270 K. W. Ranatunga and others J Physiol 585.1 The fact that, in the shortening limb of the F–V relation (F–V s relation) in Fig. 4C, the force decreases to < 0.5 P0 as the shortening velocity is increased to ∼0.1 L0 s−1 suggests a high curvature for the F–V s relation, since V 0 is around 1 L0 s−1. The temperature-dependence studies on intact mammalian fast and slow muscles, indeed, showed that the curvature of the F–V s relation would become higher (a/P0 ratio lower) as the temperature is reduced from 30–35 to 10◦C (Ranatunga, 1984). Therefore, in experiments on two ﬁbres at 8–9◦C, we obtained data for a fuller F–V s relation and they are shown in Fig. 4D; the curve drawn is a ﬁt to a Hill (1938) hyperbolic equation (P/P0 = a(V max − V )/(V + b), where P is force, V is velocity and a and b are constants) with an a/P0 ratio of 0.05; the low ratio indicates a high curvature. Such a high curvature, as obtained here, is also evident in the data reported for skinned fast (type 2) ﬁbres at 12◦C, in other previous studies (a/P0 ratios of 0.05–0.07, Bottinelli et al. 1996; Linari et al. 2003a). The data in Fig. 4D conﬁrm that, at 8–9◦C, the ﬁbre force declines to < 0.5 P0 at shortening velocities of around 0.1 L0 s−1 and the curvature of the F–V s relation is high; the high curvature would also account for the variability in the estimates for V max obtained in studies on mammalian muscle ﬁbres at the low temperatures. T-jump tension response during ramp shortening (i) The amplitude of tension rise. A constancy of the amplitude of the tension response to a standard T-jump Figure 5. Characteristics of the T-jump tension response during shortening Data during shortening in Fig. 3A are re-analysed; shortening velocity is plotted as L0 s−1 on the abscissa. Open symbols represent individual data (n = 57) and the ﬁlled symbols, the mean (± S.E.M., n = 7 or 8 per symbol). The amplitude of tension rise after a T-jump, when normalized to the pre-T-jump isometric tetanic tension, is not signiﬁcantly correlated with velocity (P > 0.05); the mean is shown by the dashed line (0.24 P0). The mean (S.E.M., n = 30) amplitude for isometric state, represented in the same manner, is plotted at zero velocity (0.17 P0). was observed when isometric force was changed by other means in previous experiments (see Discussion). Thus, in Fig. 5, the amplitude of T-jump tension rise is plotted as a ratio of the pre-shortening isometric tension of each ﬁbre; the data show some scatter and they are not correlated with shortening velocity, with a mean of ∼25% of isometric force. Thus, the increase in the amplitude obtained in Fig. 3A is due to the shortening velocity-dependent change (decrease) of the post-T-jump force and, in a given ﬁbre, a standard T-jump induces the same, ‘absolute’, increment of force at different shortening velocities. (ii) At different times, during approach to the steady state. The above results show that, at steady state, the tension during a ramp shortening is particularly sensitive to a T-jump (Fig. 3A). Figure 6A shows superimposed tension records from an experiment in which a standard T-jump was applied at different times after the onset of the same ramp shortening; a tension record to the same T-jump without shortening (isometric) is also shown. It seems that a T-jump does not induce a marked tension rise, when applied at the beginning of the ramp shortening and that the effect of a T-jump becomes prominent when it is delayed and as the pre-T-jump tension reaches a new steady level during shortening. Figure 6B shows a simple analysis made from two such experiments where the top panel shows the decrease of (pre-T-jump) tension as the ramp shortening progresses with time; the tension approaches a steady level at ∼500 ms after the beginning of the ramp. The lower panel shows that the post-T-jump (peak) tension increment is small at the beginning of a ramp but increases to a steady maximum of 20–25% when the T-jump is delayed to 500 ms. Given that T-jump produces a clear tension increment in isometric state, the above data indicate the interactive effect of the tension changes during T-jump and ramp shortening. Thus, as shown in Fig. 6C, when the tension change due to shortening is subtracted, the difference tension traces suggest that a T-jump induces a similar tension rise at different times during shortening; to what extent this behaviour is dominated by series compliance remains uncertain. (iii) At different temperatures. In the above experiments, the T-jump tension responses during shortening were examined at around 10◦C. In mammalian muscle, the isometric tension at this temperature is ∼50% of that at the body temperatures (> 30◦C) and the relative effect of a standard T-jump on isometric tension decreases as the temperature is raised (Ranatunga, 1996; Coupland & Ranatunga, 2003); moreover, the F–V s relation is temperature sensitive so that V max increases with a Q10 of > 2 and the curvature decreases (Ranatunga, 1984) in warming. Therefore, in four experiments, we examined the C 2007 The Authors. Journal compilation C 2007 The Physiological Society