Smooth Muscle Flashcards
ATP use is lower in smooth muscle than in skeletal muscle in part becauseof which of the following? A.Calcium removal from the cytoplasm does not require ATP in smooth muscle.B.Smooth muscle myosin does not require an ATP molecule for each cross-bridge cycle.C.Smooth muscle myosin completes the cross-bridge cycle more slowly than skeletal muscle myosin does.D.Smooth muscle myosin takes longer “steps” along the thin filaments than skeletal muscle myosin does.E.There is no sarcoplasmic reticulum calcium ATPase in smooth muscle.
Answer: C. Explanation:Smooth muscle contraction is regulated by a series of reactions that begins with binding of calcium to calmodulin. The calcium-calmodulin complex then binds to and activates a protein kinase called myosin light chain kinase (MLCK). MLCK catalyzes the phosphorylation of the myosin light chains. Once these lightchains are phosphorylated, myosin and actin interaction can occur and the muscle shortens and develops tension. Although 2 molecules of ATP are required to initiate contraction, one for phosphorylation of the myosin light chains and one to pivotthe myosin cross-bridge, smooth muscle is energyefficient because the rate of cross-bridge cycling is slow.
26.The arteries of some organs show a propertycalled auto-regulation. An increase in blood pressure stretches the wall of the vessel, and leads to opening of stretch sensitive Ca2+channels in the membrane of smooth muscle cells. Ca2+entry into the cells results in the muscle contraction. Calcium binding to which of the following proteins most likely initiates the smooth muscle cell contraction?A.CalmodulinB.CalsequestrinC.N2nicotinic acetylcholine receptorD.SynaptotagminE.TropomyosinF.Troponin C
Answer: A
Explanation:Smooth muscle contraction is regulated by a series of reactions that begins with binding of calcium to calmodulin. The calcium-calmodulin complex then binds to and activates a protein kinase called myosin light chain kinase (MLCK). MLCK catalyzes the phosphorylation of the myosin light chains. Once these light chains are phosphorylated, myosin and actin interaction can occur and the muscle shortens and develops tension.Calsequestrin is a calcium-binding protein found in SR which facilitates calcium storage and sequestration in the SR. Tropomyosin does not bind with calcium. Troponin C does bind with calcium, but is not expressed in smooth muscle cells. The N2nicotinic ACh receptor is a ligand-gated non-specific cationchannel found in the motor end plate of theneuromuscular junction; bindingof ACh (not calcium) to this receptor activates it. Synaptotagmin is a synaptic vesicle protein which binds to calcium. Binding of calcium to this protein catalyzes fusion of synaptic vesicles with the pre-synaptic membrane and neurotransmitter release.
- The acetylcholine receptors on smooth musclecell membranearewhich of the following types? A.ATP dependent Ca2+pumpsB.G-protein coupled receptorC.Ligand gated Ca2+channelsD.Ligand gated Na+channelsE.Na+/K+ ATPases F.Voltage gated Ca2+channels
Answer: B. Explanation:There are two main types of acetylcholine receptor –nicotinic and muscarinic. Nicotinic ACh receptors are ligand-gatedionchannels located in the neuromuscular junction and autonomic ganglia. The muscarinic receptors are all G-protein coupled receptors(GPCR)and are located in target tissues including smooth muscle cells. Binding of ACh to its Gq GPCR on smooth muscle cellsactivates PLC, increasing IP3+ DAG, Ca2+release from SR, increased intracellular Ca2+, increased MLCK activity and contraction.
The prolonged duration of smooth muscle contraction with high force generation ascompared withskeletal muscle is a primary consequence of which of the following? A.Greater amount of myosin filaments present in smooth muscle B.Higher energy requirement of smooth muscle C.Physical arrangement of actin and myosin filaments D.Slower cycling rate of the smooth muscle myosin cross-bridges E.Faster uptake of Ca2+ions following contraction
Answer: D. Explanation:The slower cycling rate of the cross-bridges in smooth muscle means that a higher percentage of cross-bridges areactive at any point in time.The more active cross-bridges there are, the greater the force that is generated. Although the relatively slow cycling rate means that it takes longer for the myosin head to attach to the actin filament, it also means that the myosin head remains attachedlonger, prolonging muscle contraction. Because of the slow cross-bridge cycling rate, smooth muscle actually requires less energy to maintain a contraction compared with skeletal muscle.
