Expression of calcium regulatory proteins in short-term hibernation and stunning in the in situ porcine heart

Effects of acute ischemia and hypoxia in young and adult calsequestrin (CSQ2) knock-out and wild-type mice

Calsequestrin (CSQ2) is the main Ca²⁺-binding protein in the sarcoplasmic reticulum of the mammalian heart. In order to understand the function of calsequestrin better, we compared two age groups (young: 4-5 months of age versus adult: 18 months of age) of CSQ2 knock-out mice (CSQ2(-/-)) and littermate wild-type mice (CSQ2(+/+)). Using echocardiography, in adult mice, the basal left ventricular ejection fraction and the spontaneous beating rate were lower in CSQ2(-/-) compared to CSQ2(+/+). The increase in ejection fraction by β-adrenergic stimulation (intraperitoneal injection of isoproterenol) was lower in adult CSQ2(-/-) versus adult CSQ2(+/+). After hypoxia in vitro (isolated atrial preparations) by gassing the organ bath buffer with 95% N₂, force of contraction in electrically driven left atria increased to lower values in young CSQ2(-/-) than in young CSQ2(+/+). In addition, after global ischemia and reperfusion (buffer-perfused hearts according to Langendorff; 20-min ischemia and 15-min reperfusion), the rate of tension development was higher in young CSQ2(-/-) compared to young CSQ2(+/+). Finally, we evaluated signs of inflammation (immune cells, autoantibodies, and fibrosis). However, whereas no immunological alterations were found between all investigated groups, pronounced fibrosis was found in the ventricles of adult CSQ2(-/-) compared to all other groups. We suggest that in young mice, CSQ2 is important for cardiac performance especially in isolated cardiac preparations under conditions of impaired oxygen supply, but with differences between atrium and ventricle. Lack of CSQ2 leads age dependently to fibrosis and depressed cardiac performance in echocardiographic studies.

The physiological significance of a coronary stenosis differentially affects contractility and mitochondrial function in viable chronically dysfunctional myocardium

The reversibility of viable dysfunctional myocardium after revascularization is variable and the reasons for this are unknown. Using 2D-DIGE, we tested the hypothesis that this could reflect the extent of molecular remodeling of myocardial tissue in the absence of infarction. Swine with a progressive left anterior descending (LAD) stenosis were studied 2 months (n = 18) or 3 months (n = 22) post-instrumentation. Coronary flow reserve (vasodilated/rest) was severely reduced at 2 months (LAD 2.6 ± 0.4 versus 5.1 ± 0.4 in normal, p < 0.05) and became critically impaired after 3 months (LAD 1.1 ± 0.2, p < 0.05 vs. 2 months). Despite progression in stenosis severity, reductions in wall thickening at 2 months (LAD 37 ± 4% vs. remote 86 ± 9%, p < 0.05) were unchanged at 3 months (LAD 32 ± 3%, p = ns). Contractile dysfunction was primarily related to reductions (LAD/normal) in contractile proteins which were not affected by stenosis severity (e.g., troponin T, 2 months 0.82 ± 0.03 vs. 0.74 ± 0.03 at 3 months, p-ns). In contrast, mitochondrial function and proteins were normal at 2 months but declined with progression to a critical stenosis (state 3 respiration at 3 months 145 ± 13 vs. 216 ± 5 ng-atoms O2 mg(-1) min(-1) at 2 months, p < 0.05). In a similar fashion, increases in stress (e.g., αB-crystalline 2.13 ± 0.2 vs. 1.17 ± 0.13 at 2 months, p < 0.05) and cytoskeletal proteins (e.g., desmin 1.63 ± 0.12 vs. 1.24 ± 0.10 at 2 months, p < 0.05) only developed with more advanced remodeling from a critical stenosis. We conclude that similar degrees of chronic contractile dysfunction can have diverse intrinsic molecular adaptations to ischemia. This spectrum of adaptations may underlie variability in the time course and extent of reversibility in viable chronically dysfunctional myocardium after revascularization.

Effects of N-n-butyl haloperidol iodide on the rat myocardial sarcoplasmic reticulum Ca(2+)-ATPase during ischemia/reperfusion

We have previously shown that N-n-butyl haloperidol iodide (F(2)), a newly synthesized compound, reduces ischemia/reperfusion (I/R) injury by preventing intracellular Ca(2+) overload through inhibiting L-type calcium channels and outward current of Na(+)/Ca(2+) exchanger. This study was to investigate the effects of F(2) on activity and protein expression of the rat myocardial sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) during I/R to discover other molecular mechanisms by which F(2) maintains intracellular Ca(2+) homeostasis. In an in vivo rat model of myocardial I/R achieved by occluding coronary artery for 30-60 min followed by 0-120 min reperfusion, treatment with F(2) (0.25, 0.5, 1, 2 and 4 mg/kg, respectively) dose-dependently inhibited the I/R-induced decrease in SERCA activity. However, neither different durations of I/R nor different doses of F(2) altered the expression levels of myocardial SERCA2a protein. These results indicate that F(2) exerts cardioprotective effects against I/R injury by inhibiting I/R-mediated decrease in SERCA activity by a mechanism independent of SERCA2a protein levels modulation.

Autonomic dysfunction in patients with Brugada syndrome: further biochemical evidence of altered signaling pathways

BACKGROUND

In patients with Brugada syndrome (BrS), life-threatening ventricular tachyarrhythmias predominantly occur during vagal stimulation at rest or during sleep. Previous imaging studies displayed an impaired autonomic function in BrS patients. However, it remains unclear whether these alterations primarily stem from a reduction of synaptic release of norepinephrine (NE) or an enhanced presynaptic reuptake. Both conditions could lead to reduced NE concentrations in the synaptic cleft. Therefore, we analyzed key components of the sympathoadrenergic signaling pathways in patients with BrS.

