The Contractile Event of Smooth Muscle

A scheme for smooth muscle contraction is shown in Fig. SM11. Contraction is initiated by the increase of Ca2+ in the myoplasm; this happens in the following ways:

1. Ca2+ may enter from the extracellular fluid through channels in the plasmalemma. These channels open, when the muscle is electrically stimulated or the plasmalemma is depolarized by excess K+.

2. Due to agonist induced receptor activation, Ca2+ may be released from the sarcoplasmic reticulum (SR). In this pathway, the activated receptor interacts with a G-protein (G) which in turn activates phospholipase C (PLC). The activated PLC hydrolyzes phosphatidyl inositol bisphosphate; one product of the hydrolysis is inositol 1,4,5-trisphosphate (IP3). IP3 binds to its receptor on the surface of SR, this opens Ca2+ channels and Ca2+ from SR is entering the myoplasm.

3. Ca2+ combines with calmodulin (CaM) and the Ca2+ -CaM complex activates MLCK, which in turn phosphorylates LC. The phosphorylated myosin filament combines with the actin filament and the muscle contracts.

scheme for smooth muscle contraction. Barany, 1996, with permission from Biochemistry of Smooth Muscle Contraction, 1996, Academic Press).

scheme for smooth muscle contraction. Barany, 1996, with permission from Biochemistry of Smooth Muscle Contraction, 1996, Academic Press).

Two books (Barany, 1996; Kao and Carsten, 1997) and a special journal issue (Murphy, 1999) are recommended for further studying the mechanism of smooth muscle contraction.and relaxation.

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