Spontaneous Oscillatory Contractions Research Articles

Observation of sarcomere chaos induced by changes in calcium concentration in cardiomyocytes.

Heating cardiomyocytes to 38-42°C induces hyperthermal sarcomeric oscillations (HSOs), which combine chaotic instability and homeostatic stability. These properties are likely important for achieving periodic and rapid ventricular expansion during the diastole phase of the heartbeat. Compared with spontaneous oscillatory contractions in cardiomyocytes, which are sarcomeric oscillations induced in the presence of a constant calcium concentration, we found that calcium concentration fluctuations cause chaotic instability during HSOs. We believe that the experimental fact that sarcomeres, autonomously oscillating, exhibit such instability due to the action of calcium concentration changes is important for understanding the physiological function of sarcomeres. Therefore, we have named this chaotic sarcomere instability that appears under conditions involving changes in calcium concentration as Sarcomere Chaos with Changes in Calcium Concentration (S4C). Interestingly, sarcomere instability that could be considered S4C has also been observed in the relaxation dynamics of EC coupling. Unlike ADP-SPOCs and Cell-SPOCs under constant calcium concentration conditions, fluctuations in oscillation amplitude indistinguishable from HSOs were observed. Additionally, like HSO, a positive Lyapunov exponent was measured. S4C is likely a crucial sarcomeric property supporting the rapid and flexible ventricular diastole with each heartbeat of the heart.

Effects of phosphomimetic mutations in the M-domain of cardiac myosin binding protein-c.

The long-term goal of the proposed research is to characterize the function of phosphorylation sites in the M-domain of Cardiac Myosin Binding Protein-C in damping spontaneous oscillatory contractions (SPOC) in skinned cardiomyocytes. The M-domain is considered the primary regulatory domain of cMyBP-C and aids in binding of cMyBP-C to both myosin and actin. Phosphorylation of the M-domain reduces affinity for both. Reduced phosphorylation in the M-domain is common in many cases of hypertrophic cardiomyopathy (HCM), and cMyBP-C mutations that reduce cMyBP-C expression are present in approximately 50% of cases of HCM. Recently, we developed a “cut-and-paste” method that allows us to remove the endogenous domains C0-C7 followed bycovalent replacement with a desired recombinant protein containing SpyCatcher. Using this method, we discovered that removal of the C0-C7 domains induced SPOC in skinned cardiomyocytes and addition of recombinant full-length protein (rC0C7Sc) damped SPOC. Here, we tested whether phosphorylation of specific sites in the M-domain (S273, S282, and S302) reduce cMyBP-C's ability to damp SPOC. Specifically, the functional effects of three recombinant proteins were tested in detergent permeabilized cardiomyocytes. These proteins contained a phosphomimetic S->D mutation at all three phosphorylation sites (DDD) and at S282 and S302 individually. Results showed that DDD, S282D and S302D damped SPOC and returned both Ca2+ sensitivity of tension and ktr back to values of endogenous cMyBP-C. Future studies will investigate recombinant proteins containing a combination of phospho-null mutations treated with PKA, in order to control which specific sites in the M-domain become phosphorylated. We seek to determine whether individual or a combination of phosphorylation in the M-domain impose limits to cMyBP-C's ability to damp SPOC. This work supported by NIH HL080367 and HL140925 and Predoctoral Fellowship from the American Heart Association 827268.

Thermogenic regulation of the simple harmonic motion of tropomyosin byweak-binding myosin cross-bridges explains spontaneous oscillatory contractions.

