DEVELOPMENTAL BIOLOGY OF THE CARDIOVASCULAR SYSTEM
The heart forms early during embryogenesis (Fig. 232-1), circulating blood, nutrients, and oxygen to the other developing organs while continuing to grow and undergo complex morphogenetic changes. Early cardiac progenitors arise within crescent-shaped fields of lateral splanchnic mesoderm under the influence of multiple signals and migrate to the midline to form the linear heart tube: a single layer of endocardium and a single layer of cardiomyocyte precursors.
A. Schematic depiction of a transverse section through an early embryo depicts the bilateral regions where early heart tubes form. B. The bilateral heart tubes subsequently migrate to the midline and fuse to form the linear heart tube. C. At the early cardiac crescent stage of embryonic development, cardiac precursors include a primary heart field fated to form the linear heart tube and a second heart field fated to add myocardium to the inflow and outflow poles of the heart. D. Second heart field cells populate the pharyngeal region before subsequently migrating to the maturing heart. E. Large portions of the right ventricle and outflow tract and some cells within the atria derive from the second heart field. F. The aortic arch arteries form as symmetric sets of vessels that then remodel under the influence of the neural crest to form the asymmetric mature vasculature. LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.
The linear heart tube undergoes asymmetric looping, that coordinates with chamber specification and multilayer growth of different regions of the heart tube to produce the presumptive atria and ventricles. Cells continue to migrate into the heart at both ends from later, or second, heart fields in pharyngeal mesoderm as looping and growth occur. These cells exhibit distinctive gene expression (e.g., Islet-1) and distinctive physiology (e.g., calcium handling), contributing to discrete areas of the adult heart, including the right atrium and the right ventricle. Different embryologic origins of cells within the right and left ventricles help explain why some forms of congenital and adult heart diseases affect regions of the heart to varying degrees.
After looping and chamber formation, a series of morphogenetic events divide the left and right sides of the heart, separate the atria from the ventricles, and form the aorta and pulmonary artery from the truncus arteriosus. Cardiac valves form between the atria and the ventricles and between the ventricles and the outflow vessels. Early in development, the single layer of myocardial cells secretes an extracellular matrix rich in hyaluronic acid, or “cardiac jelly,” which accumulates within the endocardial cushions, precursors of the cardiac valves. Signals from overlying myocardial cells trigger migration, invasion, and phenotypic changes in underlying endocardial cells, which undergo an epithelial-mesenchymal transformation to invade and populate the endocardial cushion matrix with cells. Mesenchymal cells then proliferate and form the mature valve leaflets.
The great vessels form as a series of bilaterally symmetric aortic arch arteries that remodel asymmetrically to define the mature central vasculature. Migrating neural crest cells from the dorsal neural tube orchestrate this process and are necessary for aortic arch remodeling and the septation of the truncus arteriosus. The smooth-muscle cells within the tunica media of the aortic arch, the ductus arteriosus, and the carotid arteries all derive from neural crest. By contrast, smooth-muscle within the descending aorta arises from lateral plate mesoderm, and smooth muscle of the proximal outflow tract arises from the second heart field. Neural crest cells are sensitive to both vitamin A and folic acid, and congenital heart disease involving abnormal remodeling of the aortic arch arteries can associate with maternal deficiencies of these vitamins. The shared embryologic origins of different cardiovascular cell types lead to syndromic associations between various congenital heart diseases and a range of extracardiac abnormalities.
Coronary artery formation requires the addition of yet another cell population to the embryonic heart. Epicardial cells arise in the proepicardial organ, a derivative of the septum transversum, which also contributes to the fibrous portion of the diaphragm and to the liver. Proepicardial cells contribute smooth-muscle to the coronary arteries and are required for proper coronary patterning. Other cell types within the heart, (e.g., fibroblasts) also can arise from the proepicardium.
The cardiac conduction system, which generates and propagates electrical impulses, differentiates from cardiomyocyte precursors. The conduction system is composed of slow-conducting (proximal) components, such as the sinoatrial (SA) and atrioventricular (AV) nodes, as well as fast-conducting (distal) components, including the His bundle, bundle branches, and Purkinje fibers. Precursors within the sinus venosus give rise to the SA node, whereas those within the AV canal mature into heterogeneous cell types that compose the AV node. So-called decremental conduction through the AV node delays the electrical impulses between atria and ventricles, whereas the distal conduction system rapidly delivers the impulse throughout the ventricles. Each compartment within the conduction system expresses distinct gap junction proteins and ion channels that characterize the discrete cell fates and electrical properties. Developmental defects in the conduction system can lead to clinical electrophysiologic disorders, such as congenital heart block or pre-excitation (Wolff-Parkinson-White syndrome) (Chap. 241).
