Figure 3. Effect of changing [Ca²⁺] on contraction in permeabilized muscle.

Permeabilized muscle fibre preparations were used, allowing intracellular [Ca²⁺] to be controlled simply by changing the extracellular [Ca²⁺]. The otherwise intact fibres developed active tension when [Ca²⁺] exceeded 100 nm, with maximal contraction above 1 μm. Interestingly, the relationships in striated and smooth muscles were almost identical. Reproduced with permission from Filo et al. (1965).