Cardiac Tissue PDF

CARDIAC TISSUE Fundamentals B Gopal J Babu, PhD Department of Cell Biology and Molecular Medicine Email: [email protected] Additional reading materials: Physiology of the Heart (5th Edition)- Arnold M. Katz Heart Physiology (4th Edition)- Opie Objectives 1. 2. 3. 4. 5. 6. 7. Anatomy and struct...

CARDIAC TISSUE Fundamentals B Gopal J Babu, PhD Department of Cell Biology and Molecular Medicine Email: [email protected] Additional reading materials: Physiology of the Heart (5th Edition)- Arnold M. Katz Heart Physiology (4th Edition)- Opie Objectives 1. 2. 3. 4. 5. 6. 7. Anatomy and structure Triggering cardiac contraction Calcium and contraction Metabolism Signal transduction Molecular basis of heart disease Cardiac Ischemia Part 1. Anatomy and structure Anatomy of the heart Each atrium receives blood from veins, the vena cava on the right side and the pulmonary veins on the left side, and ejects this blood into the corresponding ventricle Each ventricle ejects blood in an artery, the pulmonary artery on the right side and aorta on the left side. Reverse flow is prevented by valves The Tricuspid valve separates the right atrium from the right ventricle.  The heart is made of two sides, a right side and a left side  The right side and left side do not communicate directly, except abnormally in some forms of congenital heart disease Each side is made of an atrium and a ventricle separated by an atrioventricular valve The Pulmonic / Pulmonary valve separates the right ventricle from the pulmonary artery The Mitral (also known as the Bicuspid) valve separates the left atrium from the left ventricle. The Aortic valve separates the left ventricle from the ascending aorta Aortic valve and pulmonary valve-Semilunar valves Arrangement of Valves or His bundle Physiology of the heart by Arnold Katz Anatomy of the heart Myocardium Cardiac muscle is made of cardiac myocytes, the major part of the heart Endocardium Endothelial layer separating the myocardium from the blood Pericardium A protective sheet surrounding the myocardium 1) Lubricates the outer heart wall 2) Acts as a shock absorber, 3) Holds the heart in place, 4) Acts as a barrier to infections and 5) It helps prevent heart overexpansion Subendocardium Deep myocardial layers, adjacent to the endocardium Subepicardium Superficial myocardial layers, adjacent to the pericardium Coronary arteries Arteries from the aorta supply blood to the myocardium Capillaries Microvessels between the cardiac myocytes Pathophysiological importance of cardiac structures Myocardium Cardiomyopathy- Describes any form of dysfunctional myocardium (ischemic, hypertensive, congenital, valvular…). Heart failure- Insufficient cardiac function. Endocardium Endocarditis- Inflammation of the inner layer of the heart (bacteria, fungi). Pericardium Pericarditis- Inflammation of the pericardium, usually of viral origin. Bacterial, fungal, and other infections also can cause pericarditis. Induces strong pain that mimics a heart attack. Because the pericardium is fibrous and rigid, any effusion (pericardial effusion) will compress the myocardium (Cardiac Tamponade) and impair its function. Coronary arteries Ischemia- Obstruction of the coronary arteries leads to insufficient blood supply to the myocardium. Capillaries Angiogenesis- Myocardium submitted to chronic ischemic conditions stimulates the growth of neovessels and collaterals to improve oxygen supply. Valvular heart diseases Stenotic disease (narrowing) Insufficiency/regurgitation disease Aortic valve Aortic valve stenosis Aortic regurgitation Mitral valve Mitral valve stenosis Mitral regurgitation Tricuspid valve Tricuspid valve stenosis Tricuspid regurgitation Pulmonary valve Pulmonary valve stenosis Pulmonary regurgitation Aortic stenosis and Aortic regurgitation In aortic Stenosis, the valve doesn’t open. Common causes: congenitally abnormal, rheumatic fever, calcification of valves due to aging In aortic regurgitation, the valve opening does not close completely, causing blood to leak backward into the heart. Common causes: - high blood pressure - bacterial infection - untreated syphilis or injury - weakening of the valve tissue due to aging processes Blood Supply to the heart The first branches of the aorta are the coronary arteries, which provide the heart with blood supply.  