Carmit Levy

The sarcomeric control of energy conversion.

Abstract: The Frank‐Starling Law, Fenn Effect, and Sugas suggestions of cardiac muscle constant contractile efficiency establish the dependence of cardiac mechanics and energetics on the loading conditions. Consistent with these observations, this review suggests that the sarcomere control of contraction consists of two dominant feedbacks: (1) a cooperativity mechanism (positive feedback), whereby the number of force‐generating cross‐bridges (XBs) determines the affinity of calcium binding to the troponin regulatory protein; and (2) a mechanical (negative) feedback, whereby the filament shortening velocity affects the rate of XB turnover from the force to the non‐force generating conformation. The study explains the roles of these feedbacks in providing the adaptive control of energy consumption by the loading conditions and validates the dependence of the cooperativity mechanism on the number of strong XBs. The cooperativity mechanism regulates XB recruitment. It explains the cardiac force‐length calcium relationship, the related Frank‐Starling Law of the heart, and the adaptive control of new XB recruitment and the associated adenosine triphosphate (ATP) consumption. The mechanical feedback explains the force‐velocity relationship and the constant and high‐contractile efficiency. These mechanisms were validated by testing the force responses to large amplitude (100 nm/sarcomere) sarcomere length (SL) oscillations, in intact tetanized trabeculae (utilizing 30 μM cyclopiazonic). The force responses to large‐length oscillations lag behind the imposed oscillations at low extracellular calcium concentration ([Ca2+]0) and slow frequencies (<4 Hz, 25°C), yielding counterclockwise hystereses in the force‐length plane. The force was higher during shortening than during lengthening. The area within these hystereses corresponds to the external work generated from new XB recruitment during each oscillation, and it is determined by the delay in the force response. Characterization of the delayed response and its dependence on the SL, force, and calcium allows identification of the regulation of XB recruitment. The direct dependence of the phase on force indicates that XB recruitment is determined directly by the force (i.e., the number of strong XBs) and indirectly by SL or calcium. The suggested feedbacks determine cardiac energetics: 1) the constant and high contractile efficiency is an intrinsic property of the single XB, due to the mechanical feedback; and 2) the XBs are the myocyte sensors that modulate XB recruitment in response to length and load changes through the cooperativity mechanism.

The Adaptive Intracellular Control of Cardiac Muscle Function

Abstract: This study explores the mechanisms dominating the regulation of the biochemical energy consumption and the mechanical output of the actin‐myosin motor units, the crossbridges (Xbs), in the cardiac sarcomere. Our analytical model, which couples Xbs cycling dynamics with the kinetics of the free Ca2+ binding to troponin‐C (Tn‐C), includes two feedback mechanisms: (1) a cooperativity mechanism, whereby the amount of force generating Xbs determines the affinity of calcium binding to the regulatory protein and the force‐length relationship (FLR); and (2) a mechanical (negative) feedback, whereby the filament shortening velocity affects the rate of Xb turnover from the force‐ to the nonforce‐generating state, allows the analytical solution for the muscle force‐velocity relationship (FVR), and the linear relation between energy consumption and the generated mechanical energy. Our experimental and analytical studies of the force response to large‐amplitude sarcomere length (SL) oscillations at various frequencies and constant [Ca2+] in the isolated tetanized rat trabeculae reveal that the generated force depends on the history of contraction and establishes the validity of these two feedbacks. The cooperativity mechanism generates counterclockwise (CCW) hystereses, where the muscle generates external work; while at higher frequencies the mechanical feedback produces clockwise (CW) hystereses, where the muscle behaves as a damper. The cooperativity provides the adaptive control of the cardiac response to short‐term changes in the load by modulating Xb recruitment. The cardiac efficiency, defined as the ratio of the generated mechanical energy (i.e., external work and pseudo‐potential energy) to the sarcomere energy consumption, is determined by the mechanical feedback, reflecting an inherent property of the single Xb. The efficiency is thus independent of the number of strong Xbs and is constant and load independent.

Augmentation of Dilated Failing Left Ventricular Stroke Work by a Physiological Cardiac Assist Device