METHODS AND RESULTS

Endomyocardial biopsies were obtained from eight BrS patients (seven male; age 49 ± 15 years) and five controls (three male; age 43 ± 13 years; P = ns). The concentrations of NE, epinephrine (Epi), NE transport (NET) carrier protein, cyclic adenosine 5'monophosphate (cyclic adenosine monophosphate [cAMP]), inhibitory G-proteins (G(i1,2) α), troponin-I (TNI), and phosphorylated TNI were analyzed. Levels of NET, G(i1,2) α, TNI, Epi, and phosphorylated TNI were comparable between the groups. Compared to controls, patients with BrS showed reduced cAMP and NE concentrations.

CONCLUSIONS

The current findings expand the concept of adrenergic dysfunction in BrS: the reduction of NE in BrS could lead to an impaired stimulation of β-adrenoceptors resulting in a reduction of cAMP and alterations of the subsequent signaling pathway with potential implication for arrhythmogenesis.

Gene Expression of the Na+–Ca2+ Exchanger, SERCA2a and Calsequestrin after Myocardial Ischemia in the Neonatal Rabbit Heart

BACKGROUND

Neonatal hearts are less susceptible to developing myocardial dysfunction after hypoxia and/or ischemia than adult hearts. Differences in intracellular calcium homeostasis may be responsible for reduced calcium overload of the immature myocardium leading to the observed protection against ischemia.

OBJECTIVE

To assess differences in baseline and post-ischemic gene expression of calcium handling proteins after ischemia in neonatal and adult rabbit hearts.

METHODS

We used isolated antegrade perfused rabbit hearts (age 2 days, 28 days, n = 32), which were exposed to ischemia and hypothermia simulating myocardial stunning comparable to neonatal asphyxia. Gene and protein expression of the sodium-calcium exchanger (NCX), the sarco-endoplasmatic reticulum Ca2+-ATPase 2a (SERCA) and calsequestrin (CSQ) were measured using quantitative real-time PCR and Western blotting.

RESULTS

After ischemia and reperfusion in neonatal and adult hearts, a significant decrease in myocardial performance was recorded. At the mRNA level, significant differences in the baseline expression of NCX, SERCA and CSQ between neonatal and adult hearts were observed. In neonatal post-ischemic hearts, NCX and CSQ expression were significantly higher at the mRNA level than in controls. In contrast, SERCA expression remained unchanged in neonatal hearts and decreased in adult hearts compared to the non-ischemic controls.

CONCLUSION

These findings suggest that changes in gene expression of calcium handling proteins may be involved in the different susceptibility of neonatal compared to adult hearts to developing myocardial dysfunction after ischemia.

Myosin binding protein C is differentially phosphorylated upon myocardial stunning in canine and rat hearts — Evidence for novel phosphorylation sites

Myocardial stunning is the transient cardiac dysfunction that follows brief episodes of ischemia and reperfusion without associated myocardial necrosis. Currently, there is limited knowledge about its cellular and biochemical mechanisms. In order to better understand the underlying mechanisms of contractile dysfunction associated with the stunning, comprehensive proteomic studies using 2-D DIGE were performed using a regional stunning model in canine heart. Cardiac myosin binding protein C (cMyBP-C), a regulatory myofilament protein associated with the thick filament, and nebulette, a thin filament associated protein, were differentially expressed. Phosphoprotein specific staining indicated both protein changes were due to phosphorylation. Subsequent phosphorylation mapping of canine cMyBP-C using IMAC and MS/MS identified five phosphorylation sites, including three novel sites. In order to further evaluate this finding in a different model, cMyBP-C phosphorylation was examined in a rat model of global stunning. In the rat model, stunning was associated with increased phosphorylation of cMyBP-C at a critical calcium/calmodulin-dependent kinase II site, and the increased phosphorylation was largely inhibited when stunning was prevented by either ischemic preconditioning or reperfusion in the presence of low-calcium buffer. These data indicate cMyBP-C phosphorylation plays an important role in myocardial stunning.

Reduced sarcoplasmic reticulum Ca2+ -ATPase activity and dephosphorylated phospholamban contribute to contractile dysfunction in human hibernating myocardium

