Med Phys Pharm 551 Lecture 11: Smooth Muscle Physiology

Lecture #11: Smooth Muscle Physiology Julia Hum, PhD Primary Course Instructor Course Meets: Monday/Wednesday/Friday: 2:00-2:50pm Office Hours: Monday/Wednesday/Friday 11:00am-12:00pm (317B or WebEx)  L11: Learning Objecti...

Lecture #11: Smooth Muscle Physiology Julia Hum, PhD Primary Course Instructor Course Meets: Monday/Wednesday/Friday: 2:00-2:50pm Office Hours: Monday/Wednesday/Friday 11:00am-12:00pm (317B or WebEx)  L11: Learning Objectives 1. Reproduce the structural organization of smooth muscle. 2. Explain the organization and function of myosin and actin in smooth muscle, membrane system, and compare/contrast it with skeletal muscle. 3. Characterize the “calcium handling” that occurs for smooth muscle contraction and relaxation. 4. Relate disease states to the underlying smooth muscle physiology. 5. Compare and contrast excitation-contraction coupling in smooth and skeletal muscle. ”Take Home” Figure  Introduction to Smooth Muscle Found in all regions of the body Layered within the walls of blood vessels and airways Organ-specific adaptations: striated muscle is activated by a handful of neurotransmitters and hormones, smooth muscle is modulated by hundreds of chemical signals Plasticity of smooth muscle sarcomeres and cytoskeletal framework is key to maintain contractility during changes in hollow organ luminal volume LO2  Contractile Units of Smooth Muscle “Minisarcomeres” - densely packed actin and myosin filaments, organized into contractile units No Z-disc = No striations Thick Filaments Two heavy chains - head and neck regions The myosin heads contain ATPase activity and an actin-binding site Two pairs of light chains - essential and regulatory ht tps :// lh3.googleuser co ntent.com /pr oxy/ a1s5cfLUeaydQjJGf CR31hZhllvvKBmRp512q ZL0H yC VOKSyy8TL5JU ntz UAA PZgjh 5EocZeQsM eGYX15q Z4D xqdxMW 5sis nnBuN kK-qL2evQu d1WZtKjn p8tIJ81xqfq 6fNd64TZFRX2fPk03gF_ius eoYKvDdM OFL7jZW20O5zEU E LO1,2  Contractile Units of Smooth Muscle “Sidepolar” Model - the myosin head groups within a thick filament have a sided arrangement that allows two actin filaments to be pulled simultaneously in different directions Could account for observations that smooth muscle myocytes can shorten more than striated muscle fibers LO1,2  Contractile Units of Smooth Muscle Thin Filaments No troponin Two muscle-specific regulatory proteins - caldesmon and calponin Caldesmon - actin-associated myosin ATPase inhibitor Inhibition is relieved by high Ca2+ - calmodulin (CaM) concentrations or by phosphorylation by Ca2+-CaM–dependent protein kinase Calponin - actin-associated myosin ATPase Léguillette, Renaud & Lauzon, Anne-Marie. (2008). Molecular Mechanics of Smooth Muscle Contractile Proteins in Airway Hyperresponsiveness and Asthma. Proceedings of the American Thoracic Society. 5. 40-6. 10.1513/pats.200704-053VS. inhibitor that is regulated by Ca2+-CaM– dependent protein kinase–dependent phosphorylation LO1,2  Contractile Units of Smooth Muscle Parallel assemblies of 3–5 thick filaments, each surrounded by numerous thin filaments Variable lengths of both thick and thin filaments The filaments are anchored by α-actinin–rich dense bodies (desmosomes) found scattered throughout the sarcoplasm - functional equivalent of Z disks in skeletal muscle Dense bodies do not align like skeletal muscle Arrangement of thin filaments, thick filaments, and dense bodies does resemble a sarcomere LO1,2  Organization of Smooth Muscle The arrays are tethered to the sarcolemma by dense plaques Plaques are distributed over the entire cell surface and link