carotid artery at a fixed rate of 2.06 mL/min. At the end of the protocol, samples were cut from the center of ischemic and nonischemic regions (determined by the absence and presence of the blue dye, respectively), weighed, and counted with the reference blood samples in a well gamma counter. Blood flow was then computed and expressed in mL·min−1·g tissue−1.
We decided prospectively to exclude from the study rabbits with an ischemic risk zone of <10% of the left ventricular weight, rabbits with an ischemic regional myocardial blood flow >0.2 mL·min−1·g tissue−1 during coronary artery occlusion, or rabbits that failed to reperfuse (regional myocardial blood flow ≤0.4 mL·min−1·g tissue−1 30 minutes after release of the coronary artery snare).
Because the mean heart rate of the rabbits randomized to electrical muscle stimulation and reduction of femoral blood flow was lower than the control and the muscle stimulation without stenosis groups, an additional group was added post hoc, without randomization. These rabbits were subjected to reduction of the femoral blood flow by 55% to 65% followed by electrical stimulation of the gastrocnemius muscle at a rate of one per second, with a stimulus of 9 V and 20-ms duration for 30 minutes. Atrial pacing at a rate of 180 bpm was initiated immediately after coronary artery occlusion and continued throughout the protocol. Regional myocardial blood flow was not measured in this group because it was not feasible to achieve stable atrial pacing with a 21-gauge butterfly catheter inserted into the left atrium.
All data summary and statistical analyses were performed with SAS version 6.04. Heart rate and mean blood pressure were analyzed by repeated-measures ANOVA. Data on the necrotic and ischemic zones, femoral artery blood flow, and regional myocardial blood flow were evaluated by ANOVA. Tukey’s test was used to test for a difference among the individual means. ANCOVA was used to test for group effect on the weight of the ischemic zone versus the weight of the necrotic zone. Data are expressed as mean±SEM. Values of P≤.05 were considered statistically significant.
The rabbits used in this study were maintained in accordance with the policies of the American Heart Association (1985)16 and in accordance with the guidelines prepared by the Committee on Care and Use of Laboratory Animals of the Institute of Laboratory Animal Resources, National Research Council (Department of Health, Education, and Welfare publication 85-230). Good Samaritan Hospital is accredited by the American Association for Accreditation of Laboratory Animal Care.
Thirty-four rabbits were randomized into the first four groups. Two rabbits were excluded for regional myocardial blood flow >0.2 mL·min−1·g−1 during coronary artery occlusion. Data are reported on the remaining 32 animals, 8 in each group.
Femoral Artery Blood Flow
Femoral artery blood flow was comparable among groups at baseline (Table 1⇓). Although femoral artery blood flow did not change much in the control group, it increased to 5.96 mL/min (58±17% increase) in the muscle stimulation group compared with 4.76 mL/min (14±17%) in the control group (P<.05). Femoral artery blood flow was reduced by 60±1% and 64±1% in the stenosis and muscle stimulation+stenosis groups, respectively (P<.01 versus the control and muscle stimulation groups).
Mean blood pressure was comparable among groups throughout the duration of the protocol (P=.97) (Fig 2⇓). Throughout the entire duration of the protocol, heart rate was slower in the muscle stimulation+stenosis and stenosis groups than in the control and muscle stimulation alone groups (P=.0008 for the difference among groups), which was related primarily to postreperfusion differences (Fig 2⇓). During coronary artery occlusion, however, there was no significant difference in heart rate among groups (P=.087).
Regional Myocardial Blood Flow
Regional myocardial blood flow measurements were not available in two rabbits because of technical difficulty. All groups were equally ischemic during coronary artery occlusion (Table 2⇓). Regional myocardial blood flow in the previously ischemic zone at 30 minutes of reperfusion tended to be higher in the muscle stimulation+stenosis group, but without statistical significance. Regional myocardial blood flow in the nonischemic zone during both ischemia and reperfusion was similar among groups (Table 2⇓).
