(Circulation. 1997;96:3087-3093.)
© 1997 American Heart Association, Inc.
Articles |
Ischemic Preconditioning Does Not Protect Against Contractile Dysfunction in the Presence of Residual Flow
Studies in the Isolated, Blood-Perfused Rat Heart
From Cardiac Muscle Research Laboratories, Boston University School of Medicine, Boston, Mass.
Correspondence to Alison C. Cave, Division of Radiological Sciences, 18th Floor, Guy's Tower, Guy's Hospital, London, SE1 9RT.
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Abstract |
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Background We have previously demonstrated that ischemic preconditioning (PC) does not protect when oxygen deprivation is accompanied by a high level of perfusion (hypoxia). Since clinical ischemia can vary from mild to severe, we wished to determine whether PC could protect against injury arising from low-flow ischemia.
Methods and Results Functional recovery after 30 minutes of reperfusion was assessed in isolated, blood-perfused rat hearts (n=6 per group) subjected to (A) 30 minutes of zero-flow ischemia, (B) 30 minutes of zero-flow ischemia preceded by 3xPC (PC=5 minutes of ischemia+5 minutes of reperfusion), (C) 90 minutes of low-flow ischemia at 10% of baseline coronary flow (0.31±0.02 mL/min per gram wet wt), (D) 90 minutes of low-flow ischemia at 10% of baseline coronary flow (0.29±0.02 mL/min per gram wet wt) preceded by 3xPC. PC significantly protected against injury resulting from zero-flow ischemia (developed pressure recovered to 67±6% versus 31±12% in B and A, respectively; P<.05) but not resulting from low-flow ischemia (recovery of developed pressure was 40±8% versus 37±7% in C and D, respectively). Protein kinase C (PKC) is widely considered to be involved in the mechanism of PC such that prior activation and translocation of PKC by the PC protocol allows phosphorylation of the end-effector protein early during the subsequent ischemic insult, before loss of adenosine triphosphate occurs. However, because adenosine triphosphate content falls slowly during low-flow ischemia, PKC may be activated and translocated early enough to be active during this insult. If so, inhibition of PKC should decrease functional recovery in the control group. However, functional recovery in control groups was not decreased in the presence of the PKC inhibitor polymyxin B (50±6%), suggesting that if activation of PKC occurred during low-flow ischemia, it was not protective.
Conclusions PC does not protect against contractile dysfunction in the rat when a low level (10% of baseline flow) of ischemic perfusion remains during the prolonged insult.
Key Words: ischemia • preconditioning • perfusion
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Previously we and others have demonstrated that ischemic preconditioning significantly improves functional recovery after prolonged zero-flow ischemia in the isolated rat heart.1 2 3 However, preconditioning did not protect against contractile dysfunction when oxygen deprivation was accompanied by a high level of perfusion (hypoxia).4
The mechanism by which preconditioning provides protection against ischemia-induced injury is unclear. Initiation of protection appears to be receptor mediated,5 6 specifically by receptors linked to the second messenger protein kinase C (PKC),3 7 8 but how PKC mediates the protection is still unclear. However, it does appear that receptor occupation is important during both the preconditioning ischemia and the prolonged ischemic insult.9 The A1-specific antagonist PD 115199 completely blocked preconditioning-induced protection when it was infused either before or after the preconditioning ischemia.9 Thus the lack of protection against hypoxia-induced injury4 may be because preconditioning-induced protection requires the accumulation of a factor in the ischemic myocardium, either to exert the preconditioning protective effect (eg, adenosine) or as a factor of injury against which preconditioning affords protection (eg, protons).