The left main coronary artery is short in length. It is divided into the left anterior descending (LAD; Anterior interventricular artery) artery which travels along the intraventricular groove and the circumflex artery, which travels posteriorly along the groove between LA and ventricles. The right main coronary artery travels between the right atrium and ventricle The posterior descending artery (PDA) is a branch of the right coronary artery (80%) or can be a branch of the circumflex. These relatively narrow vessels are commonly affected by atherosclerosis and can become blocked, causing angina or a heart attack. The coronary sinus receives blood mainly from the small, middle, great, and oblique cardiac veins. It also receives blood from the left marginal and posterior ventricular veins. Pump function of the heart The heart ejects blood from the thick-walled LV to be propelled through the body, ultimately to reach peripheral circulation, where oxygen is removed to nourish the various organs and tissues. The deoxygenated venous blood flows back to the right side of the heart, to be ejected from the RV to the lungs, where it is oxygenated before it is directed toward the LA and LV The contractile unit of the heart is the cardiac myocyte EXCITATION COUPLING CONTRACTION The coupling between myocyte action potentials and contraction is called excitation-contraction (E-C) coupling The contractile unit of the heart is the cardiac myocyte Cardiac Muscle Ultrastructure The contractile function requires a tight coupling between cardiac myocytes The cardiac myocytes (CM) represent a type of striated muscle.  CMs are generally single-nucleated and have a diameter of 25mm and a length of about 100mm. Individual CMs are connected to each other by way of a specialized cell membrane called intercalated disks. Cardiac muscle synchronized electrically in an action potential- functional syncytium Three types of membrane-to-membrane contact The intercalated disk consists of three types of membrane-to-membrane contact. The predominant type of contact is the fascia adherens (FA) in which actin filaments at the ends of terminal sarcomeres insert into the fasciae adherens and thereby transmit contractile forces from cell to cell. Desmosomes - Protein complex that is linked to the sarcomeres by desmin, and which promotes force transfer. Gap Junctions, present mainly in the longitudinal portions of the interdigitations. They serve as low-resistant pathways between cells permitting cell-to-cell conduction of electrical current and small molecules. Gap Junctions Gap junctions result from the cell-cell interaction of transmembrane proteins forming the connexon, an assembly of 6 proteins called connexins. Connexin 43 is the major protein of the connexon. The contractile unit of the cardiac myocyte is the sarcomere Cardiac myocyte is composed of bundles of myofibrils that contain myofilaments. The segment between the Z-line represents the basic contractile unit of the myocyte, the sarcomere (length ranges from 1.6 to 2.2 mm, an important determinant of the force). Thick filament contains myosin, and several additional supporting proteins including titin, and MyBPC; Thin filaments contain actin, troponin complex (TnC, TnT, and TnI), and tropomyosin. Myosin filament originates from M-lines-do not attach directly to Z-line H band represents the zone containing only myosin filaments Lightly staining half, the I-band contains only thin filaments A band –overlap between myosin and actin and span the H band Composition of the sarcomere Titin: binds Z line to M line, prevents “overstretching” of the sarcomere Tropomodulin: caps actin filament and regulates its length-prevent actin monomers Nebulette: attaches actin filament to Z line MyBPC: attaches myosin to titin Z line proteins: α-actinin, desmin, CapZ protein M line proteins: Myomesin, M line protein, creatine kinase Connective Tissues Connective tissues- composed of collagen, with smaller amounts of elastin, laminin, and fibronectin. High-magnification micrograph of deep-etched replica showing the collagen fibril microthread meshwork. The intertangled network that bridges and wraps around the collagen fibrils is visible in three-dimensional array. Granules of ~8–10 nm diameter are apparent at branch points of the microfibril-microthread lattice (arrow) (353). Sarcomeres are connected to the plasma membrane Integrins are receptors that mediate the attachment between a cell and the tissues that surround the extracellular matrix (ECM). In signal transduction, integrins pass information about the chemical composition of the ECM into the cell. Therefore, they are involved in cell signaling and the regulation of cell cycle, shape, and motility. Talin is a high-molecular-weight cytoskeletal protein that binds to vinculin and integrin Vinculin is a cytoskeletal protein associated with cellcell and cell-matrix junctions. It consists of a globular head domain that contains binding sites for talin and αactinin (in the Z-line which binds actin) Cardiac contraction is triggered by the conduction system The Sino-Atrial Node is the natural pacemaker