Figure 4.

(A and B) Impact of reducing F_max on the calcium sensitivity (EC₅₀) (A) and cooperative activation (n) (B) of isolated myocardial tissue. As demonstrated, the use of blebbistatin at short (green) and long (red) SLs reduced F_max while having no significant impact on either the EC₅₀ (A) or the Hill coefficient (B), as illustrated by the shallow slopes. The use of DM-cTnC at long sarcomere lengths (gray) mimics the impact of blebbistatin, indicating that the loss of thin-filament activity at long SLs did not have a great impact on either the calcium sensitivity or the cooperativity; meanwhile, at short SLs (blue), the impact of DM-cTnC exchange on calcium sensitivity (A) and cooperative activation (Hill coefficient) (B) was severe, causing a significantly larger slope (p < 0.05) in both cases than that observed in any of the other three groups. Error bars represent the mean ±SE.