Human hibernating myocardium (HHM) is characterized by reversible contractile dysfunction during chronic ischemia. A disturbed calcium-homeostasis is a decisive factor for reduced functional capacity in heart diseases. We therefore investigated calcium-handling proteins in HHM. In 12 patients suffering from multi-vessel coronary artery disease and contractile dysfunction with indication for bypass surgery, HHM was detected preoperatively by thallium scintigraphy, radionuclide ventriculography and dobutamine echocardiography. Transmural biopsies of these regions were taken and analyzed by immunohistochemistry and electron microscopy. Furthermore, SR-calcium ATPase (SERCA2a), phospholamban (PLN), the phosphorylated forms of PLN (PLN-Ser16, PLN-Thr17) as well as sodium-calcium exchanger (NCX) and ryanodine receptor (RyR2) were investigated by RT-PCR and Western-blotting. Additionally, SERCA2a activity was measured by an enzyme-coupled assay. In all patients complete functional recovery could be documented 3 months after revascularization by repeating all preoperative investigations. In HHM maximal SERCA2a activity was significantly reduced (HHM: 424.5 +/- 33.9, control: 609.0 +/- 48.5 nmol ATP mg protein(-1) min(-1), p <or= 0.05), whereas SERCA2a protein levels were unchanged. mRNA levels (HHM: 1.36 +/- 0.08 vs. control: 0.78 +/- 0.04, p <or= 0.05) and protein amount (HHM:1.67 +/- 0.14 vs. control: 1.00 +/- 0.04, p <or= 0.05) of PLN (A1) were increased resulting in an increased PLN:SERCA2a-ratio. PLN-Ser16 (HHM: 0.60 +/- 0.08 vs. control: 1.00 +/- 0.11, p <or= 0.05) and PLN-Thr17 (HHM: 0.63 +/- 0.11 vs. control: 1.00 +/- 0.06, p <or= 0.05) phosphorylation was significantly decreased. RyR2 and NCX showed no significant alteration. In HHM a decreased activity of SERCA2a due to an impaired phosphorylation of PLN contributes to contractile dysfunction. The increase in the relative ratio of PLN/SERCA2a leads to a decreased calcium affinity of SERCA2a.

Ischemic Preconditioning by Unstable Angina Reduces the Release of CK-MB Following CABG and Stimulates Left Ventricular HSP-72 Protein Expression

BACKGROUND AND AIM

Whether the CK-MB reducing effect of ischemic preconditioning (IP) by unstable angina within 24 to 48 hours before CABG is achieved by early or by delayed preconditioning of left ventricular myocardium in humans is unknown. We investigated whether IP is associated with phosphorylation of p38 MAPK (characteristic for early preconditioning) or with increased protein expression of HSP-72 (characteristic for delayed preconditioning) at the time of CABG in patients.

METHODS

Nineteen patients were grouped according to the occurrence of ischemic episodes within 48 hours before CABG. The patients without angina were assigned to the control group (CON, n = 10) whereas patients who had experienced angina within 48 hours before CABG were assigned to the preconditioned group (IP, n = 9). The effect of IP on the CABG induced maximal release of creatine kinase (CK) and CK-MB was examined. Left ventricular biopsy specimens taken immediately before cross clamping from ischemic (ISCH) and from reference (REF) areas were processed to analyze p38 MAPK phosphorylation and HSP-72-protein expression.

RESULTS

While IP significantly reduced CK-MB (18.7 +/- 1.3 vs. 13.8 +/- 1.5 U/L, mean +/- SEM, p < 0.05), it only tended to reduce CK (292.7 +/- 32.8 vs. 274.1+/-31.1 U/L, p = NS, mean +/- SEM). CK-MB release for any given cross-clamp time was significantly reduced by IP (regression lines: CON, y= 0.4x+ 2, r= 0.8; IP, y= 0.1x+ 10, r= 0.2; p < 0.01, ANCOVA). There was no effect of IP on left ventricular p38 MAPK phosphorylation. IP increased left ventricular HSP-72-protein expression in ischemic areas when compared to reference areas (1.78 +/- 0.35 vs. 2.58 +/- 0.65, REF vs. ISCH, PhosphorImager units x10(6), mean +/- SEM, p < 0.05, ANCOVA).

CONCLUSIONS

Thus, in the human left ventricular myocardium there is a second window of protection lasting for at least 48 hours, while at that time the early phase of preconditioning has already gone.

Mechanisms of Cell Survival in Myocardial Hibernation

Myocardial hibernation represents a condition of regional ventricular dysfunction in patients with chronic coronary artery disease, which reverses gradually after revascularization. The precise mechanism mediating the regional dysfunction is still debated. One hypothesis suggests that chronic hypoperfusion results in a self-protecting downregulation in myocardial function and metabolism to match the decreased oxygen supply. An alternative hypothesis suggests that the myocardium is subject to repetitive episodes of ischemic dysfunction resulting from an imbalance between myocardial metabolic demand and supply that eventually creates a sustained depression of contractility. It is generally agreed that hibernating myocardium is submitted repeatedly to ischemic stress, and therefore one question persists: how do myocytes survive in the setting of chronic ischemia? The hallmark of hibernating myocardium is a maintained viability of the dysfunctional myocardium which relies on an increased uptake of glucose. We propose that, in addition to this metabolic adjustment, there must be molecular switches that confer resistance to ischemia in hibernating myocardium. Such mechanisms include the activation of a genomic program of cell survival as well as autophagy. These protective mechanisms are induced by ischemia and remain activated chronically as long as either sustained or intermittent ischemia persists.

Myocardial hibernation: a delicate balance

The pathophysiology of myocardial hibernation is characterized as a situation of reduced regional contractile function distal to a coronary artery stenosis that recovers after removal of the coronary stenosis. A subacute "downregulation" of contractile function in response to reduced regional myocardial blood flow exists, which normalizes regional energy and substrate metabolism but does not persist for more than 12-24 h. Chronic hibernation develops in response to one or more episodes of myocardial ischemia-reperfusion, possibly progressing from repetitive stunning with normal blood flow to hibernation with reduced blood flow. An upregulation of a protective gene program is seen in hibernating myocardium, putting it into the context of preconditioning. The morphology of hibernating myocardium is characterized by both adaptive and degenerative features.

