Length-Dependent Activation of Contractility and Ca-Transient Kinetics in Auxotonically Contracting Isolated Rat Ventricular Cardiomyocytes

Abstract

Length-dependent activation (LDA) of contraction is an important mechanism of proper myocardial function that is often blunted in diseases accompanied by deficient contractility and impaired calcium homeostasis. We evaluated how the extent of LDA is related to the decreased force in healthy rat myocardium under negative inotropic conditions that affect the calcium cycle. The length-dependent effects on auxotonic twitch and Ca-transient were compared in isolated rat ventricular cardiomyocytes at room temperature ("25C") and near-physiological temperature ("35C") in normal Tyrode and at 25°C with thapsigargin-depleted sarcoplasmic reticulum ("25C + Thap"). At the slack length, a similar negative inotropy in "35C" and "25C + Thap" was accompanied by totally different changes in Ca-transient amplitude, time-to-peak, and time-to-decline from peak to 50% amplitude. End-systolic/end-diastolic tension-sarcomere length relationships were obtained for each individual cell, and the ratio of their slopes, the dimensionless Frank-Starling Gain index, was 2.32 ± 0.16, 1.78 ± 0.09, and 1.37 ± 0.06 in "25C," "35C" and "25C + Thap," respectively (mean ± S.E.M.). Ca-transient diastolic level and amplitude did not differ between "25C" and "35C" at any SL, but in "35C" it developed and declined significantly faster. In contrast, thapsigargin-induced depletion of SERCA2a significantly attenuated and retarded Ca-transient. The relative amount of Ca 2+ utilized by troponin C, evaluated by the integral magnitude of a short-lived component of Ca-transient decline ("bump"), increased by ~25% per each 0.05 μm increase in SL in all groups. The kinetics of the Ca-TnC dissociation, evaluated by the bump time-topeak, was significantly faster in "35C" and slower in "25C + Thap" vs. "25C" (respectively, 63.7 ± 5.3 and 253.6 ± 8.3% of the value in "25C," mean ± S.E.M.). In conclusion, a similar inotropic effect can be observed in rat ventricular myocardium under totally different kinetics of free cytosolic calcium. The extent of LDA is not determined by actual peak systolic tension but is regulated by the level of peak systolic calcium and the kinetics of Ca-transient decline which, in turn, are governed by Ca-TnC dissociation and Ca 2+ reuptake by the sarcoplasmic reticulum. Altogether, these findings constitute Lookin and Protsenko Length-Dependent Activation and Ca-Transient

Work-loop contractions reveal that the afterload-dependent time course of cardiac Ca2+ transients is modulated by preload

“…The plateau in the Ca 2+ transient decay phase, or “bump,” is a morphological feature that has been observed in several animal studies ( 16 , 46 , 47 ) and predicted, to some extent, in silico ( 12 ). The bump is thought to be caused by Ca 2+ dissociation from troponin-C (TnC) during relaxation at a faster rate than it can be sequestered into the sarcoplasmic reticulum (SR) ( 46 ).…”

“…The preload dependence of Ca 2+ handling in cardiac muscle has been studied in isometrically contracting trabeculae ( 13 – 15 ) and auxotonically contracting myocytes ( 16 ), but it has not yet been examined in isolated cardiac muscle undergoing work-loop contractions. From a mechanistic standpoint, preload has a major role in modulating the ability of cardiac muscle to relax under afterloaded isotonic contractions in trabeculae ( 17 ) and myocytes ( 10 , 18 ), as well as on the mechano-Ca 2+ feedback as recently shown in silico ( 19 ).…”

Sulforaphane exposure impairs contractility and mitochondrial function in three-dimensional engineered heart tissue

“…The Frank-Starling mechanism of the heart, also known as length-dependent activation on the single cell level, is a well known adaptive mechanism that describes increasing force with increased preload [ 41 , 42 ]. Besides changes in Ca 2+ -sensitivity and positioning of the sarcomeric proteins [ 43 ], more ATP needs to be produced to further support the increase in force.…”

Active force generation contributes to the complexity of spontaneous activity and to the response to stretch of murine cardiomyocyte cultures

“…In parallel to the continuous variation of beating rate, cardiac activity is affected continuously by mechanical influences resulting from passive forces and active contractions. The typical example is the Frank–Starling mechanism, which permits, on a beat‐to‐beat basis, the stroke volume to be adjusted and optimized to diastolic filling (Lookin & Protsenko, 2019). Regarding pacemaker function, it is known that an increase in right atrial pressure causes an increase of heart rate.…”