Clinical ischemia can vary in severity, even in the setting of
myocardial infarction. For example, in most patients, after an acute
coronary occlusion, myocardial perfusion in the infarct region
is 
15% of normal coronary flow.10 We wished to
determine therefore whether preconditioning could protect against a
prolonged period of low-flow ischemia at such a level of
ischemic perfusion.10 To better simulate the
perfusion conditions and metabolic milieu in the region of
an acute myocardial infarction, we used the isolated, blood-perfused
rat heart model with low-flow ischemia where, in contrast to
hypoxia, full washout of metabolites (such as protons) produced
during low-flow ischemia does not occur. In previous studies we
have demonstrated that intracellular pH falls to 
6.1 during a
sustained period of low-flow ischemia imposed at 10% of
baseline coronary flow11 ; in addition, although
partial lactate washout occurs during the ischemic period,
there is a large peak immediately upon reperfusion when the lactate,
which has accumulated during the low-flow ischemia, is washed
out.11
Thus the initial aim of this study was to determine if ischemic preconditioning could protect against contractile dysfunction arising from a prolonged period of low-flow ischemia. Our results (see below) demonstrated that the presence of a residual level of ischemic perfusion eliminated the protection conferred by preconditioning against zero-flow ischemia. To further investigate the mechanism by which low-flow ischemia eliminated preconditioning-induced protection, we determined whether inclusion of the protective ischemic metabolite adenosine12 could restore preconditioning-induced protection and whether PKC may be activated and protective during low-flow ischemia in control hearts, thereby negating the difference between control and preconditioned hearts.
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Methods |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Perfusion Procedures
Hearts were obtained from male Sprague-Dawley rats (weight, 250 to 300 g). The isolated rat heart preparation, perfused with modified Krebs-Henseleit buffer containing bovine red blood cells at a hematocrit of 40%, was used; this methodology has been described previously.13 Rats were anesthetized with sodium pentobarbital (0.1 mL/100 g IP injection), heparinized (sodium heparin, 100 IU/100 g IV), and the heart rapidly excised. The aorta was then cannulated and coronary perfusion initiated and adjusted during the initial oxygenated perfusion to achieve a steady coronary perfusion pressure of 80 mm Hg. After equilibration, flow was then held constant and coronary perfusion pressure was determined by coronary vasomotor tone. The left ventricle was vented of thebesian drainage through an apical cannula and the pulmonary artery cannulated to allow collection of coronary sinus flow for measurement of coronary flow rate and for assessment of myocardial oxygen consumption (MVO2) and lactate production. A fluid-filled balloon connected to a Statham P23db pressure transducer (Gould) was inserted into the left ventricle to measure left ventricular developed pressure (LVDP). The balloon was filled until a left ventricular end-diastolic pressure (LVEDP) of 10 mm Hg was achieved and the balloon volume then held constant such that changes in LVEDP reflected changes in left ventricular diastolic distensibility.14 All hearts were paced throughout the experiment at 6 Hz except for the periods of zero-flow ischemia and the first 3 minutes of reperfusion thereafter. The heart was placed in a water-jacketed constant temperature chamber maintained at 37°C and submerged in saline. Hearts were excluded from study if baseline LVDP was <100 mm Hg. Low-flow ischemia was imposed by first reducing coronary flow to 20% of baseline flow for 5 minutes and then to 10% of baseline flow; this level of ischemic perfusion was then maintained for the remainder of the ischemic period. Hearts were reperfused at the baseline coronary flow rate.
All animals received humane care in compliance with the "Principles of Laboratory Animal Care" formulated by the National Society for Medical Research and the "Guide for the Care and Use of Laboratory Animals" prepared by the National Academy of Sciences and published by the National Institute of Health (NIH Publication No. 86-23, revised 1985).
Perfusion Solutions
A red blood cell perfusate similar to that described
previously13 was used, consisting of bovine red blood
cells at a final hematocrit of 40% in Krebs-Henseleit buffer. Fresh
cow blood was collected into a vessel containing 6000 units of
sodium heparin and 100 000 units of penicillin per liter. The blood
was centrifuged at 5°C at 3000 rpm for 15 minutes. The
supernatant was aspirated and the resulting packed cells were mixed 1:1
with Krebs-Henseleit buffer. The centrifugation and
resuspension steps were repeated three times. The red blood cell
suspension was thus essentially free of white blood cells and
platelets. Packed red blood cells were stored in nominally
calcium-free buffer at 4°C and washed daily before use. Cells more
than 4 days old were not used.
The modified Krebs-Henseleit buffer contained (in
mmol/L): NaCl 118, KCl 4.7, KH2PO4 1.2,
MgSO4 1.2, NaHCO3 25.5, glucose 5.5, lactate
1.0, and palmitic acid (as a source of free fatty acid) 0.4, in
combination with 4 g% bovine serum albumin (No. A7030, Sigma
Chemical Co). Essentially fatty acid–free bovine serum albumin
was first dissolved in Krebs-Henseleit buffer. Palmitic acid (No.
