Contractile Reserve and Intracellular Calcium Regulation in Mouse Myocytes From Normal and Hypertrophied Failing Hearts

Abstract

Abstract-Mouse myocyte contractility and the changes induced by pressure overload are not fully understood. We studied contractile reserve in isolated left ventricular myocytes from mice with ascending aortic stenosis (AS) during compensatory hypertrophy (4-week AS) and the later stage of early failure (7-week AS) and from control mice. ] o , 25°C), the amplitude of myocyte shortening and peak-systolic [Ca 2ϩ ] i in 7-week AS were not different from those of controls, whereas contraction, relaxation, and the decline of [Ca 2ϩ ] i transients were slower. In response to the challenge of high [Ca 2ϩ ] o , fractional cell shortening was severely depressed with reduced peak-systolic [Ca 2ϩ ] i in 7-week AS compared with controls. In response to rapid pacing stimulation, cell shortening and peak-systolic [Ca 2ϩ ] i increased in controls, but this response was depressed in 7-week AS. In contrast, the responses to both challenge with high [Ca 2ϩ ] o and rapid pacing in 4-week AS were similar to those of controls. Although protein levels of Na ϩ -Ca 2ϩ exchanger were increased in both 4-week and 7-week AS, the ratio of SR Ca 2ϩ -ATPase to phospholamban protein levels was depressed in 7-week AS compared with controls but not in 4-week AS. This was associated with an impaired capacity to increase sarcoplasmic reticulum Ca 2ϩ load during high work states in 7-week AS myocytes. In hypertrophied failing mouse myocytes, depressed contractile reserve is related to an impaired augmentation of systolic [Ca 2ϩ ] i and SR Ca 2ϩ load and simulates findings in human failing myocytes. (Circ Res. 2000;87:588-595.) Key Words: myocytes Ⅲ contractility Ⅲ sarcoplasmic reticulum Ⅲ Ca 2ϩ -ATPase Ⅲ heart failure Ⅲ hypertrophy T he regulation of contractility is under intense investigation with the use of transgenic mice. 1,2 The mouse cardiovascular system differs from that of humans and larger mammals, including the rapid heart rate, faster myofibrillar ATPase activity and sarcoplasmic reticulum (SR) Ca 2ϩ uptake, 2,3 and higher mechanical performance per unit ventricular mass. 4 To interpret findings in genetically manipulated mice, it is critical to understand the properties of normal mouse myocyte contractility and the changes induced by clinically relevant stimuli such as chronic load. We reported that mice with ascending aortic stenosis (AS) develop compensated hypertrophy (4-week AS) and the later stage of early heart failure (7-week AS). 4,5 The aim of the present study was to examine contractile reserve in myocytes from AS mice in transition from hypertrophy to early heart failure. We measured myocyte contraction and [Ca 2ϩ ] i transients in left ventricular (LV) myocytes from normal, 4-week AS, and 7-week AS mice in response to the challenge of stepped increases in [Ca 2ϩ ] o . In separate experiments, we examined the response to rapid pacing stimulation. Although only subtle abnormalities in the time course of contraction and the [Ca 2ϩ ] i transients are present at baseline, contractile reserve is depressed in ...

Cytosolic H2O2 mediates hypertrophy, apoptosis, and decreased SERCA activity in mice with chronic hemodynamic overload

“…However, because neither SERCA protein nor mRNA expression was affected by pCAT, the pCAT-induced increase in activity cannot be attributed to increased SERCA levels. In contrast with most other studies in mice with aortic constriction (15), in our study the decrease in SERCA protein was not associated with a decrease in mRNA, possibly reflecting increased protein degradation.…”

“…A major finding of this study is that decreased SERCA activity after AAC was partially prevented by pCAT. Myocardial SERCA activity is decreased in pressure overload leading to sarcoplasmic reticulum calcium depletion, abnormal calcium transients, and impaired systolic and diastolic function at the myocyte level (15,26). These abnormalities are ameliorated by transgenic overexpression of WT SERCA (14,25) and worsened in SERCA ϩ/Ϫ mice (31).…”

“…In animal models of pressure overloadinduced heart failure the activity of SERCA is decreased and contributes to depletion of sarcoplasmic reticulum calcium stores, decreased intracellular calcium transients, and impaired contractile function (12,14,15,25,26). The importance of impaired SERCA function in pressure overload is supported by the observation that increasing myocardial SERCA levels corrects calcium handling and myocardial function (9,14,22,25), whereas SERCA ϩ/Ϫ mice have accelerated progression to failure (31).…”

Enhanced calcium mobilization in rat ventricular myocytes during the onset of pressure overload-induced hypertrophy

“…At this stage, the inotropic response to sympathetic mediators is likely to be enhanced because of greater cell sensitivity to ␤-adrenoceptor stimulation. This scenario is the opposite of that reported in heart failure, when SR Ca 2ϩ pump activity and SR Ca 2ϩ content are depressed, net diastolic SR Ca 2ϩ loss is augmented, and ␤-adrenergic responses are attenuated (14,16,18,25,29,30,44,45,57,61). Some of these alterations are present during chronic, compensated hypertrophy (25,35,36,57).…”

Reduced Myocardial Sarcoplasmic Reticulum Ca 2+ -ATPase mRNA Expression and Biphasic Force-Frequency Relations in Patients With Hypertrophic Cardiomyopathy

“…Indeed, abnormal FFRs have been shown to be related to impaired Ca 2ϩ handling and/or depressed SERCA2 expression or function in patients with dilated cardiomyopathy, 12,20 as well as various experimental animal models. 21,22 In addition, several studies of myocardial samples from HCM patients after myectomy have suggested that functional abnormalities in Ca 2ϩ -handling proteins, including SERCA2, might be involved in abnormal intracellular Ca 2ϩ handling and thus, in impaired contractile performance in HCM patients. [23][24][25] SERCA2 is clearly recognized as a major determinant of myocardial contractility.…”