adjacent cells mechanically LO1,2  Membrane System of Smooth Muscle No T tubules Caveolae - linear arrays of membranes perform similar function as T-tubules Flask-shaped sarcolemmal pockets that form narrow junctions with the underlying SR Enriched in L-type Ca2+ channels Sarcoplasmic reticulum - tubular network that stores Ca2+ until contraction begins Smooth muscle SR contains two types of Ca2+-release channels: 1. Activated by Ca 2+ 2. Activated by inositol trisphosphate (IP3) LO2,3  NMJ in Smooth Muscle Mostly controlled by the autonomic nervous system (ANS) ANS neuromuscular junction is less developed than skeletal muscle ANS efferents contact multiple smooth muscle cells via a series of varicosities along the length of an axon Each of varicosities is the site of a neuromuscular junction LO2,3  E-C Coupling in Smooth Muscle Many competing signals for control of smooth muscle function Some of these signals promote contraction and others relaxation All signals converge on Ca2+ Difference between skeletal muscle - Ca2+-sensitive step transferred from the thin filament (via troponin and tropomyosin) to the thick filament (via myosin phosphorylation) Calcium sources for contraction of SM 1. Ca2+ influx across the sarcolemma 2. CICR from the SR 3. IP3-mediated Ca2+ release from the SR LO3,5  E-C Coupling in Smooth Muscle 1. Calcium influx across the sarcolemma Express two types of Ca2+ channels 1. L-type Ca2+ channels in caveolae are voltage- gated 2. Receptor-operated Ca2+channels (ROCs) ROC-mediated Ca2+ fluxes are relatively minor, but do depolarize cells to aid in Ca2+ influx and contraction via L- Zhao, Yingzi & Vanhout te, Paul & Leung, Susan. (2015). Vascular nitric oxide: Beyond eNOS. Journal of pharmacologi cal sciences. 129. 10.1016/j.jphs.2015.09.002. type Ca2+ channels LO3,5  E-C Coupling in Smooth Muscle 2. Calcium-induced calcium release (CICR) Visualized as “Ca2+sparks” in fluorescent imaging studies 3. Inositol trisphosphate Most SM express a variety of GPCRs that modulate contraction through phospholipase C and IP3 formation When intracellular IP3 concentrations rise, the IP3-gated Ca2+ channel in the SR opens, and the Ca2+ stores are released **IP3-mediated Ca2+ release and smooth muscle contraction can occur independently of an action potential or other membrane potential change** LO3,5  Membrane System of Smooth Muscle Sarcoplasmic reticulum: 1. Calcium-induced calcium-release channels - (CICR) opened by Ca2+ entering the myocyte via voltage-dependent Ca2+ channels 2. Inositol trisphosphate–gated calcium channels (IP3-gated Ca2+ channel) IP3 is a second messenger that communicates binding of chemical signals - hormones and neurotransmitters LO2,3  Contraction of Muscle: Skeletal vs Smooth Skeletal muscle – Review Rising intracellular Ca2+ concentrations cause troponin to move tropomyosin away from myosin- binding sites on the actin filament High intrinsic ATPase activity, when binding sites are exposed, contraction proceeds quickly LO2,5  Contraction of Smooth Muscle ATPase remains inactive until the regulatory light chain is phosphorylated A rise in sarcoplasmic Ca 2+ is sensed by calmodulin (CaM), which then activates myosin light-chain kinase (MLCK) MLCK phosphorylates MLC20 = Crossbridge cycling proceeds myosin phosphorylation is required for smooth muscle contraction, it is thick-filament regulated SLOW - multistep nature causes contraction that is 10–20x slower than skeletal muscle LO2,5  Relaxation of Smooth Muscle Muscle relaxation occurs when SR Ca2+ concentrations renormalize Relaxation requires a phosphatase Calcium renormalization Excitatory