title: The effect of inorganic phosphate on force generation in single myofibrils from rabbit skeletal muscle.
abstract: In striated muscle, force generation and phosphate (P(i)) release are closely related. Alterations in the [P(i)] bathing skinned fibers have been used to probe key transitions of the mechanochemical coupling. Accuracy in this kind of studies is reduced, however, by diffusional barriers. A new perfusion technique is used to study the effect of [P(i)] in single or very thin bundles (1-3 microM in diameter; 5 degrees C) of rabbit psoas myofibrils. With this technique, it is possible to rapidly jump [P(i)] during contraction and observe the transient and steady-state effects on force of both an increase and a decrease in [P(i)]. Steady-state isometric force decreases linearly with an increase in log[P(i)] in the range 500 microM to 10 mM (slope -0.4/decade). Between 5 and 200 microM P(i), the slope of the relation is smaller ( approximately -0.07/decade). The rate constant of force development (k(TR)) increases with an increase in [P(i)] over the same concentration range. After rapid jumps in [P(i)], the kinetics of both the force decrease with an increase in [P(i)] (k(Pi(+))) and the force increase with a decrease in [P(i)] (k(Pi(-))) were measured. As observed in skinned fibers with caged P(i), k(Pi(+)) is about three to four times higher than k(TR), strongly dependent on final [P(i)], and scarcely modulated by the activation level. Unexpectedly, the kinetics of force increase after jumps from high to low [P(i)] is slower: k(Pi(-)) is indistinguishable from k(TR) measured at the same [P(i)] and has the same calcium sensitivity.
authors.affiliation: Dipartimento di Scienze Fisiologiche, Università degli Studi di Firenze, Italy. chiara.tesi@unifi.it
authors.forename: C
authors.initials: C
authors.lastname: Tesi
authors.forename: F
authors.initials: F
authors.lastname: Colomo
authors.forename: S
authors.initials: S
authors.lastname: Nencini
authors.forename: N
authors.initials: N
authors.lastname: Piroddi
authors.forename: C
authors.initials: C
authors.lastname: Poggesi
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000818:Animals; D066298:In Vitro Techniques; D007700:Kinetics; D009119:Muscle Contraction / Q000187:drug effects*; D018656:Muscle Fibers, Fast-Twitch / Q000187:drug effects / Q000502:physiology; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D009210:Myofibrils / Q000187:drug effects / Q000502:physiology*; D010710:Phosphates / Q000494:pharmacology*; D011817:Rabbits
chemical_list: D010710:Phosphates; D000244:Adenosine Diphosphate