How an individual's sex and genetic background modify cardiac adaptation to increased workload is a topic of great interest. We systematically evaluated morphological and physiological cardiac adaptation in response to voluntary and forced exercise. We found that sex/gender is a dominant factor in exercise performance (in two exercise paradigms and two mouse strains) and that females of one of these strains have greater capacity to increase their cardiac mass in response to similar amounts of exercise. To explore the biochemical mechanisms for these differences, we examined signaling pathways previously implicated in cardiac hypertrophy. Ca 2ϩ /calmodulin-dependent protein kinase (CaMK) activity was significantly greater in males compared with females and increased after voluntary cage-wheel exposure in both sexes, but the proportional increase in CaMK activity was twofold higher in females compared with males. Phosphorylation of glycogen synthase kinase-3␤ (GSK-3␤) was evident after 7 days of cage-wheel exposure in both sexes and remained elevated in females only by 21 days of exercise. Despite moderate increases in myocyte enhancer factor-2 (a downstream effector of CaMK) transcriptional activity and phosphorylation of Akt with exercise, there were no sex differences. Mitogen-activated protein kinase signaling components (p38 mitogenactivated protein kinase and extracellular regulated kinase 1/2) were not different between male and female mice and were not affected by exercise. We conclude that females have increased exercise capacity and increased hypertrophic response to exercise. We have also identified sex-specific differences in hypertrophic signaling within the cardiac myocyte that may contribute to sexual dimorphism in exercise and cardiac adaptation to exercise. hypertrophy; myocyte signaling; workload; glycogen; calmodulin DESPITE INCREASING KNOWLEDGE of the mechanisms of cardiac adaptation to increased workload, there are significant sex/ gender differences that remain poorly understood. For example, in response to pathological stimuli such as systemic hypertension or aortic stenosis, women respond with more left ventricular hypertrophy and augmented contractility than men (51, 54), whereas men progress to poor contractility, chamber dilation, and wall thinning (7,11,24,39,50). Yet in the face of ischemic heart disease, females fare worse than their male counterparts (12,14,21).Physiological stimuli such as exercise can also elicit a sexually dimorphic cardiac response. Despite conflicting reports on the functional impact of exercise on the heart, it is apparent that there is differential remodeling between the sexes in response to aerobic exercise (16,49). Moreover, the sexually dimorphic cardiac response to exercise likely depends on the type of activity performed (aerobic vs. anaerobic, chronic vs. acute) as demonstrated in exercise-trained male and female rats. Male and female animals exercised by treadmill running have similar heart mass compared with sedentary controls (42, 44). In both sexes, ...

The Frank-Starling relationship of the heart has, as its molecular basis, an increase in the activation of myofibrils by calcium as the sarcomere length increases. It has been suggested that this phenomenon may be due to myofilaments moving closer together at longer lengths, thereby enhancing the probability of favorable acto-myosin interaction, resulting in increased calcium sensitivity. Accordingly, we have developed an apparatus so as to obtain accurate measurements of myocardial interfilament spacing (by synchrotron X-ray diffraction) as a function of sarcomere length (by video microscopy) over the working range of the heart, using skinned as well as intact rat trabeculas as model systems. In both these systems, lattice spacing decreased significantly as sarcomere length was increased. Furthermore, lattice spacing in the intact muscle was significantly smaller than that in the skinned muscle at all sarcomere lengths studied. These observations are consistent with the hypothesis that lattice spacing underlies length-dependent activation in the myocardium.

This study was undertaken to determine the impact of sarcomere length (SL) on the level of cooperative activation of the cardiac myofilament at physiological [Mg2+]. Active force development was measured in skinned rat cardiac trabeculae as a function of free [Ca2+] at five SLs (1.85–2.25 μm; 1 mM free [Mg2+]; 15°C). Only muscle preparations with minimal force rundown during the entire protocol were included in the analysis (average 7.2 ± 1.7%). Median SL was measured by on-line computer video micrometry and controlled within 0.01 μm. Care was taken to ensure a sufficient number of data points in the steep portion of the [Ca2+]-force relationship at every SL to allow for accurate fit of the data to a modified Hill equation. Multiple linear regression analysis of the fit parameters revealed that both maximum, Ca2+-saturated force and Ca2+sensitivity were a significant function of SL ( P < 0.001), whereas the level of cooperativity did not depend on SL ( P = 0.2). Further analysis of the [Ca2+]-force relationships revealed a marked asymmetry that, also, was not affected by SL ( P = 0.2–0.6). Finally, we found that the level of cooperativity in isolated skinned myocardium was comparable to that reported for intact, nonskinned myocardium. Our results suggest that an increase in SL induces an increase in the Ca2+ responsiveness of the cardiac sarcomere without affecting the level of cooperativity.

