Bioeng 6460 Electrophysiology and Bioelectricity Modeling of EC-Coupling and Contraction Frank B. Sachse fs@cvrti.utah.edu
Overview Quiz Excitation-Contraction Coupling Anatomy Cross Bridge Binding Coupling Experimental Studies Outline of Experiment Bioeng 6460: Electrophysiology and Bioelectricity - Page 2
Phases of Sarcomere Calcium Handling A: Actin M: Myosin Z: Z-Disc Begin of contraction Ca release Relaxation Ca uptake Triggering Filling SL: Sarcolemma TTS: Transversal tubules SPR: Sarcoplasmic Reticulum Ca 2+ Release Ca 2+ Uptake Bioeng 6460: Electrophysiology and Bioelectricity - Page 3
Calcium Handling and EC-Coupling Transversal Tubule Cell Membrane Sarcoplasmic Reticulum NCX: ATP: RyR: PLB: Troponin Mitochondrion Sodium-calcium exchanger Ionic pump driven by ATP-hydrolysis Calcium channel (ryanodine receptor) Phospholamban (Bers, Nature Insight Review Articles, 2002, modified) Bioeng 6460: Electrophysiology and Bioelectricity - Page 4
Quiz: Role of Mitochondria Work in pairs of 2 Time 5 min Mitochondria take part of the cellular calcium handling. Why were they not considered in electrophysiological models? Bioeng 6460: Electrophysiology and Bioelectricity - Page 5
Sarcomeres in Cardiac Muscle (Fawcett & McNutt 69) Sarcoplasmic reticulum Terminal cisternae Mitochondrion Transversal tubuli Z-Disc Sarcoplasmic reticulum Dyad Bioeng 6460: Electrophysiology and Bioelectricity - Page 6
Sarcomeres in Skeletal Muscle (Fawcett & McNutt 69) Sarcolemma Transversal tubules Sarcoplasmic reticulum Terminal cisternae I-Band A-Band Z-Disc Triad Bioeng 6460: Electrophysiology and Bioelectricity - Page 7
Electron Micrograph of Sarcomere in Striated Muscle A - anisotrop I - isotrop (Modified from Lodish et al., Molecular Cell Biology, 2004) Bioeng 6460: Electrophysiology and Bioelectricity - Page 8
Myofilaments (Modified from Lodish et al., Molecular Cell Biology, 2004) Bioeng 6460: Electrophysiology and Bioelectricity - Page 9
Proteins of Sarcomere 2!m Bioeng 6460: Electrophysiology and Bioelectricity - Page 10
EC-Coupling: Involved Proteins 2 µm Actin Myosin Actin TnC TnI TnT Actin Head-to-tail overlap Tm Tn: Troponin Tm: Tropomyosin A: Actin Z: Z-Disk (adapted from Gordon et al. 2001) Bioeng 6460: Electrophysiology and Bioelectricity - Page 11
Force Development: Sliding Filament Theory Cellular force development by sliding myofilaments (Huxley 1957), i.e. actin and myosin, located in sarcomere Attachment of myosin heads to actin Filament sliding Detachment Spanning of myosin heads Bioeng 6460: Electrophysiology and Bioelectricity - Page 12
Force Development: Sliding Filament Theory http://www.sci.sdsu.edu/movies/actin_myosin.html Bioeng 6460: Electrophysiology and Bioelectricity - Page 13
Actin-Myosin Interaction A Actin M Myosin ADP Adenosine diphosphate ATP Adenosine triphosphate P i Phosphate (Bers, Excitation-Contraction Coupling and Cardiac Contractile Force, 1991, modified) Bioeng 6460: Electrophysiology and Bioelectricity - Page 14
EC-Coupling Rest small Ca 2+ Stimulus Contraction Ca 2+ release from SR high Ca 2+ High concentration of intracellular Ca 2+ Force development Contraction of sarcomere/cell Bioeng 6460: Electrophysiology and Bioelectricity - Page 15
Contraction of Myocyte by Electrical Stimulation Microscopic imaging of isolated ventricular cell from guinea pig http://www-ang.kfunigraz.ac.at/ schaffer Bioeng 6460: Electrophysiology and Bioelectricity - Page 16
Measurement of Force Development in Single Cell Myocyte glued between glass plates Force transmission to measurement system Stimulator Fluid at body temperature Mechanical Fixation (variable strain) Glass plates Measured force per myocyte: 0.15-6.0 µn Bioeng 6460: Electrophysiology and Bioelectricity - Page 17
Measurement Techniques Permeabilization of sarcolemma/skinning of myocytes by saponin or Triton X-100 Direct control of intracellular concentrations of ions, drugs etc. [ Ca 2+ ] = [ Ca 2+ ] i o Transillumination of myocyte or muscle strands with laser light Diffraction pattern ~ sarcomere length Bioeng 6460: Electrophysiology and Bioelectricity - Page 18
Sarcomere Length Measurement via Laser Diffraction (Figures from Lecarpentier et al., Real-Time Kinetics of Sarcomere Relaxation by Laser Diffraction, AJP, 1985) More information: http://muscle.ucsd.edu/musintro/diffraction.shtml Bioeng 6460: Electrophysiology and Bioelectricity - Page 19
Mathematical Modeling of Myofilament Sliding (Huxley) A + M f " g A # M A + M: Anzahl Number von of bzgl. myosins Actin not ungekoppeltem bound to actin Myosin A # M: Anzahl Number Querbrücken of myosins bound (Actin/ to Myosin actin gekoppelt) f,g: Ratenkonstanten constants for für binding Bindung and bzw. unbinding Ablösung d [ A # M] ( [ ] # [ A # M] ) # g[ A # M] = f Max A # M dt Max[ A # M]: Maximale number Anzahl of an myosin Querbrücken proteins bound to actin filament Bioeng 6460: Electrophysiology and Bioelectricity - Page 20