FIGURE 3.

Equivalent myofilament compliance. (A) Superimposed length change per half-sarcomere (upper traces) and force response (lower traces) for steps of different size in a fiber activated at two pCas. (Upper) pCa 4.50; (lower) pCa 5.42. The horizontal line in each panel marks zero force. (B) T₁ relations for three pCas. The relations are obtained by plotting the extreme tension attained at the end of the length step, T₁ (relative to T₀ at pCa = 4.50, T_0,s), versus the step amplitude. Different symbols refer to different pCa values, as indicated in the inset. The lines are linear regressions fitted to the experimental points for each pCa. Fiber segment length, 3.88 mm; segment length under the striation follower, 1.08 mm; average sarcomere length, 2.58 μm; CSA, 4600 μm²; temperature, 12.1°C. (C) Relation of strain per half-sarcomere (Y₀) against the isometric force at different pCa. Solid circles are the mean values (± SE) from five fibers grouped in classes of forces (from Table 2). The solid line is the linear regression on the pooled data (open circles).