Myocardium at Risk and Infarct Size
Mean body weight, left ventricular weight, and the size of the risk region (expressed either in weight or as a fraction of the left ventricular weight) were comparable among the groups (Table 3⇓). However, the weight of the necrotic zone, the size of the necrotic zone (expressed as a fraction of the left ventricle), and infarct size (the ratio between the weight of the necrotic zone and that of the ischemic risk zone) were significantly lower in the muscle stimulation+stenosis group (P<.01 for the difference in infarct size between the muscle stimulation+stenosis versus the stenosis group and versus the muscle stimulation group; P<.05 versus the control group) (Table 3⇓).
Fig 3⇓ shows the relationship between the volume of necrosis (divided by the left ventricular weight) and the volume of the ischemic zone (divided by the left ventricular weight) by groups. ANCOVA testing for group effect performed on the fraction of the left ventricle that developed necrosis and the fraction of the left ventricle that composed the ischemic area at risk revealed a significant group effect (P=.0004). The slope of the regression line of the muscle stimulation+stenosis group was lower than that of the other three groups. Hence, for the same ischemic zone at risk, the muscle stimulation+stenosis group developed a smaller infarct size than the other three groups.
The group that was added post hoc and was subjected to gastrocnemius muscle stimulation+stenosis+atrial pacing included seven rabbits. Of seven rabbits entered into the study, one demonstrated persistent epicardial cyanosis during the reperfusion period and therefore was eliminated. Six rabbits successfully completed the protocol. The mean blood pressure of the rabbits that were subjected to atrial pacing was comparable to that of the other four groups. Heart rate, however, was maintained at 180 bpm during coronary artery occlusion and the 4 hours of reperfusion (Fig 2⇑). Mean body weight of the rabbits subjected to atrial pacing in addition to muscle stimulation+stenosis was comparable to that of the other four groups (2.43±0.06 kg). Left ventricular weight (3.88±0.09 g) and the size of the ischemic area at risk (0.34±0.05 of the left ventricular weight) were also comparable to those of the other four groups. The infarct size of these rabbits was relatively small (0.06±0.02 of the left ventricle; 0.17±0.04 of the ischemic risk zone).
The main finding in the present study is that by alteration of the supply-demand balance in a remote skeletal muscle, myocardial protection was conferred. After 30 minutes of coronary artery occlusion and 4 hours of reperfusion, infarct size expressed as a fraction of the ischemic risk zone was reduced by 65% in the rabbits subjected to reduction of flow in the femoral artery combined with rapid muscle stimulation compared with the control group (Table 3⇑). Further evidence that muscle stimulation+stenosis had an infarct size–reducing effect was provided by ANCOVA testing for group effect performed on the fraction of the left ventricle that developed necrosis and the fraction of the left ventricle that was ischemic (Fig 3⇑). A significant group effect was found. The slope of the regression line of the muscle stimulation+stenosis group was lower than that of the other three groups. Skeletal muscle stimulation (using the same rate, amplitude, and stimulus duration as in the muscle stimulation+stenosis group) without reduction of femoral blood flow, and reduction of femoral artery flow without electrical stimulation of the gastrocnemius muscle, did not alter infarct size, probably because the magnitude of the impairment in the supply-demand balance of the skeletal muscle was not severe enough to provide a preconditioning-like stimulus.
