Pacing Staircase Phenomenon in the Heart: From Bodwitch to the XXI Century

Myosin in autoinhibited off state(s), stabilized by mavacamten, can be recruited in response to inotropic interventions

Mavacamten is a FDA-approved small-molecule therapeutic designed to regulate cardiac function at the sarcomere level by selectively but reversibly inhibiting the enzymatic activity of myosin. It shifts myosin toward ordered off states close to the thick filament backbone. It remains elusive whether these myosin heads in the off state(s) can be recruited in response to physiological stimuli when required to boost cardiac output. We show that cardiac myosins stabilized in these off state(s) by mavacamten are recruitable by 1) Ca2+, 2) increased chronotropy [heart rate (HR)], 3) stretch, and 4) β-adrenergic (β-AR) stimulation, all known physiological inotropic interventions. At the molecular level, we show that Ca2+ increases myosin ATPase activity by shifting mavacamten-stabilized myosin heads from the inactive super-relaxed state to the active disordered relaxed state. At the myofilament level, both Ca2+ and passive lengthening can shift mavacamten-ordered off myosin heads from positions close to the thick filament backbone to disordered on states closer to the thin filaments. In isolated rat cardiomyocytes, increased stimulation rates enhanced shortening fraction in mavacamten-treated cells. This observation was confirmed in vivo in telemetered rats, where left-ventricular dP/dtmax, an index of inotropy, increased with HR in mavacamten-treated animals. Finally, we show that β-AR stimulation in vivo increases left-ventricular function and stroke volume in the setting of mavacamten. Our data demonstrate that the mavacamten-promoted off states of myosin in the thick filament are at least partially activable, thus preserving cardiac reserve mechanisms.

Myosin in autoinhibited off state(s), stabilized by mavacamten, can be recruited via inotropic effectors

Mavacamten is a novel, FDA-approved, small molecule therapeutic designed to regulate cardiac function by selectively but reversibly inhibiting the enzymatic activity of myosin. It shifts myosin towards ordered off states close to the thick filament backbone. It remains unresolved whether mavacamten permanently sequesters these myosin heads in the off state(s) or whether these heads can be recruited in response to physiological stimuli when required to boost cardiac output. We show that cardiac myosins stabilized in these off state(s) by mavacamten are recruitable by Ca2+, increased heart rate, stretch, and β-adrenergic (β-AR) stimulation, all known physiological inotropic effectors. At the molecular level, we show that, in presence of mavacamten, Ca2+ increases myosin ATPase activity by shifting myosin heads from the reserve super-relaxed (SRX) state to the active disordered relaxed (DRX) state. At the myofilament level, both Ca2+ and passive lengthening can shift ordered off myosin heads from positions close to the thick filament backbone to disordered on states closer to the thin filaments in the presence of mavacamten. In isolated rat cardiomyocytes, increased stimulation rates enhanced shortening fraction in mavacamten-treated cells. This observation was confirmed in vivo in telemetered rats, where left-ventricular dP/dtmax, an index of inotropy, increased with heart rate in mavacamten treated animals. Finally, we show that β-AR stimulation in vivo increases left-ventricular function and stroke volume in the setting of mavacamten. Our data demonstrate that the mavacamten-promoted off states of myosin in the thick filament are activable, at least partially, thus leading to preservation of cardiac reserve mechanisms.

Skeletal and cardiac muscle calcium transport regulation in health and disease

In healthy muscle, the rapid release of calcium ions (Ca2+) with excitation-contraction (E-C) coupling, results in elevations in Ca2+ concentrations which can exceed 10-fold that of resting values. The sizable transient changes in Ca2+ concentrations are necessary for the activation of signaling pathways, which rely on Ca2+ as a second messenger, including those involved with force generation, fiber type distribution and hypertrophy. However, prolonged elevations in intracellular Ca2+ can result in the unwanted activation of Ca2+ signaling pathways that cause muscle damage, dysfunction, and disease. Muscle employs several calcium handling and calcium transport proteins that function to rapidly return Ca2+ concentrations back to resting levels following contraction. This review will detail our current understanding of calcium handling during the decay phase of intracellular calcium transients in healthy skeletal and cardiac muscle. We will also discuss how impairments in Ca2+ transport can occur and how mishandling of Ca2+ can lead to the pathogenesis and/or progression of skeletal muscle myopathies and cardiomyopathies.

