Figure 1. Effects of P_i addition in myofibrils from rabbit fast and slow skeletal muscle.

Left, psoas muscle myofibrils (A-C); right, soleus muscle myofibrils (D-F). A, psoas myofibril 15 °C: maximal isometric force in the presence of 5 mm P_i is 0.49 of the value in control P_i-free conditions. In P_i-free and 5 mm P_i conditions, kactiv was 6.0 s⁻¹ and 11 s⁻¹, respectively. B, psoas myofibril at 5 °C: k_activ was slowed down considerably (1.5 s⁻¹); 5 mm P_i dropped force to 0.48 of the P_i-free value and increased k_activ to 4.3 s⁻¹. C, psoas myofibril at 5 °C subjected to a ‘P_i-jump’ protocol from P_i-free to 5 mm P_i (arrows mark the time of the solution change associated with the change in [P_i] from P_i-free to 5 mm and reverse). Isometric force dropped to 0.35 of P_i-free value. k_activ in P_i-free conditions: 1.4 s⁻¹; k_Pi(+), 25.7 s⁻¹; k_Pi(-), 1.7 s⁻¹. D, soleus myofibril at 20 °C: 5 mm P_i dropped force to 0.74 of P_i-free value; k_activ in P_i-free and 5 mm P_i were 3.3 s⁻¹ and 3.4 s⁻¹, respectively. E, soleus myofibril at 15 °C: force is decreased to 0.82 of P_i-free value; k_activ, in both P_i-free and 5 mm P_i was 1.7 s⁻¹. F, soleus myofibril at 15 °C subjected to a ‘P_i -jump’ protocol from P_i -free to 50 mm P_i. Isometric force dropped to 0.50 of P_i-free value. k_activ in P_i-free conditions was 1.9 s⁻¹; k_Pi(+), 6.3 s⁻¹; k_Pi(-), 1.7 s⁻¹.