The magnitude of infarct size reduction found in our study is comparable to that achieved with ischemic preconditioning produced by brief occlusion of the same coronary artery that is subjected later to a prolonged occlusion.17 18 19 20
Although mean blood pressure was comparable among the groups throughout the protocol, heart rate was slower in the stenosis group and muscle stimulation+stenosis group than in the muscle stimulation and control groups. However, during coronary artery occlusion, the difference was not statistically significant (P=.087). Moreover, mean infarct size in the stenosis without muscle stimulation group was the highest among groups (0.36±0.05), despite the lower heart rate. In the additional six rabbits that were subjected to atrial pacing at a rate of 180 bpm during coronary artery occlusion and throughout reperfusion, infarct size was also relatively small. Thus, the infarct size reduction seen in the muscle stimulation+stenosis group cannot be explained by relative bradycardia. In the latter group, atrial pacing was initiated immediately after coronary artery occlusion to eliminate the possibility of preconditioning the myocardium by atrial pacing.7 8 9 10 11
All groups were equally ischemic during coronary artery occlusion. Therefore, augmentation of collateral flow is not the mechanism of protection. The trend toward higher regional myocardial blood flow 30 minutes after reperfusion in the postischemic zone in the muscle stimulation+stenosis group is explained by the smaller infarct size. Thus, the postreperfusion hyperemia is not blunted by no-reflow.
Liauw et al21 reported that 5 hours of gracilis muscle ischemia (induced by complete reduction of blood flow) followed by 48 hours of reperfusion resulted in significant muscle salvage after 5 hours of ischemia and 48 hours of reperfusion of the contralateral gracilis muscle. Przyklenk et al13 demonstrated that four episodes of 5 minutes of left circumflex coronary artery occlusion protected the canine myocardium supplied by the left anterior descending coronary artery. After 60 minutes of left anterior descending coronary artery occlusion, infarct size was 6±2% of the ischemic zone in the dogs subjected to preceding left circumflex ischemia versus 16±5% in the controls (P<.05). McClanahan et al14 occluded the left renal artery in rabbits for 10 minutes. After 10 minutes of reperfusion, the coronary artery was ligated for 30 minutes, followed by 3 hours of reperfusion. Additional groups were subjected to preconditioning by 5 minutes of coronary artery occlusion (classic preconditioning), no preceding ischemia (controls), or permanent renal artery occlusion. Infarct size was 43±3% of the ischemic risk zone in the control rabbits, 8±2% in the classic preconditioning group (P=.0001 versus controls), and 11±2% in the transient renal ischemia group (P=.0001 versus controls). Infarct size was 31±4% (P=NS) in the group subjected to preceding permanent renal artery occlusion.14 Gho et al15 found that 15 minutes of anterior mesenteric artery occlusion in normothermic rats followed by 10 minutes of reperfusion and then 60 minutes of coronary artery occlusion resulted in reduction of infarct size to 50±3% of the risk zone compared with 68±2% of the risk zone in the control group (P<.001). A 15-minute coronary artery occlusion followed by 10 minutes of reperfusion and then 60 minutes of coronary reocclusion was equally protective (infarct size, 50±3%, P<.001), whereas 15 minutes of renal artery occlusion failed to limit infarct size (72±5%) in normothermic rats. Only in rats subjected to hypothermia (body temperature, 30°C to 31°C) did 15 minutes of renal artery occlusion protect the heart (46±6% compared with 67±3% in rats subjected to hypothermia alone, P<.01).15 However, myocardial protection by partial reduction of blood flow to a stimulated peripheral muscle has not been described previously. Moreover, in the studies by McClanahan et al14 and Gho et al,15 permanent peripheral artery occlusion without an intervening reperfusion period before coronary artery occlusion was not protective. In contrast to the previous studies, in our study there was no intervening reperfusion period. Peripheral ischemia was induced by partial stenosis of the femoral artery. Therefore, residual flow through the ischemic muscle continued and the potential mediators of protection could be continuously released into the circulation. Ischemia of the lower limbs caused by an increase of demand in muscles supplied by stenotic arteries is common in the daily life of patients with peripheral vascular disease or patients with reduced cardiac output due to severe heart failure. This type of ischemia may be more common than transient complete obstruction of mesenteric or renal arteries. Myocardial protection induced by such an ischemic event may partially explain the phenomenon of warm-up angina and especially walk-through angina.22 23 24 25 Walk-through angina refers to the paradoxical disappearance of anginal pain despite continuation of exertion.22 24 In this phenomenon, there is no intervening rest (as in the present protocol); thus, improvement of blood supply to the ischemic myocardium probably does not occur.
