Impaired sarcoplasmic reticulum function leads to contractile dysfunction and cardiac hypertrophy

Endogenous asymmetric dimethylarginine accumulation precipitates the cardiac and mitochondrial dysfunctions in type 1 diabetic rats

Myocardial mitochondrial function and biogenesis are suppressed in diabetes, but the mechanisms are unclear. Increasing evidence suggests that asymmetric dimethylarginine (ADMA) is associated with diabetic cardiovascular complications. This study was to determine whether endogenous ADMA accumulation contributes to cardiac and mitochondrial dysfunctions of diabetic rats and elucidate the potential mechanisms. Diabetic rat was induced by single intraperitoneal injection of streptozotocin (50 mg/kg). N-acetylcysteine was given (250 mg/kg/d) by gavage for 12w. Cardiac function was detected by echocardiography. Left ventricle papillary muscles were isolated to examine myocardial contractility. Myocardial ATP and mitochondrial DNA contents were measured to evaluate mitochondrial function and biogenesis. Endogenous ADMA accumulation was augmented resulting in decreased nitric oxide (NO) production and increased oxidative stress, suggesting NO synthase (NOS) uncoupling in the myocardium of T1DM rats compared with control rats. ADMA augmentation was associated with cardiac and mitochondrial dysfunctions along with myocardial uncoupling protein-2 (UCP2) upregulation and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) downregulation in T1DM rats. Exogenous ADMA could directly inhibit myocardial contractility, mitochondrial function and biogenesis in parallel with decreasing NO content and PGC-1α expression while increasing oxidative stress and UCP2 expression in papillary muscles and cardiomyocytes. Treatment with antioxidant N-acetylcysteine, also an inhibitor of NOS uncoupling, either ameliorated ADMA-associated cardiac and mitochondrial dysfunctions or reversed ADMA-induced NO reduction and oxidative stress enhance in vivo and in vitro. These results indicate that myocardial ADMA accumulation precipitates cardiac and mitochondrial dysfunctions in T1DM rats. The underlying mechanism may be related to NOS uncoupling, resulting in NO reduction and oxidative stress increment, ultimate PGC-1α down-regulation and UCP2 up-regulation.

Pre-synaptic sympathetic calcium channels, cyclic nucleotide-coupled phosphodiesterases and cardiac excitability

In sympathetic neurons innervating the heart, action potentials activate voltage-gated Ca²⁺ channels and evoke Ca²⁺ entry into presynaptic terminals triggering neurotransmitter release. Binding of transmitters to specific receptors stimulates signal transduction pathways that cause changes in cardiac function. The mechanisms contributing to presynaptic Ca²⁺ dynamics involve regulation of endogenous Ca²⁺ buffers, in particular the endoplasmic reticulum, mitochondria and cyclic nucleotide targeted pathways. The purpose of this review is to summarize and highlight recent findings about Ca²⁺ homeostasis in cardiac sympathetic neurons and how modulation of second messengers can drive neurotransmission and affect myocyte excitability in cardiovascular disease. Moreover, we discuss the underlying mechanism of abnormal intracellular Ca²⁺ homeostasis and signaling in these neurons, and speculate on the role of phosphodiesterases as a therapeutic target to restore normal autonomic transmission in disease states of overactivity.

Development of dilated cardiomyopathy in Bmal1-deficient mice

Circadian rhythms are approximate 24-h oscillations in physiology and behavior. Circadian rhythm disruption has been associated with increased incidence of hypertension, coronary artery disease, dyslipidemia, and other cardiovascular pathologies in both humans and animal models. Mice lacking the core circadian clock gene, brain and muscle aryl hydrocarbon receptor nuclear translocator (ARNT)-like protein (Bmal1), are behaviorally arrhythmic, die prematurely, and display a wide range of organ pathologies. However, data are lacking on the role of Bmal1 on the structural and functional integrity of cardiac muscle. In the present study, we demonstrate that Bmal1(-/-) mice develop dilated cardiomyopathy with age, characterized by thinning of the myocardial walls, dilation of the left ventricle, and decreased cardiac performance. Shortly after birth the Bmal1(-/-) mice exhibit a transient increase in myocardial weight, followed by regression and later onset of dilation and failure. Ex vivo working heart preparations revealed systolic ventricular dysfunction at the onset of dilation and failure, preceded by downregulation of both myosin heavy chain isoform mRNAs. We observed structural disorganization at the level of the sarcomere with a shift in titin isoform composition toward the stiffer N2B isoform. However, passive tension generation in single cardiomyocytes was not increased. Collectively, these findings suggest that the loss of the circadian clock gene, Bmal1, gives rise to the development of an age-associated dilated cardiomyopathy, which is associated with shifts in titin isoform composition, altered myosin heavy chain gene expression, and disruption of sarcomere structure.

