title: Residual force enhancement is regulated by titin in skeletal and cardiac myofibrils.
abstract: When a skeletal muscle is stretched while it contracts, the muscle produces a relatively higher force than the force from an isometric contraction at the same length: a phenomenon referred to as residual force enhancement. Residual force enhancement is puzzling because it cannot be directly explained by the classical force-length relationship and the sliding filament theory of contraction, the main paradigms in the muscle field. We used custom-built instruments to measure residual force enhancement in skeletal myofibrils, and, for the first time, in cardiac myofibrils. Our data report that residual force enhancement is present in skeletal muscles, but not cardiac muscles, and is regulated by the different isoforms of the titin protein filaments.
  
  
  
  When a skeletal muscle contracts isometrically, the muscle produces a force that is relative to the final isometric sarcomere length (SL). However, when the same final SL is reached by stretching the muscle while it contracts, the muscle produces a relatively higher force: a phenomenon commonly referred to as residual force enhancement. In this study, we investigated residual force enhancement in rabbit skeletal psoas myofibrils and, for the first time, cardiac papillary myofibrils. A custom-built atomic force microscope was used in experiments that stretched myofibrils before and after inhibiting myosin and actin interactions to determine whether the different cardiac and skeletal titin isoforms regulate residual force enhancement. At SLs ranging from 2.24 to 3.13 μm, the skeletal myofibrils enhanced the force by an average of 9.0%, and by 29.5% after hindering myosin and actin interactions. At SLs ranging from 1.80 to 2.29 μm, the cardiac myofibrils did not enhance the force before or after hindering myosin and actin interactions. We conclude that residual force enhancement is present only in skeletal muscles and is dependent on the titin isoforms.
authors.affiliation: Department of Mechanical Engineering, McGill University, 817 Sherbrooke Street West, Montreal, Quebec, Canada, H3A 2K6.
authors.forename: Nabil
authors.identifier: 0000-0002-3737-1986
authors.initials: N
authors.lastname: Shalabi
authors.affiliation: Department of Kinesiology and Physical Education, McGill University, 475 Pine Avenue West, Montreal, Quebec, Canada, H2W 1S4.
authors.forename: Anabelle
authors.initials: A
authors.lastname: Cornachione
authors.affiliation: Department of Kinesiology and Physical Education, McGill University, 475 Pine Avenue West, Montreal, Quebec, Canada, H2W 1S4.
authors.forename: Felipe
authors.identifier: 0000-0003-2493-8872
authors.initials: F
authors.lastname: de Souza Leite
authors.affiliation: Department of Mechanical Engineering, McGill University, 817 Sherbrooke Street West, Montreal, Quebec, Canada, H3A 2K6.
authors.forename: Srikar
authors.initials: S
authors.lastname: Vengallatore
authors.affiliation: Department of Kinesiology and Physical Education, McGill University, 475 Pine Avenue West, Montreal, Quebec, Canada, H2W 1S4.
authors.forename: Dilson E
authors.identifier: 0000-0001-7080-9364
authors.initials: DE
authors.lastname: Rassier
mesh_terms: D000818:Animals; D064211:Connectin / Q000502:physiology*; D005260:Female; D009210:Myofibrils / Q000502:physiology*; D010210:Papillary Muscles / Q000502:physiology; D020033:Protein Isoforms / Q000502:physiology; D016658:Psoas Muscles / Q000502:physiology*; D011817:Rabbits
chemical_list: D064211:Connectin; D020033:Protein Isoforms
keywords: Blebbistatin; cardiac; myofibril; residual force enhancement; skeletal; static tension; titin