Program of Cell Survival Underlying Human and Experimental Hibernating Myocardium

Hibernating myocardium refers to chronically dysfunctional myocardium in patients with coronary artery disease in which cardiac viability is maintained and whose function improves after coronary revascularization. It is our hypothesis that long-term adaptive genomic mechanisms subtend the survival capacity of this ischemic myocardium. Therefore, the goal of this study was to determine whether chronic repetitive ischemia elicits a gene program of survival protecting hibernating myocardium against cell death. Accordingly, we measured the expression of survival genes in hibernating myocardium, both in patients surgically treated for hibernation and in a chronic swine model of repetitive ischemia reproducing the features of hibernation. Human hibernating myocardium was characterized by an upregulation of genes and corresponding proteins involved in anti-apoptosis (IAP), growth (VEGF, H11 kinase), and cytoprotection (HSP70, HIF-1α, GLUT1). In the swine model, the same genes and proteins were upregulated after repetitive ischemia, which was accompanied by a concomitant decrease in myocyte apoptosis. These changes characterize viable tissue, because they were not found in irreversibly injured myocardium. Our report demonstrates a novel mechanism by which the activation of an endogenous gene program of cell survival underlies the sustained viability of the hibernating heart. Potentially, promoting such a program offers a novel opportunity to salvage postmitotic tissues in conditions of ischemia.

Modulation of thin filament activation by breakdown or isoform switching of thin filament proteins: physiological and pathological implications

In the heart, the contractile apparatus is adapted to the specific demands of the organ for continuous rhythmic contraction. The specialized contractile properties of heart muscle are attributable to the expression of cardiac-specific isoforms of contractile proteins. This review describes the isoforms of the thin filament proteins actin and tropomyosin and the three troponin subunits found in human heart muscle, how the isoform profiles of these proteins change during development and disease, and the possible functional consequences of these changes. During development of the heart, there is a distinctive switch of isoform expression at or shortly after birth; however, during adult life, thin filament protein isoform composition seems to be stable despite protein turnover rates of 3 to 10 days. The pattern of isoforms of actin, tropomyosin, troponin I, troponin C, and troponin T is not affected by aging or heart disease (ischemia and dilated cardiomyopathy). The evidence for proteolysis of thin filament proteins in situ during ischemia and stunning is evaluated, and it is concluded that C-terminal cleavage of troponin I is a feature of irreversibly injured myocardium but may not play a role in reversible stunning.

Novel mechanisms mediating stunned myocardium

Myocardial stunning is defined as the prolonged contractile dysfunction following an ischemic episode that does not result in necrosis, which also occurs in patients with coronary artery disease. There is also evidence to consider myocardial stunning as a fundamental component of hibernating myocardium. Various experimental approaches (from a brief episode to prolonged partial ischemia) and animal models (from rodents to large mammals) have been developed to investigate the pathogenesis of myocardial stunning. Three hypotheses to explain the mechanism, i.e. oxygen radical, Troponin I degradation, and Ca(2+), have been proposed. The first was tested primarily using large mammalian models, whereas the others were tested primarily using rodent models. Recently, the Ca(2+) handling hyothesis has been tested in a large mammalian swine model of myocardial stunning, in which both Ca(2+) and transients and L-type Ca(2+) current density were decreased. Relaxation function and phospholamban phosphorylation are also radically different in large mammalian and rodent models. In addition, troponin I degradation, which was identified as the mechanism of stunning in rodent models, was not found in stunned swine myocardium. Interestingly, the large mammalian model demonstrates that stunning elicits broad changes in gene and protein regulation, some of which have not been observed in the heart previously. The overall genomic adaptation upregulates the expression of survival genes that prevent irreversible damage. Pursuing these new concepts derived from large mammalian models of ischemia/reperfusion will provide more comprehensive mechanistic information underlying myocardial stunning and will serve to devise new therapeutic modalities for patients.

Dissociation of regional adaptations to ischemia and global myolysis in an accelerated Swine model of chronic hibernating myocardium

We tested the hypothesis that an acute critical limitation in coronary flow reserve could rapidly recapitulate the physiological, molecular, and morphological phenotype of hibernating myocardium. Chronically instrumented swine were subjected to a partial occlusion to produce acute stunning, followed by reperfusion through a critical stenosis. Stenosis severity was adjusted serially so that hyperemic flow was severely reduced yet always higher than the preocclusion resting level. After 24 hours, resting left anterior descending coronary artery (LAD) wall thickening had decreased from 36.3+/-4.0% to 25.5+/-3.7% (P<0.05), whereas resting flow had remained normal (67+/-6 versus 67+/-8 mL/min, respectively). Although peak hyperemic flow exceeded the prestenotic value, resting flow (45+/-10 mL/min) and LAD wall thickening (17.0+/-5.0%) progressively decreased after 2 weeks, when physiological features of hibernating myocardium had developed. Regional reductions in sarcoplasmic reticulum proteins were present in hibernating myocardium but absent in stunned myocardium evaluated after 24 hours. Histological analysis showed an increase in connective tissue along with myolysis (myofibrillar loss per myocyte >10%) and increased glycogen typical of hibernating myocardium in the LAD region (33+/-3% of myocytes from animals with hibernating myocardium versus 15+/-4% of myocytes from sham-instrumented animals, P<0.05). Surprisingly, the frequency of myolysis was similar in normally perfused remote regions from animals with hibernating myocardium (32+/-7%). We conclude that the regional physiological and molecular characteristics of hibernating myocardium develop rapidly after a critical limitation in flow reserve. In contrast, the global nature of myolysis and increased glycogen content dissociate them from the intrinsic adaptations to ischemia. These may be related to chronic elevations in preload but appear unlikely to contribute to chronic contractile dysfunction.