of the heart. From this node, the action potential or current spreads throughout the atria primarily through cell-to-cell conduction. This current cannot diffuse freely to the ventricles because they are separated by nonconducting connective tissues (central fibrous body) The only point available in the ventricles is the atrioventricular (AV) node, which contains specialized cells. This separation slows down the current to avoid simultaneous contraction of the atria and ventricles. The AV node distributes the current to the His (AV) bundle, which separates into one right and two left branches. The bundle branches divide into an extensive system of Purkinje fibers that conduct the current impulses at high velocity throughout the ventricles. The Purkinje fiber cells connect with ventricular myocytes, which become the final pathway for cell-to-cell conduction within the heart. Part 2. Triggering cardiac contraction Myocyte polarization  Electric changes within the myocyte initiate myocyte contraction Cardiac cells have an electrical potential across the cell membrane. It can be measured by inserting a microelectrode into the cell and measuring the electrical potential in millivolts (mV) inside the cell relative to the outside.  If measurements are taken with resting cardiac myocytes, the resting membrane potential (Em) will be -85mV. This is determined by the concentrations of positively and negatively charged ions across the cell membrane, the relative permeability of the cell membrane to these ions and ionic pumps that transport ions across the cell membrane. The conc. of Na+ and K+, are most important in determining the membrane potential K+, Na+, and Ca2+ balance of the cell at rest  K+ and Na+ determine the membrane potential at rest, whereas Ca2+ determines cell contraction.  At rest, the cell is rich in K+, and poor in Na+, the opposite being true in the extracellular milieu.  The Na+ / K+ imbalance is maintained by the Na+ / K+ pump.  At rest, the plasma membrane is impermeable to Ca2+, which creates a 103-104 fold gradient through the plasma membrane.  Intracellular Ca2+, which is low at rest maintained through Ca2+ extrusion through Na+/Ca+ exchanger, plasma membrane Ca2+ ATPase (PMCA) and sarcoplasmic reticulum Ca2+ ATPase (SERCA) The resting cardiomyocyte is negatively charged Na+ / K+ pump Na + Na+ (low conductance) In CMs, the conc. of K+ is the most important in determining the resting membrane potential. The conc. of K+ is high (150 mM) inside and low outside ( 4mM)-chemical gradient K+ The opposite situation is found for Na+ 145 mM). K+ (high conductance) The membrane potential (Em) is calculated as follows: Em = 61.5 ln (PK Ko/Ki + PNa Nao/Nai) P, conductance; o, extracellular; i, intracellular (outside- Through Na+ / K+ pump, the cell extrudes Na+ and accumulates K+ In such a cell, K+ diffuses out and leaves behind negatively charged proteins and potential differences across the membrane. Na+ tends to spontaneously enter the cell and does it slowly because its permeability is low. The Action Potential The action potential is the sequence of depolarization-repolarization that leads to cardiac cell contraction. The action potential is controlled by the influx or efflux of specific ions during a specific period. The action potential is transmitted from one cell to the next by a “domino effect”. Cardiac contraction is initiated by membrane depolarization  Cardiac contraction relies on an influx of Ca2+, but the plasma membrane is impermeable to Ca2+. To let Ca2+ come in first requires the loss of the membrane potential –membrane depolarization. Depolarization is initiated by an influx of Na+, rapidly followed by Ca2+ influx. Inflowing Ca2+ then triggers contraction by the release of endogenous Ca2+, (Ca2+ induced Ca2+ release), which leads to cardiac contraction. The five phases of the action potential 1 The action potential is controlled by the influx or efflux of specific ions during a specific period-of-time. 0 The action potential is divided into 5 phases Phase 0. Influx of Na+ (INa). Induces membrane depolarization Phase 1. Efflux of K+ (Ito). Limits the Na+ spike. Phase 2. Influx of Ca2+(ICa). Ca2+ enters the cell to trigger Ca2+-induced Ca2+ release. Phase 3. Efflux of K+ (IK). Repolarization starts Phase 4. Restoration of the resting potential 2 3 -85 mV 4 Cardiac pacemaker  The initiation of the action potential lies in the automatic pacemaker activity of the SA node, in which there is spontaneous depolarization.  