(Circulation. 1995;92:381-388.)
© 1995 American Heart Association, Inc.
Articles |
From the Cardiovascular Division, Department of Surgery, Surgical Research Center, University of Connecticut School of Medicine, Farmington, Conn, and the Department of Surgery, Baystate Medical Center, Springfield, Mass.
Correspondence to Dipak K. Das, PhD, University of Connecticut School of Medicine, Farmington, CT 06030-1110.
| Abstract |
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Methods and Results Adult Yorkshire swine were injected with amphetamine (3 mg/kg IM) (n=12), and body temperature was continuously monitored. For control studies, the pigs were injected with saline (n=12). Five swine in each group were killed after 3 hours to obtain biopsies of vital organs to measure heat-shock protein (HSP) mRNAs. After 40 hours, the remaining 7 pigs in each group were placed on cardiopulmonary bypass, and the isolated, in situ heart preparations were subjected to 1 hour of occlusion of the left anterior descending coronary artery followed by 1 hour of global hypothermic cardioplegic arrest and 1 hour of reperfusion. Postischemic myocardial performance was monitored by measuring left ventricular (LV) pressure, its dP/dt, myocardial segment shortening, and coronary blood flow. Cellular injury was examined by measurement of creatine kinase release. The antioxidant enzymes superoxide dismutase and catalase were also assayed. Amphetamine treatment was associated with the induction of mRNAs for HSP 27, HSP 70, and HSP 89 in all the vital organs, including heart, lung, liver, kidney, and brain. Amphetamine also enhanced superoxide dismutase and catalase activities in the heart. Significantly greater recovery of LV contractile functions was noticed, as demonstrated by improved recovery of LV developed pressure (61% versus 52%), LV dP/dtmax (52% versus 44%), and segment shortening (46.2% versus 10%) and reduced creatine kinase release in the amphetamine group.
Conclusions The results demonstrate that amphetamine can induce whole-body heat shock that can precondition the heart, enhancing cellular tolerance to ischemia-reperfusion injury. Amphetamine is a sympathomimetic drug that may be used for preconditioning.
Key Words: proteins genes amphetamine ischemia antioxidants
| Introduction |
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Currie and colleagues10 were probably the first group to demonstrate enhanced postischemic ventricular recovery after subjecting the heart to heat shock. The results of many recent studies11 12 13 now support the earlier observation by Currie and further imply that the heart can be preconditioned by heat shock to prevent ischemia-reperfusion injury.14
The conventional method of inducing whole-body heat shock is by warming the animal by immersing it in warm water15 or by wrapping it with an electric blanket.16 However, it is extremely difficult to raise the temperature of an internal organ such as the heart to 42°C to 43°C without causing severe burns. This is not only because the heart is protected by skin, muscle, and other tissues but also because of the ability of a live animal to maintain its own body temperature against hypothermia or hyperthermia.17 Moreover, it is practically impossible to raise the body temperature of large animals, such as pigs or dogs, by this method. In this study, we attempted to resolve this problem by inducing heat shock with a sympathomimetic drug, amphetamine, that has been known to increase the body temperature by enhancing endogenous lipolysis.18 Amphetamine was found to induce the expression of the mRNAs for HSP 27, HSP 70, and HSP 89 within 3 hours in all the vital organs of the pig, demonstrating its ability to induce whole-body heat shock.
| Methods |
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LV Global and Regional Functions
Isovolumetric LV pressure
measurements were performed with a
Millar Mikro-Tip catheter transducer (Millar Instruments, Inc), which
was placed within a 10-mL compliant balloon and inserted into the left
ventricle through an apical stab wound.19 Balloon volume
was maintained at 10 mL for each global function measurement. The LV
dP/dtmax and LVDP were also measured. LV segment
function in the LAD-distributed region was measured with two
piezoelectric crystals (Triton Technology) implanted in a
circumferential plane. The transducers were placed near the endocardium
through small epicardial punctures, and the two crystals were separated
by
1 cm. For the measurements of segment shortening, the balloon was
deflated, and measurements were made in an empty beating heart.
Tracings of segment length were recorded on a Honeywell AR-6
recorder (PPG Biochemical Systems, Honeywell, Inc) interfaced with
an oscilloscope (Tektronics, Inc) coupled to a
sonomicrometer (Triton Technology).