Effect of myocardial stunning on thiol status, myofibrillar ATPase and troponin I proteolysis

To investigate the mechanism underlying postischemic contractile dysfunction (myocardial stunning) we examined myocardial sulflhydryl group content, myofibrillar Ca2+-dependent Mg2+-ATPase activity and protein profile after global ischemia and reperfusion. The Langerdorff-perfused rabbit hearts were subjected to 15 min normothermic ischemia followed by 10 min reperfusion and myofibrils were isolated from homogenates of left ventricular tissues. Depressed contractile function during reperfusion was accompanied by a decrease in total sulfhydryl group content. However, myofibrillar protein profile was unchanged and Western immunoblotting analysis showed no significant differences in troponin I immunoreactive bands between control and stunned hearts. Likewise, myofibrillar Mg2+-ATPase activity was unaltered after ischemia and reperfusion. We conclude that myocardial stunning is not caused by altered myofibrillar function and protein degradation but may be partly due to the oxidative modification of as yet undefined proteins.

No involvement of nitric oxide in the limitation of beta-adrenergic inotropic responsiveness during ischemia

We tested whether or not endogenous nitric oxide (NO) attenuates beta-adrenergic inotropic responsiveness during normoperfusion or moderate myocardial ischemia. In 13 anesthetized pigs with a cannulated left anterior descending (LAD) coronary artery, the maximal contractile responses to intracoronary dobutamine and calcium were assessed during normoperfusion and at the end of a 90-min period of moderate ischemia (50% reduction in coronary arterial inflow) without (group 1, n = 6) and with (group 2, n = 7) prior inhibition of NO synthesis [30 mg/kg iv N(omega)-nitro-L-arginine (L-NNA)]. Contractile function was assessed by a regional work index (sonomicrometry, micromanometry, mm. mmHg). In groups 1 and 2 during normoperfusion, the maximal increase of the work index was greater with calcium than with dobutamine. At the end of ischemia in group 1, the baseline work index was decreased by approximately 50%, and the subsequent maximal increase of the work index with dobutamine, but not with calcium, was reduced compared with normoperfusion. In group 2 during normoperfusion, L-NNA did not alter the maximal increases of the work index with dobutamine or calcium. At the end of ischemia, the baseline work index was reduced by 64%, and the subsequent maximal increases of the work index with both dobutamine and calcium were reduced compared with normoperfusion; however, the response to calcium was still greater than that to dobutamine. We conclude that endogenous NO does not limit beta-adrenergic inotropic responsiveness in normoperfused or moderately ischemic porcine myocardium.

Myocyte apoptosis and reduced SR gene expression precede the transition from chronically stunned to hibernating myocardium

A systematic transition from chronic stunning to hibernation occurs as coronary flow reserve decreases to a critical level. Hibernating myocardium exhibits apoptosis-induced myocyte loss and a reduction in the expression of the sarcoplasmic reticulum (SR) Ca2+ ATPase but whether similar cellular changes occur in chronic stunning is unknown. Pigs with a chronic left anterior descending coronary artery (LAD) stenosis were studied one (n=9) or two (n=10) months after instrumentation. Anterior hypokinesis with normal levels of resting perfusion developed at each time-point, consistent with chronic stunning. After 1 month, sub-endocardial flow reserve was moderately reduced (adenosine/rest, LAD: 3.60+/-0.91 v Remote: 6.00+/-0.54, P<0.01) with no regional differences in SR protein expression, no increase in apoptosis (32+/-6 v 21+/-5 nuclei/10(6) myocyte nuclei, p-ns) and no regional myocyte loss (1976+/-44 v 1955+/-30 nuclei/mm2, p-ns). After 2 months, sub-endocardial flow reserve in chronically stunned myocardium was severely impaired (LAD: 1.41+/-0.21 v Remote: 5.59+/-0.96, P<0.01). There were small but significant reductions in LAD mRNA and protein levels for the SRCa2+ ATPase and phospholamban whereas calsequestrin was unchanged. In addition, regional myocyte apoptosis increased (127+/-24 v 55+/-9 nuclei/10(6) myocyte nuclei, P<0.01), resulting in the onset of myocyte loss (1293+/-50 v 1394+/-32 nuclei/mm2, P<0.01). Apoptosis-induced myocyte loss and reductions in SR protein expression are not invariably present in viable chronically dysfunctional myocardium. They are induced as the propensity of a region to develop reversible ischemia increases (as reflected by coronary flow reserve). The temporal progression indicates that alterations in SR protein expression and myocyte apoptosis precede the transition from chronically stunned to hibernating myocardium.

A novel mechanism for myocardial stunning involving impaired Ca(2+) handling

The mechanism of myocardial stunning has been studied extensively in rodents and is thought to involve a decrease in Ca(2+) responsiveness of the myofilaments, degradation of Troponin I (TnI), and no change in Ca(2+) handling. We studied the mechanism of stunning in isolated myocytes from chronically instrumented pigs. Myocytes were isolated from the ischemic (stunned) and nonischemic (normal) regions after 90-minute coronary stenosis followed by 60-minute reperfusion. Baseline myocyte contraction was reduced, P<0.01, in stunned myocytes (6.3+/-0.4%) compared with normal myocytes (8.8+/-0.4%). The time for 70% relaxation was prolonged, P<0.01, in stunned myocytes (131+/-8 ms) compared with normal myocytes (105+/-5 ms). The impaired contractile function was associated with decreased Ca(2+) transients (stunned, 0.33+/-0.04 versus normal, 0.49+/-0.05, P<0.01). Action potential measurements in stunned myocytes demonstrated a decrease in plateau potential without a change in resting membrane potential. These changes were associated with decreased L-type Ca(2+)-current density (stunned, -4.8+/-0.4 versus normal, -6.6+/-0.4 pA/pF, P<0.01). There were no differences in TnI, sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a), and phospholamban protein quantities. However, the fraction of phosphorylated phospholamban monomer was reduced in stunned myocardium. In rats, stunned myocytes demonstrated reduced systolic contraction but actually accelerated relaxation and no change in Ca(2+) transients. Thus, mechanisms of stunning in the pig are radically different from the widely held concepts derived from studies in rodents and involve impaired Ca(2+) handling and dephosphorylation of phospholamban, but not TnI degradation.

