Regression of isoproterenol-induced cardiac hypertrophy

Characterization of Troponin Responses in Isoproterenol-Induced Cardiac Injury in the Hanover Wistar Rat

The investigations aimed to evaluate the usefulness of cardiac troponins as biomarkers of acute myocardial injury in the rat. Serum from female Hanover Wistar rats treated with a single intraperitoneal (IP) injection of isoproterenol (ISO) was assayed for cardiac troponin I (cTnI) (ACS: 180SE, Bayer), cTnI (Immulite 2000, Diagnostic Products Corporation) and cardiac troponin T (cTnT) (Elecsys 2010, Roche). In a time-course study (50.0 mg/kg ISO), serum cTnI (ACS:180SE) and cTnT increased above control levels at 1 hour postdosing, peaking at 2 hours (cTnI, 4.30 μg/L; cTnT, 1.79 μg/L), and declined to baseline by 48 hours, with histologic cardiac lesions first seen at 4 hours postdosing. The Immulite 2000 assay gave minimal cTnI signals, indicating poor immunoreactivity towards rat cTnI. In a dose-response study (0.25 to 20.0 mg/kg ISO), there was a trend for increasing cTnI (ACS:180SE) values with increasing ISO dose levels at 2 hours postdosing. By 24 hours, cTnI levels returned to baseline although chronic cardiac myodegeneration was present. We conclude that serum cTnI and cTnT levels are sensitive and specific biomarkers for detecting ISO induced myocardial injury in the rat. Serum troponin values reflect the development of histopathologic lesions; however peak troponin levels precede maximal lesion severity.

Cardiac hypertrophy induced by sustained β-adrenoreceptor activation: pathophysiological aspects

Cardiac hypertrophy is promoted by adrenergic over-activation and represents an independent risk factor for cardiovascular morbidity and mortality. The basic knowledge about mechanisms by which sustained adrenergic activation promotes myocardial growth, as well as understanding how structural changes in hypertrophied myocardium could affect myocardial function has been acquired from studies using an animal model of chronic systemic beta-adrenoreceptor agonist administration. Sustained beta-adrenoreceptor activation was shown to enhance the synthesis of myocardial proteins, an effect mediated via stimulation of myocardial growth factors, up-regulation of nuclear proto-oncogenes, induction of cardiac oxidative stress, as well as activation of mitogen-activated protein kinases and phosphatidylinositol 3-kinase. Sustained beta-adrenoreceptor activation contributes to impaired cardiac autonomic regulation as evidenced by blunted parasympathetically-mediated cardiovascular reflexes as well as abnormal storage of myocardial catecholamines. Catecholamine-induced cardiac hypertrophy is associated with reduced contractile responses to adrenergic agonists, an effect attributed to downregulation of myocardial beta-adrenoreceptors, uncoupling of beta-adrenoreceptors and adenylate cyclase, as well as modifications of downstream cAMP-mediated signaling. In compensated cardiac hypertrophy, these changes are associated with preserved or even enhanced basal ventricular systolic function due to increased sarcoplasmic reticulum Ca(2+) content and Ca(2+)-induced sarcoplasmic reticulum Ca(2+) release. The increased availability of Ca(2+) to maintain cardiomyocyte contraction is attributed to prolongation of the action potential due to inhibition of the transient outward potassium current as well as stimulation of the reverse mode of the Na(+)-Ca(2+) exchange. Further progression of cardiac hypertrophy towards heart failure is due to abnormalities in Ca(2+) handling, necrotic myocardial injury, and increased myocardial stiffness due to interstitial fibrosis.

