Phosphate has dual roles in cross-bridge kinetics in rabbit psoas single myofibrils

In this study, we aimed to study the role of inorganic phosphate (P-i) in the production of oscillatory work and cross-bridge (CB) kinetics of striated muscle. We applied small-amplitude sinusoidal length oscillations to rabbit psoas single myofibrils and muscle fibers, and the resulting force responses were analyzed during maximal Ca2+ activation (pCa 4.65) at 15 degrees C. Three exponential processes, A, B, and C, were identified from the tension transients, which were studied as functions of P-i concentration ([P-i]). In myofibrils, we found that process C, corresponding to phase 2 of step analysis during isometric contraction, is almost a perfect single exponential function compared with skinned fibers, which exhibit distributed rate constants, as described previously. The [P-i] dependence of the apparent rate constants 2 pi b and 2 pi c, and that of isometric tension, was studied to characterize the force generation and P-i release steps in the CB cycle, as well as the inhibitory effect of P-i. In contrast to skinned fibers, P-i does not accumulate in the core of myofibrils, allowing sinusoidal analysis to be performed nearly at [P-i] = 0. Process B disappeared as [P-i] approached 0 mM in myofibrils, indicating the significance of the role of P-i rebinding to CBs in the production of oscillatory work (process B). Our results also suggest that P-i competitively inhibits ATP binding to CBs, with an inhibitory dissociation constant of -2.6 mM. Finally, we found that the sinusoidal waveform of tension is mostly distorted by second harmonics and that this distortion is closely correlated with production of oscillatory work, indicating that the mechanism of generating force is intrinsically nonlinear. A nonlinear force generation mechanism suggests that the length-dependent intrinsic rate constant is asymmetric upon stretch and release and that there may be a ratchet mechanism involved in the CB cycle.

Automated summary

This study aimed to investigate the role of inorganic phosphate (P-i) in the production of oscillatory work and cross-bridge kinetics in striated muscle. The research found that P-i competitively inhibits ATP binding to cross-bridges and rebinds to affect the production of oscillatory work. Additionally, the study showed that the sinusoidal waveform of tension is distorted by second harmonics, indicating a nonlinear force generation mechanism in muscle fibers.