P-9767, Sigma Chemicals Co) was then added to this mixture. Calcium
chloride was gradually added to the final perfusate mixture to
achieve a calcium-free concentration of 1.2 mmol/L.
Gentamicin (2 mg/L) was added to the red blood cell
perfusate to retard bacterial growth. The perfusate was
equilibrated with 20% O2/3% CO2/77%
N2 to achieve a PO2 of 140
mm Hg and a pH of 7.4.
Adenosine was dissolved in saline and infused into the aortic cannula, immediately above the heart, at 5% of the coronary flow rate to achieve a final concentration of 100 µmol/L in the blood. Similarly, the PKC inhibitor polymyxin B was dissolved in the Krebs-Henseleit buffer and also infused into the aortic cannula, immediately above the heart, at 5% of the coronary flow rate to achieve a final concentration of 50 µmol/L in the blood. We have previously demonstrated that this concentration of polymyxin B successfully abolished preconditioning-induced protection against zero-flow ischemia.7 Polymyxin B had no effect on the PCO2, PO2, or pH of the blood.
Data Collection and Analytical Procedures
Coronary blood flow was measured by timed collections of
the coronary venous effluent through the pulmonary
artery. MVO2 was calculated from the
arteriovenous oxygen content differences, derived from oxygen
saturation curves for the Krebs buffer/red blood cell suspensions over
the experimental range of pH and PO2 values.
Arterial and venous lactate concentrations were measured
with the use of the COBAS BIO system (Roche Diagnostic
Systems). Before the assay, the lactate samples were diluted 1:3 with
10% trichloroacetic acid and stored at 4°C.
Experimental Protocols
All hearts were subjected to 30 minutes of normoxic perfusion
before either (a) a further 30 minutes of normoxic perfusion (control
groups) or (b) 3x(5 minutes of ischemia and 5 minutes of
reperfusion) (preconditioned groups). Hearts were then subjected to
either 30 minutes of zero-flow global ischemia and 30 minutes
of reperfusion or 90 minutes of low-flow ischemia (10% of
baseline flow) and 30 minutes of reperfusion. Contractile function,
myocardial oxygen consumption, creatine kinase release, and lactate
release were assessed throughout.
Adenosine or polymyxin B was infused for 5 minutes before the onset of low-flow ischemia and thereafter for the remainder of the experiment.
The control and preconditioned groups subjected to zero-flow ischemia were concurrently perfused with the low-flow groups but have been previously published7 and are reported here as a comparison for the groups subjected to low-flow ischemia.
Statistical Analysis
Data are reported as mean±SEM. Data acquired sequentially
in individual hearts were tested by a two-way ANOVA for repeated
measures. If this analysis indicated a significant difference
between groups, values at specific time points were examined by a
method of least significance. A value of P<.05 was
considered significant. Differences in mean measurements between
experimental groups was tested by a two-tailed unpaired t
test or one-way ANOVA followed by the Bonferroni test.
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Determination of Whether Preconditioning Could Protect Against Injury Induced by a Prolonged Period of Low-Flow Ischemia and Reperfusion
Contractile Function
Four groups of hearts (n=6 per group) were studied: control and preconditioned hearts after 30 minutes of zero-flow ischemia and 30 minutes of reperfusion and control and preconditioned subjected to 90 minutes of low-flow ischemia and 30 minutes of reperfusion. The period of low-flow ischemia was chosen such that the percentage recovery of LVDP in the two control groups (zero- and low-flow ischemia) would be similar, that is, both control groups would suffer a similar degree of ischemic damage. No significant differences were observed between the groups in terms of preischemic absolute values of contractile function, coronary flow, or coronary perfusion pressure (Table 1
).
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In both preconditioned groups, the preconditioning protocols had
similar effects. Developed pressure fell to 60 to 70 mm Hg
(Fig 1; zero-flow data shown for comparison) and
end-diastolic pressure fell to 4.8±0.8 and 5.4±1.3
mm Hg in the zero-flow and low-flow preconditioned groups,
respectively, after the preconditioning protocol (Fig 2; zero-flow data
shown for
comparison).
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); control hearts are
represented by open circles. Solid bars represent
zero-flow ischemia; cross-hatched bars represent
low-flow ischemia; and open bars represent reperfusion.
Data are presented as mean±SEM; n=6 per group.
*P<.05.
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After the prolonged ischemic insult, preconditioning was
protective only after zero-flow ischemia (Fig 1
, left).
