Mechanisms underlying ischemic diastolic dysfunction: relation between rigor, calcium homeostasis, and relaxation rate

Cardiac contractile dysfunction, during and following ischaemia, is attenuated by low-dose dietary fish oil in rats

AIMS

Supplementing animal diets with high-dose fish oil, rich in long chain omega-3 (ω-3) docosahexaenoic acid (DHA), enhances cardiac contractile efficiency and attenuates dysfunction, attributable to ischaemia. However, it remains unclear whether smaller doses, equivalent to what is achievable via regular fish consumption in the human diet, offer similar protection.

METHODS

Male Sprague-Dawley (12-15w) rats were fed isoenergetic diets (ad libitum) containing 10% fat by weight (22% energy) for 4-5w. Control diet (CON) contained 5.5% beef tallow; 2.5% ω-6 sunflower seed oil; 2% olive oil. Fish oil diets included high-DHA tuna oil exchanged for olive oil to provide 0.32% (FO1; human equivalent EPA + DHA 570 mg/d) or 1.25% (FO2; equivalent EPA + DHA 2.3 g/d) wt/wt dose of fish oil. Anaesthetised rats (pentobarbital: 60 mg/kg i.p.) were subjected to 45 min coronary artery occlusion then reperfusion in vivo as a whole animal model of regional myocardial ischaemia, with left ventricular haemodynamic function measured by conductance catheter.

RESULTS

Ischaemia-induced reductions in rate pressure product recovered faster in the FO2 group and post-ischaemic left ventricular pressure-volume loop integrity (shifted downwards and right in CON) was partially protected in both fish oil groups.

CONCLUSION

Ischaemia-induced contractile dysfunction in rats is limited from fish oil doses equivalent to regular consumption of fish in the human diet. These observations highlight plausible and clinically relevant physiological changes that rationalise nutritional conditioning of the heart with DHA for on-going cardioprotection.

Hemodynamic characterization of a transgenic rat strain stably expressing the calcium sensor protein GCaMP2

A novel transgenic rat strain has recently been generated that stably expresses the genetically engineered calcium sensor protein GCaMP2 in different cell types, including cardiomyocytes, to investigate calcium homeostasis. To investigate whether the expression of the GCaMP2 protein itself affects cardiac function, in the present work we aimed at characterizing in vivo hemodynamics in the GCaMP2 transgenic rat strain. GCaMP2 transgenic rats and age-matched Sprague-Dawley control animals were investigated. In vivo hemodynamic characterization was performed by left ventricular (LV) pressure-volume analysis. Postmortem heart weight data showed cardiac hypertrophy in the GCaMP2 group (heart-weight-to-tibial-length ratio: 0.26 ± 0.01 GCaMP2 vs. 0.23 ± 0.01 g/cm Co, P < 0.05). We detected elevated mean arterial pressure and increased total peripheral resistance in transgenic rats. GCaMP2 transgenesis was associated with prolonged contraction and relaxation. LV systolic function was not altered in transgenic rats, as indicated by conventional parameters and load-independent, sensitive indices. We found a marked deterioration of LV active relaxation in GCaMP2 animals (τ: 16.8 ± 0.7 GCaMP2 vs. 12.2 ± 0.3 ms Co, P < 0.001). Our data indicated myocardial hypertrophy, arterial hypertension, and impaired LV active relaxation along with unchanged systolic performance in the heart of transgenic rats expressing the GCaMP2 fluorescent calcium sensor protein. Special caution should be taken when using transgenic models in cardiovascular studies.

NEW & NOTEWORTHY Genetically encoded Ca2+-sensors, like GCaMP2, are important tools to reveal molecular mechanisms for Ca2+-sensing. We provided left ventricular hemodynamic characterization of GCaMP2 transgenic rats and found increased afterload, cardiac hypertrophy, and prolonged left ventricular relaxation, along with unaltered systolic function and contractility. Special caution should be taken when using this rodent model in cardiovascular pharmacological and toxicological studies.