It might be that mediators released from ischemic limbs can attenuate myocardial ischemia. The mechanism of this type of myocardial protection was not studied in the present protocol. It remains to be determined whether the mechanisms are similar to the proposed mechanisms for the classic forms of ischemic myocardial preconditioning. Stimulation of mechanoreceptors by vigorous muscle contraction is probably not the trigger for myocardial protection, because infarct size was not limited in the group subjected to muscle stimulation alone. However, ischemic skeletal muscle or stimulated skeletal chemoreceptors may trigger release of substances via humoral or neurogenic pathways. A humoral factor, such as adenosine, bradykinin, or other substance, released from the ischemic skeletal muscle may trigger this form of protection.26 27 Myocardial protection by remote ischemia may be related to a catecholamine effect, because pretreatment with certain catecholamines can mimic the effect of preconditioning.28 29 These catecholamines may be released into the circulation from the ischemic muscle or may be released by the intramyocardial sympathetic nerve endings as a result of a stimulation of the sympathetic nervous system. Gho et al15 reported that hexamethonium (20 mg/kg IV), a ganglion blocker, abolished the protection by 15 minutes of mesenteric artery occlusion but not the protection by cardiac ischemic preconditioning. Their findings support involvement of the neurogenic pathway in the protection afforded by ischemia at a distance. However, in the studies by McClanahan et al14 and by Gho et al,15 myocardial infarct size was not reduced if the peripheral organ was not reperfused before the coronary artery occlusion. This fact may point to a release of humoral mediator from the ischemic organ by reperfusion, because activation of the nervous system may not be dependent on reperfusion. It should be determined whether this type of protection occurs in the denervated heart. Future studies using measurements of various potential mediators released after muscle stimulation and various potential blocking agents of either humoral or neurogenic pathways to inhibit the protective effect may clarify the mechanism.
In conclusion, remote ischemia of skeletal muscle induced by muscle stimulation combined with partial restriction of blood flow preconditioned the myocardium. Infarct size after such a procedure but not after muscle stimulation without restriction of blood flow or of flow restriction without muscle stimulation reduced infarct size considerably after 30 minutes of coronary artery occlusion and 4 hours of reperfusion. This form of preconditioning at a distance may be an important and common form of myocardial protection in the clinical setting, especially in patients with peripheral vascular disease. The results of this study also strengthen the concept that preconditioning at a distance can occur and is possibly related to either a hormonal or catecholamine-related mechanism.
- Received January 6, 1997.
- Revision received February 27, 1997.
- Accepted March 5, 1997.
- Copyright © 1997 by American Heart Association
Murry CE, Jennings RB, Reimer KA. Preconditioning with ischemia: a delay in lethal cell injury in ischemic myocardium. Circulation. 1986;74:1124-1136.
Ovize M, Kloner RA, Hale SL, Przyklenk K. Coronary cyclic flow variations ‘precondition’ the ischemic myocardium. Circulation. 1992;85:779-789.
Ovize M, Przyklenk K, Kloner RA. Partial coronary stenosis is sufficient and complete reperfusion mandatory for preconditioning the canine heart. Circ Res. 1992;71:1165-1173.
Koning MMG, Gho BCG, van Klaarwater E, Duncker DJ, Verdouw PD. Endocardial and epicardial infarct size after preconditioning by a partial coronary artery occlusion without intervening reperfusion: importance of the degree and duration of flow reduction. Cardiovasc Res. 1995;30:1017-1027.
Koning MMG, Simonis LAJ, de Zeeuw S, Nieukoop S, Post S, Verdouw PD. Ischaemic preconditioning by partial occlusion without intermittent reperfusion. Cardiovasc Res. 1994;28:1146-1151.
