Second messenger effects on the myosin phosphorylation system in smooth muscle.

It has been shown that with initial and tonic contractions stimulated by a cholinergic muscarinic agonist in tracheal smooth muscle, the extent of myosin heavy chain phosphorylation remains low. Cholinergic stimulation of tracheal smooth muscle results in formation of both monophosphorylated and diphosphorylated myosin light chain, although the amount of diphosphorylated light chain is substantially less than monophosphorylated light chain. Phosphorylation of the single serine site on myosin light chain by myosin light chain kinase is the primary phosphorylation that is associated with activation of smooth muscle contraction. A general scheme for the physiological regulation of smooth muscle contractility can be proposed. Upon activation of cell surface receptors by neurotransmitters or hormones, phosphoinositide metabolism is stimulated to form InsP3. The InsP3 rapidly releases Ca2+ from sarcoplasmic reticulum which then binds to calmodulin. The Ca2+/calmodulin complex binds to myosin light chain kinase which then phosphorylates myosin light chain. It appears that the rate of conversion of myosin light chain kinase from an inactive to an active enzyme may be a significant rate limiting step for the initiation of myosin light chain phosphorylation. Once the kinase is activated, phosphorylation of myosin light chain may reach maximal values within 2 s with neural stimulation, followed by the slower rate of force development. Protein kinase C does not phosphorylate myosin light chain or heavy chain during the initial or tonic phases of contraction. However, this general scheme for smooth muscle contraction does not exclude the possibility of other regulatory processes involved in sustained contractions.