Figure 4. Myofilament length-dependent activation.

A, steady state force development increased sigmoidally as a function of activating [Ca²⁺] at both short (SL = 2.2 μm; open circles) and long (SL = 2.4 μm; filled circles) sarcomere length (SL). Increased SL induced a leftward and upward shift of the relationship; maximum calcium saturated force was ∼10% higher at the longer SL. B, increased SL was associated with increased myofilament calcium sensitivity (indexed by pCa₅₀, −log [Ca²⁺]_i) at 50% activation; P < 0.05), but no change in the level of cooperative activation (indexed by the Hill coefficient). These data indicate robust myofilament length-dependent activation in the adult zebrafish cardiac sarcomere (n = 17).