End-diastolic length (EDL) and end-systolic length
(ESL) were identified on the recordings. The percent segment
shortening was calculated as [(EDL-ESL)/EDL]x100.
Baseline measurements were taken after 15 minutes of stabilization at
normothermic perfusion, consisting of LVDP, LV
dP/dtmax, myocardial segment shortening, CBF,
and CK release. The LAD was then occluded distal to the first diagonal
branch for 60 minutes. After 60 minutes of LAD occlusion, the aortic
perfusion was stopped, and 50 mL of cold (4°C) potassium crystalloid
cardioplegic solution was infused into the aortic root to arrest the
heart. Cardioplegia reinfusion was made every 15 minutes during 60
minutes of cardioplegic arrest. The LAD snare was removed just before
the second injection of cardioplegia. The myocardial temperature was
monitored by a thermistor inserted into the ventricular
septum and was maintained with cold saline between 6°C and 10°C
during cardioplegia. Normothermic reperfusion was performed
for 60 minutes after cardioplegic arrest. In each study, the heart was
paced both before and after ischemia at 120 beats per minute by
ventricular pacing. Hemodynamic and
metabolic measurements were taken after 15 minutes and 60
minutes of reperfusion (Fig 1
). At the end of each
experiment, LV biopsies were taken for biochemical determination.
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CBF and CK Release
CBF was measured by timed collection of
coronary
effluent.19 The CK activity of coronary effluent
was determined by the enzymatic assay method by use of a CK assay kit
(Sigma Diagnostics). CK release was determined by
measurement of CK accumulation over time.20
Induction of HSP mRNA Expression
Total RNA was extracted from
the tissues by the acid guanidinium
thiocyanatephenolchloroform method, and the concentration
of RNA was estimated by measurement of the optical density (OD) of the
preparation at 260 nm (1 OD=40 µg RNA/mL).21 The ratio
between the readings at 260 and 280 nm
(OD260/OD280) provided the estimate of
the purity of the isolated RNA. The isolated RNAs had a ratio between
1.94 and 2.05 (pure preparation should have a ratio of 2.0). This
preparation was further verified by running RNA on 1% agarose gel
stained with ethidium bromide. RNA (10 µg) from each sample was
subjected to 1% agarose/2.2 mol/L formaldehyde gel electrophoresis and
then transferred to nylon membrane (Gene Screen Plus, NEN Research
Products) by capillary blot as described previously.20
The membranes were baked under vacuum at 80°C for 2 hours and
prehybridized in a solution that contained 50% formamide, 6xSSPE,
5xDenhardt's reagents, 0.9% SDS, and 100 µg/mL denatured
salmon
testes DNA at 42°C for 1 hour. They were then hybridized in a
solution containing 5% formamide, 6xSSPE, 1% SDS, and 100 µg/mL of
denatured salmon testes DNA with HSP cDNA probes (StressGen) at 42°C
for 20 hours. HSP 27 probe was a 400-bp Pst I fragment of
the human cDNA, HSP 70 (inducible) probe was a 4-kb EcoRI
fragment of the human cDNA, and HSP 89-
probe was a 1.3-kb
Pst I fragment of the human cDNA. ß-Actin probe (Oncor), a
770-bp fragment of chicken cDNA, was used as a housekeeping gene. All
the cDNA probes were labeled with [
-32P]dCTP by use
of
a random-primed DNA labeling kit (Boehringer Mannheim
Biochemica). The specific activity was
0.7x109
dpm/µg.
After the membranes were washed with 2xSSC at room temperature for 5 minutes (twice), with 2xSSC and 1% SDS at 60°C for 30 minutes (twice), and with 0.1xSSC at room temperature for 30 minutes (once), autoradiograms were obtained after exposure to Kodak X-Omat RP films at -70°C (NEN Research Products). Each hybridization was repeated at least three times with different membranes. After each hybridization, the residual cDNA was removed and rehybridized with ß-actin cDNA probe, the results of which served as control. The autoradiogram was quantitatively evaluated by computerized gel scanning as described previously. The results of densitometric scanning were normalized relative to the signal obtained for ß-actin cDNA.