signaling ends - Ca2+ expelled by Ca2+ ATPases and Na+-Ca2+ exchangers Returned to the SR by a sarco(endo)plasmic reticulum Ca2+ ATPase MLCK deactivates LO2,3  Relaxation of Smooth Muscle Store refill Ca2+ transporters compete with SERCA for available Ca2+ during relaxation Stores have to be topped off with Ca2+ from the outside of the cell via a store-operated Ca2+channel (SOC) Zhao, Yingzi & Vanhout te, Paul & Leung, Susan. (2015). Vascular nitric oxide: Beyond eNOS. Journal of pharmacologi cal Critical step that must be completed for sciences. 129. 10.1016/j.jphs.2015.09.002. continued smooth muscle contractility LO2,3  Relaxation of Smooth Muscle Dephosphorylation Relaxation of smooth muscle dependent on myosin phosphatase constitutively active, always undoing the work of MLCK When sarcoplasmic Ca2+ concentrations fall Zhao, Yingzi & Vanhout te, Paul & Leung, Susan. (2015). Vascular nitric oxide: Beyond eNOS. Journal of pharmacologi cal sciences. 129. 10.1016/j.jphs.2015.09.002. and MLCK deactivates, myosin phosphatase quickly strips MLC20 of phosphate groups and the muscle relaxes LO2,3  Relaxation of Smooth Muscle Dephosphorylation Relaxation of smooth muscle dependent on myosin phosphatase constitutively active, always undoing the work of MLCK When sarcoplasmic Ca2+ concentrations fall and MLCK deactivates, myosin phosphatase quickly strips MLC20 of phosphate groups and the muscle relaxes LO2,3  Clinical Connection: IBS Irritable bowel syndrome is a GI disorder associated with intestinal cramping, increased flatulence, and altered bowel habits It has no known cause or cure Treatment: options are limited, include antispasmodics (more details on Wednesday) Natural remedies: peppermint oil Act by blocking Ca2+ channels in the smooth muscle, reducing contractility and relaxing the muscle LO4  Regulation of Smooth Muscle Skeletal muscle - force regulation occurs through control of intracellular Ca2+ concentration Smooth muscle - force is regulated via changes in MLC20 phosphorylation state MLC20 phosphorylation is dependent on both MLCK and myosin phosphatase = multiple control points 1. Rho-kinase (ROCK) 2. Protein kinase C (PKC) LO2,3  Regulation of Smooth Muscle Multiple control points of smooth muscle 1. Rho-kinase (ROCK) Serine–threonine protein kinase regulated by RhoA, a GTP- binding protein RhoA is activated indirectly following GPCR binding by norepinephrine, angiotensin II, endothelin, and others ROCK has a several targets, including the MYPT1 myosin phosphatase - phosphorylation inhibits myosin phosphatase activity and promotes contraction LO2,3  Regulation of Smooth Muscle Multiple control points of smooth muscle 2. Protein kinase C Agonists that activate phospholipase C and promote contraction via IP3 simultaneously activate PKC via diacylglycerol release PKC also phosphorylates many proteins that regulate contraction, including CPI-17 CPI-17 becomes a potent myosin phosphatase inhibitor when phosphorylated - promoting contraction LO2,3  E-C Coupling in Smooth Muscle 2. Calcium-induced calcium release (CICR) Visualized as “Ca2+sparks” in fluorescent imaging studies of SM, but their role is still being investigated 3. Inositol trisphosphate Most SM express a variety of GPCRs that modulate contraction through phospholipase C and IP3 formation When intracellular IP3 concentrations rise, the IP3-gated Ca2+ channel in the SR opens, and the Ca2+ stores are released **IP3-mediated Ca2+ release and smooth muscle contraction can occur independently of an action potential or other membrane potential change** LO3,5 Study Aid: Skeletal vs Smooth Skeletal Muscle Smooth Muscle