Calmodulin smooth muscle contraction

Another mechanism in initiating the contraction is agonist-induced contraction. It results from the hydrolysis of membrane phosphatidylinositol and the formation of inositol triphosphate (IP3)- IP3 in turn triggers the release of intracellular calcium from the sarcoplasmic reticulum and the influx of more extracellular calcium. The third mechanism in triggering the smooth muscle contraction is the increase of calcium influx through the receptor-operated channels. The increased cytosolic calcium enhances the binding to the protein, calmodulin [73298-54-1]. 

Smooth muscle contractions are subject to the actions of hormones and related agents. As shown in Figure 17.32, binding of the hormone epinephrine to smooth muscle receptors activates an intracellular adenylyl cyclase reaction that produces cyclic AMP (cAMP). The cAMP serves to activate a protein kinase that phosphorylates the myosin light chain kinase. The phosphorylated MLCK has a lower affinity for the Ca -calmodulin complex and thus is physiologically inactive. Reversal of this inactivation occurs via myosin light chain kinase phosphatase. 

Although in in vivo circumstances an intracellular free calcium increase apparently acts as the primary modulator of contraction, it can be bypassed in highly permeabilized smooth muscle preparations where the active subunit of MLCK can be introduced to phosphorylate myosin and induce contraction. The MLCK catalyzed phosphorylation of serine-19 is seen as the necessary event in the activation of smooth muscle myosin to form crossbridges. Thus, the rising phase of force during an isometric smooth muscle contraction follows an increase in the degree of phosphorylation of myosin, and that in turn follows the transient rise of (a) cytosolic free Ca, (b) Ca-calmodulin complexes, and (c) the active form of MLCK. The regulation of the intracellular calcium is discussed below. The dynam-... 

In addition to the displacement of caldesmon, smooth muscle cell contraction requires kinase-induced phosphorylation of myosin. Smooth muscle has a unique type of myosin filament called p-light chains which are the target (substrate) for MLCK, but MLCK is only active when complexed with CaCM. Myosin light chain phosphatase reverses the PKA-mediated process and when cytosolic calcium ion concentration falls, CDM is released from CaCM and re-associates with the actin. The central role of calcium-calmodulin in smooth muscle contraction is shown in Figure 7.4. 

Many of the biochemical and molecular events that are responsible for uterine smooth muscle contraction are the same as those that control other smooth muscle tissues (Fig. 62.1). Once uterine smooth muscle sensitivity has been augmented, actin and myosin must interact for contraction to occur. This interaction depends on the phosphorylation of the contractile proteins by the enzyme myosin light chain kinase (MLCK). This enzyme requires Ca++ and is active only when associated with calmodulin. Activation of the entire muscle contraction... 

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. 

As shown in Figure III-5-2, smooth muscle contraction is triggered by the influx of Ca2+ through voltage-regulated membrane channels. Ca2+ combines with calmodulin, and the complex activates myosin LC kinase, the active form of which (MLCK ) phosphorylates myosin light chains, enabling the interaction between myosin and actin. 

Caldesmon is a cytoplasmic protein with two isoform classes, one of which is found predominantly in smooth muscle cells and other cell types with partial myogenic differentiation. High-molecular-weight isoforms with molecular weights between 89 and 93 kD are capable of binding to actin, tropomyosin, calmodulin, myosin, and phospholipids, and they function to counteract actin-tropomyosin-activated myosin adenosine triphosphatase (ATPase). As such, they are mediators for the inhibition of calcium-dependent smooth muscle contraction." ... 

It has not yet been determined what this putative CCB receptor is, nor even what the full characterization of its properties are. The widespread calcium-dependent protein calmodulin has been proposed as a candidate because of its role in smooth muscle contraction and other factors. However, the evidence for this proposal so far is not established. 

Current studies suggest a vital role for CaD in the regulation of smooth muscle contraction since it inhibits the actin-activated ATPase activity of myosin (Da-browska et al., 1985 Smith and Marston, 1985 Moody et al., 1985 Sobue et al., 1985) and its subfragments (Lash et al., 1986 Chalovich et al., 1987). However, defining the precise role of CaD in cells requires an understanding of its interaction with several key proteins, namely, actin, myosin, calmodulin, and caltropin. 

MLCK plays a central role in the initiation of smooth muscle contraction and many nonmuscle motile processes owing to its Ca2+/calmodulin-dependent phosphorylation of myosin RLC. Physiological experiments with rapid, synchronous activation of smooth muscle cells demonstrate that the significant 500-ms latency for RLC phosphorylation and force development is due to the time required for increases in cytosolic Ca + concentration and activation of MLCK. Once activated, MLCK rapidly (1 s i) phosphorylates RLC in a random mechanism leading to the rapid attachment of cross-bridges and subsequent force development. Although the general properties of this cellu-... 

Both proteins bind to actin and calmodulin and have the capacity, in vitro at least, to inhibit the Mg-ATPase of smooth muscle actomyosin in a caldum-reg-ulated manner. This suggests that in smooth muscle also there is a mechanism for the regulation of modulation of contractile activity involving the thin filaments. It is clear that despite the enormous advances in knowledge there is much to be learned about the mechanism of regulation, and areas of controversy still exist. For this reason alone, a treatise such as Biochemistry of Smooth Muscle Contraction with detailed contributions by international experts is a very timely and important contribution to the literature. 

Asano M, Stull JT (1985) Effects of calmodulin antagonists on smooth muscle contraction and myosin phosphorylation. In Hidaka H, Hartshome DJ (eds) Calmodulin antagonists and cellular physiology. Orlando pp225-260... 

Just I, Selzer J, Wilm M, von Eichel-Streiber C, Mann M, Aktories K (1995) Glucosylation of Rho proteins by Clostridium difficile toxin B. Nature 375 500-503 Kamm KE, Stull JT (1985) The function of myosin and myosin light chain kinase phosphorylation in smooth muscle. Ann Rev Pharmacol Toxicol 25 593-620 Kamm KE, Stull JT (1986) Activation of smooth muscle contraction relation between myosin phosphorylation and stifftiess. Science 232 80-82 Kanamori M, Naka M, Asano M, Hidaka H (1981) Effects of N-(6-aminohexyl)-5-chloro-l-naphtalene ulfonamide and other calmodulin antagonists (calmodulin interacting scents) on calcium-induced contraction of rabbit aortic strips. J Pharmacol Exp Ther 217 494-499... 

---