Chronic nonocclusive coronary artery constriction impairs ventricular function, myocardial structure, and cardiac contractile protein enzyme activity in rats

Elevated High-Sensitivity Troponin I in the Stabilized Phase after an Acute Coronary Syndrome Predicts All-Cause and Cardiovascular Mortality in a Highly Admixed Population: A 7-Year Cohort

Background

High-sensitivity cardiac troponin I (hs-cTnI) has played an important role in the risk stratification of patients during the in-hospital phase of acute coronary syndrome (ACS), but few studies have determined its role as a long-term prognostic marker in the outpatient setting.

Objective

To investigate the association between levels of hs-cTnI measured in the subacute phase after an ACS event and long-term prognosis in a highly admixed population.

Methods

We measured levels of hs-cTnI in 525 patients 25 to 90 days after admission for an ACS event; these patients were then divided into tertiles according to hs-cTnI levels and followed for up to 7 years. We compared all-cause and cardiovascular mortality using Cox proportional hazards models and adopting a significance level of 5%.

Results

After a median follow-up of 51 months, patients in the highest tertile had a greater hazard ratio (HR) for all-cause mortality after adjustment for age, sex, known cardiovascular risk factors, medication use, and demographic factors (HR: 3.84, 95% CI: 1.92-8.12). These findings persisted after further adjustment for estimated glomerular filtration rate < 60 ml/min/1.73 m² and left ventricular ejection fraction < 0.40 (HR: 6.53, 95% CI: 2.12-20.14). Cardiovascular mortality was significantly higher in the highest tertile after adjustment for age and sex (HR: 5.65, 95% CI: 1.94-16.47) and both in the first (HR: 4.90, 95% CI: 1.35-17.82) and second models of multivariate adjustment (HR: 5.89, 95% CI: 1.08-32.27).

Conclusions

Elevated hs-cTnI levels measured in the stabilized phase after an ACS event are independent predictors of all-cause and cardiovascular mortality in a highly admixed population.

Guidelines for experimental models of myocardial ischemia and infarction

Myocardial infarction is a prevalent major cardiovascular event that arises from myocardial ischemia with or without reperfusion, and basic and translational research is needed to better understand its underlying mechanisms and consequences for cardiac structure and function. Ischemia underlies a broad range of clinical scenarios ranging from angina to hibernation to permanent occlusion, and while reperfusion is mandatory for salvage from ischemic injury, reperfusion also inflicts injury on its own. In this consensus statement, we present recommendations for animal models of myocardial ischemia and infarction. With increasing awareness of the need for rigor and reproducibility in designing and performing scientific research to ensure validation of results, the goal of this review is to provide best practice information regarding myocardial ischemia-reperfusion and infarction models.

Listen to this article’s corresponding podcast at ajpheart.podbean.com/e/guidelines-for-experimental-models-of-myocardial-ischemia-and-infarction/.

An update on cardiac troponins as circulating biomarkers in heart failure

Circulating troponins and natriuretic peptides are the only biomarkers specifically released from cardiac myocytes that can be determined with robust and sensitive analytical methods, even in healthy subjects. These intracellular proteins are released from reversibly or irreversibly damaged cardiac myocytes into the bloodstream by mechanisms that are not entirely clear. The recent introduction of a new generation of highly sensitive assays of cardiac troponin I or T has not only improved the early diagnosis of acute myocardial infarction but also suggested that there are several causes for troponin release other than acute coronary syndromes. Circulating troponins are elevated in patients with acute or chronic heart failure and are strongly associated with outcome, independently of natriuretic peptides, the benchmark biomarkers in heart failure. In the absence of further experimental evidences, the pathophysiologic basis for the elevation of circulating cardiac troponins in patients with stable chronic heart failure remains speculative.

