Regulatory mechanism of length-dependent activation in skinned porcine ventricular muscle: role of thin filament cooperative activation in the Frank-Starling relation

Abstract

Cardiac sarcomeres produce greater active force in response to stretch, forming the basis of the Frank-Starling mechanism of the heart. The purpose of this study was to provide the systematic understanding of length-dependent activation by investigating experimentally and mathematically how the thin filament “on–off” switching mechanism is involved in its regulation. Porcine left ventricular muscles were skinned, and force measurements were performed at short (1.9 µm) and long (2.3 µm) sarcomere lengths. We found that 3 mM MgADP increased Ca2+ sensitivity of force and the rate of rise of active force, consistent with the increase in thin filament cooperative activation. MgADP attenuated length-dependent activation with and without thin filament reconstitution with the fast skeletal troponin complex (sTn). Conversely, 20 mM of inorganic phosphate (Pi) decreased Ca2+ sensitivity of force and the rate of rise of active force, consistent with the decrease in thin filament cooperative activation. Pi enhanced length-dependent activation with and without sTn reconstitution. Linear regression analysis revealed that the magnitude of length-dependent activation was inversely correlated with the rate of rise of active force. These results were quantitatively simulated by a model that incorporates the Ca2+-dependent on–off switching of the thin filament state and interfilament lattice spacing modulation. Our model analysis revealed that the cooperativity of the thin filament on–off switching, but not the Ca2+-binding ability, determines the magnitude of the Frank-Starling effect. These findings demonstrate that the Frank-Starling relation is strongly influenced by thin filament cooperative activation.

Sarcomere length-dependent Ca2+ activation in skinned rabbit psoas muscle fibers: coordinated regulation of thin filament cooperative activation and passive force

“…4, we summarize the relationship between t 1/2 , pCa 50 and ΔpCa 50 obtained under various conditions, i.e., in the presence of MgADP or Pi, and cTn or sTn treatment. As found in PLV [8], a significant linear relationship was obtained in each graph, with the slope for rabbit psoas muscle being lower than that for PLV in a relationship of t 1/2 versus pCa 50 (Fig. 4b) and that of t 1/2 versus ΔpCa 50 (Fig.…”

“…Therefore, in the following experiments, we used 3 mM MgADP to enhance thin filament cooperative activation and 20 mM Pi to reduce it, based on our previous study on porcine left ventricular muscle (PLV) (cf. [8]).

Fig. 1Effects of MgADP or Pi on the rate of rise of active force in rabbit psoas muscle fibers; pCa 4.5, SL 2.0 μm.

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Sarcomere length dependent effects on the interaction between cTnC and cTnI in skinned papillary muscle strips

“…Left ventricular papillary bundles were prepared from adult (6–8 months in age) Sprague Dawley rats using previously established protocol (Terui, Shimamoto et al 2010, Rieck, Li et al 2013, Li, Rieck et al 2014). Animals were deeply anesthetized by inhalation of 3% isoflurane and hearts were quickly excised and placed into ice-cold 20 mM EGTA relaxing solution in the presence of 1.0 mM DTT, and 20 mM 2, 3-butanedione monoxime.…”

Interleukin-16 Promotes Cardiac Fibrosis and Myocardial Stiffening in Heart Failure with Preserved Ejection Fraction

“…Skinned LV muscles from mice were prepared according to previously reported methods [30]. Transverse sections of the left ventricle were skinned in relaxing solution (5 mM MgATP, 40 mM BES, 1 mM Mg 2+ , 10 mM EGTA, 1 mM dithiothreitol, 15 mM phosphocreatine, 15 U/ml creatine phosphokinase, 10 mM BDM, 180 mM ionic strength [adjusted by K-propionate], pH 7.0) containing 1% Triton X-100 overnight.…”