Progressive loss of perfusion-contraction matching during sustained moderate ischemia in pigs

It is unclear whether perfusion-contraction matching (PCM) is maintained during prolonged myocardial ischemia. In 27 anesthetized pigs, left anterior descending coronary arterial inflow was reduced to decrease an anterior work index (WI) at 5 min of hypoperfusion by 40% and then maintained at this level for 12 or 24 h. With 12 h of hypoperfusion, the myocardium remained viable in 6 of 7 pigs (with triphenyltetrazolium chloride; TTC) and with 24 h of hypoperfusion in 5 of 11 pigs (TTC, histology). The reduction in WI to 62 +/- 4 and 62 +/- 3% of baseline in the two groups was matched to the reduction of transmural blood flow (TBF; microspheres) at 5 min of hypoperfusion, averaging 59 +/- 4 and 60 +/- 2% of baseline. With prolonged hypoperfusion, WI decreased to 30 +/- 5% at 12 h and 18 +/- 3% at 24 h; TBF remained unchanged (53 +/- 4 and 54 +/- 4%). The added calcium concentration required for the half-maximal increase in WI increased from 121 +/- 25 microg/ml blood at baseline to 192 +/- 26 microg/ml blood at 12 h of hypoperfusion. Thus, with hypoperfusion for 24 h, PCM is progressively lost, and calcium responsiveness is reduced.

Short-term hibernation in adult cardiomyocytes is PO(2) dependent and Ca(2+) mediated

The mechanism of myocardial hibernation, the reversible downregulation of contractile activity on reduction of coronary flow with unchanged cardiac energetics, is presently not understood. The oxygen consumption (VO(2)), shortening fraction (DeltaL), energy status [phosphocreatine (PCr), ATP, and adenosine and lactate release], and free intracellular Ca(2+) concentration ([Ca(2+)](i)) were measured in isolated rat cardiomyocytes at precisely controlled ambient PO(2) (Oxystat). When PO(2) was reduced from 25 to 6 mmHg, VO(2) decreased by 50%, while DeltaL was downregulated from 11.2 +/- 4.1 to 7.6 +/- 4.0%, and energy status was unchanged in the steady state (observation time 12 min). Only transiently PCr decreased, and lactate and adenosine release increased. Further reduction of PO(2) (to 3 mmHg) reduced VO(2) by 80%, decreased PCr by 35%, moderately increased adenosine and lactate release, and progressively reduced DeltaL by 50% (to 5.6 +/- 3.3%). All parameters fully recovered during reoxygenation. PO(2)-dependent downregulation of DeltaL was accompanied by a progressive reduction in systolic [Ca(2+)](i) (from 512 +/- 110 to 357 +/- 91 nmol/l at 6 mmHg and to 251 +/- 69 nmol/l at 3 mmHg), whereas diastolic free [Ca(2+)](i) remained unchanged. Therefore, the mechanism of the reversible, PO(2)-dependent downregulation of contractile activity (myocardial hibernation) involves a substantial reduction of systolic calcium.

The stress-responsive MAP kinase p38 is activated by low-flow ischemia in the in situ porcine heart

Stress-responsive p38 MAP kinase is activated by phosphorylation during global and severe regional myocardial ischemia. However, it is unknown whether or not moderate, low-flow ischemia also activates p38 MAP kinase. Therefore, we investigated p38 MAP kinase activation in an established model of short-term hibernation and stunning. In anesthetized swine, coronary blood flow into the left anterior descending coronary artery was decreased in order to reduce regional contractile function by identical with 50%. Transmural myocardial biopsies were taken before (controls) and during ischemia as well as after reperfusion. Creatine phosphate content, after an early ischemic reduction, recovered to control values at 90 min ischemia. The expression of phospholamban, SERCA2a, calsequestrin, and troponin inhibitor was unchanged under these conditions (Northern and Western blotting). At 8 min of ischemia, however, p38 MAP kinase was activated to 221% of the pre-ischemic value as judged by its elevated phosphorylation state. Then, it returned to control values by 85 min ischemia. We conclude that low-flow ischemia transiently activates the stress-responsive p38 MAP kinase which might act to trigger cardioprotective events.

