As in other muscle types, calcium ions are the trigger for the activation of contraction. Depolarization opens calcium channels, allowing calcium ions to enter the cell, and these may in turn release further calcium by activating calcium-release channels in the sarcoplasmic reticulum. An alternative route

Fig. 11.9. A contractile unit of smooth muscle, showing how the filaments could slide past each other during contraction. From Squire (1986).

for calcium release is via the production of inositoltrisphosphate (IPj) as a result of activation of G-protein-coupled receptors. The IP3 combines with IP3 receptors in the sarcoplasmic reticulum membrane and so releases calcium ions into the cytoplasm.

The action of calcium ions in smooth muscle differs from that in skeletal and heart muscle. There is no troponin in the thin filaments, and the principal action of calcium is to combine with calmodulin to activate the enzyme myosin light chain kinase, which then phosphorylates one of the light chains in the myosin molecule, MLC20. This phosphorylation of MLC20 is the necessary trigger for formation of cross-bridges, the splitting of ATP, and contraction (Fig. 11.8).

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