Exercise elicits coordinated multi-organ responses including skeletal muscle, vasculature, heart and lung. In the short term, the output of the heart increases to meet the demand of strenuous exercise. Long term exercise instigates remodeling of the heart including growth and adaptive molecular and cellular re-programming. Signaling pathways such as the insulin-like growth factor 1/PI3K/Akt pathway mediate many of these responses. Exercise-induced, or physiologic, cardiac growth contrasts with growth elicited by pathological stimuli such as hypertension. Comparing the molecular and cellular underpinnings of physiologic and pathologic cardiac growth has unveiled phenotype-specific signaling pathways and transcriptional regulatory programs. Studies suggest that exercise pathways likely antagonize pathological pathways, and exercise training is often recommended for patients with chronic stable heart failure or following myocardial infarction. Herein, we summarize the current understanding of the structural and functional cardiac responses to exercise as well as signaling pathways and downstream effector molecules responsible for these adaptations.

Nebulin is a giant filamentous protein that is coextensive with the actin filaments of the skeletal muscle sarcomere. Nebulin mutations are the main cause of nemaline myopathy (NEM), with typical adult patients having low expression of nebulin, yet the roles of nebulin in adult muscle remain poorly understood. To establish nebulin's functional roles in adult muscle, we studied a novel conditional nebulin KO (Neb cKO) mouse model in which nebulin deletion was driven by the muscle creatine kinase (MCK) promotor. Neb cKO mice are born with high nebulin levels in their skeletal muscles, but within weeks after birth nebulin expression rapidly falls to barely detectable levels Surprisingly, a large fraction of the mice survive to adulthood with low nebulin levels (<5% of control), contain nemaline rods and undergo fiber-type switching toward oxidative types. Nebulin deficiency causes a large deficit in specific force, and mechanistic studies provide evidence that a reduced fraction of force-generating cross-bridges and shortened thin filaments contribute to the force deficit. Muscles rich in glycolytic fibers upregulate proteolysis pathways (MuRF-1, Fbxo30/MUSA1, Gadd45a) and undergo hypotrophy with smaller cross-sectional areas (CSAs), worsening their force deficit. Muscles rich in oxidative fibers do not have smaller weights and can even have hypertrophy, offsetting their specific-force deficit. These studies reveal nebulin as critically important for force development and trophicity in adult muscle. The Neb cKO phenocopies important aspects of NEM (muscle weakness, oxidative fiber-type predominance, variable trophicity effects, nemaline rods) and will be highly useful to test therapeutic approaches to ameliorate muscle weakness.

Abstract-Hypertrophic cardiomyopathy (HCM) is the most common form of sudden death in young competitive athletes.However, exercise has also been shown to be beneficial in the setting of other cardiac diseases. We examined the ability of voluntary exercise to prevent or reverse the phenotypes of a murine model of HCM harboring a mutant myosin heavy chain (MyHC). No differences in voluntary cage wheel performance between nontransgenic (NTG) and HCM male mice were seen. Exercise prevented fibrosis, myocyte disarray, and induction of "hypertrophic" markers including NFAT activity when initiated before established HCM pathology. If initiated in older HCM animals with documented disease, exercise reversed myocyte disarray (but not fibrosis) and "hypertrophic" marker induction. In addition, exercise returned the increased levels of phosphorylated GSK-3␤ to those of NTG and decreased levels of phosphorylated CREB in HCM mice to normal levels. Exercise in HCM mice also favorably impacted components of the apoptotic signaling pathway, including Bcl-2 (an inhibitor of apoptosis) and procaspase-9 (an effector of apoptosis) expression, and caspase-3 activity. Remarkably, there were no differences in mortality between exercised NTG and HCM mice. Thus, not only was exercise not harmful but also it was able to prevent and even reverse established cardiac disease phenotypes in this HCM model. (Circ Res. 2006;98:540-548.)Key Words: apoptosis Ⅲ exercise Ⅲ hypertrophic cardiomyopathy Ⅲ remodeling T reatment strategies for hypertrophic cardiomyopathy (HCM) depend on clinical symptoms and stratification of sudden death risk with little evidence supporting prophylactic treatment of asymptomatic HCM. 1 It is becoming increasingly evident that cardiac rehabilitation for patients with other forms of cardiac disease such as ischemic heart disease, hypertension, and congestive heart failure (CHF) includes regular exercise, particularly aerobic exercise, and that exercise reduces cardiovascular morbidity and mortality in these subjects. 2,3 Yet there are no reports in the literature that discuss the impact of mild, monitored exercise as a therapeutic measure for HCM patients. Exercise rehabilitation in HCM patients, however, is somewhat counterintuitive considering the high incidence of sudden cardiac death in young athletes and that both the American Heart Association and the European Society of Cardiology strongly encourage preparticipation cardiovascular screening of HCM for young competitive athletes. 4,5 However, we hypothesized that voluntary cage wheel running as a form of mild exercise would improve and/or prevent the pathologic cardiac phenotype in a murine model of HCM. Voluntary cage wheel was chosen as the exercise intervention because it eliminates physical and psychological stressors associated with forced exercise paradigms that may exacerbate the HCM syndrome, and we have previously demonstrated cardiac adaptation to voluntary cage wheel exercise. 6 The particular HCM mouse model developed by our group 7 is ideal to addres...