Prognostic utility of cardiac troponin in heart failure: a novel role for an established biomarker

Many individual variables are predictive of an increased risk of mortality and morbidity in heart failure. These include a range of data from patient demographics, clinical findings, comorbidities and invasive and noninvasive parameters. Some of these markers, for example the B-type natriuretic peptides, have been identified as independently predictive in large, robust, multivariable analyses. However, many markers have had less vigorous scrutiny and were identified in small cohorts after only univariate or limited multivariable analyses. Recently, cardiac troponins have emerged as potential biomarkers for patients with heart failure. In this article, we consider the utility of cardiac troponins in heart failure and propose what role they may play in improving the risk stratification of this disease.

Cardiopulmonary resuscitation in a rat model of chronic myocardial ischemia

Our group has developed a rat model of cardiac arrest and cardiopulmonary resuscitation (CPR). However, the current rat model uses healthy adult animals. In an effort to more closely reproduce the event of cardiac arrest and CPR in humans with chronic coronary disease, a rat model of coronary artery constriction was investigated during cardiac arrest and CPR. Left coronary artery constriction was induced surgically in anesthetized, mechanically ventilated Sprague-Dawley rats. Echocardiography was used to measure global cardiac performance before surgery and 4 wk postsurgery. Coronary constriction provoked significant decreases in ejection fraction, increases in left ventricular end-diastolic volume, and increases left ventricular end-systolic volume at 4 wk postintervention, just before induction of ventricular fibrillation (VF). After 6 min of untreated VF, CPR was initiated on three groups: 1) coronary artery constriction group, 2) sham-operated group, and 3) control group (without preceding surgery). Defibrillation was attempted after 6 min of CPR. All the animals were resuscitated. Postresuscitation myocardial function as measured by rate of left ventricular pressure increase at 40 mmHg and the rate of left ventricular pressure decline was more significantly impaired and left ventricular end-diastolic pressure was greater in the coronary artery constriction group compared with the sham-operated group and the control group. There were no differences in the total shock energy required for successful resuscitation and duration of survival among the groups. In summary, this rat model of chronic myocardial ischemia was associated with ventricular remodeling and left ventricular myocardial dysfunction 4 wk postintervention and subsequently with severe postresuscitation myocardial dysfunction. This model would suggest further clinically relevant investigation on cardiac arrest and CPR.

Impaired resting perfusion in viable myocardium distal to chronic coronary stenosis in rats

Chronic coronary artery stenosis results in patchy necrosis in the dependent myocardium and impairs global and regional left ventricular (LV) function in rats in vivo. The aim of the present study was to compare regional myocardial blood flow (RMBF) and function (F) in poststenotic myocardium by using magnetic resonance imaging (MRI) and to compare MRI blood flow changes to histological alterations to assess whether RMBF in the viable poststenotic tissue remains normal. MRI was performed in 11 anesthetized Wistar rats with 2-wk stenosis of the left coronary artery. Postmortem, the extent of fibrotic tissue was quantified. Poststenotic RMBF was significantly reduced to 2.21 ± 0.30 ml·g−1·min−1 compared with RMBF in the remote myocardium (4.05 ± 0.50 ml·g−1·min−1). A significant relationship between the poststenotic RMBF (%remote area) and the poststenotic F (%remote myocardium) was calculated ( r = 0.61, P < 0.05). Assuming perfusion in scar tissue to be 32 ± 5% of perfusion of remote myocardium, as measured in five additional rats, and that in remote myocardium to be 114 ± 25% of that in normal myocardium, as assessed in five sham rats, the calculated perfusion in partially fibrotic tissue samples (35.7 ± 5.2% of analyzed area) was 2.88 ± 0.18 ml·g−1·min−1, whereas measured MRI perfusion was only 1.86 ± 0.24 ml·g−1·min−1 ( P < 0.05). These results indicate that resting perfusion in viable poststenotic myocardium is moderately reduced. Alterations in global and regional LV function are therefore secondary to both patchy fibrosis and reduced resting perfusion.