The automaticity relies on specific cells in the SA node, the Pacemaker cells or P cells  The P cells have no true resting potential but instead generate regular, spontaneous action potentials. Currents in the SA node The shape of the action potential in the SA node is different from the shape described in the conduction system and in myocytes because the ion channels expressed in the P cells are different from other cardiac cells. ICa is an inward Ca2+ current depolarizing the P cell through a T-type Ca2+ channel (T for transient). Catecholamines (hormones released during stress) through the β-adrenergic receptor accelerate the depolarization of ICa and therefore accelerate heart rate (positive chronotropy). IK is a rectifier potassium current that repolarizes the cells after the Ica. Acetylcholine through the muscarinic receptor increases the repolarization of IK and therefore decreases heart rate (negative chronotropy). If (“funny current”) is an inward sodium current specific for the SA node that destabilizes the resting potential and therefore underlies the automaticity of the P cells. The lowest potential of P cells is –65 mV, compared to -85 mV in cardiac myocytes Part 3. Calcium and Contraction Contraction and Relaxation Cycle Systole = the ventricle contracts Diastole = the ventricle relaxes Chordae tendinae: 80% collagen and 20% elastin, prevent prolapse of the valve The sinoatrial (SA) node, which starts cardiac conduction, contracts causing atrial contraction. Atrioventricular valves are open. The atria empty blood into the ventricles. Semilunar (aortic) valves close  The ventricles contract  Atrioventricular valves close and semilunar valves open  Blood flows to either the pulmonary artery or the aorta. Calcium cycling in the heart Excitation-Contraction Coupling  In cardiac E-C coupling, a small amount of Ca2+ enters through the L-type Ca2+ channel (LTCC) during membrane depolarization  This Ca2+ influx triggers a large-scale release of Ca2+ from the Sarcoplasmic Reticulum Membrane via ryanodine receptor (RyR)  Released Ca2+ binds to myofibrillar proteins, TnC-induce muscle contraction.  Relaxation is initiated by the reuptake of Ca2+  This process is called Ca2+ induced Ca2+ release The Ca2+ induced Ca2+ release is coupled by the T tubules T tubules are tube-like invaginations of the plasma membrane inside the cardiac myocytes, where the DHP receptors (L-type Ca2+ channels) are highly expressed. Inside the cell, the T tubule is wrapped by the extremity of the sarcoplasmic reticulum (or cisternae), which expresses the ryanodine receptor. Three advantages of the T tubule (coupling, coupling, coupling) Excitation-contraction coupling- The T tubules increase the surface of the plasma membrane by 30%, which increases the density of DHP receptors and therefore the speed of EC coupling. Receptor coupling- The tight coupling between DHP receptors on the T tubule and ryanodine receptors on the sarcoplasmic reticulum optimizes the coupling between Ca2+ entry (through the DHP receptor) and Ca2+ release (through the ryanodine receptor). SR coupling- The deep invaginations of the T tubule inside the cell allow a synchronized release of Ca2+ from the entire sarcoplasmic reticulum compartment DHP = Dihydropyridine (e.g., nifedipine) Mechanisms of Ca2+ influx in the cardiac cytosol Ca2+ is released to the cytosol through two channels  Ca2+ crosses the plasma membrane through the DHP receptor (L-type channel). The inward current (ICa) through this channel is driven by the 103-fold gradient of [Ca2+] through the plasma membrane.  Ca2+ crosses the sarcoplasmic reticulum membrane through the Ryanodine receptor. These two channels are called “receptors” because they bind specific drugs. Mechanisms of Ca2+ reuptake Ca2+ is extruded from the cytosol through pumps and exchangers  Sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA). Pumps back Ca2+ inside the SR. This represents 90% of reuptake. SERCA function requires ATP to pump Ca2+ against the gradient. SERCA is inhibited by phospholamban (PLN) and the inhibitory effect is relieved upon phosphorylation of PLN by PKA or CaMKII The sarcolemmal Na+/Ca2+ exchanger (NCX). Pumps Ca2+ out of the cell through an exchange with Na+ (1 Ca to 3 Na). It represent 5% of calcium removal. The NCX does not use ATP because Na+ follows its spontaneous electrochemical gradient, which provides the energy. The mitochondrial Ca2+ uniporter (MCU). Pumps Ca2+ into the mitochondria, which represents about 2% of reuptake. The sarcolemmal Ca2+ ATPase. Pumps back Ca2+ in the extracellular milieu, which represents 1% of the reuptake mechanisms. This pump requires ATP to overcome the Ca2+ gradient. Composition of the sarcomere Thick filament: Myosin (Myosin heavy chains and light chains) Thin filaments: actin, tropomyosin, troponin complex Z line: Titin- binds the Z line to the M line, prevents “overstretching” of the sarcomere Tropomodulin: caps actin filament and