End‐diastolic force pre‐activates cardiomyocytes and determines contractile force: role of titin and calcium

Abstract

Titin functions as a molecular spring, and cardiomyocytes are able, through splicing, to control the length of titin. We hypothesized that together with diastolic [Ca2+], titin‐based stretch pre‐activates cardiomyocytes during diastole and is a major determinant of force production in the subsequent contraction. Through this mechanism titin would play an important role in active force development and length‐dependent activation. Mutations in the splicing factor RNA binding motif protein 20 (RBM20) result in expression of large, highly compliant titin isoforms. We measured single cardiomyocyte work loops that mimic the cardiac cycle in wild‐type (WT) and heterozygous (HET) RBM20‐deficient rats. In addition, we studied the role of diastolic [Ca2+] in membrane‐permeabilized WT and HET cardiomyocytes. Intact cardiomyocytes isolated from HET left ventricles were unable to produce normal levels of work (55% of WT) at low pacing frequencies, but this difference disappeared at high pacing frequencies. Length‐dependent activation (force–sarcomere length relationship) was blunted in HET cardiomyocytes, but the force–end‐diastolic force relationship was not different between HET and WT cardiomyocytes. To delineate the effects of diastolic [Ca2+] and titin pre‐activation on force generation, measurements were performed in detergent‐permeabilized cardiomyocytes. Cardiac twitches were simulated by transiently exposing permeabilized cardiomyocytes to 2 µm Ca2+. Increasing diastolic [Ca2+] from 1 to 80 nm increased force development twofold in WT. Higher diastolic [Ca2+] was needed in HET. These findings are consistent with our hypothesis that pre‐activation increases active force development. Highly compliant titin allows cells to function at higher diastolic [Ca2+].

Large-Scale Contractility Measurements Reveal Large Atrioventricular and Subtle Interventricular Differences in Cultured Unloaded Rat Cardiomyocytes

“…These differences may partly contribute to the contraction properties of LV and RV myofilaments described by Belin et al (2011) , in addition to the differences in myofilament phosphorylation they observed. Titin isoform composition could play a role in the kinetics and extent of contraction, as we recently showed ( Najafi et al, 2019 ). Furthermore, LV versus RV differences have been found in terms of ion channel make-up and regulation ( Gaborit et al, 2007 ; Molina et al, 2016 ; Zaitsev et al, 2019 ).…”

“…Titin isoform composition could play a role in the kinetics and extent of contraction, as we recently showed (Najafi et al, 2019). Furthermore, LV versus RV differences have been found in terms of ion channel make-up and regulation (Gaborit et al, 2007;Molina et al, 2016;Zaitsev et al, 2019).…”

Impact of connective tissue dysplasia on heart adaptation to exercise stress in young athletes

“…Global LV function was estimated by means of transthoracic echocardiography (TTE) with the use of ultrasound system "Samsung Medison HM70" (Samsung, Korea), at rest and immediately after test (within 3 minutes of recovery period) according to ACC/AHA Guidelines for the Clinical Application of Echocardiography (2013) [19]. The following parameters were chosen as measures of global function: end-diastolic volume (EDV, ml), stroke volume (SV, ml), ejection fraction (EF, %), end-diastolic index (EDI, ml/m2), stroke index (SI, ml/m2) [20][21][22][23].…”

Echocardiographic Assessment of Myocardial Deformation during Exercise

“…A significant addition to the longstanding knowledge on oblique and circumferential fibers was provided by Lunkenheimer et al, who provided evidence for the existence of transmural myofibres that may be of fundamental relevance to the regulation of forces associated with normal myocardial contraction and relaxation [19]. Finally, there is important structural diversity on the myocyte level that contributes to the overall elasticity of the cardiomyocyte, as revealed by different isoforms of the giant protein titin, which may influence myocardial deformation in systole and diastole, not least during exercise [20,21]. Collectively, the current knowledge indicates a non-uniform, complex mesh of diverse cardiac myofibre arrangements which may be grouped in sheets and laminae, influencing the electrical activation sequence of the heterogeneously distributed autonomic nerves in the heart (Figure 1, [22]).…”