The high-force region of the force-velocity relation in frog skinned muscle fibres

The force-velocity relation has been studied during calcium-induced contracture of chemically skinned fibres from the semitendinosus muscle of Rana temporaria with special interest focused on the high-load region. The force-velocity curve was hyperbolic at low and intermediate loads but departed below the hyperbola as the load exceeded about 80% of the isometric force (P_o). The force intercept (P*_o) of the hyperbola derived from force-velocity data truncated at 0.78 P_o was higher than P_o (P*_o /P_o = 1.14±0.04). At submaximum Ca²⁺ concentration, where the isometric force of the fibre was 65–75% of the maximum value, the force-velocity data still departed below the hyperbola at high loads (P_o /P_o = 1.09±0.04). The departure of the force-velocity data from the hyperbola at high force was also found at high ionic strength (250 mM), but not at low ionic strength (150 mM) (P_o /P_o = 1.09±0.03 and 0.98±0.03, respectively). The force-velocity relations derived under different experimental conditions could be fitted well by a modified version of Hill's (1938) hyperbolic equation (Edman 1988) using similar numerical values of k₁ and k₂ in the latter equation. The results indicate that the force-velocity relation in skinned muscle fibres is biphasic, and that the two curvatures, as in intact muscle fibres, are closely related to one another. Furthermore the evidence supports the hypothesis that the altered shape of the force-velocity relation at high loads is not related to the force level per se but rather to the speed of shortening of the contractile system (Edman 1992).