Adenosine and cardioprotection during ischaemia and reperfusion--an overview

Adenosine is a local hormone, with numerous tissue-specific biological functions. In the myocardium, adenosine is released in small amounts at constant basal rate during normoxia. During ischaemia the production of adenosine increases several fold due to breakdown of adenosine triphosphate (ATP). Increased production of adenosine causes coronary vasodilatation. Thus, adenosine couples myocardial metabolism and flow during ischaemia and is called a homeostatic or "retaliatory metabolite". Furthermore, adenosine has electrophysiological effects in supraventricular tissue, causing a decrease in heart rate. In 1985 it was discovered that adenosine also exerts cardioprotective effects directly on cardiomyocytes. The aim of this review is to give an overview of the role of adenosine as a directly cytoprotective agent during myocardial ischaemia and reperfusion. We will focus on its effects on the myocytes, elicited by stimulation of adenosine receptors in sarcolemma, which triggers intracellular signalling systems. We will also address the new aspect that adenosine can influence regulation of gene expression. There is evidence that the myocardium is capable of endogenous adaptation in response to ischaemia, namely "hibernation" and early and late phases of "preconditioning". Endogenous substances produced during ischaemia probably trigger these responses. We will discuss the role of adenosine in these different settings. Adenosine can be given exogenously through intravasal routes; however, this review will also focus on the effects of endogenously produced adenosine. We will discuss pharmacological ways to increase endogenous levels of adenosine, and the effects of such interventions during ischaemia and reperfusion. Finally, we will review results from studies in humans together with relevant experimental studies, and indicate potential therapeutic implications of adenosine.

Less afterload sensitivity in short-term hibernating than in acutely ischemic and stunned myocardium

Short-term hibernating myocardium is characterized by reduced contractile function during persistent moderate ischemia, the recovery of metabolic parameters, and the absence of necrosis. To study the afterload dependence of regional wall excursion in short-term hibernating myocardium, in 11 enflurane-anesthetized swine the left anterior descending coronary artery was cannulated and hypoperfused for 90 min to reduce anterior systolic wall thickening (WT, sonomicrometry) by 60%. Under control conditions, at 5 and 90 min ischemia the descending thoracic aorta was acutely constricted to increase left ventricular (LV) pressure by 30 mmHg. Under control conditions, increased LV pressure resulted in decreased WT [i.e., a negative slope of the relationship between WT and LV end-systolic pressure: -11.2 +/- 4.2 (SD) microm/mmHg]. This slope was further significantly decreased at 5 min ischemia (-26.5 +/- 8.8 microm/mmHg) but returned toward control values in short-term hibernating myocardium at 90 min ischemia (-17.2 +/- 6.6 microm/mmHg). At 30 min reperfusion, the slope was once more significantly decreased (-27.8 +/- 8.1 microm/mmHg). In conclusion, WT in short-term hibernating myocardium is less afterload dependent than in acutely ischemic and reperfused myocardium.

Biochemical mechanism(s) of stunning in conscious dogs

The mechanism(s) underlying contractile dysfunction in cardiac stunning is not completely understood. The expression and/or the phosphorylation state of cardiac Ca(2+) homoeostasis-regulating proteins might be altered in stunning. We tested this hypothesis in a well-characterized model of stunning. Conscious dogs were chronically instrumented, and the left anterior descending artery (LAD) was occluded for 10 min. Thereafter, reperfusion of the LAD was initiated. Tissues from reperfused LAD (stunned) and Ramus circumflexus (control) areas were obtained when left ventricular regional wall thickening fraction had recovered by 50%. Northern and Western blotting revealed no differences in the expression of the following genes: phospholamban, calsequestrin, sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a, and the inhibitory subunit of troponin I (TnI). However, the phosphorylation state of TnI and phospholamban were reduced in the LAD area. Fittingly, cAMP levels were reduced by 28% (P < 0.05). It is concluded that the contractile dysfunction in cardiac stunning might be mediated in part by decreased levels of cAMP and subsequently a reduced phosphorylation state of phospholamban and TnI.

Regional alterations in SR Ca(2+)-ATPase, phospholamban, and HSP-70 expression in chronic hibernating myocardium

We sought to identify mechanisms for chronic dysfunction in hibernating myocardium. Pigs were instrumented with a left anterior descending artery stenosis for 3 mo. Angiography demonstrated high-grade stenoses and hibernating myocardium with 1) severe anterior hypokinesis (P < 0.001 vs. shams), 2) reduced subendocardial perfusion [0.73 +/- 0.05 (SE) vs. 1.01 +/- 0.06 ml. min(-1). g(-1) in normal, P < 0.001], and 3) critically reduced adenosine flow (1.0 +/- 0.17 vs. 3.84 +/- 0.26 ml. min(-1). g(-1) in normal, P < 0.001). Histology did not reveal necrosis. Northern blot analysis of hibernating myocardium demonstrated regional downregulation in mRNAs for sarcoplasmic reticulum (SR) proteins phospholamban (0.76 +/- 0.08 vs. 1.07 +/- 0.06, P < 0.02) and SR Ca(2+)-ATPase (0.83 +/- 0.06 vs. 1.02 +/- 0.06, P < 0.05) with no change in calsequestrin (1.08 +/- 0.06 vs. 0.96 +/- 0.05, P = not significant). Heat shock protein (HSP)-70 mRNA was regionally induced in hibernating myocardium (2.4 +/- 0.3 vs. 1.0 +/- 0.11, P < 0.01). Directionally similar changes were confirmed by Western blot analysis of respective proteins. Our results indicate that hibernating myocardium exhibits a molecular phenotype that on a regional basis is similar to end-stage ischemic cardiomyopathy. This supports the hypothesis that SR dysfunction from reversible ischemia may be an early defect in the progression of left ventricular dysfunction.

