New perspectives on the role of SERCA2's Ca2+ affinity in cardiac function

AbstractCardiomyocyte relaxation and contraction are tightly controlled by the activity of the cardiac sarco(endo)plasmic reticulum (SR) Ca2+ transport ATPase (SERCA2a). The SR Ca2+-uptake activity not only determines the speed of Ca2+ removal during relaxation, but also the SR Ca2+ content and therefore the amount of Ca2+ released for cardiomyocyte contraction. The Ca2+ affinity is the major determinant of the pump's activity in the physiological Ca2+ concentration range. In the heart, the affinity of the pump for Ca2+ needs to be controlled between narrow borders, since an imbalanced affinity may evoke hypertrophic cardiomyopathy. Several small proteins (phospholamban, sarcolipin) adjust the Ca2+ affinity of the pump to the physiological needs of the cardiomyocyte. It is generally accepted that a chronically reduced Ca2+ affinity of the pump contributes to depressed SR Ca2+ handling in heart failure. Moreover, a persistently lower Ca2+ affinity is sufficient to impair cardiomyocyte SR Ca2+ handling and contractility inducing dilated cardiomyopathy in mice and humans. Conversely, the expression of SERCA2a, a pump with a lower Ca2+ affinity than the housekeeping isoform SERCA2b, is crucial to maintain normal cardiac function and growth. Novel findings demonstrated that a chronically increased Ca2+ affinity also may trigger cardiac hypertrophy in mice and humans. In addition, recent studies suggest that some models of heart failure are marked by a higher affinity of the pump for Ca2+, and hence by improved cardiomyocyte relaxation and contraction. Depressed cardiomyocyte SR Ca2+ uptake activity may therefore not be a universal hallmark of heart failure

53 * altered CaMKII and ROS microdomains favor sparks in orphaned RyR after myocardial infarction

Purpose: In ventricular myocytes of rodents, ryanodine receptors (RyRs) are typically organized at Z-lines where the sarcoplasmic reticulum forms dyads with T-tubules (TTs). In large mammals, TT density is lower and not all RyRs are coupled to the TTs. Recently, we have shown that non-coupled RyRs lack a CaMKII and ROS-dependent microdomain modulation during rate adaptation. Here we examine how this microdomain modulation may be altered in ischemic cardiomyopathy where the fraction of non-coupled RyRs is increased. Methods: Using an established pig model of chronic ischemia and myocardial infarction (MI, N=7), we studied myocytes adjacent to the MI and compared these to myocytes from SHAM pigs (N=5) using whole-cell voltage clamp with Fluo-4 as a [Ca2+]i indicator and confocal line scan imaging. Spontaneous Ca2+ sparks were recorded during a 15s period following stimulation and assigned to different subcellular regions categorized as coupled or non-coupled RyR using a specific algorithm. All data were obtained in at least 3 different animals. Results: In myocytes from SHAM, we confirmed the specific modulation of coupled, but not of non-coupled, RyR as seen in control hearts, i.e. an increase of spark frequency at 2 Hz stimulation that is dependent on CaMKII and NOX2-generated ROS (#sparks/100 μm/s in SHAM: 0.8 ± 0.1 at 0.5 Hz to 2.3 ± 0.2 at 2 Hz, vs. 1.5 ± 0.2 with AIP at 2 Hz, and 1.1 ± 0.2 with gp91 ds-tat peptide at 2 Hz, p-value < 0.05 vs. control, n=7-14 in each group). In MI myocytes this modulation of coupled RyR was absent (#sparks/100 μm/s at 2 Hz not different from at 0.5 Hz and no effect of AIP and gp-91 ds-tat peptide). The fraction of non-coupled RyR was larger in MI (orphaned RyR) and their spark frequency at 2 Hz was significantly higher compared to SHAM. In contrast to SHAM, the response of these orphaned RyR was sensitive to CaMKII inhibition (AIP) (at 2 Hz #sparks/100 μm/s was 5.3 ± 0.5 at baseline vs. 3.1 ± 0.6 with AIP, p-value < 0.05, n=8-10 in each group). A significant reduction in spark frequency was observed in these orphaned RyRs after global ROS scavenging (NAC) (2 Hz #sparks/100 μm/s was 5.2 ± 0.5 at baseline vs. 2.9 ± 0.6 with NAC, p-value < 0.05, n=9-10 in each group) and after mitochondrial ROS inhibition using mitoTEMPO (at 2 Hz #sparks/100 μm/s was 2.9 ± 0.4 at baseline vs. 1.1 ± 0.3 with mitoTEMPO, p-value < 0.001, n=8 in each group) while NOX2 inhibition had no effect (gp91 ds-tat peptide, n=10–14). Conclusion: After MI there is a novel RyR microdomain organization favoring sparks in orphaned RyR, possibly related to mitochondrial ROS production

ERS is founding member of a new Alliance for Biomedical Research in Europe.

The Alliance for Biomedical Research in Europe (BioMed Alliance) was founded on December 9, 2010 by the four leading European medical academic societies: the European Association for the Study of Diabetes (EASD), the European Society of Cardiology (ESC), the European Respiratory Society (ERS) and the European CanCer Organisation (ECCO)..

Exercise training does not improve cardiac function in compensated or decompensated left ventricular hypertrophy induced by aortic stenosis

There is ample evidence that regular exercise exerts beneficial effects on left ventricular (LV) hypertrophy, remodeling and dysfunction produced by ischemic heart disease or systemic hypertension. In contrast, the effects of exercise on pathological LV hypertrophy and dysfunction produced by LV outflow obstruction have not been studied to date. Consequently, we evaluated the effects of 8 weeks of voluntary wheel running in mice (which mitigates post-infarct LV dysfunction) on LV hypertrophy and dysfunction produced by mild (mTAC) and severe (sTAC) transverse aortic constriction. mTAC produced similar to 40% LV hypertrophy and increased myocardial expression of hypertrophy marker genes but did not affect LV function, SERCA2a protein levels. apoptosis or capillary density. Exercise had no effect on global LV hypertrophy and function in mTAC but increased interstitial collagen, and ANP expression. sTAC produced similar to 80% LV hypertrophy and further increased ANP expression and interstitial fibrosis and, in contrast with mTAC, also produced LV dilation. systolic as well as diastolic dysfunction, pulmonary congestion, apoptosis and capillary rarefaction and decreased SERCA2a and ryanodine receptor (RyR) protein levels. LV diastolic dysfunction was likely aggravated by elevated passive isometric force and Ca(2+)-sensitivity of myofilaments. Exercise training failed to mitigate the sTAC-induced LV hypertrophy and capillary rarefaction or the decreases in SERCA2a and RyR. Exercise attenuated the sTAC-induced increase in passive isometric force but did not affect myofilament Ca(2+)-sensitivity and tended to aggravate interstitial fibrosis. In conclusion, exercise had no effect on LV function in compensated and decompensated cardiac hypertrophy produced by LV outflow obstruction, suggesting that the effect of exercise on pathologic LV hypertrophy and dysfunction depends critically on the underlying cause. (C) 2011 Elsevier Ltd. All rights reserved