Dichloroacetate selectively improves cardiac function and metabolism in female and male rainbow trout

Cardiac tissue from female rainbow trout demonstrates a sex-specific preference for exogenous glucose and glycolysis, impaired Ca(2+) handling, and a greater tolerance for hypoxia and reoxygenation than cardiac tissue from male rainbow trout. We tested the hypothesis that dichloroacetate (DCA), an activator of pyruvate dehydrogenase, enhances cardiac energy metabolism and Ca(2+) handling in female preparations and provide cardioprotection for hypoxic male tissue. Ventricle strips from sexually immature fish with very low (male) and nondetectable (female) plasma sex steroids were electrically paced in oxygenated or hypoxic Ringer solution with or without 1 mM DCA. In the presence of 5 mM glucose, aerobic tissue from male trout could be paced at a higher frequency (1.79 vs. 1.36 Hz) with lower resting tension and less contractile dysfunction than female tissue. At 0.5 Hz, DCA selectively reduced resting tension below baseline values and lactate efflux by 75% in aerobic female ventricle strips. DCA improved the functional recovery of developed twitch force, reduced lactate efflux by 50%, and doubled citrate in male preparations after hypoxia-reoxygenation. Independent of female sex steroids, reduced myocardial pyruvate dehydrogenase activity and impaired carbohydrate oxidation might explain the higher lactate efflux, compromised function of the sarcoplasmic reticulum, and reduced mechanical performance of aerobic female tissue. Elevated oxidative metabolism and reduced glycolysis might also underlie the beneficial effects of DCA on the mechanical recovery of male cardiac tissue after hypoxia-reoxygenation. These results support the use of rainbow trout as an experimental model of sex differences of cardiovascular energetics and function, with the potential for modifying metabolic phenotypes and cardioprotection independent of sex steroids.

Pathophysiology of diastolic dysfunction in chronic heart failure

Chronic heart failure is a disease with high morbidity and mortality, and its incidence is increasing rapidly worldwide. New therapies are needed that can halt or even reverse the progression of heart failure, but little progress has been made in the last 20 years. This is partly due to the fact that chronic heart failure is a heterogeneous disease with many different etiologies and clinical phenotypes. At present, a pathophysiological concept to unify these different phenotypes is missing. A prominent pathophysiological feature of chronic heart failure is diastolic dysfunction, which is almost universally present in heart failure patients. This review will examine the role and mechanisms of diastolic dysfunction in heart failure. We will study diastolic dysfunction at different levels of complexity of organization: the cardiovascular system, the heart as an organ, the myocardium as a tissue, the myocyte as a cell and the molecular aspects of diastolic dysfunction.

The effects of angiotensin II signaling pathway in the systolic response to acute stretch in the normal and ischemic myocardium

Acute myocardial stretch elicits a biphasic increase in contractility: an immediate increase, known as Frank-Starling mechanism (FSM), followed by a progressive increase, regarded as slow force response (SFR). In this study, we characterized the contractile response to acute stretch from 92 to 100% Lmax in rabbit papillary muscles (n=86) under normoxic and ischemic conditions, and its modulation by angiotensin II signaling pathway. Under normoxia, the FSM was independent of Na(+)/H(+)-exchanger, reverse mode of Na(+)/Ca(2+)-exchanger (r-NCX), AT1 receptor, AT2 receptor and PKC. Regarding the SFR, it was mediated by AT1 receptor activation and its downstream effectors PKC, Na(+)/H(+)-exchanger and r-NCX. Ischemia negatively impacted on the FSM and abolished the SFR, with the muscles exhibiting a time-dependent decline in contractility. Under ischemic conditions, FSM was not influenced by AT1 and AT2 receptors or PKC activation. AT1 receptor antagonism rescued the progressive deterioration in contractility, an effect partially dependent on AT2 receptor activation.

Transient Ca2+ depletion of the sarcoplasmic reticulum at the onset of reperfusion

AIMS

Myocardial stunning is a contractile dysfunction that occurs after a brief ischaemic insult. Substantial evidence supports that this dysfunction is triggered by Ca2+ overload during reperfusion. The aim of the present manuscript is to define the origin of this Ca2+ increase in the intact heart.

METHODS AND RESULTS

To address this issue, Langendorff-perfused mouse hearts positioned on a pulsed local field fluorescence microscope and loaded with fluorescent dyes Rhod-2, Mag-fluo-4, and Di-8-ANEPPS, to assess cytosolic Ca2+, sarcoplasmic reticulum (SR) Ca2+, and transmembrane action potentials (AP), respectively, in the epicardial layer of the hearts, were submitted to 12 min of global ischaemia followed by reperfusion. Ischaemia increased cytosolic Ca2+ in association with a decrease in intracellular Ca2+ transients and a depression of Ca2+ transient kinetics, i.e. the rise time and decay time constant of Ca2+ transients were significantly prolonged. Reperfusion produced a transient increase in cytosolic Ca2+ (Ca2+ bump), which was temporally associated with a decrease in SR-Ca2+ content, as a mirror-like image. Caffeine pulses (20 mM) confirmed that SR-Ca2+ content was greatly diminished at the onset of reflow. The SR-Ca2+ decrease was associated with a decrease in Ca2+ transient amplitude and a shortening of AP duration mainly due to a decrease in phase 2.