Excitation-Contraction Coupling in the Goldfish (Carassius auratus) Intact Heart

Cardiac physiology of fish models is an emerging field given the ease of genome editing and the development of transgenic models. Several studies have described the cardiac properties of zebrafish (Denio rerio). The goldfish (Carassius auratus) belongs to the same family as the zebrafish and has emerged as an alternative model with which to study cardiac function. Here, we propose to acutely study electrophysiological and systolic Ca²⁺ signaling in intact goldfish hearts. We assessed the Ca²⁺ dynamics and the electrophysiological cardiac function of goldfish, zebrafish, and mice models, using pulsed local field fluorescence microscopy, intracellular microelectrodes, and flash photolysis in perfused hearts. We observed goldfish ventricular action potentials (APs) and Ca²⁺ transients to be significantly longer when compared to the zebrafish. The action potential half duration at 50% (APD₅₀) of goldfish was 370.38 ± 8.8 ms long, and in the zebrafish they were observed to be only 83.9 ± 9.4 ms. Additionally, the half duration of the Ca²⁺ transients was also longer for goldfish (402.1 ± 4.4 ms) compared to the zebrafish (99.1 ± 2.7 ms). Also, blocking of the L-type Ca²⁺ channels with nifedipine revealed this current has a major role in defining the amplitude and the duration of goldfish Ca²⁺ transients. Interestingly, nifedipine flash photolysis experiments in the intact heart identified whether or not the decrease in the amplitude of Ca²⁺ transients was due to shorter APs. Moreover, an increase in temperature and heart rate had a strong shortening effect on the AP and Ca²⁺ transients of goldfish hearts. Furthermore, ryanodine (Ry) and thapsigargin (Tg) significantly reduced the amplitude of the Ca²⁺ transients, induced a prolongation in the APs, and altogether exhibited the degree to which the Ca²⁺ release from the sarcoplasmic reticulum contributed to the Ca²⁺ transients. We conclude that the electrophysiological properties and Ca²⁺ signaling in intact goldfish hearts strongly resembles the endocardial layer of larger mammals.

Hypothermia elongates the contraction-relaxation cycle in explanted human failing heart decreasing the time for ventricular filling during diastole

Targeted temperature management is part of the standardized treatment for patients in cardiac arrest. Hypothermia decreases cerebral oxygen consumption and induces bradycardia; thus, increasing the heart rate may be considered to maintain cardiac output. We hypothesized that increasing heart rate during hypothermia would impair diastolic function. Human left ventricular trabeculae obtained from explanted hearts of patients with terminal heart failure were stimulated at 0.5 Hz, and contraction-relaxation cycles were recorded. Maximal developed force (Fmax), maximal rate of development of force [(dF/d t)max], time to peak force (TPF), time to 80% relaxation (TR80), and relaxation time (RT = TR80 − TPF) were measured at 37, 33, 31, and 29°C. At these temperatures, stimulation frequency was increased from 0.5 to 1.0 and to 1.5 Hz. At 1.5 Hz, concentration-response curves for the β-adrenergic receptor (β-AR) agonist isoproterenol were performed. Fmax, TPF, and RT increased when temperature was lowered, whereas (dF/d t)max decreased. At all temperatures, increasing stimulation frequency increased Fmax and (dF/d t)max, whereas TPF and RT decreased. At 31 and 29°C, resting tension increased at 1.5 Hz, which was ameliorated by β-AR stimulation. At all temperatures, maximal β-AR stimulation increased Fmax, (dF/d t)max, and maximal systolic force, whereas resting tension decreased progressively with lowering temperature. β-AR stimulation reduced TPF and RT to the same extent at all temperatures, despite the more elongated contraction-relaxation cycle at lower temperatures. Diastolic dysfunction during hypothermia results from an elongation of the contraction-relaxation cycle, which decreases the time for ventricular filling. Hypothermic bradycardia protects the heart from diastolic dysfunction and increasing the heart rate during hypothermia should be avoided.