Microvascular adaptation to coronary stenosis in the rat heart in vivo: a serial magnetic resonance imaging study

Changes in the microcirculation may compensate for the reduction of perfusion supplied by the stenotic vessel. The objective of this study was to determine functional adaptive processes in the microcirculation by magnetic resonance imaging (MRI) during coronary stenosis in the rat heart. Left coronary artery (LAD) narrowing (cross-sectional area 49.8 +/- 3.5%; n = 14) or sham operation (n = 10) was induced in rats. Myocardial perfusion and relative intracapillary blood volume (RBV) at rest and during vasodilatation 1 and 2 weeks after surgery were quantified using MRI. Coronary stenosis in vivo was verified by 3D MR angiography. Foci of fibrosis were found in the poststenotic myocardium. In this area, perfusion at rest was significantly reduced (1.79 +/- 0.11 ml/g/min, p < 0.001) despite a maintained perfusion reserve during adenosine (3.12 +/- 0.20 ml/g/min compared to the remote myocardium (3.07 +/- 0.12 and 5.24 +/- 0.24 ml/g/min, respectively) and the sham operated group (3.22 +/- 0.07 and 5.28 +/- 0.24 ml/g/min, respectively). Poststenotic RBV at rest (12.63 +/- 0.42 %) and during vasodilatation (22.42 +/- 0.81 %) were not significantly different (p > 0.05) from RBV of the remote myocardium (12.92 +/- 0.33 and 23.32 +/- 0.52 %, respectively). Coronary stenosis in the rat leads to foci of tissue injury with impaired perfusion at rest despite a partially maintained perfusion reserve distal to the stenosis. RBV remains constant in order to maintain blood supply. These functional changes reflect adaptive processes that may compensate for ischemic tissue loss.

Ischemic cardiomyopathy in pigs with two-vessel occlusion and viable, chronically dysfunctional myocardium

A chronic left anterior descending coronary artery (LAD) stenosis leads to the development of hibernating myocardium with severe regional hypokinesis but normal global ventricular function after 3 mo. We hypothesized that two-vessel occlusion would accelerate the progression to hibernating myocardium and lead to global left ventricular (LV) dysfunction and heart failure. Pigs were instrumented with a fixed 1.5-mm constrictor on the proximal LAD and circumflex arteries. After 2 mo, there were no overt signs of right-heart failure and triphenyl tetrazolium chloride infarction was trivial (1.4 +/- 0.1% of the LV). Compared with shams, regional function [myocardial systolic excursion (DeltaWT); 2.1 +/- 0.3 vs. 4.6 +/- 0.4 mm, P < 0.05] and resting perfusion (0.90 +/- 0.13 vs. 1.32 +/- 0.09 ml small middle dot min(-1) small middle dot g(-1), P < 0.05) were reduced, consistent with hibernating myocardium. Pulmonary systolic (45.9 +/- 3.3 vs. 36.5 +/- 2.2 mmHg, P < 0.05) and wedge pressures (19.1 +/- 1.6 vs. 11.2 +/- 0.9 mmHg, P < 0.05) were increased with global ventricular dysfunction (ejection fraction 43 +/- 2 vs. 50 +/- 2%, P < 0.05). Early LV remodeling was present with increased cavity size and mass. Reductions in sarcoplasmic reticulum Ca(2+)-ATPase and phospholamban were confined to the dysfunctional LAD region with no change in calsequestrin. Thus combined stenoses of the LAD and circumflex arteries accelerate the development of hibernating myocardium and result in compensated heart failure.

