Lecture 5 - Smooth Muscle

\Lecture 5 - smooth muscle lecture 2 Smooth muscle are either phasic or Tonic Phasic smooth muscle are normally visceral smooth muscle, and contract in a wave like force Tonic - Present in most blood vessels / trachea. This controls vascular tone. These different modes of activity have led to difference in the mechanisms of influencing Force/. Generally, tonic smooth muscles uses CA sensitisation mechanisms. And phasic muscle uses regulation of CA Smooth muscle can be described as either Single Or multi unit. Single Unit - The whole muscle contracts and relaxes together. It has coordinated activity and all visceral smooth miscues are single units. The action propagates to neighbouring myocytes via gap junction Single-unit visceral smooth muscle is myogenic. It can contract regularly without input from a motor neurone or hormones. A few can act as a pace maker generating rhythm. Multi unit Blood vessels, respiratory tract, ciliary muscle rely on nervous excitation to stimulate them - each cell is innervated individually. Multi units have very few if any gap junctions as it is only activating a single myocyte , Regulation of Force We can view contraction as being the balance of activity. (MLCK and MLCP) Ca-Calmodulin -0 MLCK At low calcium levels the smooth muscle is relaxed as MLCK is dissociated from calmodulin. When Ca rises from 100nm to around 1000nm, 4 calcium ions bind to calmodulin with high affinity Ca-calmodlin rapidly associated with MLCK, causes a conformational change and exposes the catalytic site of the enzyme. Active MLCK uses ATP to catalyse transfer of P to Her 19 on regulatory light chain in the myosin head. MLCP - myosin light chain phosphatase. The light chains are de-phosphorylated by MLCP. This causes the relaxation of smooth muscle. Calmodulin at low calcium leaves causes relaxation by dissociating from MLCK heavy chain MLCP is a heterotrimer: Catalytic subunit - PP1c Targeting subunit - MYPT1 Regulatory subunit - M20 MLCP is tightly bound to myosin. Activated by cyclic nucleotides e.g. cGMO Inhibited by various mechanisms e.g. rho A This is important for altering calcium sensitivity Lecture 6 - Smooth muscle lecture 3 The relationship between calcium and force is sygmodal relationship. Ca sensitivity of myofilaments Agonists (stimulants) not only increase calcium but also alter the reaction between CA and Force i.e. sensitivity. (Ca-sensitisation most important for Multi unit smooth muscle) Calcium sensitisation - For the same amount of calcium we are getting more force Calcium de-sensitisation - For the same amount of calcium we are getting less force Mechanism are changing calcium sensitivity 1. MLCP (the most important way MLCP can be phosphorylated by a kinase e.g. Rho-kinase, PKC, CPI-17. When MLCP is phosphorylated is activity is decreased. This will increase force. This is mainly used in blood vessel, slowly to develop tone. Rho is a member of Ras family of small G proteins. Inactive when bound to GDP and active when bound to GTP. GEF bring this activation about. GDP —> GTP. MLCP - There is drug that is Y-27632 inhibits ROCK. This was given to hypertensive ravins —> normotensive CPI-17 is a small MLCP inhibitor. It is endogenous and activated when P e.g. by PKC and Rho A. P-CPI-17 can directly inhibit MLCP as it catalytic site, PPIc thereby sensitive SM force. N.b. PKC is activated by DAG, and is formed when PIP2 is hydrolysed by phospholipase C and D. 2. MLCK MLCK can be phosphorylated by various kinases e.g. PKC, PKa When MLCK is bound to phosphate is has a 20 fold decrease affinity for calcium calmodulin,. This is going to decrease force, m this is an example of desensitisation. This pathway is used by Beta-adrenergic agonists that relax smooth muscle, I.e. in Asthma inhalers. Thin filament regulation: Thin filaments have caldesmon (CaD) and Calponin (CaP). Both bind to actin and inhibit myosin ATPase activity, probably by competing with myosin head for actin binding site, and slowing product release from actomyosin. At rest, thin filaments are switched off, regulation of which CaD and CaP are under physiological control. Phosphorylation of CaD and CaP by MAPK or PKS, then they no longer inhibit myosin ATPase Thin filament sensitisation mechanisms • • Phosphorylation of caldesmon and calponis decreases their inhibition on myosin-actin interaction, thus increasing force. This is an example of calcium sensitisation. MAP Kinase is activated when phosphorylated by tyrosine kinase. Other mechanisms: Can agonists induce changes in actin polymerisation and thereby affect force. The actin filaments and cytoskeleton of smooth muscle are dynamic. During contraction actin polymerisation is triggered, to help maintain force and also better transmit force to extracellular matrix. Schubert et al proposed that “the AMP-related Kinase (AMPK) indices independent dilation of resistance arteries by interfering with actin filament formation.