Antioxidative Enzymes
The tissue homogenate was prepared by
homogenization of weighed amounts of tissue in a
measured volume of ice-cold Tris-sucrose buffer containing 0.25
mol/L sucrose, 10 mmol/L Tris-HCl, 1 mmol/L EDTA, and 0.5 mmol/L DTT,
pH 7.5. After centrifugation at 1000g for 15
minutes, the supernatant was centrifuged at 10 000g
for 20 minutes to obtain the mitochondrial pellet that was used as a
source of Mn-SOD9 The supernatant was again subjected to
centrifugation at 105 000g for 1 hour. The
supernatant was used to assay Cu/Zn-SOD. Catalase was estimated as
described previously.9 SOD activity was assayed by its
inhibitory action on the superoxide-dependent reduction
of ferricytochrome by xanthinexanthine oxidase.13
The final concentrations in the assay medium (total volume, 1 mL) were
cytochrome C 100 µmol/L, hypoxanthine 100 µmol/L, Tris/HCl 10
mmol/L, and enzyme protein 50 to 80 µg. The reaction was initiated by
addition of 8 mU xanthine oxidase. Catalase was estimated by
measurement of the decreases in the absorbance of
H2O2 at 240 nm.21 Final
concentrations in the assay medium (total volume, 1 mL) were phosphate
buffer 35 mmol/L, pH 7.2, Triton X-100 0.02%, and enzyme protein 30 to
50 µg. The reaction was started by the addition of 30 µL of 1%
H2O2 and monitored by the decrease in
absorbance of H2O2 at 240 nm.
Statistical Analysis
All results were expressed as
mean±SEM. Student's t
test was performed for comparison of differences between the two
groups, and a two-way ANOVA for repeated measures followed by
Bonferroni's correction was performed to compare the differences
within the group by use of computerized statistical analysis
software (PRIMER, ©1988, McGraw-Hill Inc). A difference
was considered significant only at values of P<.05.
| Results |
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The
rectal temperature of pigs before amphetamine injection ranged from
37°C to 39°C. One hour after the amphetamine injection, the rectal
temperature increased markedly, to a range of 41.5°C to 42.5°C.
This hyperthermic state lasted up to 1 hour, gradually decreased to
40°C after 2 hours, and finally returned to the basal temperature in
another 3 to 4 hours (Fig 2
). The animals were slightly
hyperactive after the amphetamine injection for a period of 3 to 6
hours, and then they behaved normally.
|
Improved Postischemic LV Function
The baseline values for
global and regional functions were
comparable between amphetamine and control groups
(Table
). LAD occlusion caused dyskinesis in the LAD
region, and the dyskinesis persisted throughout the duration of 60
minutes of occlusion in all hearts. Then the hearts were rapidly
arrested with cold cardioplegia, and the heart temperature was
maintained between 6°C and 10°C in both groups. During reperfusion,
the heat-shock group showed better recovery of LVDP and LV
dP/dtmax, as shown in Figs 3
and 4
.
The recovery of LVDP was significant after 60
minutes of reperfusion, 72% versus 52% (amphetamine versus control,
P<.05), whereas LV dP/dtmax became
significant after 15 minutes of reperfusion (58% for amphetamine and
40% for control, P<.05). This difference was maintained
during the rest of reperfusion such that at the end of 60 minutes of
reperfusion, the recovery of LV dP/dtmax for the
amphetamine group was 60% versus 44% for the control group
(P<.05) (Fig 4
) when determined at the end of
reperfusion.
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The baseline percent segment shortening was considered
100% (actual
value varied between 1 and 2 mm), which is shown in Fig 5
.
Percent segment shortening in both groups showed
steady recovery during reperfusion: 46.2% recovery was observed in the
amphetamine group compared with 10% in the control group
(P<.05) at 60 minutes of reperfusion.
|
CBF and CK Release
The change of CBF was not significantly
different between the two
groups (Fig 6
); responsive hyperemia was
observed during reperfusion in both groups. CK, which reflects cellular
injury and membrane permeability, was significantly increased during
reperfusion in both groups, but the CK release was significantly higher
at 15 and 60 minutes of reperfusion in the control group compared with
the heat-shock group (P<.05) (Fig 7
).
