Force Responses and Sarcomere Dynamics of Cardiac Myofibrils Induced by Rapid Changes in [Pi]

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 [P^sub i^] were investigated in calcium-activated cardiac myofibrils. Rapid increases in [P^sub i^] 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^sub +Pi(2)^) had a similar [P^sub i^]-dependence, indicating that the kinetics of the major force decay (phase 2) upon rapid increase in [P^sub i^] is determined by sarcomere dynamics. In contrast, no "give" was observed during phase 1 after rapid [P^sub i^]-increase (rate constant k^sub +Pi(1)^) and during the single-exponential force rise (rate constant k^sub -Pi^) after rapid [P^sub i^]-decrease. The values of k^sub +Pi(1)^ and k^sub -Pi^ were similar to the rate constant of mechanically induced force redevelopment (k^sub TR^) and Ca^sup 2+^-induced force development (k^sub ACT^) measured at same [P^sub i^]. These results indicate that the major phase 2 of force decay upon a P^sub i^-jump does not reflect kinetics of the force-generating step but results from sarcomere "give". The other phases of P^sub i^-induced force kinetics that occur in the absence of "give" yield the same information as mechanically and Ca^sup 2+^-induced force kinetics (k^sub +Pi(1)^ ~ k^sub -Pi^ ~ k^sub TR^ ~ k^sub ACT^). Model simulations indicate that P^sub i^-induced force kinetics neither enable the separation of P^sub i^-release from the rate-limiting transition f into force states nor differentiate whether the "force-generating step" occurs before, along, or after the P^sub i^-release..

Medienart:	Artikel

Erscheinungsjahr:	2017
Erschienen:	2017

Enthalten in:	Zur Gesamtaufnahme - volume:112
Enthalten in:	Biophysical journal - 112(2017), 2, Seite 356-367

Sprache:	Englisch

Beteiligte Personen:	Robert Stehle [VerfasserIn]

Links:	Volltext search.proquest.com

Themen:	Biophysics Hypothesis testing Phosphates Reaction kinetics Simulation

doi:	10.1016/j.bpj.2016.11.005

funding:
Förderinstitution / Projekttitel:

PPN (Katalog-ID):	OLC1988244064