Recovery of LVDP was 67±6% and 31±12% in preconditioned and control
groups, respectively. No preconditioning-induced protection was
observed after low-flow ischemia (Fig 1, right) in which
recovery of LVDP was 37±7% and 40±8% in preconditioned and control
groups, respectively. Similarly, a marked protection against
diastolic dysfunction was observed in the preconditioned
group after zero-flow ischemia (Fig 2, left); this protection
was completely lost after low-flow ischemia (Fig 2, right).
Lactate Release
Lactate release after the prolonged period of zero-flow
ischemia was significantly decreased in the preconditioned
group compared with its control (Fig 3, left). In contrast, no
reduction in total lactate release was seen in preconditioned hearts
after low-flow ischemia and reperfusion (Fig 3, right).
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Can Infusion of Adenosine, an Endogenous
Initiator of Preconditioning, Restore Preconditioning-Induced
Protection Against Injury Sustained During Low-Flow Ischemia?
Liu and colleagues6 have demonstrated that
adenosine is required not only to initiate
preconditioning-induced protection in the rabbit but also to mediate
the protection. If an adenosine antagonist is
infused after the preconditioning cycles of ischemia and
reperfusion but immediately before the sustained ischemic
period, protection is lost. This demonstrates that adenosine
receptors must be occupied during both the preconditioning protocol and
the prolonged ischemic insult for preconditioning to be
effective. Furthermore, Headrick15 recently reported that
pretreatment with cyclohexyladenosine mimicked preconditioning
and that the competitive adenosine receptor
antagonist 8-(p-sulfophenyl) theophylline
abolished preconditioning in the isolated rat heart. Although the role
of adenosine in preconditioning in the rat heart is
controversial,16 17 18 it is possible that the failure to
observe preconditioning-induced protection after low-flow
ischemia was because the necessary mediator, adenosine,
was washed out.
Contractile Function
No significant differences were observed between the control and
preconditioned groups in terms of preischemic absolute
values of contractile function, coronary flow, or
coronary perfusion pressure (Table 2). No prolonged changes were
observed on the start of adenosine infusion, although a
transient dip of about 10 mm Hg in coronary perfusion
pressure was observed. No heart rate effects were observed (pacing was
continued throughout the protocol).
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As in the previous groups, the preconditioning protocol resulted in a
fall in developed pressure to 73±4 mm Hg (Fig 4) and
end-diastolic pressure to 5.0±0.6 mm Hg after the
third episode of ischemia and
reperfusion.
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As in the previous groups, no preconditioning-induced protection was
observed either during or after the prolonged low-flow ischemic
insult (Fig 4). Recovery of LVDP was 37±7% and 40±8% in
preconditioned and control groups, respectively. Thus the presence of
adenosine did not restore the protective effect of
preconditioning during low-flow ischemia.
Does the PKC Inhibitor Polymyxin B Decrease Contractile
Recovery After Low-Flow Ischemia and Reperfusion in
Control Hearts?
An alternative explanantion for the absence of
preconditioning-induced protection against injury sustained during
low-flow ischemia was that PKC was activated during the
initial few minutes of low-flow ischemia. In contrast to
zero-flow ischemia, phosphorylation potential
may remain high enough, for long enough, in low-flow ischemia
for PKC to phosphorylate the appropriate end-effector
protein. If this were the case, it is possible that under conditions of
low-flow ischemia, all hearts, that is, control and
preconditioned, are effectively "preconditioned." To test this
theory, we repeated the control group protocol (90 minutes of low-flow
ischemia and 30 minutes of reperfusion) in the presence of the
PKC inhibitor polymyxin B (50 µmol/L). If PKC
was activated and protective against injury sustained during
low-flow ischemia, functional recovery should be decreased in
the presence of polymyxin B compared with control hearts perfused in
the absence of polymyxin B. In initial studies when polymyxin B was
dissolved in the buffer, coronary flow rate was significantly
reduced. This had particular consequences for the low-flow insult
because the low-flow ischemia was imposed at 10% of baseline
coronary flow rate. In addition, preischemic
function was significantly decreased. To avoid this, polymyxin B was
infused at 5% of the coronary flow rate into the aortic
cannula, immediately above the heart, starting 5 minutes before the
low-flow insult, and the low-flow ischemic flow rate was
therefore calculated on the basis of baseline flow in the absence of
polymyxin B. No significant differences in ischemic flow rate
were then observed compared with a second currently performed control
group (Table 3).
