Myosin phosphorylation effects

One should note overall, that while some of these suggested mechanisms may in the future prove to have a role in the control of smooth muscle contraction, in chemically skinned preparations maximum force development follows activation by the MLCK active subunit in extremely low Ca " ion concentrations. The conclusion can hardly be avoided that phosphorylation alone is sufficient for activation, and if another mechanism is involved, it is not necessary for the initial genesis of force. If such mechanisms are operative, then they might be expected to run in parallel or consequent to myosin phosphorylation. A possible example of this category of effect is that a GTP-dependent process (G-protein) shifts the force vs. Ca ion concentration relationship to lower Ca ion concentrations. This kind of mechanism calls attention to the divergence of signals along the intracellular control pathways. 

An alternative mechanism by which cAMP may act to inhibit platelet function was proposed by Hathaway et al. (81). In this model (Figure 12), it is suggested that an increase in cAMP results in inhibition of myosin phosphorylation and consequent inhibition of platelet contractile activity (since unphos-phorylated myosin cannot interact with actin). Thus, it was proposed that cAMP causes the activation of a protein kinase which in turn phosphorylates myosin kinase. In the phosphory-lated form, myosin kinase is less capable of binding calmodulin and therefore is not as effective in phosphorylating myosin. 

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... 

Barsotti RJ, Ikebe M, Hartshome DJ (1987) Effects of Ca, Mg ", and myosin phosphorylation on skinned smooth muscle fibers. Am J Physiol 252 C543-C554 Bengur AR, Robinson EA, Appella E, Sellers JR (1987) Sequence of the sites phospho-rylated by protein kinase C in the smooth muscle myosin li t chain. J Biol Chem 262 7613-7617... 

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... 

The regulation of myosin-I activity is not well understood. As mentioned above, the heavy chain is phosphorylated in Acanthamoeba but the regulatory effect of this process is unclear. Myosin-I from Acanthamoeba, Dictyostelium, brush-bor-... 

When MLCK is phosphorylated by cAMP activated protein kinase, it itself is harder to activate. Molecule for molecule, being phosphorylated does not diminish the effectiveness of MLCK in catalyzing the phosphorylation of myosin. However, phosphorylated MLCK has a much smaller affinity for the Ca-calmodulin complex, which activates it, than the uninhibited, nonphosphorylated form. Thus, phosphorylation of MLCK by protein kinase decreases the number of activated MLCK... 

The general picture of muscle contraction in the heart resembles that of skeletal muscle. Cardiac muscle, like skeletal muscle, is striated and uses the actin-myosin-tropomyosin-troponin system described above. Unlike skeletal muscle, cardiac muscle exhibits intrinsic rhyth-micity, and individual myocytes communicate with each other because of its syncytial nature. The T tubular system is more developed in cardiac muscle, whereas the sarcoplasmic reticulum is less extensive and consequently the intracellular supply of Ca for contraction is less. Cardiac muscle thus relies on extracellular Ca for contraction if isolated cardiac muscle is deprived of Ca, it ceases to beat within approximately 1 minute, whereas skeletal muscle can continue to contract without an extraceUular source of Ca +. Cyclic AMP plays a more prominent role in cardiac than in skeletal muscle. It modulates intracellular levels of Ca through the activation of protein kinases these enzymes phosphorylate various transport proteins in the sarcolemma and sarcoplasmic reticulum and also in the troponin-tropomyosin regulatory complex, affecting intracellular levels of Ca or responses to it. There is a rough correlation between the phosphorylation of Tpl and the increased contraction of cardiac muscle induced by catecholamines. This may account for the inotropic effects (increased contractility) of P-adrenergic compounds on the heart. Some differences among skeletal, cardiac, and smooth muscle are summarized in... 

Effect of protein-bound Ca TpC 4Ca antagonizes Tpl inhibition of F-actin-myosin interaction (allows F-actin activation of ATPase) Calmodulin 4Ca activates myosin light chain kinase that phosphorylates myosin p-light chain. The phosphorylated p-light chain no longer inhibits F-actin-myosin interaction (allows F-actin activation of ATPase). 

Figure 9.25 Control of the Krebs q/cle and myosin-ATPase by direct effects of Ccf ions and the resultant effects on electron transfer and oxidative phosphorylation in muscle. The stimulation of the Krebs cycle by ions results in an increase in the NADH/NAD concentration ratio, which stimulates electron transfer. The stimulation of myosin-ATPase by Ca lowers the ATP/ADP concentration ratio, which also stimulates electron transfer. The Ca ions are released from the sarcoplasmic reticulum in muscle in response to nervous stimulation. In addition, generation of ADP by myosin ATPase increases the ADP concentration, which stimulates the cycle. Note that a lack of oxygen will prevent generation of ATP (Chapter 13).

Figure 22.12 Regulation of actin-myosin interaction in smooth muscle via the light-chain kinase and phosphatase and effect on blood pressure. ions bind to calmodulin and the complex stimulates the conversion of inactive myosin light chain kinase (MLCK) to active MLCK which then phosphorylates the light chain. This results in activation of the cross-bridge cycle. The overall effect is vasoconstriction of the arteriole, which increases blood pressure.

Smooth muscle effects. The opposing effects on smooth muscle (A) of a-and p-adrenoceptor activation are due to differences in signal transduction (p. 66). This is exemplified by vascular smooth muscle (A). ai-Receptor stimulation leads to intracellular release of Ca + via activation of the inositol tris-phosphate (IP3) pathway. In concert with the protein calmodulin, Ca + can activate myosin kinase, leading to a rise in tonus via phosphorylation of the contractile protein myosin. cAMP inhibits activation of myosin kinase. Via the former effector pathway, stimulation of a-receptors results in vasoconstriction via the latter, P2-receptors mediate vasodilation, particularly in skeletal muscle - an effect that has little therapeutic use. 

Current evidence indicates that PLN phosphorylation appears to be dominant over troponin I phosphorylation (Li, et al., 2000). The faster SR Ca uptake by phospho-rylated PLN also contributes to increased SR Ca load, which is available for subsequent release, resulting in an inotropic effect. The increased ICa by PKA activation also contributes to the inotropic effects of the (i-AR agonists. The myofilament effects of PKA appear to be almost entirely attributable to troponin I phosphorylation (vs. myosin binding protein C) because substitution of troponin I with a non-phosphorylatable troponin I abolishes myofilament effects of PKA (Kentish, et al., 2001 Pi, et al., 2002). 

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