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Induction of the Expression of HSP mRNAs by
Amphetamine
To examine whether amphetamine could induce heat-shock
genes,
biopsies from heart, lung, liver, kidney, and brain were examined for
the expression of the mRNAs for HSPs. Northern blots were hybridized
with different probes for HSP 27, HSP 70, and HSP 89. ß-Actin served
as control. Amphetamine induced the expression of the mRNA for all the
HSPs tested. The observed HSP 27, HSP 70, and HSP 89 mRNA had signals
at 0.9, 2.4, and 3.0 kb, respectively, compared with the RNA marker
(28S and 18S) (results not shown). After
autoradiography, the hybridization signals were
quantified by scanning densitometry. Each hybridization signal was
normalized relative to the signals obtained for ß-actin signal.
These results are depicted in Figs 8 through
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Stimulation of SOD and Catalase Enzyme Activities
Forty hours
of amphetamine treatment significantly enhanced SOD
and catalase enzyme activities in the heart (Fig 11
).
Thus, 40 hours of pretreatment with amphetamine increased the
myocardial catalase activity by 63.7% (control, 13.25±1.9 versus
amphetamine, 21.28±1.6; P<.005). The activity of Cu/Zn-SOD
was increased by 43.1% (control, 5.41±0.75 versus amphetamine,
7.73±0.63; P<.04), and Mn-SOD was increased by 50%
(control, 19.59±1.01 versus amphetamine, 40.0±0.9
nmol · min-1 · mg protein-1;
P<.05) by amphetamine treatment compared with control.
|
| Discussion |
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In the present study, we used the sympathomimetic drug amphetamine to elevate the body temperature. It is well known that amphetamine stimulates cellular metabolism by accelerating endogenous lipolysis.18 In agreement, we demonstrated marked elevation of body temperature within hours, which is instrumental for the induction of heat-shock response. Amphetamine is one of the most potent sympathomimetic amines with respect to stimulation of the central nervous system. Prevention and reversal of fatigue by amphetamine has been studied extensively in experimental laboratories, military field studies, and athletics.24 The optimal therapeutic dose (3 mg/kg) was established after injection of different groups of pigs with several doses of amphetamine ranging from 0.5 to 10 mg/kg. Amphetamine at 10 mg/kg caused ventricular fibrillation in the pigs, whereas 0.5 to 1 mg/kg was quite ineffective in elevating the body temperature. The therapeutic dose used in this experiment caused a minimal amount of restlessness but was enough to induce the heat-shock response. Amphetamine caused the induction of the expression of HSP genes in all the vital organs, including heart, lung, liver, brain, kidney, and intestine, indicating that this drug can cause a whole-body heat-shock response. Thus, induction of heat shock by amphetamine seems to be superior to other commonly used methods, such as direct heating of the body or immersing the animals in a hot-water bath.
In addition to enhancing endogenous lipolysis, resulting in
the elevation of body temperature, amphetamine also stimulates the
-receptors and provokes the release of
catecholamines. This may induce a preconditioning
phenomenon resulting in myocardial preservation. However, it is well
established that such a preconditioning effect can last for only a few
hours. We specifically selected a time of 40 hours after amphetamine
treatment to study myocardial preservation to eliminate any
sympathomimetic or preconditioning effects of the drug. A growing body
of evidence now suggests that myocardial protection is observed after
24 to 48 hours (called a second window of protection or adaptation) by
a mechanism likely to be mediated by the expression of heat-shock,
antioxidant, and/or other stress proteins.
The induction of the expression of HSPs has been observed in a variety of cells and tissues in response to many diseases, including oxidant stress, ischemia-reperfusion, inflammation, and fever.25 26 It is generally believed that the expression of HSPs represents the acute response of the cell to the pathophysiological condition in an attempt to adapt itself to that particular disease state. Thus, heat shock can induce the expression of specific stress-related genes, including heat-shock genes, that are often translated into heat-shock proteins and other stress proteins that are protective against the subsequent injury to the cells and tissues.25 26 27 The major HSPs have been classified into four families: the HSP 90 family (83 to 90 kD), the HSP 70 family (66 to 78 kD), the HSP 60 family, and the HSP 27 family (15 to 30 kD).28 These stress proteins are functionally different and therefore are likely to differ in their ability to enhance cellular tolerance to subsequent stress.
