Figure 4. [P_i] dependence of tension, stiffness and initial tension recovery.

A, the mean (± s.e.m.) steady tension (•) and the mean (± s.e.m.) stiffness (○) recorded at different levels of [P_i] are shown. Data are from five fibres in each of which data were collected at each level of [P_i]. Stiffness was determined as the tension change (i.e. as ΔT = T₁ - T₀) to a standard stretch (0.4-0.5 % L₀)) and both the steady tension and the stiffness are plotted as ratios of that in the control solution (no added P_i). The abscissa is the [P_i] within active fibres calculated as given in Methods (see Coupland et al. 2001). A hyperbolic curve was fitted to each set of data. Note that both stiffness and tension are depressed with added P_i, but the tension depression is greater than the stiffness change. Consequently, the average stiffness/tension ratio (ΔT/T₀) increases with added [P_i], as shown by the (×) symbols and the dotted line. The average ratio for 25 mm [P_i] is 1.34, which compares with the ratio of the T₁ slopes (1.33) given in Fig. 3. B, the initial recovery of tension (the decay in stretches and rise in releases, between T₁ and T₂) was analysed by fitting a bi-exponential curve (see Fig. 3A). The two rate constants (for phases 2a and 2b) for different [P_i] are shown as means (± s.e.m.) where, rate 2a (□, ○ - upper ordinate) and rate 2b (▪, • - lower ordinate) are plotted separately for stretches (□, ▪) and releases (○, •). For clarity, plus or minus error bars are not shown for some data points. The slower phase 2b rate (▪, continuous curve) from stretches (only) is correlated with [P_i] and the hyperbolic curve gives a maximum rate of 129 s⁻¹, minimum of 13 s⁻¹ and half-maximum [P_i] of 11 mm (continuous curve through the points, r = 0.77). Others show no correlation with [P_i] (dotted and dashed curves). (The hyperbolic relation was also found for reciprocal half-time data from stretches; it corresponded to a maximum rate of 760 s⁻¹, a minimum rate of 99 s⁻¹ and a half-maximal [P_i] of 11 mm. The data from releases showed no correlation with [P_i] (P > 0.05)). C, the amplitudes of the two exponential components for stretches (□, ▪) and releases (○, •) are plotted separately against [P_i]; each amplitude is plotted as a ratio of steady force (T₀). Both amplitudes of release and of phase 2b from stretches show no correlation (dotted and dashed lines, P > 0.05), but the amplitude of phase 2a from stretches is positively correlated (□, continuous line, P < 0.05).