regulates its lengthprevent actin monomers MyBPC: attaches myosin to titin Nebulette: attaches actin filament to Z line Z line: α-actinin, desmin, CapZ protein (a.k.a. b-actinin, which Stabilizes the actin filaments-protects from assembly and disassembly, and signaling) M line: Myomesin, M line protein, Creatine kinase The contractile proteins Cardiac contraction is performed by contractile proteins- myosin (heavy and light chains), actin, tropomyosin, troponin, and myosin binding proteins. The organization of these contractile proteins is maintained in the structure of the sarcomere Myosin is the molecular motor. It is a dimeric molecule containing a filamentous tail and a globular head, which contain ATPase and actinbinding site. Actin, a small globular protein forms a doublestranded macromolecular helix that makes the backbone of thin filaments. Actin slide to shorten the sarcomere. Cardiac contraction is controlled by regulatory proteins  Tropomyosin prevents actin-myosin interaction in the relaxation phase. Troponin complex regulates actin-myosin interaction in the contraction phase. Troponin C binds Ca2+ to activate contraction Troponin T moves tropomyosin to release its inhibitory effect Troponin I regulates Ca2+ release to initiate relaxation Actin-myosin interaction The crossbridge cycle The crossbridge represents the attachment of the myosin heads to a binding site on the actin filament. The crossbridge cycle consists of the repetitive attachment and detachment of myosin heads to and from actin F Explained by Sliding Filament Theory The crossbridge cycle (sliding filament theory) The crossbridge is initiated by the binding of Ca2+ (released from the SR) to TnC and is followed by four steps:  The myosin head attaches to actin  The power stroke bends the myosin head and actin slides  ATP binds to the myosin head, which is thereby released from actin  ATP is hydrolyzed, which energizes the myosin head to bind actin COCKED CROSS BRIDGE POWER STROKE Molecular basis of the crossbridge cycle  During diastole, ATP binds to myosin, which cannot interact with actin because tropomyosin is in the way. ATP is rapidly hydrolyzed by the myosin ATPase, but ADP and Pi remain in the nucleotide pocket. Upon Ca2+ stimulation, the troponin complex moves tropomyosin. The energy released by ATP hydrolysis and Pi released from the pocket create a strong actin-myosin binding. De-energized myosin bends its head to come back to a resting position and thereby creates the power stroke that slides the actin filament. The change in conformation releases ADP. ATP binds to the myosin head, relaxes the actinmyosin bond and ATP hydrolysis re-extends the myosin head. ATP is rapidly hydrolyzed by the myosin ATPase. A new cycle can be repeated if TnC still binds Ca2+. If Ca2+ reuptake by SERCA has begun, tropomyosin comes back to its original position and the actinmyosin complex remains in a relaxed state. Part 4. Metabolism Energy cost The energy requirement of the heart is reflected by dense vascularization and an abundance of mitochondria (30%). The heart consumes a vast quantity of energy. It cycles ~6 kg of ATP/day Energy is obtained from the oxidation of fat, carbohydrates, and to a minor extent from proteins. The energy made from a given amount of O2 is similar for all the substrates- fat ~9 cal/gm, carbohydrates ~4 cal/gm. However, more O2 is consumed by the oxidation of fat. This can be estimated by measuring the consumption of O2 by the heart Cardiac efficiency= external work/ energy equivalent of O2 consumed Cardiac pathology is associated with a reduction in cardiac energy status-involves impaired energy generation and/or inefficient energy utilization Determinants of cardiac metabolism WORKLOAD Cardiac myocytes have an exceptionally high metabolic rate because their primary function is to contract repetitively. Cardiac muscle contracts 1 to 3 times per second. Due to this constant contractility, the heart requires a considerable amount of ATP to feed the ATPase activities of the myosin head (50%), SERCA (25%), and the Na+ / K+ pump (15%) SUBSTRATES To sustain its energy supply, the heart is a “metabolic omnivore”, using every substrate that provides ATP. Substrates interact with each other to coordinate metabolic activity. The two main substrates for the heart are fatty acids in fasting conditions and glucose in fed conditions. The heart also uses lactate during exercise. OXYGEN A sufficient supply of ATP can be achieved only through oxidative phosphorylation, which requires an aerobic metabolism. In conditions of oxygen deprivation (ischemia), the heart must shift toward an anaerobic metabolism, which provides far less ATP per mole of substrate. Glucose metabolism Glucose concentration