CONCLUSION

To the best of our knowledge, this is the first study in which SR-Ca2+ transients are recorded in the intact heart, revealing a previously unknown participation of SR on cytosolic Ca2+ overload upon reperfusion in the intact beating heart. Additionally, the associated shortening of phase 2 of the AP may provide a clue to explain early reperfusion arrhythmias.

Glucocorticoids improve calcium cycling in cardiac myocytes after cardiopulmonary bypass

BACKGROUND

Glucocorticoids can reduce myocardial dysfunction associated with ischemia and reperfusion injury following cardiopulmonary bypass (CPB) and circulatory arrest. The hypothesis was that maintenance of cardiac function after CPB with methylprednisolone therapy results, in part, from preservation of myocyte calcium cycling.

METHODS

Piglets (5-7 kg) underwent CPB and 120 min of hypothermic circulatory arrest with (CPB-GC) or without (CPB) methylprednisolone (30 mgkg(-1)) administered 6h before and at CPB. Controls (No-CPB) did not undergo CPB or receive glucocorticoids (n=6 per treatment). Myocardial function was monitored in vivo for 120 min after CPB. Calcium cycling was analyzed using rapid line-scan confocal microscopy in isolated, fluo-3-AM-loaded cardiac myocytes. Phospholamban phosphorylation and sarco(endo)plasmic reticulum calcium-ATPase (SERCA2a) protein levels were determined by immunoblotting of myocardium collected 120 min after CPB. Calpain activation in myocardium was measured by fluorometric assay.

RESULTS

Preload recruitable stroke work in vivo 120 min after reperfusion decreased from baseline in CPB (47.4±12 versus 26.4±8.3 slope of the regression line, P<0.05), but was not different in CPB-GC (41±8.1 versus 37.6±2.2, P=0.7). In myocytes isolated from piglets, total calcium transient time remained unaltered in CPB-GC (368±52.5 ms) compared with controls (434.5±35.3 ms; P=0.07), but was prolonged in CPB myocytes (632±83.4 ms; P<0.01). Calcium transient amplitude was blunted in myocytes from CPB (757±168 nM) compared with controls (1127±126 nM, P<0.05) but was maintained in CPB-GC (1021±155 nM, P>0.05). Activation of calpain after CPB was reduced with glucocorticoids. Phospholamban phosphorylation and SERCA2a protein levels in myocardium were decreased in CPB compared with No-CPB and CPB-GC (P<0.05).

CONCLUSIONS

The glucocorticoid-mediated improvement in myocardial function after CPB might be due, in part, to prevention of calpain activation and maintenance of cardiac myocyte calcium cycling.

Doppler strain imaging closely reflects myocardial energetic status in acute progressive ischemia and indicates energetic recovery after reperfusion

BACKGROUND

Capitalizing on mechanoenergetic coupling, we investigated whether strain echocardiography can noninvasively estimate the ratio of adenosine triphosphate (ATP) to adenosine diphosphate (ADP), a marker of energetic status during acute myocardial ischemia and reperfusion.

METHODS

Twenty-eight pigs were divided into 7 groups (1 baseline, 4 ischemic, and 2 reperfusion). Ischemia was induced by left anterior descending coronary artery occlusion. Longitudinal systolic lengthening (SL) and postsystolic shortening (PSS) strain were measured by echocardiography. The ATP/ADP ratio was obtained from myocardial biopsies in the ischemic and control regions.

RESULTS

SL and PSS strain and the ATP/ADP ratio progressively decreased (P < .05) with increased duration (12, 40, 120, and 200 minutes) of ischemia. A mathematical formula (ATP/ADP = -0.97 + 0.25 x PSS strain + 0.20 x SL strain) estimated best the ATP/ADP ratio (r = 0.94, P < .05). Reperfusion after 12 but not after 120 minutes of ischemia significantly improved the ATP/ADP ratio and decreased SL and PSS strain.

CONCLUSIONS

Strain echocardiography closely reflected changes and enabled the noninvasive estimation of the ATP/ADP ratio. A higher ATP/ADP ratio is associated with functional improvement after reperfusion.

