Reversal of the cross‐bridge force‐generating transition by photogeneration of phosphate in rabbit psoas muscle fibres.

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Abstract
1. Orthophosphate (P(i), 0.1‐2.0 mM) was photogenerated within the filament lattice of isometrically contracting glycerinated fibres of rabbit psoas muscle at 10 and 20 degrees C. The P(i) was produced by laser flash photolysis of the photolabile compound 1‐(2‐nitrophenyl)ethylphosphate (caged P(i)). Caged P(i) caused a depression of tension that was much smaller than that caused by P(i). 2. Photolysis of caged P(i) produced a decline in isometric force composed of four phases: phase I, a lag phase (e.g. 1‐4 ms at 10 degrees C) during which force did not change; phase II, an exponential decline by as much as 20% of the pre‐pulse force; phase III, a partial force recovery (0‐3% of the pre‐pulse force); and phase IV, a further slow (0.5‐3 s) decline to the steady value. Phases I, III and IV were largely independent of [P(i)] and are likely to be indirect effects caused by the caged P(i) photolysis. 3. Both the rate and amplitude of phase II depended markedly on [P(i)]. The amplitude of phase II was similar to the reduction of steady‐state force by P(i). The rate of phase II increased with increasing temperature and [P(i)]. At high [P(i)] the rate began to saturate, and approached limits of 123 s‐1 at 10 degrees C and 194 s‐1 at 20 degrees C. 4. The rate of phase II was independent of sarcomere overlap, while the amplitude was proportional to tension at partial filament overlap. A control experiment using caged ATP showed that phase II was not produced by the photolytic by‐products or the light pulse. The results suggest that phase II is associated with the force‐generating transition of the cross‐bridge cycle. 5. Sinusoidal length oscillations at 0.5 and 2 kHz were used to measure muscle stiffness during phase II. Stiffness declined in a single exponential phase, with the same time course as phase II of the tension transient. The change in stiffness was 83 +/‐ 6% (mean +/‐ S.E.M., n = 10, 0.5 kHz) of the change in tension when both signals were normalized to their pre‐flash values. 6. Analysis of the data shows that two steps are involved in force generation and P(i) release. The non‐force exerting AM‐ADP‐P(i) cross‐bridge state first isomerizes to form a force‐exerting cross‐bridge state (AM'‐ADP‐P(i)). P(i) is then released to form a second force‐generating state, AM'‐ADP.(ABSTRACT TRUNCATED AT 400 WORDS)