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From Table 3, it can be seen that percentage recovery of LVDP in the
control group in the absence (55±12%) and presence of polymyxin B
(56±1%) was similar.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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In this study, we have demonstrated that in the isolated, blood-perfused rat heart, preconditioning does not protect against contractile dysfunction when residual flow is present during the sustained insult. This failure to demonstrate protection was not due to a washout of the ischemic metabolite adenosine nor due to a protective effect of PKC, which may have been activated early enough during low-flow ischemia (in the control group) to phosphorylate the end-effector protein. In a previous study, we have also demonstrated that preconditioning was unable to protect against contractile dysfunction resulting from a prolonged period of hypoxia,4 that is, continued coronary flow with hypoxic buffer.
The reason for this loss of protection against hypoxic or low-flow ischemia–induced injury is unknown but may be related to effects of tissue perfusion or washout. In the previous study, where the sustained insult was hypoxia, flow was maintained at normoxic levels and thus complete washout of all ischemic metabolites probably occurred.4 In contrast, in the current study, washout of ischemic metabolites was undoubtedly incomplete. In the isolated, blood-perfused rat heart model, flow rates are at a physiological level of between 2 and 3 mL/min per gram wet wt. Consequently, when coronary flow rate is reduced to 10% of baseline level, the residual flow is only 0.2 to 0.3 mL/min per gram wet wt. In a previous study, we have demonstrated that under these conditions, washout of protons is incomplete.11 Intracellular pH decreased to 6.1, a similar level to that observed during zero-flow ischemia.2 Similarly, only a partial washout of lactate occurs; upon reperfusion, a large lactate washout peak was observed during the first minute of reperfusion, indicating that a significant portion of the lactate produced during ischemia remains in the tissue until reperfusion. Thus it is likely that a significant proportion of other ischemic metabolites, such as adenosine, also remain in the tissue. Despite this, however, preconditioning was unable to protect against injury sustained during a low-flow insult.
To investigate the influence of tissue washout further, we infused
100 µmol/L of adenosine during the sustained
period of ischemia. Adenosine was chosen because it is
one of the few ischemic metabolites known to initiate
preconditioning but that could be infused without metabolic
cost during low-flow ischemia. If, for example, an
1-agonist such as phenylephrine had been
infused, contractile function would have been stimulated and at this
level of perfusion the degree of relative ischemia exacerbated.
Furthermore, Headrick15 recently reported that the
adenosine A1-receptor agonist
N6-cyclohexyladenosine mimicked
preconditioning and the competitive adenosine receptor
antagonist 8-(p-sulfophenyl) theophylline
abolished preconditioning in the isolated rat heart.15 It
should be noted that these results are in contrast to others in the
literature.16 17 18 The infusion of adenosine had no
contractile effects and only caused a transient vasodilation that had
recovered before the initiation of low-flow ischemia. The
infusion of adenosine, however, failed to restore
preconditioning-induced protection in this model. Thus if the washout
of an ischemic metabolite is responsible for the loss of
preconditioning in the rat, it is unlikely to be adenosine.
Several studies have demonstrated that glycolysis is slowed in preconditioned hearts during the sustained ischemic period.4 19 However, we have recently demonstrated that a reduction in lactate release (as a measure of glycolytic rate) is not a prerequiste for preconditioning-induced protection.7 Thus the lack of a significant reduction in lactate production in preconditioned hearts subjected to low-flow ischemia is unlikely to account for the lack of protection. Furthermore, these data are complicated by the fact that during zero-flow ischemia, lactate production will arise from glycogen breakdown, whereas during low-flow ischemia it will arise from glucose in the perfusate.
Another possible cause for the loss of protection against either
ischemia or hypoxia may be that PKC may be
activated early enough during the sustained insult to
phosphorylate the end-effector protein. Since the
translocation of PKC into the cell membrane requires 10
minutes20 and kinase activity parallels translocation,
there is an 10-minute delay from activation of PKC to
phosphorylation of a substrate. During zero-flow
ischemia, therefore, high-energy phosphate stores will have
been significantly depleted, thus limiting any protective effect.
