The Frank-Starling law of the heart describes the interrelationship between end-diastolic volume and cardiac ejection volume, a regulatory system that operates on a beat-to-beat basis. The main cellular mechanism that underlies this phenomenon is an increase in the responsiveness of cardiac myofilaments to activating Ca 2+ ions at a longer sarcomere length, commonly referred to as myofilament length dependent activation. This review focuses on what molecular mechanisms may underlie myofilament length dependency. Specifically, the roles of inter-filament spacing, thick and thin filament based regulation, as well as sarcomeric regulatory proteins are discussed. Although the "Frank-Starling law of the heart" constitutes a fundamental cardiac property that has been appreciated for well over a century, it is still not known in muscle how the contractile apparatus transduces the information concerning sarcomere length to modulate ventricular pressure development. KeywordsFrank-Starling Law of The Heart; Length-Tension Relationship; Sarcomere length; Regulation Frank-Starling's Law of the HeartOver a century ago, Otto Frank in Germany and Ernest Starling in England reported on the relationship between the extent of ventricular filling and pump function of the heart, a phenomenon collectively referred to as Frank-Starling's Law of the Heart. A modern view of this phenomenon [1] (illustrated in Figure 1) holds that there is a unique relationship between end-systolic volume and end-systolic pressure in the heart that is solely determined by contractile state. As a consequence, for a given contractile state, ventricular stroke volume is i) proportional to diastolic filling (i.e. preload), and ii) stroke volume can be maintained in the face of increased aortic pressures (i.e. afterload) simply by increasing preload as illustrated by the two pressure-volume loops in Figure 1. Contractile state, within this framework, can be viewed as any factor that alters end-systolic pressure independently of end-systolic volume and Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Figure 1). The ESPVR-slope is a very useful index of cardiac contractility that can be measured in situ by various methods; a convenient and popular approach is the use of the pressure-volume conductance catheter [2]. The cellular mechanisms that underlie the ESPVR are discussed in the following section. NIH Public Access Relationship between whole heart property and myofilament length dependent activationPump function of the heart is intimately related to force generation, active shortening, and regulation of card...

The Frank-Starling mechanism of the heart is due, in part, to modulation of myofilament Ca 2+ sensitivity by sarcomere length (SL) [length-dependent activation (LDA)]. The molecular mechanism(s) that underlie LDA are unknown. Recent evidence has implicated the giant protein titin in this cellular process, possibly by positioning the myosin head closer to actin. To clarify the role of titin strain in LDA, we isolated myocardium from either WT or homozygous mutant (HM) rats that express a giant splice isoform of titin, and subjected the muscles to stretch from 2.0 to 2.4 μm of SL. Upon stretch, HM compared with WT muscles displayed reduced passive force, twitch force, and myofilament LDA. Time-resolved small-angle X-ray diffraction measurements of WT twitching muscles during diastole revealed stretch-induced increases in the intensity of myosin (M2 and M6) and troponin (Tn3) reflections, as well as a reduction in cross-bridge radial spacing. Independent fluorescent probe analyses in relaxed permeabilized myocytes corroborated these findings. X-ray electron density reconstruction revealed increased mass/ordering in both thick and thin filaments. The SL-dependent changes in structure observed in WT myocardium were absent in HM myocardium. Overall, our results reveal a correlation between titin strain and the Frank-Starling mechanism. The molecular basis underlying this phenomenon appears not to involve interfilament spacing or movement of myosin toward actin but, rather, sarcomere stretch-induced simultaneous structural rearrangements within both thin and thick filaments that correlate with titin strain and myofilament LDA. myofilament length-dependent activation | small-angle X-ray diffraction | rat | passive force | fluorescent probes T he Frank-Starling law of the heart describes a cardiac regulatory control mechanism that operates on a beat-to-beat basis (1). There is a unique relationship between ventricular endsystolic volume and end-systolic pressure that is determined by cardiac contractility. As a result, ventricular stroke volume is directly proportional to the extent of diastolic filling. In conjunction with heart rate and contractility, the Frank-Starling mechanism constitutes a major determinant of cardiac output. Although the Frank-Starling mechanism has been well established for well over a century, the molecular mechanisms underlying this phenomenon are not resolved (1). At the cellular level, an increase in sarcomere length (SL) results in an immediate increase in twitch force development. Existing data, mostly derived from permeabilized isolated myocardium, strongly support the notion that this phenomenon is due to an increase in the Ca 2+ responsiveness of the cardiac contractile apparatus, a phenomenon termed "myofilament lengthdependent activation" (LDA) (1).The mechanism by which the mechanical strain signal is transduced by the cardiac sarcomere is not known. We have recently demonstrated that LDA develops within a few milliseconds following a change in SL (2), a finding suggestive of a mole...