is high in the circulating blood, particularly in the fed state, and glucose utilization is stimulated by insulin. Glucose is carried into the cardiac myocyte through specific glucose transporters (GLUT). The cardiac myocyte expresses the ubiquitous GLUT1 and the insulinregulated GLUT4. Inside the cardiac cell, glucose phosphorylated to prevent any exit. is immediately Phosphorylated glucose can be stored in the form of glycogen or degraded through the glycolytic pathway that ends up with pyruvate. A minor component enters the pentose-phosphate pathway. Pyruvate is taken in the mitochondria, where it is oxidized through the tricarboxylic acid (Krebs) cycle The regulatory steps of glycolysis are  Glucose uptake through specific transporters (GLUT) Phosphohexose isomerase  Glucose phosphorylation through hexokinase (HK)  Hexose phosphate phosphorylation through phosphofructokinase (PFK-1)  Triose phosphate oxidation through glyceraldehyde-3-phosphate dehydrogenase (GAPDH)  Pyruvate oxidation through pyruvate dehydrogenase (PDH)  Glycolysis can be fed from glycogen, which is regulated by glycogen phosphorylase *6 PDH Fatty acid metabolism Fatty acid concentration is high in the circulating blood, particularly in the fasted state. Protein-bound fatty acids are taken by lipoprotein lipase and cross the plasma membrane. Inside the cardiac cell, fatty acids can be stored in the form of triglycerides. Most of fatty acids are transferred to the mitochondria upon binding to the specific carrier carnitine by carnitine palmitoyl transferase (CPT). Inside the mitochondria, fatty acids are oxidized by the β-oxidation chain that produces acetyl-CoA to feed the tricarboxylic acid (Krebs) cycle. When fatty acids are oxidized, glucose oxidation is inhibited, and glucose taken up by the cardiomyocyte is converted into glycogen. Substrate interactions FASTED STATE FED STATE The heart permanently adapts its substrate utilization according to the plasma concentration of substrates. It is not an “all-or-nothing” phenomenon, the substrates constantly interact Metabolic remodeling and the development of heart failure Pathological hypertrophy in response to mechanical overload (hypertension, valvular disease, or postmyocardial infarction)- Decreases in fatty acid oxidation (FAO) and increases in glycolysis. This fetal-like metabolic profile decreases the capacity for ATP synthesis, consistent with the energy starvation model. Obese with diabetes mellitus leads to an upregulation of FAO with a concomitant decrease in glucose oxidation (Glc ox). This lipid overload condition impairs cardiac efficiency. Part 5. Signal transduction Catecholamines  Catecholamines are the major mediators of the cardiac response to increased work (positive chronotropy).  Catecholamines are synthesized from the amino acid tyrosine  Epinephrine (adrenaline) is released by the adrenal medulla (systemic). Binds to β-adrenergic receptors. Has a global cardio-vascular effect (“fight or flight”). Norepinephrine (noradrenaline), a paracrine factor is released by the stellate ganglion (a sympathetic ganglion formed by the fusion of the inferior cervical ganglion and the first thoracic ganglion. Stellate ganglion is located at the level of C7 (7th cervical vertebrae). It binds to β-adrenergic receptors in heart. Catecholamines activate a G protein The G protein is a GTPase that spontaneously degrades GTP into GDP. When binding GDP, the G protein is in an inactive, trimeric state. β-receptor stimulation displaces GDP by GTP and the subunit α translocates to adenylate cyclase. Inactivation of the system is performed by the GTPase hydrolysis of GTP into GDP and reunification of the trimeric protein. Function of β-adrenergic receptors Cyclic AMP releases the catalytic subunit of PKA from the regulatory subunit, which activates the enzyme Cyclic AMPdependent protein kinase (PKA) c Active PKA Targets of PKA Increases the heart rate (the heart beats faster) L-type Ca2+ channel MLC-2 Increased inotropy (contraction) Phospholamban Troponin I Increased lusitropy (relaxation) Glucose transporters PFK-2, the enzyme producing fructose 2,6-bisphosphate Glycogen synthase kinase, phosphorylase kinase cAMP-responsive element binding protein (CREB) Increased production of ATP from glucose Increased heart mass (hypertrophy) Acetylcholine Acetylcholine controls cardiac activity at rest The chief function of acetylcholine is to act as an antagonist of catecholamines, by reducing the formation of cAMP in response to catecholamine stimulation. Acetylcholine acts by two main mechanisms: Blocks the effects of catecholamines on cardiac contraction (inotropy) Slow down heart rate by inhibiting the sino-atrial node (chronotropy) Acetylcholine