Voluntary cage wheel exercise has been used extensively to determine the physiological adaptation of cardiac and skeletal muscle in mice. In this study, we tested the effect of different loading conditions on voluntary cage wheel performance and muscle adaptation. Male C57Bl/6 mice were exposed to a cage wheel with no-resistance (NR), low-resistance (LR), or high-resistance (HR) loads for 7 wk. Power output was elevated (3-fold) under increased loading (LR and HR) conditions compared with unloaded (NR) exercise training. Only unloaded (NR) exercise induced an increase in heart mass, whereas only loaded (LR and HR) exercise training induced an increase in skeletal (soleus) muscle mass. Moreover, unloaded and loaded exercise training had a differential impact on the cross-sectional area of muscle fibers, depending on the type of myosin heavy chain expressed by each fiber. The biochemical adaptation of the heart was characterized by a decrease in genes associated with pathological (but not physiological) cardiac hypertrophy and a decrease in calcineurin expression in all exercise groups. In addition, transcriptional activity of myocyte enhancer factor-2 (MEF-2) was significantly decreased in the hearts of the LR group as determined by a MEF-2-dependent transgene driving the expression of beta-galactosidase. Phosphorylation of glycogen synthase kinase-3beta, protein kinase B (Akt), and p70 S6 kinase was increased only in the hearts of the NR group, consistent with the significant increase in cardiac mass. In conclusion, unloaded and loaded cage wheel exercise have a differential impact on cage wheel performance and muscle (cardiac and skeletal) adaptation.

Abstract-The increase in myofilament Ca2ϩ responsiveness on an increase in sarcomere length (SL) is, in part, the cellular basis for Frank-Starling's law of the heart. It has been suggested that a decrease in myofilament lattice spacing (LS) in response to an increase in SL underlies this phenomenon. This hypothesis is supported by previous studies in which reduced muscle width induced by osmotic compression was associated with an increase in Ca 2ϩ sensitivity, mimicking those changes observed with an increase in SL. To evaluate this hypothesis, we directly measured LS by synchrotron x-ray diffraction as function of SL in skinned rat cardiac trabeculae bathed in 0% to 6% dextran solutions (MW 413 000). We found that EC 50 , [Ca 2ϩ ] at which force is half-maximal, at SL between 1.95 and 2.25 m did not vary in proportion to LS when 3% or 6% dextran solutions were applied. We also found that moderate compression (1% dextran) of skinned trabeculae at SLϭ2.02 m reduced LS (LSϭ42.29Ϯ0.14 nm) to match that of uncompressed fibers at a long SL (SLϭ2.19 m; LSϭ42.28Ϯ0.15 nm). Whereas increasing SL from 2.02 to 2.19 m significantly increased Ca 2ϩ sensitivity as indexed by the EC 50 parameter (2.87Ϯ0.11 mol/L to 2.52Ϯ0.12 mol/L), similar reduction in myofilament lattice spacing achieved by compression with 1% dextran did not alter Ca 2ϩ sensitivity (2.87Ϯ0.10 mol/L) at the short SL. We conclude that alterations in myofilament lattice spacing may not be the mechanism that underlies the sarcomere length-induced alteration of calcium sensitivity in skinned myocardium.