NEW & NOTEWORTHY Decreasing temperature increases the duration of the contraction-relaxation cycle in the human ventricular myocardium, significantly reducing the time for ventricular filling during diastole. During hypothermia, increasing heart rate further reduces the time for ventricular filling and in some situations increases resting tension further impairing diastolic function. Modest β-adrenergic receptor stimulation can ameliorate these potentially detrimental changes during diastole while improving contractile force generation during targeted temperature management.

SERCA is critical to control the Bowditch effect in the heart

The Bowditch effect or staircase phenomenon is the increment or reduction of contractile force when heart rate increases, defined as either a positive or negative staircase. The healthy and failing human heart both show positive or negative staircase, respectively, but the causes of these distinct cardiac responses are unclear. Different experimental approaches indicate that while the level of Ca2+ in the sarcoplasmic reticulum is critical, the molecular mechanisms are unclear. Here, we demonstrate that Drosophila melanogaster shows a negative staircase which is associated to a slight but significant frequency-dependent acceleration of relaxation (FDAR) at the highest stimulation frequencies tested. We further showed that the type of staircase is oppositely modified by two distinct SERCA mutations. The dominant conditional mutation SERCAA617T induced positive staircase and arrhythmia, while SERCAE442K accentuated the negative staircase of wild type. At the stimulation frequencies tested, no significant FDAR could be appreciated in mutant flies. The present results provide evidence that two individual mutations directly modify the type of staircase occurring within the heart and suggest an important role of SERCA in regulating the Bowditch effect.

Post-extrasystolic Potentiation: Link between Ca(2+) Homeostasis and Heart Failure?

Post-extrasystolic potentiation (PESP) describes the phenomenon of increased contractility of the beat following an extrasystole and has been attributed to changes in Ca(2+) homeostasis. While this effect has long been regarded to be a normal physiological phenomenon, a number of reports describe an enhanced potentiation of the post-extrasystolic beat in heart failure patients. The exact mechanism of this increased PESP is unknown, but disruption of normal Ca(2+) handling in heart failure may be the underlying cause. The use of PESP as a prognostic marker or therapeutic intervention have recently regained new attention, however, the value of the application of PESP in the clinic is still under debate. In this review, the mechanism of PESP with regard to Ca(2+) in the normal and failing heart will be discussed and the possible diagnostic and therapeutic role of this phenomenon will be explored.

Dissociation of Calcium Transients and Force Development following a Change in Stimulation Frequency in Isolated Rabbit Myocardium

As the heart transitions from one exercise intensity to another, changes in cardiac output occur, which are modulated by alterations in force development and calcium handling. Although the steady-state force-calcium relationship at various heart rates is well investigated, regulation of these processes during transitions in heart rate is poorly understood. In isolated right ventricular muscle preparations from the rabbit, we investigated the beat-to-beat alterations in force and calcium during the transition from one stimulation frequency to another, using contractile assessments and confocal microscopy. We show that a change in steady-state conditions occurs in multiple phases: a rapid phase, which is characterized by a fast change in force production mirrored by a change in calcium transient amplitude, and a slow phase, which follows the rapid phase and occurs as the muscle proceeds to stabilize at the new frequency. This second/late phase is characterized by a quantitative dissociation between the calcium transient amplitude and developed force. Twitch timing kinetics, such as time to peak tension and 50% relaxation rate, reached steady-state well before force development and calcium transient amplitude. The dynamic relationship between force and calcium upon a switch in stimulation frequency unveils the dynamic involvement of myofilament-based properties in frequency-dependent activation.

