Muscles Skeletal ~ Cardiac ~ Smooth Seminar No. 12 - Chapter 19 - Thick filament is the myosin aggregate of cca 350 monomers Myosin monomer Thin filament -- Actin * globular monomer (G-actin) makes a double helix (F-actin) * F-actin has other accessory proteins attached: * tropomyosin (double helix) * troponin C -- binds calcium ions * troponin I -- inhibits interaction actin-myosin * troponin T -- binds to tropomyosin and other troponins Q. Which signal molecule triggers the contraction of skeletal muscles? A. Events on neuromuscular junctions * junction consists from nerve terminal separated from postsynaptic region by the synaptic cleft * acetylcholine is released from synaptic vesicles and binds to nicotinic receptors in muscle cell membrane TH depolarization of membrane and T-tubules * T-tubules are connected with sarcoplastic reticulum (SR) TH Ca^2+ ions are released from SR (where are associated with calsequestrin protein) * calcium ions then bind to troponin C TH contraction Q. What is the Ca^2+ concentration? *in ICF - sarcoplasm during resting state *in ICF - sarcoplasm during contraction *in ECF - blood plasma The concentration of calcium ions in body fluids Skeletal muscle: relaxation/contraction cycle * Relaxation (scheme on p. 109) * troponin I inhibits actin-myosin interaction * ATP molecule (attached to myosin head) has been hydrolyzed TH chemical energy is conserved in myosin head conformation * concentration of calcium ions in sarcoplasm is extremely low (10^-8 M) Skeletal muscle: relaxation/contraction cycle * after release of Ca^2+ from SR TH myosin-ADP-P[i] complex binds to actin * ADP and P[i] are liberated from myosin head, actin filament is pulled by cca 10 nm towards to sarcomere centre TH chemical energy is transformed to mechanical work * new ATP molecule binds to myosin head TH dissociation of actin-myosin complex * the liberation of Ca^2+ ions from troponin C and hydrolysis of ATP leads to relaxation Q. What is the effect of botulinum toxin on the neuromuscular junction? A. (see scheme on p. 135) * Botulinum toxin is produced by bacterium Clostridium botulinum. The toxin is a two-chain polypeptide with a heavy chain joined by a disulphide bond to a light chain. * The light chain is a protease that attacks one of the fusion proteins at a neuromuscular junction, preventing vesicles from anchoring to the membrane to release acetylcholine. By inhibiting acetylcholine release, the toxin interferes with nerve impulses and causes paralysis of muscles (botulism). * no action potential is generated TH permanent relaxation Medical uses of botulinum toxin * Currently, Botox (= trade name) is finding enormous potential in several therapeutic areas including the treatment of migraine headaches, cervical dystonia (a neuromuscular disorder involving the head and neck), blepharospasm (involuntary contraction of the eye muscles), and severe primary axillary hyperhidrosis (excessive sweating). * Other uses of botulinum toxin include urinary incontinence, anal fissure, spastic disorders associated with injury or disease of the central nervous system including trauma, stroke, multiple sclerosis, or cerebral palsy and focal dystonias affecting the limbs, face, jaw etc. Q. What are ATP sources for maximal work: *during the first 10 sec *after 1 min *after 10 min ATP sources for muscle contraction * During the first 10 sec -- ATP itself and creatine phosphate present in muscle cell * After 1 min -- mainly anaerobic glycolysis glucose (r) 2 lactate + 2 ATP * After 10 min -- aerobic oxidation of glc + FA glucose (r) 2 pyruvate (r) 2 acetyl-CoA (r) 38 ATP stearic acid (r) 9 acetyl-CoA (r) 146 ATP Skeletal muscles contain red (slow) and white (fast) fibers Skeletal muscles contain red (slow) and white (fast) fibers Maximal intesity of muscle work (scheme on p. 94) * anaerobic phase * 30 sec -- 2 min * working muscles use glucose TH metabolized to lactate * lactate goes to liver TH substrate of gluconeogenesis * small portion of lactate becomes metabolic fuel for resting muscles and myocard Prolonged muscle work/exercise (scheme, p. 94) * working muscles are adapted to aerobic metabolism of glucose and FA * resting muscles utilize FA and KB * glycerol from lipolysis is the substrate for liver gluconeogenesis Q. What is the yield of ATP during: *aerobic glycolysis *anaerobic glycolysis A. Q. Explain the cause of rigor mortis. A. Rigor mortis is a recognizable sign of death (L. mors, mortis, f.) that is caused by a chemical