Figure 1. Average twitch force – extracellular [Ca²⁺]_o relationships.

Twitch stress (force divided by cross-sectional area) was measured in electrically stimulated right ventricular trabeculae isolated from wild-type (WT; closed circles; n=7), and giant titin homozygous mutant (HM; closed squares; n=8) rats at bath [Ca²⁺]_o ranging between 0.1 and 2.0 mM. Data were fit to a modified Hill equation (see also Table 1). Presence of the giant isoform in the HM muscles was associated with lower twitch tress (at all [Ca²⁺]_o). In contrast, neither the apparent sensitivity to extracellular Ca²⁺ (EC₅₀), nor the level of cooperativity (Hill coefficient) was affected by titin isoform (see Table 1). Systolic SL=2.2 μm; stimulus frequency=1 Hz; temperature 25 °C.