As noted above, smooth-muscle cells in various types of artery derive from different sources. Some upper-body arterial smooth-muscle cells derive from the neural crest, whereas lower-body arteries generally recruit smooth-muscle cells from neighboring mesodermal structures during development. Bone marrow–derived endothelial progenitors may aid repair of damaged or aging arteries. In addition, multipotent vascular stem cells resident in vessel walls may give rise to the smooth-muscle cells that accumulate in injured or atheromatous arteries (Chaps. 92 and 473).
Blood vessels participate in physiologic function and play roles in disease biology in virtually every organ system. The smallest blood vessels—capillaries—consist of a monolayer of endothelial cells on a basement membrane, adjacent to a discontinuous layer of smooth-muscle-like cells known as pericytes (Fig. 232-2A). Arteries typically have a trilaminar structure (Fig. 232-2B–E). The intima consists of a monolayer of endothelial cells continuous with those of the capillaries. The middle layer, or tunica media, consists of smooth-muscle cells, in veins, the media can contain just a few layers of smooth-muscle cells (Fig. 232-2B). The outer layer, the adventitia, consists of looser extracellular matrix with fibroblasts, mast cells, and nerve terminals. Larger arteries have their own vasculature, the vasa vasorum, which nourishes the tunica media.
Schematics of the structures of various types of blood vessels. A. Capillaries consist of an endothelial tube in contact with a discontinuous population of pericytes. B. Veins typically have thin medias and thicker adventitias. C. A small muscular artery features a prominent tunica media. D. Larger muscular arteries have a prominent media with smooth-muscle cells embedded in a complex extracellular matrix. E. Larger elastic arteries have cylindrical layers of elastic tissue alternating with concentric rings of smooth-muscle cells.
Arteriolar muscle tone regulates blood pressure and flow through arterial beds (Fig. 232-2C). Medium-size muscular arteries also contain prominent smooth muscle layers (Fig. 232-2D) that participate in atherosclerosis. Larger elastic arteries have a highly structured tunica media with concentric bands of smooth-muscle cells, interspersed with strata of elastin-rich extracellular matrix (Fig. 232-2E). Larger arteries form an internal elastic lamina between intima and media while an external elastic lamina partitions media from surrounding adventitia.
The endothelium forms the interface between tissues and the blood compartment, regulating the passage of molecules and cells. The ability of endothelial cells to serve as a selectively permeable barrier fails in vascular diseases, including atherosclerosis, hypertension, and renal disease, as well as in pulmonary edema, sepsis and other situations of “capillary leak.”
The endothelium also participates in the local regulation of vascular tone and blood flow. Endogenous substances produced by endothelial cells such as prostacyclin, endothelium-derived hyperpolarizing factor, nitric oxide (NO), and hydrogen peroxide (H2O2) provide tonic vasodilatory stimuli under physiologic conditions in vivo (Table 232-1). Impaired production or excess catabolism of NO impairs endothelium-dependent vasodilator function contributing to pathologic vasoconstriction. Measurement of flow-mediated dilatation can assess endothelial vasodilator function in humans (Fig. 232-3). Endothelial cells also produce potent vasoconstrictor substances such as endothelin. Excessive production of reactive oxygen species, such as superoxide anion (O2−), by endothelial or smooth-muscle cells under pathologic conditions (e.g., excessive exposure to angiotensin II), can promote local oxidative stress and inactivate NO.
TABLE 232-1Endothelial Functions in Health and Disease ||Download (.pdf) TABLE 232-1 Endothelial Functions in Health and Disease
|Homeostatic Properties ||Dysfunctional Properties |
|Optimize balance between vasodilation and vasoconstriction ||Impaired dilation, vasoconstriction |
|Antithrombotic, profibrinolytic ||Prothrombotic, antifibrinolytic |
|Anti-inflammatory ||Proinflammatory |
|Antiproliferative ||Proproliferative |
|Antioxidant ||Prooxidant |
|Permselectivity ||Impaired barrier function |
Assessment of endothelial function in vivo using blood pressure cuff occlusion and release. Upon deflation of the cuff, an ultrasound probe monitors changes in diameter (A) and blood flow (B) of the brachial artery (C). (Reproduced with permission of J. Vita, MD.)
