title: Residual force depression in single sarcomeres is abolished by MgADP-induced activation.
abstract: The mechanisms behind the shortening-induced force depression commonly observed in skeletal muscles remain unclear, but have been associated with sarcomere length non-uniformity and/or crossbridge inhibition. The purpose of this study was twofold: (i) to evaluate if force depression is present in isolated single sarcomeres, a preparation that eliminates sarcomere length non-uniformities and (ii) to evaluate if force depression is inhibited when single sarcomeres are activated with MgADP, which biases crossbridges into a strongly-bound state. Single sarcomeres (n = 16) were isolated from rabbit psoas myofibrils using two micro-needles (one compliant, one rigid), piercing the sarcomere externally adjacent to the Z-lines. The sarcomeres were contracted isometrically and subsequently shortened, in both Ca(2+)- and MgADP-activating solutions. Shortening in Ca(2+)-activated samples resulted in a 27.44 ± 9.04% force depression when compared to isometric contractions produced at similar final sarcomere lengths (P < 0.001). There was no force depression in MgADP-activated sarcomeres (force depression = -1.79 ± 9.69%, P =  0.435). These results suggest that force depression is a sarcomeric property, and that is associated with an inhibition of myosin-actin interactions.
authors.affiliation: Department of Kinesiology and Physical Education, McGill University.
authors.forename: Neal
authors.initials: N
authors.lastname: Trecarten
authors.affiliation: Department of Kinesiology and Physical Education, McGill University.
authors.forename: Fabio C
authors.initials: FC
authors.lastname: Minozzo
authors.affiliation: Department of Kinesiology and Physical Education, McGill University.
authors.forename: Felipe S
authors.initials: FS
authors.lastname: Leite
authors.affiliation: 1] Department of Kinesiology and Physical Education, McGill University [2] Departments of Physics and Physiology, McGill University.
authors.forename: Dilson E
authors.initials: DE
authors.lastname: Rassier
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D011817:Rabbits; D012518:Sarcomeres / Q000187:drug effects / Q000502:physiology*
chemical_list: D000244:Adenosine Diphosphate; D002118:Calcium