A sarcomeric regulatory role for weak-binding, disordered cross-bridges (dXbs) of the myosin disordered-relaxed state is described which explains spontaneous oscillatory contractions (SPOCs). The simple harmonic (SH) theory proposes that as tropomyosin (Tm) interacts with thin filament the random vibrational motion begins to vibrate harmonically with the actin repeat interval (i.e., simple harmonic motion, SHM). This interaction increases as the thick filament electrostatic force increases as lattice spacing decreases by stretch or osmotic compression, causing the SHM to increase with displacement of Tm from thin filament OFF to ON position, exposing sites for cross-bridge formation and force generation. Troponin inhibits this process with decreasing calcium. A thermogenic mechanism is implicit in the SH theory whereby the random/Brownian motion of molecules that interact with Tm modulate the random motion of Tm which, in turn, is regulated harmonically. Heat and internal work from external sources and hydrolysis of ATP are primary sources for Brownian motion of sarcomeric molecules including dXbs which are likely a major source for modulating the SHM thermogenicly. Initially, at partial activation, by calcium or ADP, the increase in dXbs causes thermogenic vibrational energy and SHM of Tm to increase. Thermogenic activation continues throughout the slow contraction phase of SPOCs. Subsequent rapid elongation/relaxation phase occurs as the thermogenic process reaches its limit, as 1) dXbs decrease as force generating cross-bridges increase, 2) contractile efficiency increases (i.e., less heat/internal work) as shortening velocity decreases, and 3) thermogenic excitation by dXbs decreases as lattice spacing increases. The inhibitory force of troponin now overcomes the SHM of Tm which rapidly returns to initial partial OFF/ON conditions and the cycle repeats. Propagation of SPOC waves and implications for the regulatory mechanism of MyBP-C are discussed.

The C5C7 domains of cMyBP-C do not alter myofilament calcium sensitivity of tension.

Cardiac myosin binding protein-C (cMyBP-C) regulates muscle contraction and relaxation. Most of the functional effects of cMyBP-C have been attributed to its N’-terminal domains C0-C2. Yet, specific functional roles of its central domains (C3, C4, C5, C6, and C7), if any, are unknown. Structural studies show that C5 contains of a 28 amino acid insertion that is unstructured and hypothesized to contribute to protein-protein interactions. In this study, we aimed to determine the functional effects of C5, C6 and C7 on myofilament force mechanics. We used the novel Spy-C3 “cut and paste” mouse model that allows for the replacement of recombinant cMyBP-C protein within its location in the sarcomere. The homozygous Spy-C3 mouse has a 20 amino acid cassette inserted between C7 and C8 with a tobacco etch virus protease (TEVp) recognition site followed by a “SpyTag.” Detergent-permeabilized cardiomyocytes were attached to a force transducer and exposed to TEVp to cut and wash out the C0-C7 domains. Recombinant C5-C7 domains expressing SpyCatcher (sc) were then ligated to SpyTag at the position of endogenous cMyBP-C. Results showed that cutting cMyBP-C with TEVp led to a significant rightward shift in the tension-pCa curve (repeated measures one-way ANOVA, p < 0.0001), an increase in the rate of cross-bridge detachment determined by stretch activation (Krel) (p = 0.0003), and the presence of spontaneous oscillatory contractions (SPOC) compared to before TEVp treatment. However, replacement with recombinant protein C5C7sc did not rescue the functional effects following loss of C0-C7. These data may indicate that domains C5-C7 alone do not contribute to cMyBP-C's role in sensitizing the myofilament and regulating rates of cross-bridge cycling. This work was supported by NIH HL080367 (SPH), AHA Predoctoral Fellowship (NE), and an AHA Postdoctoral Fellowship (AG).

Thermodynamics and Kinetics of a Binary Mechanical System: Mechanisms of Muscle Contraction

Biological motors function at the interface of biology, physics, and chemistry, and it remains unsettled what rules from which disciplines account for how these motors work. Myosin motors are enzymes that catalyze the hydrolysis of ATP through a mechanism involving a switch-like myosin structural change (a lever arm rotation) induced by actin binding that generates a small displacement of an actin filament. In muscle, individual myosin motors are widely assumed to function as molecular machines having mechanical properties that resemble those of muscle. In a fundamental departure from this perspective, here, I show that muscle more closely resembles a heat engine with mechanical properties that emerge from the thermodynamics of a myosin motor ensemble. The transformative impact of thermodynamics on our understanding of how a heat engine works guides a parallel transformation in our understanding of how muscle works. I consider the simplest possible model of force generation: a binary mechanical system. I develop the mechanics, energetics, and kinetics of this system and show that a single binding reaction generates force when muscle is held at a fixed length and performs work when muscle is allowed to shorten. This creates a network of thermodynamic binding pathways that resembles many of the characteristic mechanical and energetic behaviors of muscle including the muscle force-velocity relationship, heat output by shortening muscle, four phases of a muscle tension transient, spontaneous oscillatory contractions, and force redevelopment. Analogous to the thermodynamic (Carnot) cycle for a heat engine, isothermal and adiabatic binding and detachment reactions create a thermodynamic cycle for muscle that resembles cardiac pressure-volume loops (i.e., how the heart works). This paper provides an outline for how to re-interpret muscle mechanic data using thermodynamics - an ongoing effort that will continue providing novel insights into how muscle and molecular motors work.

