Relaxation phosphorylation, myosin light

Figure 19-3 Mechanism of nitrovasodilators Nitric oxide (NO) formed in smooth muscle from nitrovasodilators or from endothelial cells (EDRF) activates guanylate cyclase (GC ). GC activates cGMP-dependent protein kinases that phosphorylate myosin light-chain kinase (MLCK), causing its inactivation and subsequent muscle relaxation (see also Fig. 19-2)...

Pharmacomechanical mechanisms for relaxation include (1) G kinase-dependent increases in the activity of sarcoplasmic reticulum Ca pumps (SERCA). Ca pumps on the plasma membrane may also be stimulated (not shown). This increase in Ca sequestration and extrusion decreases [Ca +Jj and induces relaxation as shown in the foregoing. (2) Some agents appear to decrease 1,4,5-1P3 formation and may relax smooth muscle by decrease Ca + release (not shown) (3) Finally, increases in [cAMP] activate cAMP-dependent protein kinase (A kinase), which could phosphorylate myosin light chain kinase and decrease its Ca sensitivity (this mechanism has not been demonstrated in intact smooth muscle). 

Barany M, Barany K (1993a) Dissociation of relaxation and myosin light chain phosphorylation in porcine uterine muscle. Arch Biochem Biophys 305 202-204 Barany M, Barany K (1993b) Calponin phosphorylation does not accompany contraction of various smooth muscles. Biochim Biophys Acta 1179 229-233 Barany K, Barany M (1996a) Myosin light chains. In Bdrany M (ed) Biochemistry of smooth muscle contraction. Academic Press, New York, pp 21-35 Barany K, Barany M (1996b) Protein phosphorylation during contraction and relaxation. In Barany M (ed) Biochemistry of smooth muscle contraction. Academic Press, New York, pp 321-339... 

In the presence of calcium, the primary contractile protein, myosin, is phosphorylated by the myosin light-chain kinase initiating the subsequent actin-activation of the myosin adenosine triphosphate activity and resulting in muscle contraction. Removal of calcium inactivates the kinase and allows the myosin light chain to dephosphorylate myosin which results in muscle relaxation. Therefore the general biochemical mechanism for the muscle contractile process is dependent on the avaUabUity of a sufficient intraceUular calcium concentration. 

If MLCK activates contraction by increasing myosin phosphorylation, then an increase in the activity of myosin light chain phosphatase, MLCP, by decreasing the fraction of myosin which is phosphorylated, should lead to relaxation from the active (contractile) state. Cyclic adenosine monophosphate (AMP) is a strong inhibitor of smooth muscle contraction and it has been suggested that activation of MLCP could result from its phosphorylation via cAMP activated protein kinase (see Figure 5). 

Hydralazine causes direct relaxation of arteriolar smooth muscle. The arteriolar vasodilatation produced by hydralazine requires an intact endothelium. Therefore, one proposed mechanism of action is that hydralazine liberates nitric oxide from the endothelium (similar to the nitrates), which in turn increases cGMP to ultimately prevent the phosphorylation of myosin light chain (which is required for smooth muscle contraction) resulting in arteriolar vasorelaxation. 

An enhancement of ATPase action comes through the phosphorylation of myosin light chains (MW 18,000). The phosphorylation is achieved because the high cellular [Ca2+] activates myosin kinase, an enzyme that contains calmodulin, a Ca2+-binding subunit. Phosphorylation of myosin is absolutely required for smooth muscle contraction, though not for the contraction of skeletal or cardiac muscle, because smooth muscle has no troponin. Thus, whereas contraction and relaxation in skeletal and cardiac muscle are achieved principally via the action of Ca2+ on troponin, in smooth muscle they must depend solely on the Ca2+-dependent phosphorylation of myosin. In skeletal and cardiac muscle, once the stimulus to the sarcolemma is removed, [Ca2+] in sarcoplasm drops rapidly back to 10 7 or 10 8 M via various Ca2+ pump mechanisms present in the sarcoplasmic reticulum, and tropomyosin can once again interfere with the myosin-actin interaction. 

Pj-selective agonists such as prenalterol (Figure 10.5) or dobutamine are used to stimulate heart contractility and heart rate in patients that exhibit too low blood pressure. P2-Selective agents such as terbutaline find application for relaxation and dilatation of the bronchi in the treatment of asthma or chronic obstmctive lung disease, and in the suppression of premature labour. The mechanism of P-receptor-induced relaxation of smooth muscle - cAMP-mediated phosphorylation and inhibition of myosin light chain kinase - has been described before. 

Figure 11.6. Mechanisms of smooth muscle relaxation induced by cyclic GMP and protein kinase G (PKG). a PKG phospho-lydates and thereby aetivates myosin light ehain phosphatase, b PKG also phosphorylates the IP3 reeeptor, which resnlts in reduced efflux of Ca from the endoplasmie reticulum. Ca is involved in smooth musele eell eontraetion via calmodulin and myosin light chain kinase.

It has been established for several years that the major mechanism for regulation of contraction in smooth muscle is myosin phosphorylation (Hart-shorne, 1987). Phosphorylation of the two 20,000-dalton light chains of myosin (LC20) activates the actin-dependent ATPase activity of myosin and this initiates the contractile response. Dephosphorylated myosin is associated with relaxed muscle. In this scheme there are two key enzymes the myosin light chain kinase (MLCK) and the myosin light chain phosphatase (MLCP). Obviously a balance of these two activities determines the level of myosin phosphorylation. 

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