Original article Stretch increases the force by decreasing cross-bridge weakening rate in the rat cardiac trabeculae Moran Yadid, Amir Landesberg ⁎ Biomedical Engineering, Technion–Israel Institute of Technology, Haifa 32000, Israel abstract article info Article history: Received 3 August 2010 Received in revised form 20 September 2010 Accepted 21 September 2010 Available online 27 September 2010 Keywords: Force enhancement Cardiac mechanics Force–velocity relationship Cross-bridge cycling Cardiac energetics Stretch increases the force and decreases energy consumption in skeletal muscles. Cardiac muscle response to stretch has been scarcely investigated, and the underlying mechanisms remain elusive. We hypothesized that stretch increases the force by modulating the cross-bridge (XB) cycling rate. Trabeculae (n = 10) were isolated from rat right ventricles. Sarcomere length was measured by laser diffraction and controlled by a fast servomotor. The number of strong XBs was assessed by measuring the dynamic stiffness. Ramp stretches at different velocities (V SL ≤ 2.17 μm/s) and onset times were imposed on sarcomeric isometric contractions. Stretches yielded identical increase in the stress and stiffness, implying that stretch increases force by increasing the number of XBs. A unique linear relationship was observed between the instantaneous normalized stress and stiffness for all the stretch velocities (1.01 ± 0.15, R 2 = 0.98 ± 0.04), suggesting that the force per XB is constant for all stretch velocities. The increase in the stress during stretch normalized by the instantaneous isometric stress was denoted as the normalized stress enhancement (σ E ). The normalized stiffness enhancement (K E ) was defined accordingly. The rates of σ E and K E development depended linearly on the stretch velocity (7.06 ± 1.03 and 6.57 ± 1.17 μm −1 , respectively). Moreover, it was independent of the stretch onset time, indicating that it is not dominated by XB recruitment processes, since the number of available XBs and XB recruitment vary with time during the twitch. These observations strongly suggest that stretch decreases the rate of strong XB turnover to the weak conformation in a velocity-dependent manner. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Stretching cardiac muscles has important implications. In patho- logical conditions such as ischemic heart diseases, the myocardium is mechanically inhomogeneous [1–5]. The malfunctioning weak regions are stretched during contraction, and consequently, the cardiac output decreases [1–5]. Stretch also affects myocytes survival by inducing programmed cell death [6]. Stretching cardiac fiber may also elicit calcium waves and triggered arrhythmia [7,8]. However, the mechanisms that determine the cardiac mechanical response to stretch are still elusive. Thus, the study aims to investigate the isolated cardiac muscle response to stretch and the underlying mechanisms. An increase in the generated force during stretch well above the isometric force is a well-established phenomenon in the skeletal muscles. This phenomenon, denoted as ‘force enhancement during stretch’, was observed in whole skeletal muscles [9] and single skeletal fibers [10]. Furthermore, it was found that stretch reduces the rate of energy consumption below the isometric rate in skeletal muscles [11–14]. It was suggested that the work done on the muscle during stretch is stored in the elastic cross-bridges (XBs), and the amount of absorbed energy grows with the stretch velocity [12]. Although these phenomena are well known in the skeletal muscles, the underlying mechanisms remain controversial. The classical Huxley's model of muscle contraction postulates that stretch increases the mean XB strain, and consequently, it increases the unitary force per attached XB [15]. However, this classical model predicts that stretch increases the rate of XB detachment and decreases the number of attached XBs. Other studies suggest that stretch promotes recruitment of additional XBs that are in the pre- power stroke conformation [16–18]. There are only few studies on the effects of stretch on cardiac mechanics [19], which is the focus of this study. We hypothesize that force enhancement during stretch is due to the dependence of XB cycling rate on the filament sliding velocity [20–22]. The XB cycles between two main biochemical conformations, denoted as the weak and strong conformations [23]. XB transitions between weak and strong conformations relate to the biochemical rate-limiting steps of nucleotide binding and release. ATP hydrolysis and phosphate release are required for XB weak-to-strong transition [23]. While filament shortening increases the rate of XB turnover from strong to weak conformation, denoted as XB weakening rate, filament lengthening Journal of Molecular and Cellular Cardiology 49 (2010) 962–971 ⁎ Corresponding author. Tel.: +972 4 8294143; fax: +972 4 8294599. E-mail address: amir@bm.technion.ac.il (A. Landesberg). 0022-2828/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.yjmcc.2010.09.016 Contents lists available at ScienceDirect Journal of Molecular and Cellular Cardiology journal homepage: www.elsevier.com/locate/yjmcc