Chronic heart failure: a report from the Dartmouth Diastole Discourses

On February 28, 2004, a group of investigators and clinician scientists with expertise in the field of heart failure met for 2 days in Woodstock, VT, to discuss the status of the current heart failure nomenclature and to seek a consensus on whether and how this nomenclature might be changed. Despite a wide diversity of expressed opinions, several areas of discussion ultimately led to the consensus expressed in this document.

Role of p90 ribosomal S6 kinase-mediated prorenin-converting enzyme in ischemic and diabetic myocardium

BACKGROUND

Epidemiological data strongly indicate that diabetes increases the incidence of heart failure. Although the benefit of angiotensin-converting enzyme inhibitor (ACE-I) treatment during and after myocardial infarction has been found to be greater in diabetics than nondiabetics and activation of the renin-angiotensin system (RAS) has been implicated, the molecular basis of these actions remains unclear.

METHODS AND RESULTS

We generated transgenic mice with cardiac-specific overexpression of wild-type p90 ribosomal S6 kinase (WT-p90RSK-Tg) and a dominant-negative form of p90RSK (DN-p90RSK-Tg). Recovery of cardiac function after ischemia/reperfusion in WT-p90RSK-Tg isolated mouse hearts was significantly impaired. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry revealed specific induction of prorenin-converting enzyme (PRECE) in WT-p90RSK-Tg mice. mRNA induction of PRECE was confirmed with serial angiotensinogen protein reduction after perfusion in WT-p90RSK-Tg mice, suggesting an increase of angiotensinogen cleavage and subsequent RAS activation in WT-p90RSK-Tg mice. We investigated the role of the RAS in WT-p90RSK-Tg animals after ischemia/reperfusion with the use of an ACE-I (captopril) and an angiotensin II type 1 receptor blocker (olmesartan). We did not observe any effect of these inhibitors in non-Tg littermate controls, thus corroborating other reports in rodents. In contrast, both captopril and olmesartan significantly improved cardiac function and reduced infarct size in WT-p90RSK-Tg mice. At 8 months of age, WT-p90RSK-Tg mice developed cardiac dysfunction. p90RSK activity and PRECE mRNA were both increased by streptozotocin-induced hyperglycemia in non-Tg littermate controls, whereas DN-p90RSK-Tg animals exposed to streptozotocin did not have PRECE induction.

CONCLUSIONS

This study demonstrates the critical role of p90RSK in hyperglycemia-mediated myocardial PRECE induction, which may explain the augmentation of the RAS in diabetic hearts and provide an alternative therapeutic approach to treat diabetic cardiomyopathy.

Relationship among diastolic intraventricular pressure gradients, relaxation, and preload: impact of age and fitness

Diastolic intraventricular pressure gradients (IVPGs) are a measure of the ability of the ventricle to facilitate its filling using diastolic suction. We assessed 15 healthy young but sedentary subjects, aged <50 yr (young subjects; age, 35 ± 9 yr); 13 healthy but sedentary seniors, aged >65 yr with known reductions in ventricular compliance (elderly sedentary subjects; age, 70 ± 4 yr); and 12 master athletes, aged >65 yr, previously shown to have preserved ventricular compliance (elderly fit subjects; age, 68 ± 3 yr). Pulmonary capillary wedge pressure (PCWP) and echocardiography measurements were performed at baseline, during load manipulation by lower body negative pressure at −15 and −30 mmHg, and after saline infusion of 10 and 20 ml/kg (elderly) or 15 and 30 ml/kg (young). IVPGs were obtained from color M-mode Doppler echocardiograms. Baseline IVPGs were lower (1.2 ± 0.4 vs. 2.4 ± 0.7 mmHg, P < 0.0001), and the time constant of pressure decay (τ0) was longer (60 ± 10 vs. 46 ± 6 ms, P < 0.0001) in elderly sedentary than in young subjects, with no difference in PCWP. Although PCWP changes during load manipulations were similar ( P = 0.70), IVPG changes were less prominent in elderly sedentary than in young subjects ( P = 0.02). Changes in stroke volume and IVPGs during loading manipulations correlated ( r = 0.96, P = 0.0002). PCWP and τ0 were strong multivariate correlates of IVPGs ( P < 0.001, for both). IVPG response to loading interventions in elderly sedentary and elderly fit subjects was similar ( P = 0.33), despite known large differences in ventricular compliance. The ability to regulate IVPGs during changes in preload is impaired with aging. Preserving ventricular compliance during aging by lifelong exercise training does not prevent this impairment.