The induction of heat shock has been demonstrated to be protective against ischemic injury in several models. For example, Currie and associates10 reported that heat shock induced by physical heating improves the recovery of LV contractile force after 30 minutes of ischemia in rat hearts. The expression of heat-shock protein and induction of catalase activity were also demonstrated in their experiment. Heat-shock pretreatment also improved the function in hypothermically preserved kidney transplants.29 Rapid expression of heat-shock proteins has been shown in liver and heart subjected to ischemic insult.30 31 Knowlton and coworkers,15 from the results of their study of the enhanced postischemic ventricular recovery after the heart was exposed to brief periods of ischemia, also suggested that induction of HSP 70 might play a role in myocardial stunning and preconditioning. A recent study by Donnelly and associates11 indicated a cardioprotective effect of whole-body hyperthermia against ischemic injury. In agreement with these results, our laboratory also demonstrated that perfusing an isolated heart with warm blood could protect it against ischemia-reperfusion injury.8 Contrary to that, in a study by Wall et al,32 the beneficial effects of heat shock were not found after severe myocardial ischemia, although the HSP synthesis was enhanced. Yellon and coworkers33 also demonstrated the induction of HSP 72 after whole-body heat shock, but without any cardioprotection. To the contrary, irrespective of the species differences, almost all the investigators found cardioprotective effects of heat shock when isolated heart was used as the experimental model.8 10 11 As mentioned earlier, it is extremely difficult to raise the temperature of an internal organ such as the heart. Warming the large animals does not usually raise the myocardial temperature high enough to induce heat shock, whereas in the isolated heart model this can be easily achieved by perfusing the heart at 42°C to 43°C.
The results of the present study demonstrate that pretreating the hearts with amphetamine elevated the body temperature to 42°C to 43°C; induced whole-body heat shock, as evidenced by the induction of the expression of HSP genes; improved postischemic LV global as well as regional contractile functions; and reduced myocardial cellular injury after LAD artery occlusion and hypothermic cardioplegic arrest. Sixty minutes of regional ischemia followed by 60 minutes of hypothermic cardioplegic arrest was designed to mimic the clinical event of acute coronary occlusion (thrombosis) leading to ischemia followed by revascularization.
The exact mechanisms of the protective effects of HSPs against cellular injury remain unknown. The results of our study also demonstrated activation of two antioxidant enzymes, SOD and catalase. A recent study from our laboratory demonstrated activation of SOD when an isolated in situ pig heart was subjected to heat shock by 42°C blood cardioplegia.8 In another recent study, mRNAs for catalase and Mn-SOD simultaneously with the mRNAs of HSPs were induced after four brief episodes of ischemia.21 In contrast, Karmazyn et al34 found no detectable change in mRNA for catalase after heat shock or during recovery after heat shock, but they demonstrated an increase in catalase enzyme activity at 24 and 48 hours after the induction of heat-shock response. The authors concluded that catalase activity is either translationally or posttranslationally regulated. Several recent studies also suggest that heat shock enhances the cellular antioxidative defense system.35 In another related study, heat shock increased the tolerance of polymorphonuclear neutrophils to oxidative injury mediated by H2O2.36 Substantial evidence supports the formation of oxygen free radicals and development of oxidative stress in ischemic-reperfused myocardium.37 38 Ischemia and reperfusion also cause the inactivation of natural antioxidant enzymes such as SOD and catalase. Thus, the stimulation of SOD and catalase could reflect a mechanism of myocardial adaptation to oxidative stress.39 Although it is tempting to speculate that heat shockmediated stimulation of antioxidant enzymes is instrumental for the enhanced postischemic functional recovery of the heart, further study is required to confirm such a possibility.
In summary, we have demonstrated that a sympathomimetic drug such as amphetamine can induce the heat-shock response, inducing the expression of several HSP genes. This study has some relevance to the concept of warm heart surgery, in which heart is directly subjected to heat shock by warm blood cardioplegia. However, the myocardial preservation by warm blood cardioplegia resembles the preconditioning phenomenon, whereas induction of the antioxidant defense system through heat shock by amphetamine reflects the adaptive response. Amphetamine, either directly or indirectly, through the stimulation of the antioxidant defense system, can adapt the heart to ischemic stress and improve postischemic ventricular recovery in a setting of coronary revascularization for acute myocardial infarction.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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| References |
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