Force enhancement by PEG during ramp stretches of skeletal muscle

We have investigated the effects of a stronger actomyosin bond on force ( P-s) during rapid stretch of active permeabilized rabbit psoas muscle fibers as a function of temperature from 5 to 30degreesC. The strength of the actomyosin bond is enhanced by addition of polyethylene glycol ( PEG), especially in pre- powerstroke states [ Chinn et al. ( 2000) Biophys J 49: 437 - 451]. We have hypothesized that such states produce much of the force when activated muscles are stretched [ Getz et al. ( 1998) Biophys J 75: 2971 - 2983]. Addition of PEG to activated fibers produced a small increase in isometric tension, P-o ( 50 - 90 kN/ m(2)), which was approximately independent of temperature. In contrast PEG produced a dramatic increase in P-s at low temperatures ( 200 - 300 kN/ m(2)), but a modest increase at higher temperatures ( 70 - 90 kN/ m(2)). We also measured P-s and P-o in solutions containing the phosphate analog aluminum fluoride ( AlF4,) with and without PEG. In the absence of PEG, AlF4 reduced P-o much more than P-s. Addition of PEG did not enhance P-o, but enhanced P-s significantly. The contrasting effects of PEG on P-s and P-o, and the effect of temperature can be explained by a model in which stretch force is produced by prepowerstroke cross- bridges whose maximum distension is increased by PEG, and isometric force is produced by strongly bound cross- bridges whose bond strength is also enhanced by PEG, but to a lesser extent.