We have explored the role of the giant elastic protein titin in the Frank‐Starling mechanism of the heart by measuring the sarcomere length (SL) dependence of activation in skinned cardiac muscles with different titin‐based passive stiffness characteristics. We studied muscle from the bovine left ventricle (BLV), which expresses a high level of a stiff titin isoform, and muscle from the bovine left atrium (BLA), which expresses more compliant titin isoforms. Passive tension was also varied in each muscle type by manipulating the pre‐history of stretch prior to activation. We found that the SL‐dependent increases in Ca2+ sensitivity and maximal Ca2+‐activated tension were markedly more pronounced when titin‐based passive tension was high. Small‐angle X‐ray diffraction experiments revealed that the SL dependence of reduction of interfilament lattice spacing is greater in BLV than in BLA and that the lattice spacing is coupled with titin‐based passive tension. These results support the notion that titin‐based passive tension promotes actomyosin interaction by reducing the lattice spacing. This work indicates that titin may be a factor involved in the Frank‐Starling mechanism of the heart by promoting actomyosin interaction in response to stretch.

Atrial fibrillation (AF) is the most common supraventricular arrhythmia that, for unknown reasons, is linked to intense endurance exercise. Our studies reveal that 6 weeks of swimming or treadmill exercise improves heart pump function and reduces heart-rates. Exercise also increases vulnerability to AF in association with inflammation, fibrosis, increased vagal tone, slowed conduction velocity, prolonged cardiomyocyte action potentials and RyR2 phosphorylation (CamKII-dependent S2814) in the atria, without corresponding alterations in the ventricles. Microarray results suggest the involvement of the inflammatory cytokine, TNFα, in exercised-induced atrial remodelling. Accordingly, exercise induces TNFα-dependent activation of both NFκB and p38MAPK, while TNFα inhibition (with etanercept), TNFα gene ablation, or p38 inhibition, prevents atrial structural remodelling and AF vulnerability in response to exercise, without affecting the beneficial physiological changes. Our results identify TNFα as a key factor in the pathology of intense exercise-induced AF.

Rapid electrical conduction in the His-Purkinje system tightly controls spatiotemporal activation of the ventricles. Although recent work has shed much light on the regulation of early specification and morphogenesis of the His-Purkinje system, less is known about how transcriptional regulation establishes impulse conduction properties of the constituent cells. Here we show that Iroquois homeobox gene 3 (Irx3) is critical for efficient conduction in this specialized tissue by antithetically regulating two gap junction-forming connexins (Cxs). Loss of Irx3 resulted in disruption of the rapid coordinated spread of ventricular excitation, reduced levels of Cx40, and ectopic Cx43 expression in the proximal bundle branches. Irx3 directly represses Cx43 transcription and indirectly activates Cx40 transcription. Our results reveal a critical role for Irx3 in the precise regulation of intercellular gap junction coupling and impulse propagation in the heart. development | electrophysiology | transcription factor