Alteration of mitochondrial function in a model of chronic ischemia in vivo in rat heart

The aim of this study was to investigate mitochondrial alterations in an animal model of chronic myocardial ischemia in rats obtained by surgical constriction of the left coronary artery. Resting coronary blood flow was measured using the fluorescent microsphere technique. Contractile function, defined by rate-pressure product, and myocardial oxygen consumption were measured in a Langendorff preparation. The mitochondrial function was evaluated on permeabilized skinned fibers. Three weeks after surgery, ischemic hearts showed a significant decrease in coronary blood flow compared with sham. Hemodynamic measurements showed a significant systolic and diastolic dysfunction. Alterations in mitochondrial function in ischemic hearts were mainly characterized by a significant decrease in the maximal velocity and apparent half-saturation constant for ADP, loss of the stimulatory effect of creatine, and a stimulatory effect of exogenous cytochrome c. These functional alterations were supported by structural alterations characterized by mitochondrial clustering and swelling associated with membrane rupture. We conclude that the alterations in systolic function after chronic ischemia are supported by severe modifications of mitochondrial structure and function.

Ischemic cardiomyopathy and the cellular renin-angiotensin system

BACKGROUND

Ischemic cardiomyopathy produced by non-occlusive coronary artery constriction is characterized by left ventricular failure and right ventricular dysfunction, but whether the local renin-angiotensin system (RAS) is implicated in myocyte dysfunction and cell death remains unclear.

METHODS

Changes in single-cell mechanics, the localization of the various constituents of RAS in the myocardium, and the effects of angiotensin II (Ang II) stimulation on myocyte performance and cell death were measured.

RESULTS

Chronic ischemia is coupled with alterations in the mechanical properties and calcium (Ca2+) transients of the remaining viable myocytes. The abnormalities in myocyte mechanics consist of depression in peak shortening and velocity of shortening. Moreover, peak systolic Ca2+ is significantly decreased in the cells. In vitro stimulation with Ang II ameliorates myocyte function and systolic Ca2+. Additionally, adult myocytes express genes for renin, angiotensinogen, angiotensin-converting enzyme (ACE), and Ang II receptors. Renin, ACE, and Ang II receptors mRNAs increase under the setting of impaired coronary perfusion. Similarly, the percentage of myocytes containing renin, Ang I, and Ang II increases as well. In vitro studies of neonatal and adult ventricular myocytes indicate that Ang II triggers programmed myocyte cell death and this phenomenon is mediated by activation of the AT1 receptor sub-type. Importantly, the AT1-receptor blocker, losartan, completely inhibits apoptosis.

CONCLUSIONS

These multiple observations are consistent with the notion that Ang II may exert 3 separate functions on the heart: (1) stimulation of myocyte hypertrophy, (2) amelioration of myocyte contractile performance, and (3) activation of the suicide program of myocytes.

Animal models of chronic heart failure

Chronic heart failure is associated with multiple pathophysiological alterations and adaptations, such as marked anatomic and biochemical changes of the myocardium, left ventricular dysfunction and dilatation, increased systemic vascular resistance, and activation of neurohumoral and cytokine systems. The use of animal models has provided a new insight into the complex pathogenesis of this syndrome and supplemented clinical experience. However, all of the animal models used have advantages and limitations, and the transfer from experimental to human heart failure needs critical evaluation. The current review will focus upon new aspects of rat and rabbit models of heart failure.

Myocyte degeneration and cell death in hibernating human myocardium

OBJECTIVES

The aim of this study was to analyze the morphologic characteristics of myocyte degeneration leading to replacement fibrosis in hibernating myocardium by use of electron microscopy and immunohistochemical techniques.

BACKGROUND

Data on the ultrastructure and the cytoskeleton of cardiomyocytes in myocardial hibernation are scarce. Incomplete or delayed functional recovery might be due to variable degree of cardiomyocyte degeneration in hibernating myocardium.

METHODS

In 24 patients, regional wall motion abnormalities were analyzed by use of the centerline method before and 6 +/- 1 months after coronary artery bypass surgery. Preoperative technetium-99m sestamibi uptake was measured by single-photon emission computed tomography for assessing regional perfusion. Fluorine-18 fluorodeoxyglucose uptake was measured by positron emission tomography to assess glucose metabolism. Transmural biopsy specimens were taken during coronary artery bypass surgery from the center of the hypocontractile area of the anterior wall.