Tachycardia-induced diastolic dysfunction and resting tone in myocardium from patients with a normal ejection fraction

OBJECTIVES

The purpose of this study is to evaluate tachycardia-induced relaxation abnormalities in myocardium from patients with a normal ejection fraction.

BACKGROUND

Diastolic dysfunction and left ventricular (LV) hypertrophy are closely linked. Tachycardia can induce heart failure symptoms in otherwise asymptomatic patients. To study the effects of tachycardia on myocardial contractility and relaxation, we evaluated the effects of increasing pacing rates in myocardial biopsy samples obtained from patients with a normal ejection fraction.

METHODS

LV biopsy samples were obtained during coronary bypass surgery. Myocardial strip preparations were electrically paced at rates from 60 to 180 beats/min. Diastolic resting tone was assessed by cross-bridge deactivation. Calcium transporting systems were functionally examined, and myofilament calcium sensitivity was studied.

RESULTS

Incomplete relaxation developed in 7 preparations, with increased diastolic tension development at increasing pacing rates. This was absent in the remaining 7 preparations. Incomplete relaxation was found to be associated with increased LV mass and left atrial volume. Cross-bridge deactivation showed that these preparations also had a significant resting tone. Additional functional analyses suggest that incomplete relaxation is associated with disproportionately elevated cellular calcium loads due to a reduced sarcolemmal calcium extrusion reserve.

CONCLUSIONS

Tachycardia-induced incomplete relaxation was associated with increased LV mass and left atrial volumes. We also found a disproportionately increased calcium load at high rates and a substantial resting tone due to diastolic cross-bridge cycling. These observations may play a role in reduced exercise tolerance and tachycardia-induced diastolic dysfunction.

Left ventricular contractile function is preserved during prolonged exercise in middle-aged men

We examined left ventricular (LV) performance before, during, and following prolonged exercise (EX) in 12 healthy middle-aged men [means ± SE: age = 43.5 ± 1.9 yr; maximal O2 uptake (V̇o2max) = 51.7 ± 1.5 ml·kg−1·min−1]. Subjects cycled for 120 min at 65% V̇o2max (75% of maximal heart rate). Two-dimensional echocardiography (ECHO) to determine tissue-Doppler longitudinal myocardial strain and strain rate, LV ejection fraction (EF), end-diastolic (EDV), end-systolic (ESV), and stroke volume (SV) at baseline and after 5, 30, and 120 min of EX and following 30 min of recovery. In addition, hematocrit and plasma norepinephrine (NE) were measured. From baseline to 5 min of EX, there were significant increases in LV longitudinal strain (−23.20 ± 0.87 to −27.63 ± 1.07%; P < 0.01), strain rate (−1.50 ± 0.15 to −2.08 ± 0.14 s−1; P < 0.01), and EF (56.3 ± 2.2 to 77.1 ± 1.0%; P < 0.05) with continued increases by both at 30 min of exercise vs. SV, EDV, and ESV, which remained constant. After 120 min of EX, HR and NE increased further with reductions in SV, cardiac output, and systolic blood pressure without changes in strain or strain rate. EDV decreased after 120 min of EX (−9.2- vs. 30-min value; P = 0.05) along with a hemoconcentration (baseline = 41.3 ± 1.0 vs. EX = 45.1 ± 1.2%; P < 0001) and significant reduction in body mass despite a mean fluid consumption of 1.8 ± 0.2 liters throughout EX. After 30 min of recovery, LV longitudinal strain was depressed relative to baseline (−23.20 ± 0.87 to −19.57 ± 1.21%; P < 0.01). The reduction in LV SV during prolonged EX occurred without changes in the LV contractile state and is likely secondary to reduced LV preload. A reduction in LV contractility despite a reduced afterload following exercise may be due to factors unique to the recovery period and do not appear to contribute to a reduction in SV during prolonged exercise.