Role of CaMKII and ROS in rapid pacing-induced apoptosis

Tachycardia promotes cell death and cardiac remodeling, leading to congestive heart failure. However, the underlying mechanism of tachycardia- or rapid pacing (RP)-induced cell death remains unknown. Myocyte loss by apoptosis is recognized as a critical factor in the progression to heart failure and simulation of tachycardia by RP has been shown to increase the intracellular levels of at least two potentially proapoptotic molecules, Ca(2+) and reactive oxygen species (ROS). However, whether these molecules mediate tachycardia- or RP-induced cell death has yet to be determined. The aim of this study was to examine the subcellular mechanisms underlying RP-induced apoptosis. For this purpose rat ventricular myocytes were maintained quiescent or paced at 0.5, 5 and 8Hz for 1hr. RP at 5 and 8Hz decreased myocyte viability by 58±3% and 75±6% (n=24), respectively, compared to cells maintained at 0.5Hz, and increased caspase-3 activity and Bax/Bcl-2 ratio, indicative of apoptosis. RP-induced cell death and apoptosis were prevented when pacing protocols were conducted in the presence of either the ROS scavenger, MPG, or nifedipine to reduce Ca(2+) entry or the CaMKII inhibitors, KN93 and AIP. Consistently, myocytes from transgenic mice expressing a CaMKII inhibitory peptide (AC3-I) were protected against RP-induced cell death. Interestingly, tetracaine and carvedilol used to reduce ryanodine receptor (RyR) diastolic Ca(2+) release, and ruthenium red used to prevent Ca(2+) entry into the mitochondria prevented RP-induced cell death, whereas PI3K inhibition with Wortmannin exacerbated pacing-induced cell mortality. We conclude that CaMKII activation and ROS production are involved in RP-induced apoptosis. Particularly, our results suggest that CaMKII-dependent posttranslational modifications of the cardiac ryanodine receptor (RyR) leading to enhanced diastolic Ca(2+) release and mitochondrial Ca(2+) overload could be the underlying mechanism involved. We further show that RP simultaneously activates a protective cascade involving PI3K/AKT signaling which is however, insufficient to completely suppress apoptosis.

Potential antiarrhythmic effect of methyl 3,4,5-trimethoxycinnamate, a bioactive substance from roots of polygalae radix: suppression of triggered activities in rabbit myocytes

3,4,5-Trimethoxycinnamic acid (TMCA), methyl 3,4,5-trimethoxycinnamate (M-TMCA) and p-methoxycinnamic acid (PMCA) have been identified as the major bioactive components in the serum collected from rats treated with oral administration of Polygalae Radix ("YuanZhi," the roots of Polygala tenuifolia WILLD.), a traditional Chinese medicine used to relieve insomnia, anxiety and heart palpitation. The present study was designed to investigate its direct electrophysiological effects on isolated ventricular myocytes from rabbits. Whole-cell configuration of the patch-clamp technique was used to measure action potential (AP) and membrane currents in single ventricular myocytes enzymatically isolated from adult rabbit hearts. Ca(2+) transients were recorded in myocytes loaded with the Ca(2+) indicator Fluo-4AM. Among three bioactive substances of Polygala metabolites, only M-TMCA (15-30 µM) significantly shortened action potential duration at 50% and 90% repolarization (APD(50) and APD(90)) in cardiomyocytes in a concentration-dependent and a reversible manner. M-TMCA also inhibited L-type calcium current (I(Ca,L)), but showed effect on neither transient outward potassium current (I(to)) nor steady-state potassium current (I(K,SS)). Furthermore, M-TMCA abolished isoprenaline plus BayK8644-induced early afterdepolarizations (EADs) and suppressed delayed afterdepolarizations (DADs) and triggered activities (TAs). This potential anti-arrhythmic effects were likely attributed by the inhibition of I(Ca,L) and the suppression of intracellular Ca(2+) transients, which consequently suppress the generation of transient inward current (I(ti)). These findings suggest that M-TMCA may protect the heart from arrhythmias via its inhibitory effect on calcium channel.