Alterations in myocardial ultrastructure and protein expression after a single injection of isoproterenol

Immunochemical and electron microscopic characterization of rat myocardium was conducted 2 h and 3 weeks after a single injection of isoproterenol in rats. The relative content of several myospecific proteins (KRP--kinase-related protein, desmin), cytoskeletal proteins (tubulin, vinculin, myosin light chain kinase--MLCK) and extracellular matrix protein fibronectin was determined by immunoblotting. Two hours after injection of 50 mg/kg isoproterenol a destruction of some cardiomyocytes, contracture of myofibrils and mild edema of intercellular space was observed. The content of all the studied proteins except KRP decreased below control levels. This situation sustained 3 weeks after injection and paralleled alterations in cardiomyocyte ultrastructure. Areas of myofibrillar contracture and lysis were noted, glycogen granules were sparse; mitochondria contained arrow-like inclusions that are characteristic for calcium overload, also huge mitochondria contacting each other by specialized intermitochondrial contacts were detected. Clumps of unripe elastic fibers in enlarged intercellular space were combined with increased deposition of collagens type I and III forming areas of fibrosis. The smaller dosage of isoproterenol (10 mg/kg) rendered no significant damage in the acute postinjection period but 3 weeks later it induced the thickening of extracellular matrix around cardiac cells and the increase in KRP and tubulin content by 26 and 32%, correspondingly. MLCK levels remained depressed throughout the experiment. The rise in KRP expression was also observed after the addition of isoproterenol to cultured chicken embryo cardiomyocytes. Obtained results indicate that even a single injection of isoproterenol creates long lasting structural alterations in cardiac muscle accompanied by the increased expression of extracellular matrix proteins and several sarcoplasmic proteins apparently involved in hypertrophic response of cardiomyocytes.

Cellular adaptations of the myocardium to chronic exercise

The heart responds positively to programs of chronic dynamic exercise. Hallmark adaptations of the heart include a training bradycardia, increases in end-diastolic dimension and maximal stroke volume, and a general improvement in ventricular performance and contractile function. Of considerable clinical significance are the general observations that chronic exercise renders the myocardium less susceptible to the deleterious effects of acute ischemic episodes and can effectively prevent and/or reverse many of the cardiac functional deficits that are known to occur in settings of chronic hypertension, advanced age, and myocardial infarction. In the text that follows, information gathered over the last 25 to 30 years has been reviewed in an attempt to identify cellular myocardial adaptations, both known and hypothetical, that are responsible for the observed effects of chronic dynamic exercise on the function and morphology of the heart in both normal and selected pathophysiologic settings. Finally, a variety of unresolved issues regarding the ability of chronic exercise to elicit adaptive cardiocyte responses has been identified. In so doing, it is hoped that creative thought and future work in the area will be stimulated.

Control of myocardial tissue components and cardiocyte organelles in pressure-overload hypertrophy of the cat right ventricle

Previous studies have demonstrated that there is a disproportionate increase in connective tissue in right ventricular myocardium subjected to pressure-overload hypertrophy associated with depressed cardiac contractility. While the myocardium is primarily responsive to load, the aim of the present study was to determine whether catecholamines also modulate the response of myocardial tissue components and cardiocyte organelles in pressure-overload-induced cardiac hypertrophy. Four experimental groups of cats were examined: a sham-operated control group, a group which had their pulmonary arteries banded in order to induce a pressure overload, a group which had been subjected to the same pressure overload, but in addition had beta-adrenoceptor blockade produced prior to and during the pressure overloading, and a group which had been subjected to the same pressure overload, but in addition had alpha-adrenoceptor blockade produced prior to and maintained during the pressure overloading. As in our previous study, there was a significant and equivalent degree of right ventricular hypertrophy in all experimental groups with pressure overload when assessed either as the ratio of right ventricular weight to body weight or as cardiocyte cross-sectional area. At the light microscopic level, the disproportionate increase in the volume density of myocardial connective tissue seen in banded animals was completely prevented by either alpha- or beta-adrenoceptor blockade. At the electron microscopic level, there was a reduction in the mitochondrial and myofibrillar volume fractions following beta-adrenoceptor blockade. The results of this study provide evidence for a modulatory role of catecholamines in the control of myocardial connective-tissue proliferation in pressure-overload-induced cardiac hypertrophy. There is also evidence to support the role of the adrenergic nervous system in regulating cardiocyte subcellular organelles, independent of the regulation of cardiocyte size.