Smooth muscle activation other kinases

The major relaxing transmitters are those that elevate the cAMP or cGMP concentration (Fig. 3). Adenosine stimulates the activity of cAMP kinase. The next step is not clear, but evidence has been accumulated that cAMP kinase decreases the calcium sensitivity of the contractile machinery. In vitro, cAMP kinase phosphorylated MLCK and decreased thereby the affinity of MLCK for calcium-calmodulin. However, this regulation does not occur in intact smooth muscle. Possible other substrate candidates for cAMP kinase are the heat stable protein HSP 20, (A heat stable protein of 20 kDa that is phosphorylated by cGMP kinase. It has been postulated that phospho-HSP 20 interferes with the interaction between actin and myosin allowing thereby smooth muscle relaxation without dephosphorylation of the rMLC.) Rho A and MLCP that are phosphorylated also by cGMP kinase I (Fig. 3). 

Preincubation of the cells with genistein invariably blocked the Ca + response evoked by either phenylephrine or serotonin (Figs. 4C and 4D). Though not shown, the inhibitory effects of genistein were reversible as replacement of the medium with genistein-free PSS resulted in recovery of responsiveness to both phenylephrine and serotonin. Previous studies showed that the concentration of genistein used in these experiments virtually abolished the tyrosine kinase activity of pp60<= in extracts from smooth muscle and other... 

Of the several kinase activities which are important in smooth muscle, myosin light chain kinase, MLCK, is the one responsible for activation of the actin-myosin system to in vivo levels. MLCK is present in the other nonmuscle cell types which have the actin-myosin contractile system and all of these are probably activated in a manner similar to smooth muscle rather than by way of the Ca -troponin mechanism of striated muscle. MLCK from smooth muscle is about 130 kDa and is rather variable in shape. It is present in smooth muscle in 1-4 pM concentrations and binds with an equally high affinity to both myosin and actin. Thus, most MLCK molecules are bound to actin. Myosin light chain serine-19 is the primary target of smooth muscle myosin light chain kinase. 

MLCK itself is phosphorylated by cyclic-AMP activated protein kinase, (protein kinase A) and cyclic-GMP activated protein kinase, (protein kinase G). Protein kinase A will phosphorylate MLCK at two sites and protein kinase G at one in some cases and two in others. These differences seem to be important in how the individual smooth muscle cells are regulated. 

Figure 6. A hypothetical scheme for the control of the number of active crossbridges in smooth muscle. Following the activation of a smooth muscle by an agonist, the concentrations of intermediates along the main route begins to build up transiently. This is shown by the thickened arrows. Also, cAMP is generated which is universally an inhibitor in smooth muscle. Cyclic AMP in turn combines with protein kinase A, which accounts for most of its action. The downstream mechanisms, however, are not well worked out and at least three possibilities are likely in different circumstances. First, protein kinase A is known to catalyze the phosphorylation of MLCK, once phosphorylated MLCK has a relatively lower affinity for Ca-calmodulin so that for a given concentration of Ca-calmodulin, the activation downstream is reduced. The law of mass action predicts that this inhibition should be reversed at high calcium concentrations. Other cAMP inhibitory mechanisms for which there is evidence include interference with the SR Ca storage system, and activation of a MLC phosphatase.

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... 

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... 

Uterine relaxation is mediated in part through inhibition of MLCK. This inhibition results from the phosphorylation of MLCK that follows the stimulation of myometrial (3-adrenoceptors relaxation involves the activity of a cyclic adenosine monophosphate (cAMP) mediated protein kinase, accumulation of Ca++ in the sarcoplasmic reticulum, and a decrease in cytoplasmic Ca. Other circulating substances that favor quiescence of uterine smooth muscle include progesterone, which increases throughout pregnancy, and possibly prostacyclin. Progesterone s action probably involves hyperpolarization of the muscle cell membrane, reduction of impulse conduction in muscle cells, and increased calcium binding to the sarcoplasmic reticulum. 

Mechanism of action of nitrates, nitrites, and other substances that increase the concentration of nitric oxide (NO) in vascular smooth muscle cells. Steps leading to relaxation are shown with heavy arrows. MLCK, activated myosin light-chain kinase [see Figure 12-1]. GC, activated guanylyl cyclase PDE, phosphodiesterase eNOS, endothelial nitric oxide synthase. 

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