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 μM in diameter; 5°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 μM to 10 mM (slope −0.4/decade). Between 5 and 200 μM P i, the slope of the relation is smaller (∼ −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.