Calcium and strontium contractile activation properties of single skinned skeletal muscle fibres from elderly women 66–90 years of age

The single freshly skinned muscle fibre technique was used to investigate Ca2+- and Sr2+-activation properties of skeletal muscle fibres from elderly women (66–90 years). Muscle biopsies were obtained from the vastus lateralis muscle. Three populations of muscle fibres were identified according to their specific Sr2+-activation properties: slow-twitch (type I), fast-twitch (type II) and hybrid (type I/II) fibres. All three fibre types were sampled from the biopsies of 66 to 72 years old women, but the muscle biopsies of women older than 80 years yielded only slow-twitch (type I) fibres. The proportion of hybrid fibres in the vastus lateralis muscle of women of circa 70 years of age (24%) was several-fold greater than in the same muscle of adults (< 10%), suggesting that muscle remodelling occurs around this age. There were no differences between the Ca2+- and Sr2+-activation properties of slow-twitch fibres from the two groups of elderly women, but there were differences compared with muscle fibres from young adults with respect to sensitivity to Ca2+, steepness of the activation curves, and characteristics of the fibre-type dependent phenomenon of spontaneous oscillatory contractions (SPOC) (or force oscillations) occurring at submaximal levels of activation. The maximal Ca2+ activated specific force from all the fibres collected from the seven old women use in the present study was significantly lower by 20% than in the same muscle of adults. Taken together these results show there are qualitative and quantitative changes in the activation properties of the contractile apparatus of muscle fibres from the vastus lateralis muscle of women with advancing age, and that these changes need to be considered when explaining observed changes in women’s mobility with aging.

Real-time imaging of cellular forces using optical interference

Important dynamic processes in mechanobiology remain elusive due to a lack of tools to image the small cellular forces at play with sufficient speed and throughput. Here, we introduce a fast, interference-based force imaging method that uses the illumination of an elastic deformable microcavity with two rapidly alternating wavelengths to map forces. We show real-time acquisition and processing of data, obtain images of mechanical activity while scanning across a cell culture, and investigate sub-second fluctuations of the piconewton forces exerted by macrophage podosomes. We also demonstrate force imaging of beating neonatal cardiomyocytes at 100 fps which reveals mechanical aspects of spontaneous oscillatory contraction waves in between the main contraction cycles. These examples illustrate the wider potential of our technique for monitoring cellular forces with high throughput and excellent temporal resolution.

The M Domain of Cardiac Myosin Binding Protein-C Is Necessary to Damp SPOC in Spy-C Cardiomyocytes

The long-term goal of the proposed research is to characterize the function of the motif (“M-domain”) of cardiac myosin binding protein C (cMyBP-C) in damping spontaneous oscillatory contractions (SPOC) in skinned cardiomyocytes. The motif aids in binding of cMyBP-C to both myosin and actin, and changes to its phosphorylation affect cMyBP-C affinity for both. Reduced phosphorylation of the motif is common in many cases of hypertrophic cardiomyopathy (HCM), and cMyBP-C mutations that reduce cMyBP-C expression are present in approximately 50% of cases of HCM.

Making waves: A proposed new role for myosin-binding protein C in regulating oscillatory contractions in vertebrate striated muscle.