Blebbistatin (BLEB) is a recently discovered compound that inhibits myosin-II ATPase activity. In this study, we tested BLEB in intact and skinned isolated rat cardiac trabeculae, rat intact myocytes, and single rabbit psoas myofibrils. BLEB (10 muM) reduced twitch force and cell shortening that was reversed by exposure to light. BLEB treatment of skinned trabeculae in the dark (1 hr) reduced Ca(2+)-activated force (EC(50) = 0.38 +/- 0.03 muM). Rapid (<5 ms) BLEB application in Ca(2+)-activated rabbit myofibrils reduced force proportional to [BLEB]. Two-photon Indo1-AM ratio-metric confocal line-scan microscopy revealed no impact of BLEB on the cytosolic Ca(2+) transient. BLEB reduced contractile force in skinned trabeculae without affecting tension-dependent myofilament ATPase activity. We conclude that BLEB specifically uncouples cardiac myofilament activation from Ca(2+) activation without affecting EC coupling or cross-bridge cycling parameters. This agent could be useful to uncouple myofilament contractility from electrical events that lead to sarcoplasmic reticulum Ca(2+) release in the cardiac myocyte (uncoupling agent) However, the compound is very sensitive to light, a property that limits its application to mechanistic physiological studies.

Flight in insects--which constitute the largest group of species in the animal kingdom--is powered by specialized muscles located within the thorax. In most insects each contraction is triggered not by a motor neuron spike but by mechanical stretch imposed by antagonistic muscles. Whereas 'stretch activation' and its reciprocal phenomenon 'shortening deactivation' are observed to varying extents in all striated muscles, both are particularly prominent in the indirect flight muscles of insects. Here we show changes in thick-filament structure and actin-myosin interactions in living, flying Drosophila with the use of synchrotron small-angle X-ray diffraction. To elicit stable flight behaviour and permit the capture of images at specific phases within the 5-ms wingbeat cycle, we tethered flies within a visual flight simulator. We recorded images of 340 micros duration every 625 micros to create an eight-frame diffraction movie, with each frame reflecting the instantaneous structure of the contractile apparatus. These time-resolved measurements of molecular-level structure provide new insight into the unique ability of insect flight muscle to generate elevated power at high frequency.

Rationale Baseline contractility of mouse hearts is modulated in a PI3Kγ-dependent manner by type 4 phosphodiesterases (PDE4), which regulate cAMP levels within microdomains containing the sarcoplasmic reticular (SR) calcium-ATPase (SERCA2a). Objective To determine whether PDE4D regulates basal cAMP levels, phospholamban (PLN) phosphorylation and SERCA2a activity in SR microdomains. Methods & Results We assessed myocardial function in PDE4D-deficient (PDE4D−/−) and littermate wild-type (WT) mice at 10-12 weeks of age. Baseline cardiac contractility in PDE4D−/− mice was elevated in vivo and in Langendorff perfused hearts, while isolated PDE4D−/− cardiomyocytes showed increased Ca2+ transient amplitudes and SR Ca2+content, but unchanged ICa(L), compared to WT. The PKA inhibitor, Rp-cAMPS, lowered Ca2+ transient amplitudes and SR Ca2+ content in PDE4D−/− cardiomyocytes to WT levels. The PDE4 inhibitor rolipram (ROL) had no effect on cardiac contractility, Ca2+ transients or SR Ca2+ content in PDE4D−/− preparations but increased these parameters in WT hearts to levels indistinguishable from those in PDE4D−/−. The functional changes in PDE4D−/− myocardium were associated with increased PLN phosphorylation (pPLN) but not RyR2 receptor phosphorylation. ROL increased pPLN in WT cardiomyocytes to levels indistinguishable from those in PDE4D−/− cardiomyocytes. In murine and failing human hearts, PDE4D co-immunoprecipitated with SERCA2a but not with RyR2. Conclusions PDE4D regulates basal cAMP levels in SR microdomains through its interactions with SERCA2a-PLN. Since Ca2+ transient amplitudes are reduced in failing human myocardium, these observations may have therapeutic implications for patients with heart failure.