Topic 16: Cardiac Muscle Properties Structural tissue cellular molecular Functional electrophysiological mechanical Experimental 2-Mar-1999: This filled the time very comfortably; could even have added a little more. It was very hastily put together and could do with some polishing and filling out in places especially regarding the testing (Ming's thesis?). It was based on this original outline: Cardiac Muscle mechanics: -Differences between cardiac and skeletal muscle -structural cell geometry mitochondria myosin isoforms regulatory proteins titin cell-cell connections: intercalated disks branching fiber/sheet architecture -functional functional syncytium (action potential propagation) cardiac action potential plateau - refractoriness can not be tetanized spontaneous rhythmicity pacemaker activity higher resting tension can not be stretched as much no descending limb -experimental difficult to test requires sophisticated preparations and techniques standard preparations: isolated papillary muscle (nonuniform) Sara isolated trabeculae (Henk) techniques: spots laser diffraction (grating equation) see Ming's thesis previously throught to have higher SE, tethering effects Cardiac Muscle uniaxial data isometric length dependendent activation isotonic quick release (ter Keurs) Hill's equation fit (Julius) shortening deactivation (ter Keurs, Julius) tension recovery calcium transient (Julius) Calcium handling in the cell (Markus Meyer/Bers)

Myocytes Rod-shaped Striated 80-100 m long 15-25 m diameter

Myocyte Connections Myocytes connect to an average of 11 other cells (half end-to-end and half side-to-side) Functional syncytium Myocytes branch (about 12-15º) Intercalated disks gap junctions connexons connexins

Muscle Fiber Architecture Muscle Band Torrent-Guasp, 1995 Streeter et al. (1969) endocardium midwall epicardium Continuum

Laminar Sheet Structure x1050 x510 x145

Cleavage Planes Circumferential-Radial sections Longitudinal-Radial sections

Cleavage Planes are Produced by Sheets

Myocyte Ultrastructure Sarcolemma Mitochondria (M) ~30% Nucleus (N) Myofibrils (MF) Sarcoplasmic Reticulum and T-tubule network

Cardiac Myofilaments Slow cardiac myosin isoforms Cardiac troponin Titin

Cardiac Action Potential Plateau phase 2 Refractory period absolute relative Calcium current Cardiac muscle can not be tetanized

Excitation-Contraction Coupling Calcium-induced calcium release Calcium current Na+/Ca2+ exchange Sarcolemmal Ca2+ pump SR Ca2+ ATP-dependent pump  Click image to view animation of calcium cycling http://www.meddean.luc.edu/lumen/DeptWebs/physio/bers.html

Muscle testing Tissue preparation right ventricular muscle isolated rat trabecula 4000x200x90 mm Force transducers piezoresistive 1 V/mN capacitive 0.1 V/mN Displacement variable mutual inductance transducer Sarcomere length, L 1 mW He-Ne laser (l = 632 nm) L = kl/d Diffraction spacing, d linear photodiode array lateral effect photodetector

Cardiac Muscle Testing Cardiac muscle is much more difficult to test than skeletal muscle: tissue structure is complex and 3-D long uniform preparations with tendons attached are not available the best preparations are isolated papillary muscles (which hold atrioventricular valves closed during systole) and isolated trabeculae, which are more uniform but very small cardiac muscle branching scatters light making laser diffraction more difficult intact cardiac muscle can not be tetanized so it must be tested dynamically or artificially tetanized

Isometric testing At constant muscle length, muscle preparation shortens in the middle at the expense of lengthening at the damaged ends

Isometric Testing 2.1 2.0 1.9 Sarcomere length, mm Sarcomere isometric Muscle isometric 2.0 Tension, mN time, msec 200 100 300 500 700 600 400 1.0

Isometric Length-Tension Curve Peak developed isometric twitch tension (total-passive) High calcium Low calcium sarcomere isometric muscle isometric Passive

Length-Dependent Activation Isometric peak twitch tension in cardiac muscle continues to rise at sarcomere lengths >2 mm due to sarcomere-length dependent increase in myofilament calcium sensitivity

Isotonic Testing Isovelocity release experiment conducting during a twitch Cardiac muscle force-velocity relation corrected for viscous forces of passive cardiac muscle which reduce shortening velocity

Cardiac Muscle: Summary Cardiac muscle fibers (cells) are short and rod-shaped but are connected by intercalated disks and collagen matrix into a spiral-wound laminar fibrous architecture The cardiac sarcomere is similar to the skeletal muscle sarcomere Cardiac muscle has a very slow twitch but it can not be tetanized because the cardiac action potential has a refractory period Calcium is the intracellular trigger for cardiac muscle contraction Cardiac muscle testing is much more difficult than skeletal muscle: laser diffraction has been used in trabeculae Cardiac muscle has relatively high resting stiffness (titin?) The cardiac muscle isometric length-tension curve has no real descending limb