1 Quiz 05 Results Question 1: Multiple Choice Average Score 0.39583 points Which one of the following statements about cross-bridge cycling is true? Correct Percent Answered The detachment of the myosin head from actin is the Ca2+-dependent step of the cross-bridge cycle. 3.125% Hydrolysis of ATP is followed immediately by release of the reaction products, ADP and inorganic phosphate. 11.458% The binding affinity of the myosin head for actin is lowest when there are no nucleotides bound to myosin, higher when ADP is bound, and highest when ATP is bound. 6.25% All else being equal, a myosin isoform with a longer neck region, i.e., the part between the head and the shaft of the thick filament, will cause faster filament sliding than a myosin isoform with a shorter neck region. 79.167% Question 2: Multiple Choice Average Score 0.45833 points Caldesmon is a regulatory protein in smooth muscle that binds to tropomyosin and actin in thin filaments. When caldesmon binds to thin filaments, it inhibits the myosin ATPase. This action of caldesmon is inhibited when caldesmon is phosphorylated by protein kinase C or when caldesmon binds Ca2+-calmodulin. Imagine that a hormone binding to its receptor on a smooth muscle cell activates the mechanisms shown in the slide entitled “DAG/IP3 System” in the Cell Communication lecture. What would be the likely effect of this hormone on contraction of the smooth muscle cell? Correct Percent Answered The hormone would inhibit contraction. 8.333% The hormone would stimulate contraction. 91.667% The effect of the hormone cannot be predicted because of its opposite effects on caldesmon and myosin light chain kinase. 0% Don't choose this one. 0% Question 3: Multiple Choice Average Score 0.42188 points Rho kinase is a regulatory enzyme in smooth muscle cells. Rho kinase phosphorylates myosin light chain phosphatase, inhibiting its activity. Which one of the following would most likely result from administration of a drug that inhibits Rho kinase? Correct Percent Answered increased cytoplasmic Ca2+ concentration 1.042% relaxation of vascular smooth muscle 84.375% increased cross-bridge cycling 11.458% elevation of blood pressure 3.125%2 Question 4: Essay In a laboratory, an isolated strip of smooth muscle tissue was placed in a chamber such that measurements could be made of force and the concentration of intracellular free calcium ions, [Ca2+]in. Two experiments were done: Experiment 1: The muscle was depolarized with high potassium saline solution such that [Ca2+]in increased ten-fold. A certain amount of force was generated. Experiment 2: In normal saline solution, the muscle was exposed to serotonin, a neurotransmitter that normally functions to stimulate smooth muscle contraction. The dose of serotonin was adjusted such that it also caused a ten-fold increase in [Ca2+]in. Results: The smooth muscle strip produced much more force in the second experiment. The Question: By applying what you know about the control of contraction in smooth muscle cells, briefly explain two things: a) how an increase in ([Ca2+]in) triggers contraction in smooth muscle, and b) why the muscle produced more force in the second experiment even though the increase in [Ca2+]in was the same. Given Answers An increase in Ca2+in triggers contraction in smooth muscle when calcium ions bind to calmodulin. After this, the Ca-calmodulin complex activates myosin light chain kinase which is responsible for phosphorylating the myosin light chain. This mechanism results in a conformational change in myosin that allows cross-bridge cycling. b) Despite the increase in Ca2in being the same in both experiments, the muscle produced more force when exposed to nuerotransmitter serotonin. Serotonin binds to its receptor beginning an intracellular signal transduction cascade. This leads to an increase in Ca2+in. In addition to this, myosin light chain phosphotase is inhibited resulting in more force in the second experiment with the same increase in Ca2+. The inhibition of the enzyme myosin light chain phosphotase which is responsible for dephosphorylating myosin will amplify the response of myosin light chain kinase resulting in a larger force produced by the smooth muscle exposed to serotonin even though there are equal increases in Ca2+in in both experiments. Calcium ions bind to calmodulin, in which the whole Ca-calmodulin complex binds to myosin light chain kinase. Ca-Cal-Myosin LC Kinase then phosphorylate the myosin light chain which can enter crossbeidge cycling to generate force and contraction in smooth muscles. Since the binding of calcium ions and calmodulin directly activates the the function of myosin LC Kinase, an increase of intracellular calcium ions thus triggers contration in smooth muscle. b) Serotonin binding in smooth muscle cells may cause a series of signaling cascade in the cells. Apart from increasing the concentration of intracecullar calcium ions concentration, serotonin is also capable of inhibiting the effects of Myosin Light Chain Phosphatase, a protein that dephosphorylates myosin head and causes muscle relaxation. Since serotonin potentially inhibits Myosin LC Phosphatase, the sole effects on smooth muscles would be actions from the increased calcium concentration that activiates Myosin LC Kinase. In this way, for the same increase of calcium ion intracellularly, the contraction effects will not be "canceled out" by the actions of Myosin LC Phosphatase, resulting in a greater force generated. When action potentials reach muscle cells, there's a release of calcium from the sarcoplasmic reticulum that causes the muscles to relax. Thus, an increase in calcium would have a similar contraction as an effect, causing the myosin and actin to form a cross bridge which results in the contraction. b) The presence of serotonin must trigger a change in the calcium exchange, most likely in the presence of the emitted calcium; if the calcium remains in the cytoplasm to trigger the myosin-actin cross bridge, the force resulting will the stronger. Serotonin most likely inhibits the removal of calcium, leaving it present more than it would be otherwise. a. b. To generate more force even though the increase in Ca in was the same, inhibition of myosin light chain phosphatase shouldve occured.3 Increase in [Ca 2+] in triggers contraction in smooth muscle because calcium binds to calmodulin which then activates