The effect of inorganic phosphate on force generation in single myofibrils from rabbit skeletal muscle

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 mu M 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 mu M to 10 mM (slope -0.4/decade). Between 5 and 200 mu M P-i, the slope of the relation is smaller (similar to -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.