BME 501 Advanced Topics in Biomedical Systems Spring 2014 Dr. KayBME 501 Lecture Notes – Apr 14 Excitation-Contraction Coupling • Cardiac Myocyte • Mechanism of Contraction • Ca2+ Cycle• Myocyte – Roughly cylindrical cells – 10−20 μm wide and 50−100 μm long – Some branched – Single, central nucleus • Adjacent myocytes attached end-to-end • Intercalated disc: stepped attachment between myocytes • Two types of junctions in intercalated disc – Gap junction/nexus: transmit electrical signal – Desmosome: provides mechanical strength Cardiac MyocyteCardiac Myocyte• Gap junctions/nexuses – Adjacent myocytes approach each other closely, within 2−4 nm – Transmit ionic currents (electrical excitation) between myocytes – Formed by hollow tubes, connexons, spanning cell membrane – Pairs of connexons from opposing myocytes unite end-to-end – Joined hemi-tubes create continuous channel – Increased intracellular acidity and [Ca2+] cause closure of some connexons; poor electrical coupling facilitates arrhythmia Cardiac Myocyte• Desmosomes – Provide mechanical strength by riveting myocytes together – Composed of cadherin, a transmembrane glycoprotein that spans 25 nm-wide space between myocytes – Cytoskeletal cables, intermediate/desmin filaments (made up of keratin), anchored internally to desmosomes – Desmin filaments run through myocyte, providing tensile strength Cardiac Myocyte• Myocyte packed with myofibrils, long contractile bundles ~1 μm wide • Myofibril composed of many basic contractile units, sarcomeres • Sarcomeres joined end-to-end, aligned in register across cell • Resting length of sarcomere is 1.8−2.0 μm Cardiac Myocyte• Sarcomere made up of filamentous proteins between 2 thin partitions, Z lines/discs • Two kinds of contractile filaments between Z lines – Thick myosin filament – Thin actin filament • Actin and myosin filaments interdigitate/interlock • Spring-like filaments of titin run from Z line to Z line – Align myosin filaments – Contribute to elasticity of heart wall Cardiac Myocyte• Thick myosin filaments – 1.6 μm long by 11 nm wide – Multiple filaments are arranged in parallel in central part of sarcomere – Each filament consists of ~400 myosin molecules – Myosin molecule resembles golf club with double head – “Handle” of myosin molecule lies along filament axis with double head protruding from side of filament Cardiac Myocyte• Thin actin filaments – 1.05 μm long by 6 nm wide – Interposed between myosin filaments – One end free in A band, other end attached to Z line – Filamentous actin (F-actin) is polymer of globular actin (G-actin) subunits bonded side-by-side – Thin filament consists of 2 F-actin strings, arranged as a 2-stranded helix – Groove of double helix contains tropomyosin, a regulatory protein – Regulatory complex composed of troponins attached to tropomyosin and actin at regular intervals – Tropomyosin-troponin complex plays key role in regulating contraction initiation Cardiac MyocyteCardiac Myocyte• Transverse tubules (T-tubules) – At each Z line, surface of cell membrane (sarcolemma) invaginated into set of narrow transverse tubules – Possess Na+ and Ca2+ channels, just like rest of sarcolemma – Transmit electrical excitation rapidly into interior of cell – Help activate numerous myofibrils virtually simultaneously – Well-developed in ventricular myocytes; scanty in atrial and Purkinje cells Cardiac Myocyte• Sarcoplasmic reticulum (SR) – Contained within muscle cytoplasm (sarcoplasm) – Closed set of anastomosing tubes, 20−60 nm wide, that surrounds myofibrils – Passes very close to sarcolemma of T-tubules or cell surface in places, but does not open to extracellular space – Contains store of Ca2+ that are partially released into sarcoplasm when cell electrically excited Cardiac Myocyte• SR Contains 2 functionally distinct regions, with different roles in Ca2+ handling – Junctional SR (Ca2+ release) – Network SR (Ca2+ uptake) • Junctional SR – Approaches to within 15 nm of sarcolemma of T-tubule/cell surface, forming a ‘diad’ – Lumen contains store of Ca2+ loosely attached to storage protein, calsequestrin – Ca2+-release channels extend from junctional SR towards sarcolemma of T-tubule or cell surface – Ca2+-release channels also called ryanodine receptors (RyR) or calcium-induced calcium release (CICR) channels Cardiac Myocyte• Network SR – Comprised of tubes that run over myofibrils – Actively takes up sarcoplasmic Ca2+ via abundant Ca2+-ATPase pumps – Ca2+-pump activity regulated by inhibitory protein, phospholamban Cardiac Myocyte• Sarcomeres shorten by a sliding-filament mechanism – Thin actin filaments slide into spaces between thick myosin filaments (or, vice versa) – Both I bands shorten, A band stays same length • Filaments propelled past each other by repeated formation, rotation, and breaking of biochemical bonds (crossbridges) between thin and thick filaments • Crossbridges formed by heads of myosin molecules attaching to thin actin filament Mechanism of Contraction• Each myosin head acts as an independent force generator • Swiveling action of hundreds of myosin heads per thick filament produce substantial force • Similar to rowing: – Boat (thick filament) moves up river (thin filament) – Boat propelled by rowers’ repeatedly placing, moving and withdrawing oar blades (myosin heads) Mechanism of Contraction• When myocytes excited electrically, Ca2+ released from junctional SR via Ca2+-release channels • Released Ca2+ activates contractile machinery by causing myosin-binding sites to be revealed on thin actin filament • Each actin subunit has a binding site for a myosin head • Binding sites blocked at rest by tropomyosin molecule Mechanism of Contraction• Tropomyosin – 42 nm-long protein – Lies in groove of F-actin double helix – Overlies seven G-actin subunits – Each tropomyosin molecule has a troponin complex attached to one end • Troponin complex composed of 3 units – Troponin C: binds Ca2+ – Troponin I: inhibits binding of myosin head – Troponin T: binds complex to tropomyosin Mechanism of Contraction• Electrical excitation of muscle cell causes release of Ca2+ from SR store • Produces rise of free Ca2+ concentration in sarcoplasm • Free Ca2+ binds to troponin C, causing conformational change of troponin complex