is the main modulator of cardiac contractile activity at rest (negative chronotropy), whereas catecholamines are the main modulator of cardiac activity when workload increases (positive chronotropy) Acetylcholine binds the muscarinic receptor Acetylcholine is released from the vagal nerve and binds to its specific receptor, the muscarinic receptor, which is inhibited by atropine. The receptor is coupled to a trimeric protein Gi. The binding of acetylcholine to the receptor dissociates the trimeric Gi protein in a Giα subunit and a β−γ complex. In the myocardium, the α subunit binds to adenylate cyclase and decreases its enzymatic activity, whereas the β−γ complex inhibits Gsα. The combined action decreases the β-adrenergic stimulation and thereby decreases cardiac contractility. In the SA node, the Gi β−γ complex opens a K+ channel that inhibits the rate of spontaneous depolarization and thereby decreases the heart rate (negative chronotropy) Part 6. Molecular basis of heart disease Cardiac hypertrophy Hypertrophy is the mechanism by which cells increase in size. This is opposed to hyperplasia, which is the mechanism leading to increased cell number. Increased cardiac workload requires an increase in the contractile capacity of the heart. Because of their limited mitotic capacity, if any, cardiac myocytes respond to increased workload by hypertrophy. A hypertrophied cardiac myocyte accumulates more sarcomeres, which improves its contractile capacity but also increases its energy needs Physiological hypertrophy- Chronic exercise, pregnancy Pathological hypertrophy- Pressure or volume overload Pathological Cardiac Hypertrophy Gross specimen of the heart with concentric left ventricular hypertrophy. Hypertrophy is initially an adaptive mechanism by which an increased contractile demand is matched by an increased number of sarcomeres in the cardiac cell. Chronically, however, this increased contractile capacity becomes maladaptive and leads to cardiac dysfunction or heart failure. Chronic cardiac hypertrophy therefore represents the most common cause and origin of heart failure. Causes of cardiac overload Hypertensive heart Normal heart Hypertension- Increased stiffness in conductance vessels and/or increased peripheral vascular resistance requires a higher ventricular systolic pressure development. Myocardial infarction- The irreversible damage of a part of the myocardium requires the remaining myocytes to work more. Causes of cardiac overload Valve dysfunction- Aortic stenosis increases developed pressure. Aortic and mitral regurgitations increase cardiac volume. Aortic valve should have three tissue leaflets-should open-and-close with a tight seal. Prevents blood in the aorta from returning to the left ventricle. Many patients diagnosed with aortic stenosis were born with a bicuspid aortic valve. Congenital diseases- Abnormal communications between cardiac cavities require a higher blood flow. Genetic mutations in contractile proteins require more sarcomeres. Tetralogy of Fallot (ToF) Atrial Septal Defect (ASD) A birth defect with 4 parts Congenital Diseases Ventricular Septal Defect (VSD) Patent ductus arteriosus (PDA) An abnormal opening between the aorta and the pulmonary artery. Mechanisms associated with Cardiac hypertrophy Cardiac hypertrophy results from an accumulation of sarcomeres in the cardiac cell, which is made possible by the coordinated activation of different mechanisms Increased protein synthesis, necessary to accumulate more sarcomeric proteins Adaptation of gene expression, to upregulate the transcription of genes encoding contractile proteins Activation of signaling pathways, to trigger the adaptation of gene expression (eg, calcineurin), the increased capacity of protein synthesis (eg, PI-3-kinase), or both (eg, MAP kinases). Activation of sensors and receptors, to activate the signaling pathways in response to overload (eg, G proteins or integrin). Hypertrophy promotes angiogenesis and fibrosis  Because the hypertrophied myocytes increase the space between capillaries, this stimulates neocapillarization, which in turn improves oxygen supply to the hypertrophied muscle The interstitium will be filled with denser collagen produced from interstitial fibroblasts. This collagen maintains a better tension of the myocardium during systole but impairs its relaxation during diastole Part 7. Cardiac Ischemia Definition of myocardial ischemia Ischemic heart disease is caused by an imbalance between the myocardial blood flow (reduced O2 supply) and the metabolic demand of the myocardium. Reduction in coronary blood flow is related to progressive atherosclerosis with increasing occlusion of coronary arteries resulting in myocardial dysfunction. The most