During low-flow ischemia, however, we have previously
demonstrated that adenosine triphosphate (ATP) and
phosphocreatine (PCr) falls far more slowly than during zero-flow
ischemia; after 60 minutes, ATP and PCr content was still
41±3% and 36±3% of baseline levels, respectively.11
Thus PKC, activated and translocated early during the low-flow
ischemic period, could probably phosphorylate its
substrate even in control hearts. Control hearts would thus effectively
be "preconditioned." In support of this argument, both
hypoxia and low-flow ischemia are able to initiate the
preconditioning pheneomenon in the isolated rat heart,21
and we have previously demonstrated that the PKC inhibitor
polymyxin B is able to abolish preconditioning in the isolated rat
heart.7 If control hearts are indeed
"preconditioned" by low-flow ischemia, then functional
recovery should be decreased in the presence of polymyxin B. However,
this was not the case; functional recovery was almost identical in the
presence and absence of polymyxin B. Thus it is unlikely that the lack
of preconditioning-induced protection against low-flow ischemia
is a result of an early activation of PKC during the sustained low-flow
ischemic period.
Study Limitations
The residual ischemic flow of 0.3 mL/min per gram wet
wt in our isolated rat heart study must be interpreted in the context
of prior work that has demonstrated that preconditioning decreases
infarct size in many species in which collateral flow
exists.19 22 23 However, in such experimental studies,
animals were excluded from study when the residual collateral flow was
>0.15 mL/min per gram wet wt22 23 or 0.12 mL/min gram wet
wt.19 In fact, collateral flow in the subendocardial
region, where the infarct develops, was considerably less than this
(0.04 mL/min per gram wet wt).22 Thus preconditioning
can protect against low-flow ischemia but only when the degree
of ischemia is at the requisite level of severity. Our results
would suggest that preconditioning cannot protect against injury
induced by less severe degrees of ischemia. Similar
observations were made by Murry et al,19 who observed no
preconditioning-induced protection against necrosis in the
subepicardial zone in the dog heart, in which collateral flow ranged
from 0.22 to 0.37 mL/min per gram wet wt. Murry et al19
hypothesized that the failure of preconditioning to protect these areas
may be either because the preconditioning protocol failed to
effectively precondition these areas, that is, the higher collateral
flow in these areas would result in less damage, or because
preconditioning-induced protection is only effective in severely
ischemic tissue. Our results would suggest that the latter
explanation is the correct one. However, the failure of preconditioning
to protect against mild to moderate ischemia does not negate
its proven protection against necrosis arising from severe
ischemia.
Preconditioning can only delay cell death. Thus if the ischemic duration is greatly extended,19 preconditioning-induced protection is overwhelmed. However, if the rate of development of ischemic damage is slowed, for example, with hypothermia24 or cardioplegic arrest,25 preconditioning can protect over ischemic durations as long as 200 minutes. Consequently, the protocol used in the current study was carefully chosen such that control hearts recovered to a similar extent in both groups. Therefore, since injury develops slower in the presence of residual flow, hearts subjected to low-flow ischemia had to be exposed to a longer (90 minutes) sustained insult than the hearts subjected to zero-flow ischemia (30 minutes). In fact, even under conditions of normothermic ischemia, preconditioning has been shown to protect against zero-flow ischemia insults of up to 90 minutes in length.26 27 28 Thus we are confident that preconditioning did not "wear off," nor was the protection overwhelmed during the prolonged period of low-flow ischemia. Furthermore, we have not measured PKC activity in this study, nor do we have any direct evidence that polymyxin B inhibited PKC activity. However, in previous studies using the same model, we have demonstrated that polymyxin B abolished preconditioning-induced protection.7
Conclusions
We have demonstrated that in the presence of a residual flow
of 0.29±0.02 mL/min per gram, preconditioning failed to protect
against contractile dysfunction assessed upon reperfusion. The failure
of preconditioning to protect against this insult was not a result of
the washout of adenosine or a result of the activation of PKC.
It remains to be determined whether the loss of preconditioning-induced
protection in the presence of residual flow during the sustained insult
reduces the therapeutic potential of the preconditioning
phenomenon.
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Acknowledgments |
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Dr Cave was supported by the Massachusetts Affiliate of the American Heart Association. The authors gratefully acknowledge the advice of and discussions with Drs Franz Eberli and Thomas Suter.
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Footnotes |
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E mail a.cave@umds.ac.uk
Received December 31, 1996; revision received May 9, 1997; accepted May 28, 1997.
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References |
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