Myosin-binding protein C (MyBP-C) is a critical regulator of muscle performance that was first identified through its strong binding interactions with myosin, the force-generating protein of muscle. Almost simultaneously with its discovery, MyBP-C was soon found to bind to actin, the physiological catalyst for myosin's activity. However, the two observations posed an apparent paradox, in part because interactions of MyBP-C with myosin were on the thick filament, whereas MyBP-C interactions with actin were on the thin filament. Despite the intervening decades since these initial discoveries, it is only recently that the dual binding modes of MyBP-C are becoming reconciled in models that place MyBP-C at a central position between actin and myosin, where MyBP-C alternately stabilizes a newly discovered super-relaxed state (SRX) of myosin on thick filaments in resting muscle and then prolongs the "on" state of actin on thin filaments in active muscle. Recognition of these dual, alternating functions of MyBP-C reveals how it is central to the regulation of both muscle contraction and relaxation. The purpose of this Viewpoint is to briefly summarize the roles of MyBP-C in binding to myosin and actin and then to highlight a possible new role for MyBP-C in inducing and damping oscillatory waves of contraction and relaxation. Because the contractile waves bear similarity to cycles of contraction and relaxation in insect flight muscles, which evolved for fast, energetically efficient contraction, the ability of MyBP-C to damp so-called spontaneous oscillatory contractions (SPOCs) has broad implications for previously unrecognized regulatory mechanisms in vertebrate striated muscle. While the molecular mechanisms by which MyBP-C can function as a wave maker or a wave breaker are just beginning to be explored, it is likely that MyBP-C dual interactions with both myosin and actin will continue to be important for understanding the new functions of this enigmatic protein.

Nanoscopic changes in the lattice structure of striated muscle sarcomeres involved in the mechanism of spontaneous oscillatory contraction (SPOC)

Muscles perform a wide range of motile functions in animals. Among various types are skeletal and cardiac muscles, which exhibit a steady auto-oscillation of force and length when they are activated at an intermediate level of contraction. This phenomenon, termed spontaneous oscillatory contraction or SPOC, occurs devoid of cell membranes and at fixed concentrations of chemical substances, and is thus the property of the contractile system per se. We have previously developed a theoretical model of SPOC and proposed that the oscillation emerges from a dynamic force balance along both the longitudinal and lateral axes of sarcomeres, the contractile units of the striated muscle. Here, we experimentally tested this hypothesis by developing an imaging-based analysis that facilitates detection of the structural changes of single sarcomeres at unprecedented spatial resolution. We found that the sarcomere width oscillates anti-phase with the sarcomere length in SPOC. We also found that the oscillatory dynamics can be altered by osmotic compression of the myofilament lattice structure of sarcomeres, but they are unchanged by a proteolytic digestion of titin/connectin—the spring-like protein that provides passive elasticity to sarcomeres. Our data thus reveal the three-dimensional mechanical dynamics of oscillating sarcomeres and suggest a structural requirement of steady auto-oscillation.

A Novel "Cut and Paste" Method for In Situ Replacement of cMyBP-C Reveals a New Role for cMyBP-C in the Regulation of Contractile Oscillations.

cMyBP-C (cardiac myosin-binding protein-C) is a critical regulator of heart contraction, but the mechanisms by which cMyBP-C affects actin and myosin are only partly understood. A primary obstacle is that cMyBP-C localization on thick filaments may be a key factor defining its interactions, but most in vitro studies cannot duplicate the unique spatial arrangement of cMyBP-C within the sarcomere. The goal of this study was to validate a novel hybrid genetic/protein engineering approach for rapid manipulation of cMyBP-C in sarcomeres in situ. We designed a novel cut and paste approach for removal and replacement of cMyBP-C N'-terminal domains (C0-C7) in detergent-permeabilized cardiomyocytes from gene-edited Spy-C mice. Spy-C mice express a TEVp (tobacco etch virus protease) cleavage site and a SpyTag (st) between cMyBP-C domains C7 and C8. A cut is achieved using TEVp which cleaves cMyBP-C to create a soluble N'-terminal γC0C7 (endogenous [genetically encoded] N'-terminal domains C0 to C7 of cardiac myosin binding protein-C) fragment and an insoluble C'-terminal SpyTag-C8-C10 fragment that remains associated with thick filaments. Paste of new recombinant (r)C0C7 domains is achieved by a covalent bond formed between SpyCatcher (-sc; encoded at the C'-termini of recombinant proteins) and SpyTag. Results show that loss of γC0C7 reduced myofilament Ca2+ sensitivity and increased cross-bridge cycling (ktr) at submaximal [Ca2+]. Acute loss of γC0C7 also induced auto-oscillatory contractions at submaximal [Ca2+]. Ligation of rC0C7 (exogenous [recombinant] N'-terminal domains C0 to C7 of cardiac myosin binding protein-C)-sc returned pCa50 and ktr to control values and abolished oscillations, but phosphorylated (p)-rC0C7-sc did not completely rescue these effects. We describe a robust new approach for acute removal and replacement of cMyBP-C in situ. The method revealed a novel role for cMyBP-C N'-terminal domains to damp sarcomere-driven contractile waves (so-called spontaneous oscillatory contractions). Because phosphorylated (p)-rC0C7-sc was less effective at damping contractile oscillations, results suggest that spontaneous oscillatory contractions may contribute to enhanced contractility in response to inotropic stimuli.