Vegh A, Szekeres L, Parratt JR. Transient ischaemia induced by rapid cardiac pacing results in myocardial preconditioning. Cardiovasc Res. 1991;25:1051-1053.
Szilvássy Z, Ferdinandy P, Bor P, Jakab I, Lonovics J, Koltai M. Ventricular overdrive pacing induced anti-ischemic effect: a conscious rabbit model of preconditioning. Am J Physiol. 1994;266:H2033-H2041.
Koning MMG, Gho BCG, van Klaarwater E, Opstal RLJ, Duncker DJ, Verdouw PD. Rapid ventricular pacing produces myocardial protection by nonischemic activation of KATP+ channels. Circulation. 1996;93:178-186.
Iwamoto T, Bai X-J, Downey HF. Preconditioning with supply-demand imbalance limits infarct size in dog heart. Cardiovasc Res. 1993;27:2071-2076.
Przyklenk K, Bauer B, Ovize M, Kloner RA, Whittaker P. Regional ischemic ‘preconditioning’ protects remote virgin myocardium from subsequent sustained coronary occlusion. Circulation. 1993;87:893-899.
McClanahan TB, Nao BS, Wolke LJ, Martin BJ, Mertz TE, Gallagher KP. Brief renal occlusion and reperfusion reduces myocardial infarct size in rabbits. FASEB J. 1993;7:A118. Abstract.
Gho BCG, Schoemaker RG, van den Doel MA, Duncker DJ, Verdouw PD. Myocardial protection by brief ischemia in noncardiac tissue. Circulation. 1996;94:2193-2200.
Position of the American Heart Association on Research Animal Use. A statement for health professionals by the task force appointed by the board of directors of the American Heart Association. Circulation. 1985;71:849A-850A.
Van Winkle DM, Thornton J, Downey DM, Downey JM. The natural history of preconditioning: cardioprotection depends on duration of transient ischemia and time to subsequent ischemia. Coron Artery Dis. 1991;2:613-619.
Goto M, Liu Y, Yang XM, Ardell JL, Cohen MV, Downey JM. Role of bradykinin in protection of ischemic preconditioning in rabbit hearts. Circ Res. 1995;77:611-621.
Toombs CF, Moore TL, Shebuski RJ. Limitation of infarct size in the rabbit by ischaemic preconditioning is reversible with glibenclamide. Cardiovasc Res. 1993;27:617-622.
Liu GS, Thornton J, Van Winkle DM, Stanley AW, Olsson RA, Downey JM. Protection against infarction afforded by preconditioning is mediated by A1 adenosine receptors in rabbit heart. Circulation. 1991;84:350-356.
Liauw SK, Rubin BB, Lindsay TF, Romaschin AD, Walker PM. Sequential ischemia/reperfusion results in contralateral skeletal muscle salvage. Am J Physiol. 1996;270:H1407-H1413.
Marber MS, Joy MD, Yellon DM. Is warm-up in angina ischaemic preconditioning? Br Heart J. 1994;72:213-215.
MacAlpin RN, Kattus AA. Adaptation to exercise in angina pectoris: the electrocardiogram during treadmill walking and coronary angiographic findings. Circulation. 1966;33:183-201.
Tomai F, Danesi A, Perino M, Gaspardone A, Ghini AS, Cascarano MT, Chiariello L, Gioffrè PA. Mechanisms of the warm-up phenomenon. Eur Heart J. 1996;17:1022-1027.
Miura T, Iimura O. Infarct size limitation by preconditioning: its phenomenological features and the key role of adenosine. Cardiovasc Res. 1993;27:36-42.
Bankwala Z, Hale SL, Kloner RA. α-Adrenoceptor stimulation with exogenous norepinephrine or release of endogenous catecholamines mimics ischemic preconditioning. Circulation. 1994;90:1023-1028.