change in the muscles, causing the limbs of the corpse to become stiff (L. rigor, oris, m.) and difficult to move or manipulate. Assuming mild temperatures, rigor usually sets in about 3-4 hours after clinical death, with full rigor being in effect at about 12 hours. ATP supply from metabolic reactions is exhausted, the muscles remain contracted for ever. Cardiac muscles - Contraction * three different sources of Ca^2+: ECF, SR, mitochondria * extracellular calcium enters muscle cells via VOC (voltage operated channels) ^* Ca^2+ bind to: Cardiac muscles - Relaxation * Ca^2+ ions are liberated from troponin C and removed from sarcoplasm * there are four systems how to vanish Ca^2+ in sarcoplasm * Ca^2+-ATPase in SR * Ca^2+-ATPase in sarcolemma * Na^+/Ca^2+-exchanger (antiport) in sarcolemma * Ca^2+ re-entry to mitochondria ^ Autoregulation in cardiac muscle * see scheme on page 110 * intracellular calcium is in the complex with protein calmodulin: Ca^2+-CM * Ca^2+-CM stimulates all Ca^2+-pumps which decrease [Ca^2+] in sarcoplasm * the increase of intracellular [Ca^2+] triggers contraction but, at the same time, stimulates relaxation processes Modulatory effect of cAMP on cardiac muscles * cAMP is the second messenger produced after the activation of G[s]-protein-linked-receptors (b-adrenergic receptors) * such receptors are activated by catecholamines -- nor/adrenaline * cAMP activates protein kinase A * protein kinase A catalyzes the phosphorylation of: calciductin of VOC TH influx of Ca^2+ TH contraction Ca^2+-ATPase in sarcolemma TH eflux of Ca^2+ TH relaxation Ca^2+-ATPase in SR TH eflux of Ca^2+ TH relaxation cAMP as the second messenger (compare p. 136) Q. Which parameters are used as the best markers of myocardial infarction (MI)? Markers of MI (updated table from p. 25) Metabolic background of MI * ischemia (lack of oxygen in tissues) leads to anaerobic metabolism TH Glc is converted to lactate * lactate accumulates in ICF and alters intracellular environment TH prolonged acidosis causes irreversible cell damage (necrosis) * permeability of cell membrane increases TH cytoplasmatic/mitochondrial/contractile proteins are released into ECF * the best markers of MI are: myoglobin, CK-MB, cardial troponins (T or I) -- a triple combination is recommended * LD isoforms are no longer used Creatine kinase (CK) -- see p. 23 * Dimer, two different chains (M -- muscle, B -- brain) * Three isoenzymes: MM (muscle), MB (heart), BB (brain) * Major isoenzyme in blood is MM (95 %) * MB form in blood: 0 -- 6 % * BB in blood: traces (BB cannot pass across blood-brain barrier) * MB isoenzyme is a marker of myocardial infarction Smooth muscles - Contraction * source of Ca^2+: ECF (VOC, ROC), SR * there is no troponine C, but two other regulatory proteins binding calcium -- calmodulin + caldesmon * calcium-calmodulin complex (Ca^2+-CM) activates MLCK (myosin light chain kinase) * activated MLCK catalyzes the phosphorylation of myosin * phosphorylated myosin is capable to make complex with actin TH contraction Smooth muscles - Relaxation * MLC-phosphatase catalyzes the hydrolysis of phosphorylated myosin: MLC-P + H[2]O (r) P[i] + MLC * MLC does not bind to actin TH relaxation The influence of cAMP on smooth muscles * cAMP activates protein kinase A (PK-A) * PK-A phosphorylates MLC-kinase: MLCK (r) MLCK-P * MLCK-P is inactive, does not phosphorylates MLC TH no interaction between actin and myosin TH relaxation The influence of NO on smooth muscles * nitric oxide (NO) is a relaxant of smooth muscles (e.g. arterial myocytes) [* ] activates guanylate cyclase in cytosol: GTP (r) cGMP + PP[i ]* cGMP activates protein kinase G (PK-G) * PK-G phosphorylates MLC-kinase: MLCK (r) MLCK-P * MLCK-P is inactive, does not phosphorylate MLC TH no interaction between actin and myosin TH relaxation NO releasing compounds * Endogenous: L-arginine (the imino nitrogen of guanidine part) * Exogenous: organic nitrates = esters of nitric acid (R-O-NO[2]) organic nitrites = esters of nitrous acid (R-O-N=O) sodium nitroprusside = a complex of Fe^3+ with CN^- and NO NO originates from imino nitrogen of L-arginine Organic nitrates (alkyl nitrates) Organic nitrites (alkyl nitrites) Other NO releasing compounds Other metabolic pathways of NO Q. What effect on smooth muscle contractility is caused by a signal molecule acting through: a[1]-adrenergic receptors a[2]-adrenergic receptors b-adrenergic receptors A. Actions mediated through adrenergic receptors (Harper, Ch. 49)