A chemical kinetic model for Ca2+ induced spontaneous oscillatory contraction of myocardium

The Ca2+ induced Spontaneous Oscillatory Contraction (Ca-SPOC) of cardiac myofibrils oscillate with a period similar to resting heartbeat of several animal species, and its auto-oscillatory properties set the basic rhythm of cardiac contraction. To explain the dynamics of Ca-SPOC, the present paper constructs a novel chemical kinetical model based upon the cooperative behavior between the two heads of myosin II dimer, also considering the reaction-diffusion effect of ATP inside myocardial fibers. The simulation results show that the concentration of ATP inside myocardial fibers oscillates over time under some special conditions, together with the proportions of myosin II dimers in different states periodically changing with time, which contributes to produce the sustained oscillations of contractive tension. These results indicate that the SPOC of muscles may be partly due to chemical oscillation involved in the actomyosin ATPase cycle, which has been ignored by the previous theoretical studies.

Effect of myofibril passive elastic properties on the mechanical communication between motor proteins on adjacent sarcomeres

Rapid sarcomere lengthening waves propagate along a single muscle myofibril during spontaneous oscillatory contraction (SPOC). In asynchronous insect flight muscles, SPOC is thought to be almost completely synchronized over the entire myofibril. This phenomenon does not require Ca2+ regulation of the dynamics of the motor proteins, and cannot be explained simply by the longitudinal mechanical equilibrium among sarcomeres in the myofibril. In the present study, we rationalize these phenomena by considering the lateral mechanical equilibrium, in which two tensions originating from the inverse relationship between sarcomere length and lattice spacing, along with the lattice alignment, play important roles in the mechanical communication between motor proteins on adjacent filaments via the Z-disc. The proposed model is capable of explaining various SPOC phenomena based on the stochastic power-stroke mechanism of motor proteins, which responds to temporal changes in longitudinal mechanical load.

Mechanochemical model for myosin II dimer that can explain the spontaneous oscillatory contraction of muscle**Project supported by Research Program of Science and Technology at Universities of Inner Mongolia Autonomous Region, China (Grant Nos. NJZY16493 and NJZC17458).

The spontaneous oscillatory contraction (SPOC) of myofibrils is the essential property inherent to the contractile system of muscle. Muscle contraction results from cyclic interactions between actin filament and myosin II which is a dimeric motor protein with two heads. Taking the two heads of myosin II as an indivisible element and considering the effects of cooperative behavior between the two heads on rate constants in the mechanochemical cycle, the present work proposes the tenstate mechanochemical cycle model for myosin II dimer. The simulations of this model show that the proportion of myosin II in different states periodically changes with time, which results in the sustained oscillations of contractive tension, and serves as the primary factor for SPOC. The good fit of this model to experimental results suggests that the cooperative interaction between the two heads of myosin II dimer may be one of the underlying mechanisms for muscle contraction.

Sarcomeric Auto-Oscillations in Single Myofibrils From the Heart of Patients With Dilated Cardiomyopathy.