likely cause of myocardial ischemia is an obstruction of coronary arteries by atherosclerosis resulting in: Angina- The coronary artery is partially occluded. The residual flow is insufficient to meet the energy needs of the myocardium. Myocardial infarction- The coronary artery is totally occluded. There is no residual flow at all. The damaged myocardium will die. Atherosclerosis Atherosclerosis is the leading cause of cardiovascular disease. It is a progressive disease characterized by the accumulation of cholesterol-rich lipids (atheroma) in the intima of large arteries and coronary arteries. In coronary arteries, the development of the atheroma reduces the artery lumen diameter and reduces cardiac blood perfusion. Progression of atherosclerosis can lead to the occlusion of the diseased coronary artery and stop blood perfusion. The atherosclerotic plaque is made of a lipid pool (atheroma) recovered by fibrous tissue (fibrous cap). The plaque protrudes in the lumen of the coronary artery as the atheroma accumulates Normal artery Atherosclerotic artery Fibrous cap Lipid pool (atheroma) Atherosclerosis Atherosclerosis is a slow and progressive building up of plaque, fatty substances, cholesterol, cellular waste products, calcium and fibrin in the inner lining of an artery. This building up of plaque may lead to thickening and hardening of the arteries, subsequently blocking the blood flow either partially or totally in an artery. Types of Atherosclerosis Coronary Artery Disease (CAD): When plaque builds up in the coronary arteries, supply of oxygen rich blood to the heart is reduced leading to chest pain and ultimately a heart attack. Carotid Artery Disease or Cerebrovascular Disease: When plaque builds up in the carotid arteries, the supply of oxygen rich blood to the brain is reduced leading to a stroke. Peripheral Arterial Disease (PAD): When plaque builds up in the arteries supplying blood to the leg, arms, and pelvis, the oxygen-rich blood supply to these parts is restricted leading to numbness, pain, and dangerous infections. Abdominal Angina and Bowel Infarction: Atherosclerosis leads to the narrowing of the arteries supplying blood to the intestines causing abdominal pain and is called abdominal angina. Complete or sudden blockage of blood supply to the intestines leads to bowel infarction. Factors reducing coronary blood flow Decreased aortic diastolic pressure Increased intraventricular pressure and myocardial contraction Coronary artery stenosis, which can be further subdivided into the following etiologies: Fixed coronary stenosis Acute plaque change (rupture, hemorrhage) Coronary artery thrombosis (blood clot inside a blood vessel) Vasoconstriction Aortic valve stenosis and regurgitation (reverse flow of blood from aorta to LV during contraction) Increased right atrial pressure Symptoms of cardiac ischemia Pain- Due to the stimulation of nerve endings by adenosine release. Crushing pain on the left part of the chest, extending typically to the left arm, and creating a feeling of anxiety (“angina”) Syncope- transient loss of consciousness (fainting). Due to arrhythmias, vagal stimulation or massive myocardial dysfunction. Nausea- Due to an irritation of the diaphragm by an inflammatory reaction. Sometimes, nausea/vomiting is the only symptom of a heart attack. Irregular heartbeats- Signs of arrhythmias created by ischemia of the conduction system. Dyspnea- Difficulty to breathe. Due to the accumulation of blood in the right circulation following a dysfunction of the left ventricle. This can be followed by pulmonary edema (effusion of plasma inside the lungs) Functional consequences of ischemia Cardiac ischemia induces both systolic and diastolic cardiac dysfunction Systolic dysfunction is characterized by impaired contraction. Contraction decreases because the lack of oxygen and blood flow leads to an accumulation of end-products (inorganic phosphate, protons, lactate…) and a loss of K+ that impair the function of contractile proteins. Diastolic dysfunction is characterized by impaired relaxation. The lack of ATP impairs the function of the myosin ATPase, SERCA and the Na+/ K+ ATPase. Impaired myosin ATPase leads to impaired relaxation of the myofilament Impaired SERCA activity leads to increased [Ca2+] in diastole, and therefore, to impaired relaxation Impaired Na+/ K+ pump activity leads to accumulation of cytosolic Na+ Consequences of myocardial infarction The lost myocytes are replaced by non-contractile and dense connective tissue that stretches the ventricle. Consequently, wall stress is increased, and contraction is impaired. This progressively leads to a dilation of the ventricle followed by impairment of cardiac function, or heart failure.