Left ventricular hypertrabeculation/noncompaction with and without neuromuscular disorders

BACKGROUND

Left ventricular hypertrabeculation/noncompaction (LVHT) is frequently associated with neuromuscular disorders (NMD). It is unknown, whether LVHT patients differ according to the presence or absence of NMD. Aim of the study was to assess, if clinical, ECG or echocardiographic findings differ between LVHT patients with and without NMD.

METHODS

Included were all patients, in whom LVHT was diagnosed between June 1995 and February 2003 in one echocardiographic laboratory. All patients underwent a cardiologic examination and were invited for a neurologic investigation.

RESULTS

Of 77 patients with LVHT (19 female, mean age 52 years), 59 were investigated neurologically. Eleven were neurologically normal, 21 had a definite NMD (metabolic myopathy, n=15; Leber's hereditary optic neuropathy, n=3; myotonic dystrophy, n=2 and Becker muscular dystrophy, n=1). The remaining 27 had a NMD of unknown etiology. Neurologically normal patients had more often anginal chest pain than patients with definite NMD (64% vs. 14%, P=0.0042) or NMD of unknown etiology (64% vs. 26%, P=0.0157). Neurologically normal patients were more often in NYHA class 0 or I than patients with NMD of unknown etiology (64% vs. 26%, P=0.0289) and had a thinner interventricular septum than patients with NMD (10.6 mm vs. 12.8 mm, P=0.0253).

CONCLUSIONS

Cardiac abnormalities are hardly different between patients with and without NMD.

The Impact of Prolonged Exercise in a Cold Environment upon Cardiac Function

PURPOSE

The purpose of the present study was to examine the impact of cold exposure coupled with prolonged exercise upon postexercise left ventricular (LV) function and markers of myocardial damage.

METHODS

colon; Eight highly trained male athletes (mean +/- SD; age: 28.2 +/- 8.8 yr; height: 1.78 +/- 0.07 m; body mass: 74.9 +/- 7.6 kg; VO2max: 65.6 +/- 7.0 mL x kg(-1) x min(-1)) performed two 100-mile cycle trials, the first in an ambient temperature of 0 degrees C, the second in an ambient temperature of 19 degrees C. Echocardiographic assessment was completed and blood samples drawn before, immediately postexercise, and 24-h postexercise. Left ventricular systolic (stroke volume [SV], ejection fraction [EF], and systolic blood pressure/end systolic volume ratio [SBP/ESV]) and diastolic (early [E] to late [A] filling ratio [E:A]) parameters were calculated. Serum was analyzed for creatine kinase isoenzyme MB (CK-MBmass) and cardiac troponin T (cTnT). cTnT was analyzed descriptively whereas other variables were assessed using two-way repeated-measures ANOVA.

RESULTS

No significant change was observed in systolic function across time or between trials. A significant difference between trials was observed in E:A immediately after exercise (1.4 +/- 0.4 [19 degrees C] vs 1.8 +/- 0.3 [0 degrees C]) (P < 0.05). CK-MBmass was significantly elevated immediately after exercise in both trials (P < 0.05). Positive cTnT concentrations were observed in two subjects immediately after the 19 degrees C trial (0.012 microg x L(-1) and 0.034 microg x L(-1)).

CONCLUSIONS

Cycling 100 miles in an ambient temperature of 19 degrees C is associated with an acute change in diastolic filling that is not observed after prolonged exercise at 0 degrees C. Prolonged exercise is associated with minimal cardiac damage in some individuals; it appears that this is a separate phenomenon to the change in diastolic filling.

Cardiac Systolic and Diastolic Dysfunction After a Cholesterol-Rich Diet

Background— Although hypercholesterolemia is a well-established risk factor for coronary artery disease, little is known regarding its direct effects on cardiac function.

Methods and Results— We examined the effects of cholesterol feeding (0.5%) on cardiac function in rabbits. After 10 weeks, both systolic shortening and diastolic relaxation rates were impaired without any change in aortic pressure or ventricular hypertrophy. However, sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA)-2 mRNA levels were reduced within 4 days after initiation of cholesterol feeding. After this effect, SERCA-2 protein and SERCA-mediated Ca uptake into sarcoplasmic reticulum vesicles were impaired, and the ratio of MHC-β to MHC-α mRNA increased 5-fold. Suppression of the SERCA-2 message correlated temporally with enrichment of the cardiac sarcolemma with cholesterol.

