An asymmetry in the phosphate dependence of tension transients induced by length perturbation in mammalian (rabbit psoas) muscle fibres

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Abstract
The effects of inorganic phosphate (Pi, a product released during ATP hydrolysis in active muscle) on tension transients induced by length perturbation (∼0.3 ms) were examined in chemically skinned (0.5 % Brij), maximally Ca2+‐activated rabbit psoas muscle fibres at 10 °C (ionic strength 200 mm, pH 7.1). In one type of experiment, the tension transients induced by length release and stretch of a standard amplitude (0.4‐0.5 % of Lo, muscle fibre length) were examined at a range of added [Pi] (range 3–25 mm). The steady active tension was depressed ∼45 % with 25 mm added Pi. The initial tension recovery (from T1, extreme tension reached after length step, to T2, tension after quick recovery) was analysed by half‐time measurement and also by exponential curve fitting ‐ extracting a fast (phase 2a) and a slow (phase 2b) component. The tension decay after a stretch became faster with increased [Pi], whereas the quick tension rise induced by a length release was insensitive to added Pi. Consequently, the asymmetry in the speed of tension recovery from stretch and release was reduced at high [Pi]. A plot of the phase 2b rate (or 1/half‐time) of tension decay after stretch versus[Pi] was approximately hyperbolic and showed saturation at higher [Pi] levels. In a second type of experiment, the tension transients induced by length steps of different amplitudes were examined in control (no added Pi) and in the presence of 25 mm added Pi. Over a range of length step amplitudes (up to 1 % L0), the tension decay after stretch was consistently faster in the presence of Pi than in the control; this was particularly pronounced in phase 2b. The rate of tension rise after length release remained high but similar in the presence and absence of added Pi. These observations indicate that a stretch and release perturb different molecular steps in the crossbridge cycle. The Pi sensitivity of tension decay (phase 2b) after stretch is similar to that seen using other perturbations (e.g. [Pi] jumps, hydrostatic pressure jumps and temperature jumps and sinusoidal length oscillations). The results indicate that the Pi‐sensitive force generation identified in previous studies is strain sensitive (as expected), but it is seen only with respect to positive strain (stretches).