Effects of complete heart block on myocardial function, morphology, and energy metabolism in the rat

AIMS

Severe sustained bradycardia may cause acute and possibly chronic congestive heart failure (CHF). The aim of this study was to investigate acute and chronic effects of complete heart block (CHB) on cardiac function, morphology, and creatine (Cr) metabolism.

METHODS AND RESULTS

CHB was induced in male Sprague-Dawley rats (approximately 250 g, n = 11) by means of electrocautery applied to the region of AV node and were compared with controls (n = 15). The rats were investigated at 1, 3, and 12 weeks after CHB induction with transthoracic echocardiography. Invasive haemodynamic assessment of left and right ventricular pressures was performed at 12 weeks. After the sacrifice, the hearts were freeze-clamped for analysis of myocardial Cr, and high energy phosphometabolites. The efficacy of operative procedure was 54%. The peri-operative mortality rate was 20%. Heart rate (HR) decreased by approximately 50% (P < 0.01) while stroke volume (SV) increased 2.5 times (P < 0.01) in the CHB rats. Cardiac index remained unchanged. The rats with CHB grew normally and were in no apparent distress. Filling pressures in left and right ventricles were normal. The CHB rats developed marked cardiomegaly with biventricular dilatation and eccentric left ventricular hypertrophy (P < 0.01). There was no change in the myocardial content of Cr and high energy phosphometabolites.

CONCLUSION

Rats with CHB are compensating for reduction in HR with increased SV without haemodynamic and biochemical characteristics of CHF. This model may be useful to study the effects of CHB and bradycardia on myocardial structure, function, electrophysiology, and metabolism as well as for studies of cell therapy for reparation of AV conductance.

Effects of halothane, sevoflurane and desflurane on the force-frequency relation in the dog heart in vivo

PURPOSE

Frequency potentiation is the increase in force of contraction induced by an increased heart rate (HR). This positive staircase phenomenon has been attributed to changes in Ca2+ entry and loading of intracellular Ca2+ stores. Volatile anesthetics interfere with Ca2+ homeostasis of cardiomyocytes. We hypothesized that frequency potentiation is altered by volatile anesthetics and investigated the influence of halothane (H), sevoflurane (S) and desflurane (D) on the positive staircase phenomenon in dogsin vivo.

METHODS

Dogs were chronically instrumented for measurement of left ventricular (LV) pressure and cardiac output. Heart rate was increased by atrial pacing from 120 to 220 beats·min^-1 and the LV maximal rate of pressure increase (dP/ dt_max) was determined as an index of myocardial performance. Measurements were performed in conscious dogs and during anesthesia with 1.0 minimal alveolar concentrations of each of the three inhaled anesthetics.

RESULTS

Increasing HR from 120 to 220 beats·min^-1 increased dP/dt_max from 3394 ± 786 (mean ± SD) to 3798 ± 810 mmHg sec-1 in conscious dogs. All anesthetics reduced dP/dt_max during baseline (at 120 beatss·min^-1: H, 1745 ± 340 mmHgs·sec^-1; S, 1882 ± 418; D, 1928 ± 454, allP < 0.05vs awake) but did not influence the frequency potentiation of dP/dtmax (at 220 beatss·min^-1: H, 1981 ± 587 mmHgs·sec^-1; S, 2187 ± 787; D, 2307 ± 691). The slope of the regression line correlating dP/dt_max and HR was not different between awake and anesetized dogs. Increasing HR did not influence cardiac output in awake or anesthetized dogs.

CONCLUSION

These results indicate that volatile anesthetics do not alter the force-frequency relation in dogs in vivo.