Elevated levels of endogenous adenosine alter metabolism and enhance reduction in contractile function during low-flow ischemia: associated changes in expression of Ca(2+)-ATPase and phospholamban

Adenosine has several potentially cardioprotective effects including vasodilatation, reduction in heart rate and alterations in metabolism. Adenosine inhibits catecholamine-induced increase in contractile function mainly through inhibition of phosphorylation of phospholamban (PLB), the main regulatory protein of Ca(2+)-ATPase in sarcoplasmic reticulum (SR), and during ischemia it reduces calcium (Ca2+) overload. In this study we examined the effects of endogenous adenosine on contractile function and metabolism during low-flow ischemia (LFI) and investigated whether endogenous adenosine can alter expression of the Ca(2+)-ATPase/PLB-system and other Ca(2+)-regulatory proteins. Isolated blood-perfused piglet hearts underwent 120 min 10% flow. Hearts were treated with either saline, the adenosine receptor blocker (8)-sulfophenyl theophylline (8SPT, 300 micromol/l) or the nucleoside transport inhibitor draflazine (1 micromol/l). During LFI, 8SPT did not substantially influence metabolic or functional responses. However, draflazine enhanced the reduction in heart rate, contractile force and MVO(2), with less release of H+ and CO2. Before LFI there were no significant differences between groups for any of the proteins (Ca(2+)-ATPase, ryanodine-receptor, Na+/K(+)-ATPase) or mRNAs (Ca(2+)-ATPase, PLB, calsequestrin, Na+/Ca(2+)-exchanger) measured. At end of LFI mRNA-level of PLB was higher in draflazine-treated hearts compared to both other groups (P<0.01 vs both). Also, at end of LFI protein-level of Ca(2+)-ATPase was lower in draflazine-treated hearts (P<0.05 vs both), and a parallel trend towards a lower mRNA-level was seen (P=0.11 vs saline and P=0.43 vs 8SPT). During LFI tissue Ca2+ tended to rise in saline- and 8SPT-treated hearts but not in draflazine-treated hearts (at end of LFI, P=0.01 vs 8SPT). We conclude that the amount of adenosine normally produced during LFI does not substantially influence function and metabolism. However, increased endogenous levels by draflazine enhance downregulation of function and reduce signs of anaerobic metabolism. At end of LFI associated changes in expression of PLB and Ca(2+)-ATPase were seen. The functional significance was not determined in the present study. However, altered protein-levels might influence Ca(2+)-handling in sarcoplasmic reticulum and thus affect contractile force and tolerance to ischemia.

Absence of troponin I degradation or altered sarcoplasmic reticulum uptake protein expression after reversible ischemia in swine

The findings of troponin I (TnI) proteolysis (in isolated rat hearts) and induction of selected sarcoplasmic reticulum (SR) calcium-regulatory genes (after repetitive total coronary occlusions in swine) have given rise to the hypothesis that the time course of functional recovery of stunned myocardium reflects the resynthesis of reversibly damaged proteins. Although stunning occurs after brief total occlusions and prolonged partial occlusions (ie, short-term hibernation), the time course of functional recovery varies from a few hours to several days, suggesting that the severity of protein damage or mechanisms responsible for the dysfunction may differ. To study this, we examined SR gene expression and TnI degradation in stunned myocardium produced by 10-minute total left anterior descending coronary artery (LAD) occlusions (n=4) or 1-hour partial LAD occlusions, in which flow was reduced to approximately 50% of control values for 60 minutes (n=6) in swine. One hour after reperfusion, LAD wall thickening was severely depressed in both models despite normal perfusion and no triphenyltetrazolium chloride evidence of necrosis. Normal myocardium exhibited TnI immunoreactivity at 31 kDa and a weak secondary band at 27 kDa. Irreversible injury or calpain activation in vitro produced a marked increase in the intensity of the 27-kDa band, consistent with TnI degradation. Stunned myocardium demonstrated no change in the 31- or the 27-kDa band, and the percentage of the 27- to 31-kDa band remained constant after 10-minute total occlusions (LAD, 5.9+/-0.9%; normal, 4.9+/-1.6%) and 1-hour partial occlusions (LAD, 8.5+/-1.9%; normal, 7.3+/-1.4%) and in sham controls (LAD, 10.9+/-1.5%; normal, 9.8+/-1.4%). Northern analysis showed no alterations in TnI or SR gene expression, but the stress protein HSP-70 was variably induced. Thus, stunned myocardium occurs without TnI degradation or altered SR gene expression, indicating that additional mechanisms are responsible for the reversible dysfunction after single episodes of regional ischemia in swine.

Hibernating myocardium

Decreased myocardial contraction occurs as a consequence of a reduction in blood flow. The concept of hibernation implies a downregulation of contractile function as an adaptation to a reduction in myocardial blood flow that serves to maintain myocardial integrity and viability during persistent ischemia. Unequivocal evidence for this concept exists in scenarios of myocardial ischemia that lasts for several hours, and sustained perfusion-contraction matching, recovery of energy and substrate metabolism, the potential for recruitment of inotropic reserve at the expense of metabolic recovery, and lack of necrosis are established criteria of short-term hibernation. The mechanisms of short-term hibernation, apart from reduced calcium responsiveness, are not clear at present. Experimental studies with chronic coronary stenosis lasting more than several hours have failed to continuously monitor flow and function. Nevertheless, a number of studies in chronic animal models and patients have demonstrated regional myocardial dysfunction at reduced resting blood flow that recovered upon reperfusion, consistent with chronic hibernation. Further studies are required to distinguish chronic hibernation from cumulative stunning. With a better understanding of the mechanisms underlying short-term hibernation, it is hoped that these adaptive responses can be recruited and reinforced to minimize the consequences of acute myocardial ischemia and delay impending infarction. Patients with chronic hibernation must be identified and undergo adequate reperfusion therapy.