RESULTS

The myocytes showed varying signs of mild-to-severe degenerative changes and an increased degree of fibrosis. Immunohistochemical analysis demonstrated disruption of the cytoskeletal proteins titin and alpha-actinin. Electron microscopy of the cell organelles and immunohistochemical analysis of the cytoskeleton showed a similarity in the degree of degenerative alterations. Group 1 (n = 11) represented patients with only minor structural alterations, whereas group 2 (n = 13) showed severe morphologic degenerative changes. Wall motion abnormalities showed postoperative improvements, and nuclear imaging revealed a perfusion-metabolism mismatch without significant differences between the groups.

CONCLUSIONS

Long-term hypoperfusion causes different degrees of morphologic alterations leading to degeneration. Preoperative analysis of regional contractility and perfusion-metabolism imaging does not distinguish the severity of morphologic alterations nor the functional outcome after revascularization. The insufficient act of self-preservation in hibernating myocardium may lead to a progressive structural degeneration with an incomplete and delayed recovery of function after restoration of blood flow.

Role of regulatory proteins (troponin-tropomyosin) in pathologic states

In vertebrate striated muscle, troponin-tropomyosin is responsible, in part, not only for transducing the effect of calcium on contractile protein activation, but also for inhibiting actin and myosin interaction when calcium is absent. The regulatory troponin (Tn) complex displays several molecular and calcium binding variations in cardiac muscles of different species and undergoes genetic changes with development and in various pathologic states. Extensive reviews on the role of tropomyosin (Tm) and Tn in the regulation of striated muscle contraction have been published describing the molecular mechanisms involved in contractile protein regulation. In our studies, we have found an increase in Mg2+ ATPase activity in cardiac myofibrils from dystrophic hamsters and in rats with chronic coronary artery narrowing. The abnormalities in myofibrillar ATPase activity from cardiomyopathic hamsters were largely corrected by recombining the preparations with a TnTm complex isolated from normal hamsters indicating that the TnTm may play a major role in altered myocardial function. We have also observed down regulation of Ca2+ Mg2+ ATPase of myofibrils from hypertrophic guinea pig hearts, myocardial infarcted rats and diabetic-hypertensive rat hearts. In myosin from diabetic rats, this abnormality was substantially corrected by adding troponin-tropomyosin complex from control hearts. All of these disease models are associated with decreased ATPase activities of pure myosin and in the case of rat and hamster models, shifts of myosin heavy chain from alpha to beta predominate. In summary, there are three main troponin subunit components which might alter myofibrillar function however, very few direct links of molecular alterations in the regulatory proteins to physiologic and pathologic function have been demonstrated so far.

The management of heart failure: a matter of definition?

The term heart failure has become a label for more than one clinical entity. For many years heart failure has been used to denote patients with various heart diseases who have begun to suffer from fluid retention, pulmonary venous hypertension, or systemic venous hypertension, either alone or in combination. More recently, the term heart failure has been applied to the combination of effort intolerance and reduced left ventricular contractility due to ischemic heart disease or other myocardial disease. Comparison of the results of epidemiological studies and therapeutic trials is complicated by variation in the composition of the patient populations selected for study. Drug treatment of heart failure remains fairly empirical. Distinction should be made between immediate or prognostic benefits related to the etiological diagnosis, and benefits related specifically to prevention and relief of, for example, fluid retention, rhythm disturbances, or ventricular hypertrophy. The response of individual patients to several forms of drug treatment, including digoxin, ACE inhibitors, and beta-blockade, is unpredictable. Prospective identification of patients liable to respond well to these drugs is not yet possible, but would greatly assist the choice of treatment. At present, trial of therapy is required in each patient to establish benefit and to avoid long-term treatment of nonresponders.