title: Force Responses and Sarcomere Dynamics of Cardiac Myofibrils Induced by Rapid Changes in [Pi].
abstract: The second phase of the biphasic force decay upon release of phosphate from caged phosphate was previously interpreted as a signature of kinetics of the force-generating step in the cross-bridge cycle. To test this hypothesis without using caged compounds, force responses and individual sarcomere dynamics upon rapid increases or decreases in concentration of inorganic phosphate [Pi] were investigated in calcium-activated cardiac myofibrils. Rapid increases in [Pi] induced a biphasic force decay with an initial slow decline (phase 1) and a subsequent 3-5-fold faster major decay (phase 2). Phase 2 started with the distinct elongation of a single sarcomere, the so-called sarcomere "give". "Give" then propagated from sarcomere to sarcomere along the myofibril. Propagation speed and rate constant of phase 2 (k+Pi(2)) had a similar [Pi]-dependence, indicating that the kinetics of the major force decay (phase 2) upon rapid increase in [Pi] is determined by sarcomere dynamics. In contrast, no "give" was observed during phase 1 after rapid [Pi]-increase (rate constant k+Pi(1)) and during the single-exponential force rise (rate constant k-Pi) after rapid [Pi]-decrease. The values of k+Pi(1)and k-Piwere similar to the rate constant of mechanically induced force redevelopment (kTR) and Ca2+-induced force development (kACT) measured at same [Pi]. These results indicate that the major phase 2 of force decay upon a Pi-jump does not reflect kinetics of the force-generating step but results from sarcomere "give". The other phases of Pi-induced force kinetics that occur in the absence of "give" yield the same information as mechanically and Ca2+-induced force kinetics (k+Pi(1)∼ k-Pi∼ kTR∼ kACT). Model simulations indicate that Pi-induced force kinetics neither enable the separation of Pi-release from the rate-limiting transition f into force states nor differentiate whether the "force-generating step" occurs before, along, or after the Pi-release.
authors.affiliation: Institute of Vegetative Physiology, University of Cologne, Köln, Germany. Electronic address: robert.stehle@uni-koeln.de.
authors.forename: Robert
authors.initials: R
authors.lastname: Stehle
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D006168:Guinea Pigs; D007700:Kinetics; D055595:Mechanical Phenomena / Q000187:drug effects*; D010710:Phosphates / Q000494:pharmacology*; D012518:Sarcomeres / Q000187:drug effects* / Q000378:metabolism*
chemical_list: D010710:Phosphates