The role of orthophosphate in crossbridge kinetics in chemically skinned rabbit psoas fibres as detected with sinusoidal and step length alterations.

The role of orthophosphate (Pi) ions in crossbridge kinetics was investigated in chemically skinned rabbit psoas fibres in the presence of saturating Ca2+. The muscle length was altered sinusoidally, and the resulting tension time courses were analysed in terms of three exponential processes (A), (B) and (C). Experiments were also performed with step length changes, and the resulting tension transients were correlated with the results of sinusoidal analysis. It was shown that addition of a low millimolar concentration of Pi increased both the rate constant and magnitude of process (B), which resulted in a dramatic increase in the oscillatory power output. The Pi effect was greater at higher oscillation amplitude and at higher MgATP concentration. At 5 mM MgATP, the amplitude effect became saturated at a 6 nm length change per crossbridge, whereas the Pi effect did not become saturated in the concentration range tested (0-16 mM). An introduction of MgADP to the activating saline resulted in a decrease of all rate constants, and these effects were opposite to MgATP. The effect of Pi resembled neither MgADP nor MgATP. Based on these observations, all the crossbridge reactions except for one (ADP desorption reaction) were eliminated as the possible site of action of Pi ions, supposing that Pi affects only one specific site in the crossbridge cycle. Other mechanisms, which might account for the Pi effects, are the presence of parallel hydrolysis pathways and the presence of multiple sites of action of the Pi ions.