Myocardial hypoperfusion/reperfusion tolerance with exercise training in hypertension

The purpose of this study was to examine whether exercise training, superimposed on compensated-concentric hypertrophy, could increase myocardial hypoperfusion-reperfusion (H/R) tolerance. Female Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) (age: 4 mo; N = 40) were placed into a sedentary (SED) or exercise training (TRD) group (treadmill running; 25 m/min, 1 h/day, 5 days/wk for 16 wk). Four groups were studied: WKY-SED ( n = 10), WKY-TRD ( n = 10), SHR-SED ( n = 10), and SHR-TRD ( n = 10). Blood pressure and heart rate were determined, and in vitro isolated heart performance was measured with a retrogradely perfused, Langendorff isovolumic preparation. The H/R protocol consisted of a 75% reduction in coronary flow for 17 min followed by 30 min of reperfusion. Although the rate-pressure product was significantly elevated in SHR relative to WKY, training-induced bradycardia reduced the rate-pressure product in SHR-TRD ( P < 0.05) without an attenuation in systolic blood pressure. Heart-to-body weight ratio was greater in both groups of SHR vs. WKY-SED ( P < 0.001). Absolute and relative myocardial tolerance to H/R was greater in WKY-TRD and both groups of SHR relative to WKY-SED ( P < 0.05). Endurance training superimposed on hypertension-induced compensated hypertrophy conferred no further cardioprotection to H/R. Postreperfusion 72-kDa heat shock protein abundance was enhanced in WKY-TRD and both groups of SHR relative to WKY-SED ( P < 0.05) and was highly correlated with absolute left ventricular functional recovery during reperfusion ( R2= 0.86, P < 0.0001). These data suggest that both compensated hypertrophy associated with short-term hypertension and endurance training individually improved H/R and that increased postreperfusion 72-kDa heat shock protein abundance was, in part, associated with the cardioprotective phenotype observed in this study.

Sprint training improves postischemic, left ventricular diastolic performance

We examined the effects of sprint training on left ventricular diastolic stiffness during normoxia and after ischemia-reperfusion (I/R). Thirty-seven, male Sprague-Dawley rats, weighing 150-175 g at the initiation of the experiment, were randomly assigned to a sedentary, control group (n = 20) or to a high-intensity, sprint-trained group (n = 17). Animals were trained 5 days/wk on a motor-driven treadmill for 6 wk. High-intensity sprint training consisted of running five 1-min sprints at 75 m/min, 15% grade, interspersed with 1-min active recovery runs at a speed of 20 m/min, 15% grade. Langendorff-derived isolated heart performance was measured before and after 20 min of no-flow ischemia followed by 30 min of reperfusion. Isolated myocytes were harvested from a subset of postischemic hearts. Sprint training reduced Langendorff-derived LV chamber stiffness (P < 0.05) and induced a rightward shift in the LV pressure-volume relationship during both normoxic perfusion and after I/R. LV developed pressure after I/R was also better preserved in hearts obtained from sprint-trained animals (P < 0.05), a result that is in part related to a lower postischemic LV chamber stiffness in sprint-trained hearts. The putative impact of sprint training on postischemic LV chamber stiffness was masked by glycolytic inhibition with iodoacetate, suggesting that glycolysis was involved in the better postischemic recovery observed in sprint-trained hearts. There was a tendency for enhanced postischemic cardiomyocyte shortening in sprint-trained cardiomyocytes compared with control. The rate of myocyte relaxation, i.e., time for 50% relaxation of the Ca(2+) transient amplitude, was similar between groups. These data suggest that additional mechanisms unrelated to Ca(2+) were involved in sprint-induced protection from ischemia-reperfusion-induced LV diastolic dysfunction.

What mechanisms underlie diastolic dysfunction in heart failure?

Abnormalities of diastolic function are common to virtually all forms of cardiac failure. However, their underlying mechanisms, precise role in the generation and phenotypic expression of heart failure, and value as specific therapeutic targets remain poorly understood. A growing proportion of heart failure patients, particularly among the elderly, have apparently preserved systolic function, and this is fueling interest for better understanding and treating diastolic abnormalities. Much of the attention in clinical and experimental studies has focused on relaxation and filling abnormalities of the heart, whereas chamber stiffness has been less well studied, particularly in humans. Nonetheless, new insights from basic and clinical research are helping define the regulators of diastolic dysfunction and illuminate novel targets for treatment. This review puts these developments into perspective with the major aim of highlighting current knowledge gaps and controversies.