Figure 9.

Model for activation, potentiation and post-tetanic potentiation in tarantula striated muscle. (a) One helix of thick filament in relaxed state showing three adjacent interacting-head motifs (bare zone is at the top): most free heads (blue) are Ser35 constitutively mono-phosphorylated (yellow spheres) and interact with their partner blocked heads (green). These heads have relatively weak contacts with the thick filament and are therefore in equilibrium with a detached state that occasionally moves out (double arrows; “swaying” heads; light/dark blue). In a twitch, when Ca²⁺ concentration increases, detached swaying heads can attach to thin filaments producing single or summated twitch force (b), which reverses when Ca²⁺ concentration is lowered (a). If Ca²⁺ concentration remains high (c), free head Ser45 can become phosphorylated (red triangles) so that these heads are permanently released. Blocked head Ser45 can now be phosphorylated (red triangles) and released for actin interaction, along with their partner free heads (see text). These released blocked and free heads lead to force potentiation, the level depending on the number of heads released (2 levels of potentiation ratios, i.e. the actual number vs. the initial number of heads producing force, are shown 4:3 and 6:3). If Ca²⁺ concentration decreases, relaxation is achieved (d) by switching off of the thin filaments. If Ca²⁺ concentration increases again without a long delay (e), the recruited blocked heads, as well as swaying heads, will be immediately available to produce high twitch force (post-tetanic potentiation). When Ca²⁺ concentration remains low long enough to switch off MLCK, MLCP would dephosphorylate Ser45-phosphorylated heads, returning to relaxation (a).