Abstract: A novel physiological cardiac assist device (PCAD), otherwise known as the LEVRAM assist device, which is synchronized with the heartbeat, was developed to assist the left ventricle (LV) in chronic heart failure (CHF). The PCAD utilizes a single cannula, which is inserted in less than 15 s through the apex of the beating LV by means of a specially designed device. Blood is withdrawn from the LV into the PCAD in diastole and is injected back to the LV, through the same cannula, during the systolic ejection phase, thereby augmenting stroke volume (SV) and stroke work (SW). CHF with dilated LV was induced in sheep by successive intracoronary injections of 100‐μm beads. The sheep (92.2 ± 25.9 kg, n= 5) developed stable CHF with increased LV end‐diastolic diameter (69.4 ± 3.3 mm) and end‐diastolic volume (LVEDV = 239 ± 32 mL), with severely reduced ejection fraction (23.8 ± 7.6%), as well as mild‐to‐moderate mitral regurgitation. The sheep were anesthetized, and the heart was exposed by left thoracotomy. Pressure was measured in the LV and aorta (Millar). The SV was measured by flow meters and the LV volume by sonocrystals. Assist was provided every 10 regular beats, and the assisted beats were compared with the preceding unassisted beats, at the same LVEDV. The PCAD displaced 13.6 ± 3.4 mL, less than 8% of LVEDV. Added SW was calculated from the assisted and control pressure‐volume loops. The efficiency, defined as an increase in SW divided by the mechanical work of the PCAD, was 85.4 ± 16.9%. We conclude that the PCAD, working with a small displaced blood volume in synchrony with the heartbeat, efficiently augments the SW of the dilated failing LV. The PCAD is suggested for use as a permanent implantable device in CHF.

Effects of synchronized cardiac assist device on cardiac energetics.

Abstract:  A novel physiological cardiac assist device (PCAD), the LEV RAM assist device, which is synchronized with the failing heart ejection, was developed to improve the failing heart systolic and diastolic functions and cardiac energetics. The PCAD uses a single short cannula, which is inserted into the beating left ventricle (LV) by means of a specially designed device. Blood is ejected from the PCAD into the LV after the opening of the aortic valve and augments the cardiac stroke work. The same amount of blood is withdrawn from the LV into the PCAD, through the same cannula, during the diastole. The study aims to test the effects of the PCAD on cardiac energetics and coronary blood flow. Adult normal sheep were anesthetized and the heart was exposed by left thoracotomy. Pressures transducers (Millar Instruments, Inc., Houston, TX) were inserted into the LV and aorta. LV volume was measured by sonocrystals (Sonometrics Corp., London, Ontario, Canada) and impedance catheter (CD Lycom, Argonstrat 116 Zoetermeer, 2718 SP The Netherlands). Flowmeters (transonic) measured the cardiac output (CO) and the coronary arteries (left anterior descending (LAD) and circumflex) flows. A thin cannula was inserted into the coronary sinus and the oxygen content of the LV and the coronary sinus were determined (AVOXimeter‐1000). Pressure‐volume loops, myocardial energetics, and coronary flow were measured. The displaced PCAD volume was 11 mL. Four different levels of assist were studied by changing the frequency of the assist: (1) assist beat after three successive regular beats [1:4], (2) assist every third beat [1:3], (3) alternate assist and normal beat [1:2], and (4) continuous assist [1:1]. Cardiac output (CO) and stroke volume (SV) increased proportionally with increasing frequency of assist. Systolic mechanical efficiency of the PCAD was above 90%. Simultaneously, the PCAD decreased the end‐diastolic volume (EDV; diastolic unloading). The PCAD increased coronary flow and decreased cardiac arterial–venous O2 difference. We conclude that the PCAD efficiently augments CO and stroke work, decreases preload, and decreases the coronary arterial–venous O2 difference; all these may expedite cardiac reverse remodeling, and promote recovery of function and eventual easy explanation of the device.

The Mechanoelectric Feedback: A Novel “Calcium Clamp” Method, Using Tetanic Contraction, for Testing the Role of the Intracellular Free Calcium

Abstract:  Mechanical perturbations affect the membrane action potential, a phenomenon denoted as the mechanoelectric feedback (MEF), and may elicit cardiac arrhythmias. Two plausible mechanisms were suggested to explain this phenomenon: (i) stretch‐activated channels (SACs) within the cell membrane and (ii) modulation of the action potential by the intracellular Ca2+ (the Calcium hypothesis). The intracellular Ca2+ varies mainly due to the effects of the mechanical perturbations on the affinity of troponin for calcium. The present study concentrates on the unique experimental methods that allow differentiating between the effects of SAC and Ca2+ on the action potential. This is achieved by controlling the sarcomere lengths (SLs) independently of the intracellular Ca2+ concentration, in the intact fiber. A dedicated experimental setup allowed simultaneous measurements of the membrane potential and the mechanical performance (Force and SL). The action potential was measured by voltage‐sensitive dye (Di‐4‐ANEPPS). The SL was measured by laser diffraction technique and was controlled by a fast servomotor. The intracellular Ca2+ was controlled (calcium clamp) by imposing stable tetanic contractions at various extracellular calcium concentrations ([Ca2+]0s). Tetanus was obtained by 8 Hz stimulation in the presence of cyclopiazonic acid (CPA) (30 μM). Isolated trabeculae from a rats right ventricle were studied at different SLs and [Ca2+]0s. The experimental data strongly support the calcium hypothesis. Although the action potential duration (APD) decreases at longer SL, the [Ca2+]0 has a significantly larger effect on the APD. The APD decreases as the [Ca2+]0 increases. Understanding the underlying mechanism opens new research avenues for the development of therapeutic modalities for cardiac arrhythmias.