Biophys J, June 2000, p. 3081-3092, Vol. 78, No. 6

The Effect of Inorganic Phosphate on Force Generation in Single Myofibrils from Rabbit Skeletal Muscle

Dipartimento di Scienze Fisiologiche, Università degli Studi di Firenze, I-50134 Firenze, Italy

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.

Biophys J, June 2000, p. 3081-3092, Vol. 78, No. 6
© 2000 by the Biophysical Society   0006-3495/00/06/3081/12  $2.00

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