Conclusions— These data demonstrate that dietary hypercholesterolemia induces a “cholesterol cardiomyopathy” characterized by systolic and diastolic dysfunction. These alterations were independent of vascular disease and demonstrate a dietary link to cardiac dysfunction.

Left ventricular hypertrabeculation/noncompaction

In normal human hearts the left ventricle (LV) has up to 3 prominent trabeculations and is, thus, less trabeculated than the right ventricle. Rarely, more than 3 prominent trabeculations can be found at autopsy and by various imaging techniques in the LV. For this abnormality, different synonyms are used such as spongy myocardium, LV noncompaction, and LV hypertrabeculation (LVHT). In this review it is stated that: (1) LVHT has a higher prevalence than previously thought and the prevalence of LVHT seems to increase with the improvement of cardiac imaging; (2) because LVHT is most frequently diagnosed primarily by echocardiography, echocardiographers should be aware and trained to recognize this abnormality; (3) LVHT is frequently associated with other cardiac and extracardiac, particularly neuromuscular, disorders; (4) there are indications that the cause of LVHT is usually a genetic one and quite heterogeneous; and (5) controversies exist about diagnostic criteria, nomenclature, prognosis, origin, pathogenesis, and the necessity to classify LVHT as a distinct entity and cardiomyopathy by the World Health Organization.

Accelerated onset of heart failure in mice during pressure overload with chronically decreased SERCA2 calcium pump activity

We recently developed a mouse model with a single functional allele of Serca2 (Serca2+/-) that shows impaired cardiac contractility and relaxation without overt heart disease. The goal of this study was to test the hypothesis that chronic reduction in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)2 levels in combination with an increased hemodynamic load will result in an accelerated pathway to heart failure. Age-matched wild-type and Serca2+/- mice were subjected to 10 wk of pressure overload via transverse aortic coarctation surgery. Cardiac hypertrophy and heart failure were assessed by echocardiography, gravimetry/histology, hemodynamics, and Western blotting analyses. Our results showed that approximately 64% of coarcted Serca2+/- mice were in heart failure compared with 0% of coarcted wild-type mice (P < 0.05). Overall, morbidity and mortality were greatly increased in Serca2+/- mice under pressure overload. Echocardiography assessment revealed a significant increase in left ventricular (LV) mass, and LV hypertrophy in coarcted Serca2+/- mice converted from a concentric to an eccentric pattern, similar to that seen in human heart failure. Coarcted Serca2+/- mice had decreased contractile/systolic and relaxation/diastolic performance and/or function compared with coarcted wild-type mice (P < 0.05), despite a similar duration and degree of pressure overload. SERCA2a protein levels were significantly reduced (>50%) in coarcted Serca2+/- mice compared with noncoarcted and coarcted wild-type mice. Our findings suggest that reduction in SERCA2 levels in combination with an increased hemodynamic load results in an accelerated pathway to heart failure.

Time-dependent changes of the susceptibility of cardiac contractile function to hypoxia-reoxygenation after myocardial infarction in rats

In this study we analyzed the susceptibility of contractile function of the myocardium to hypoxia-reoxygenation after infarction. For this purpose, the contractility of isolated papillary muscles from rats was studied at high oxygen tension (pO2 80 kPa) and during hypoxia (pO2 3 kPa) with subsequent reoxygenation at variable intervals between 15 h and 9 weeks after permanent ligation of the left coronary artery. Hypoxic exposure reduced the contractile performance of the preparations to a similar extent in both groups. Notably, the contractility and, in particular, the relaxation rates recovered more completely from hypoxia in the hypertrophied myocardium of rats with coronary artery ligation than in sham-operated (SO) animals. The recovery of contractile function was improved maximally between 6 and 9 weeks after myocardial infarction (MI). The lower sensitivity of the (post)ischemic myocardium to hypoxia-reoxygenation correlated with enhanced left ventricular glutathione peroxidase (GSH-Px) activity (15 h to 9 weeks post-MI) and 2-3-fold increased expression levels (15 h to 6 weeks post-MI) of the 72 kDa heat shock protein (HSP72) in the papillary muscles. These findings suggest that the greater antioxidant potential and, possibly, stimulation of HSPs contribute to the sustained tolerance of the myocardium to hypoxia-reoxygenation injury after infarction.