Left ventricular wall motion is depressed in patients with dilated cardiomyopathy (DCM). However, whether or not the depressed left ventricular wall motion is caused by impairment of sarcomere dynamics remains to be fully clarified. We analyzed the mechanical properties of single sarcomere dynamics during sarcomeric auto-oscillations (calcium spontaneous oscillatory contractions [Ca-SPOC]) that occurred at partial activation under the isometric condition in myofibrils from donor hearts and from patients with severe DCM (New York Heart Association classification III-IV). Ca-SPOC reproducibly occurred in the presence of 1 μmol/L free Ca2+ in both nonfailing and DCM myofibrils, and sarcomeres exhibited a saw-tooth waveform along single myofibrils composed of quick lengthening and slow shortening. The period of Ca-SPOC was longer in DCM myofibrils than in nonfailing myofibrils, in association with prolonged shortening time. Lengthening time was similar in both groups. Then, we performed Tn (troponin) exchange in myofibrils with a DCM-causing homozygous mutation (K36Q) in cTnI (cardiac TnI). On exchange with the Tn complex from healthy porcine ventricles, period, shortening time, and shortening velocity in cTnI-K36Q myofibrils became similar to those in Tn-reconstituted nonfailing myofibrils. Protein kinase A abbreviated period in both Tn-reconstituted nonfailing and cTnI-K36Q myofibrils, demonstrating acceleration of cross-bridge kinetics. Sarcomere dynamics was found to be depressed under loaded conditions in DCM myofibrils because of impairment of thick-thin filament sliding. Thus, microscopic analysis of Ca-SPOC in human cardiac myofibrils is beneficial to systematically unveil the kinetic properties of single sarcomeres in various types of heart disease.

Dynamic properties of bio-motile systems with a liquid-crystalline structure

ABSTRACTBio-motile systems have liquid-crystalline structures. This review first describes the contractile system of striated muscle having a smectic liquid crystalline structure. We here report the muscle's auto-oscillatory property named spontaneous oscillatory contraction (SPOC) [1], and a mathematical model to explain its mechanism [2, 3]. Also, sarcomere dynamics observed during heartbeat are described. The second topic is the micromechanics of the meiotic spindle, a bipolar assembly of microtubules with chromosomes [4]. The third topic is the demonstration of a contractile actin ring spontaneously formed inside a water-in-oil droplet, which can be considered as an artificial cell model [5].

Model simulation of the SPOC wave in a bundle of striated myofibrils.

SPOC (spontaneous oscillatory contraction) is a phenomenon observed in striated muscle under intermediate activation conditions. Recently, we constructed a theoretical model of SPOC for a sarcomere, a unit sarcomere model, which explains the behavior of SPOC at each sarcomere level. We also constructed a single myofibril model, which visco-elastically connects the unit model in series, and explains the behaviors of SPOC at the myofibril level. In the present study, to understand the SPOC properties in a bundle of myofibrils, we extended the single myofibril model to a two-dimensional (2D) model and a three-dimensional (3D) model, in which myofibrils were elastically connected side-by-side through cross-linkers between the Z-lines and M-lines. These 2D and 3D myofibril models could reproduce various patterns of SPOC waves experimentally observed in a 2D sheet and a 3D bundle of myofibrils only by choosing different values of elastic constants of the cross-linkers and the external spring. The results of these 2D and 3D myofibril models provide insight into the SPOC properties of the higher-ordered assembly of myofibrils.

Spontaneous oscillatory contraction (SPOC) in cardiomyocytes.