An experimental drug is being tested as a potential therapeutic treatment for asthma. Preclinical studies have shown that this drug induces the relaxation of tracheal smooth muscle cells pre-contracted with acetylcholine. Which of the following mechanisms of action is most likely to induce this effect? A.Decreased affinity of troponin C for Ca2+B.Decreased plasma membrane K+permeability C.Inhibition of the sarcoplasmic reticulum Ca2+-ATPase D.Increased plasma membrane Na+permeability E.Stimulation of adenylate cyclase
Answer: E. Explanation:The stimulation of either adenylate or guanylate cyclase induces smooth muscle relaxation. The cyclic nucleotides produced by these enzymes stimulate cAMP-and cGMP-dependent kinases, respectively. Activation of cAMP-PKA pathways inhibitsmyosin light chain kinase, and in this way promotes relaxation. In contrast, either a decrease in K+ permeability or an increase in Na+permeability results in membrane depolarization and contraction. Likewise, inhibition of the sarcoplasmic reticulum Ca2+-ATPase would also favor muscle contraction. Smooth muscle does not express troponin.
In contraction of gastrointestinal smooth muscle, whichof the following events occurs after binding of Ca2+to calmodulin?A.Calcium-induced calcium-releaseB.Depolarization of the sarcolemmal membraneC.Increased intracellular Ca2+concentrationD.Increased myosin-light-chain kinaseactivity E.Opening of ligand-gated Ca2+channels
Answer: D. Explanation:The steps that produce contraction in smooth muscle occur in the following order:(1) Ca2+enters the cell from ECFowing to a number of mechanisms (depolarization of the sarcolemmal membrane, which opens voltage-gated Ca2+channels, opening of ligand-gated Ca2+channels)(2) increased intracellular Ca2+concentration (3) calcium-induced calcium-release from SR(4) binding of Ca2+to calmodulin (5) increased myosin-lightchain kinase; (6) phosphorylation of myosin (7) binding of myosin to actin (8) cross-bridge cycling, which produces contraction.
A 26 year-old male medical student is walking briskly to lecture while eating abreakfast bagel. What is the major difference in the contractile responses occurring in his smooth versus skeletal muscle? A.The mechanism of force generation B.The nature of the contractile proteinsC.The role of calcium in initiating contraction D.The source of calcium for contraction E.The type of energy used during contraction
Answer: C
Explanation:The greatest difference in excitation-contraction coupling between skeletal muscle and smooth muscle involves the role of calcium in initiating contraction. In smooth muscle, calcium binds to and activates calmodulin, which, by activating myosin light chain kinase, catalyzes the phosphorylation of the myosin light chains. Once the light chains are phosphorylated, myosin cross-bridges bind to actin on the thin filaments, which initiates contraction. In skeletal muscle, calcium binds to troponin, which removes the tropomyosin-mediated inhibition of the actin-myosin interaction. Once the inhibition is removed, cross-bridge cycling (and contraction) begins. In both smooth and skeletal muscles, the cycling cross bridges generates force. ATP provides the energy for the cycling of cross bridges in bothmuscles. In skeletal muscle, activator calcium comes exclusively from the SR, whereas in SM, calcium can come from both the SR and ECF.
Which of the following will most likely cause relaxation of the vascular smooth muscle? A.Activation of myosin light-chain kinase B.Activation of myosin light-chain phosphatase C.Opening of the voltage-gated Ca2+channels in the cell membraneD.Phosphorylation of myosinlight chains E.Phosphorylation of myosin light-chain phosphatase
Answer: B. Explanation:Opening of voltage-gated calcium channels in the membrane of SM cells will permit calcium entry. Increased intracellular calcium is a stimulus for contraction, not relaxation (A). Activation of myosin light chain kinase (MLCK) leads to phosphorylation of myosin light chains. Phosphorylated myosin light chains can bind with actin and undergo cross-bridge cycling, and thus contraction (B, C). Activated myosin light chain phosphatase dephosphorylates myosin light chains, inhibiting myosin and actin interaction and thus promoting relaxation (D). Phosphorylation of MLCP inactivates this enzyme, thereby promoting contraction (E).
