Figure 7.

Increase in calcium sensitivity in intact muscle normalized tension–pCa relationships deduced from the twitch simulations versus those observed in intact and demembranated muscles. MUSICO simulations of the tension–pCa relationships matched the observations in intact muscles by Gao et al. (1994) and Janssen et al. (2002). The muscle tension is normalized by maximum tension at fully activated muscle (T/Tmax). The parameters for MUSICO simulations are the same as in twitch simulations at 22.5°C shown in Fig. 6. However, the sensitivity in intact muscles was significantly higher than in demembranated (Demem.) muscles. The experiments performed on the same muscle before and after demembranation showed a somewhat smaller increase in the muscle sensitivity (gray arrow) than between Kreutziger et al. (2011) demembranated muscle (red arrow). Interestingly, the intact muscle tension–pCa relationships show only small differences between the Gao et al. (1994) and Janssen et al. (2002) observations. Change in [Mg²⁺] from 1.2 mM (gray dashed line) to the physiological value of 0.5 mM increased calcium sensitivity (green dotted line) toward that observed in intact trabeculae (green arrows; Gao et al., 1994).