SPOC (spontaneous oscillatory contraction) is a characteristic state of the contractile system of striated (skeletal and cardiac) muscle that exists between the states of relaxation and contraction. For example, Ca-SPOCs occur at physiological Ca2+ levels (pCa ∼6.0), whereas ADP-SPOC occurs in the virtual absence of Ca2+ (pCa ≥ 8; relaxing conditions in the presence of MgATP), butin the presence of inorganic phosphate (Pi) and a high concentration of MgADP. The concentration of Mg-ADP necessary for SPOC is nearly equal to or greater than the MgATP concentration for cardiac muscle and is several times higher for skeletal muscle. Thus, the cellular conditions for SPOC are broader in cardiac muscle than in skeletal muscle. During these SPOCs, each sarcomere in a myofibril undergoes length oscillation that has a saw-tooth waveform consisting of a rapid lengthening and a slow shortening phase. The lengthening phase of one half of a sarcomere is transmitted to the adjacent half of the sarcomere successively, forming a propagating wave (termed a SPOC wave). The SPOC waves are synchronized across the cardiomyocytes resulting in a visible wave of successive contractions and relaxations termed the SPOC wave. Experimentally, the SPOC period (and therefore the velocity of SPOC wave) is observed in demembranated cardiomyocytes and can be prepared from a wide range of animal hearts. These periods correlate well with the resting heartbeats of a wide range of mammals (rat, rabbit, dog, pig and cow). Preliminary experiments showed that the SPOC properties of human cardiomyocytes are similar to the heartbeat of a large dog or a pig. This correlation suggests that SPOCs may play a fundamental role in the heart. Here, we briefly summarize a range of SPOC parameters obtained experimentally, and relate them to a theoretical model to explain those characteristics. Finally, we discuss the possible significance of these SPOC properties in each and everyheartbeat.

Depressed Contractility at Low-Load Spontaneous Oscillatory Contractions in Human Hypertrophic Cardiomyopathy with MYBPC3 Mutations

We examined the role of missense and nonsense myosin-binding protein mutations in human tissue of patients with hypertrophic cardiomyopathy in an essentially unloaded isotonic system by partial Ca2+-activation, i.e. Ca-spontaneous oscillary contractions (SPOC). Despite considerable literature suggesting that hypercontractility is a feature of HCM-causing mutations, we observe: (1) prolonged durations of both the lengthening (p<0.0001) and shortening (p<0.001) phases of the SPOC cycle in MYBPC3 mutants; (2) depressed contractility where the rates of both lengthening (P<0.01) and shortening (p<0.05) were reduced; however (3) the amplitude of the SPOC cycles did not vary between mutated MYBPC3 and healthy donor samples under essentially unloaded isotonic conditions. We found no difference between MYBPC3 samples containing missense or nonsense mutations. Unexpectedly, principal component analysis demonstrated that the contractile properties of human derived cardiomyocytes under low-load conditions were distinctively different for mutations in MYBPC3 and troponin genes. We conclude that, at least under the isotonic Ca-activated SPOC conditions: (1) MYBPC3 mutants display depressed contractility; (2) this effect is independent of the gene variant but is consistent with a mechanism of haplotype insufficiency (rather than “poison peptide” effect); and (3) HCM mutations display different phenotypes when examined under conditions of low strain.

Gastric contractility in experimental gastroschisis

Background/purposeThe mechanism of fetal gastric dilation in gastroschisis is controversial. This study was designed to characterize changes in the contractile profile of strips of stomach from rats following experimental gastroschisis. MethodsPregnant Wistar rats were operated on day 18.5. Fetuses were divided into three groups: gastroschisis (G), sham (S), and control (C). On day 21.5, gastric fundus and antrum strips were obtained and suspended to a force transducer connected to a digital data acquisition system. They were submitted to increasing concentrations of carbachol (CCh) and weighed at the end of each procedure. Frequency and amplitude of each contraction were evaluated. ResultsUnder basal conditions, spontaneous oscillatory contractions of antrum and fundus strips of G, S, and C were similar (P>0.05; ANOVA). However, cumulative concentrations of CCh (0.01–100μM) produced different effects in all groups and were characterized by a significant increase in amplitude and frequency of spontaneous contractions in antral smooth muscle and a sustained increase in tonus in fundic strips. Upon analysis, no significant difference in frequency or amplitude was noted in antral tissues comparing C to G and to S (P>0.05). No significant contractility difference was noted in fundic smooth muscle (comparing all groups, P>0.05), with the CCh-induced curve following a typical sigmoidal format, dependent on increasing concentrations (P<0.001). ConclusionsGastric contractile responses to CCh are preserved in experimental gastroschisis. These results do not support the theory that gastric dilation occurs secondary to intestinal inflammation alone.