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The force-velocity relation was determined in fully activated skinned fibres from frog muscle at concentrations of the substrate, magnesium adenosine triphosphate (MgATP), ranging from 10 microM to 10 mM. The ionic strength of the solutions was 200 mM, temperature 0-5 degrees C, pH 7.1. The activation procedure of Moisescu (1976) was used to raise the calcium concentration rapidly in the interior of the fibres. A re-phosphorylating system (creatine kinase and creatine phosphate) was used to maintain the MgATP concentration in the fibres. Isotonic releases were performed using a fast servo-controlled motor and tension transducer. Releases to a pre-determined tension level relative to the isometric tension were made using a novel normalizing circuit. In some of the experiments changes of sarcomere length were recorded using the diffraction device described in the preceding paper (Goldman & Simmons, 1984). There was satisfactory agreement between velocities determined from the total length change and the sarcomere length change. The isometric tension showed a biphasic dependence on MgATP concentration. Tension increased with MgATP concentration from 1 microM to reach a peak at about 30-100 microM and decreased by about 20% from the value at the peak with further increase in the MgATP concentration to 5 mM (about the physiological concentration). At 5 mM-MgATP, the isometric tension was approximately the same as in intact fibres, if allowance is made for the increase in cross-sectional area that occurs when the surface membrane is removed. The maximum velocity of shortening, Vmax, was obtained by fitting the force-velocity relation using Hill's (1938) equation. Vmax showed a roughly hyperbolic dependence on MgATP concentration, with a Km of 0.47 mM. At 5 mM-MgATP, the value of Vmax was 2.16 muscle lengths per second, which is similar to that of intact fibres. a/P0, the parameter of Hill's (1938) equation that is related to the curvature of the force-velocity relation, showed a slight decrease with increasing MgATP concentration. Its value at 5 mM-MgATP of 0.16 is somewhat lower than found for intact fibres. The results are discussed in terms of a simple model based on the biochemical cycle of hydrolysis of ATP by actomyosin in solution. The decrease of tension from about 30 microM to higher concentrations of MgATP can be related to the dissociating effect of MgATP on actomyosin. The increase of isometric tension from 1 to 30 microM-MgATP is discussed in terms of two types of rigor attachment of cross-bridges which support different amounts of tension.(ABSTRACT TRUNCATED AT 400 WORDS)
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