Normal endothelium exhibits limited interaction with circulating leukocytes, but when activated by bacterial products such as endotoxin or by proinflammatory cytokines released during infection or injury, endothelial cells express an array of adhesion molecules that selectively bind various classes of leukocytes in different pathologic conditions. The adhesion molecules and chemokines generated during acute bacterial infection tend to recruit granulocytes, while in chronic inflammatory diseases such as tuberculosis or atherosclerosis, the adhesion molecules expressed favor monocyte recruitment. Endothelial cells participate in the pathophysiology of many immune-mediated diseases. Complement-mediated lysis of endothelial cells is an example of immunologically mediated tissue injury. The presentation of foreign histocompatibility complex antigens by endothelial cells in solid-organ allografts can promote allograft arteriopathy, while immune-mediated endothelial injury also plays a role in thrombotic thrombocytopenic purpura or hemolytic-uremic syndrome.
The endothelium also regulates the balance between thrombosis and hemostasis through a highly tuned set of regulatory pathways. When activated by inflammatory cytokines, bacterial endotoxin, or angiotensin II, for example, endothelial cells can produce substantial quantities of the major inhibitor of fibrinolysis, plasminogen activator inhibitor 1 (PAI-1). Thus, in pathologic circumstances, the endothelial cell may promote local thrombus accumulation rather than combat it. Inflammatory stimuli also induce the expression of the potent procoagulant tissue factor, a contributor to disseminated intravascular coagulation in sepsis.
Endothelial cells regulate the growth of subjacent smooth-muscle cells. For example, heparan sulfate glycosaminoglycans elaborated by endothelial cells can inhibit smooth-muscle proliferation, and in the setting of injury endothelial cells produce growth factors and chemoattractants, such as platelet-derived growth factor, which cause the migration and proliferation of vascular smooth-muscle cells. Dysregulation of these growth-stimulatory molecules may promote smooth-muscle accumulation in atherosclerotic lesions.
Vascular Smooth-Muscle Cell
Contraction and relaxation of vascular smooth-muscle cells in muscular arteries determines blood pressure, regional flow and the afterload experienced by the left ventricle (see below). Venous tone regulates the capacitance of the venous tree and so influences ventricular preload. Smooth-muscle cells in the adult vessel seldom replicate in the absence of arterial injury or inflammatory activation, but proliferation and migration of arterial smooth-muscle cells contributes to arterial stenoses in atherosclerosis, arteriolar remodeling in hypertension, and the hyperplastic response of arteries injured by percutaneous intervention. In the pulmonary circulation, smooth-muscle migration and proliferation underlie the vascular disease that occurs in sustained high-flow states such as left-to-right shunts in congenital heart disease.
Smooth-muscle cells secrete the bulk of vascular extracellular matrix. Excessive production of collagen and glycosaminoglycans contributes to the remodeling, altered biomechanics and physiology of arteries affected by hypertension or atherosclerosis. In larger elastic arteries, such as the aorta, the ability to store the kinetic energy of systole promotes tissue perfusion during diastole. Arterial stiffness associated with aging or disease, evident in a widening pulse pressure, increases left ventricular afterload and portends a poor outcome.
Like endothelial cells, vascular smooth-muscle cells do not merely respond to vasomotor or inflammatory stimuli elaborated by other cell types, but can themselves serve as a source of such stimuli. For example, when exposed to proinflammatory stimuli, smooth-muscle cells elaborate cytokines and other mediators which drive thrombosis and fibrinolysis as well as proliferation.
Vascular Smooth-Muscle Cell Contraction
Vascular smooth-muscle cells contract as cytoplasmic calcium concentration rises due to transmembrane influx and triggered release from intracellular calcium stores (Fig. 232-4). In vascular smooth-muscle cells, voltage-dependent L-type calcium channels open with membrane depolarization. Local changes in intracellular calcium concentration, termed calcium sparks, can trigger release from intracellular stores which results in more contraction and higher vessel tone (see below). Opposing currents balance the effects of individual ionic fluxes, promoting homeostasis which is tightly regulated by neural and metabolic influences.
Regulation of vascular smooth-muscle cell calcium concentration and actomyosin ATPase-dependent contraction. AC, adenylyl cyclase; Ang II, angiotensin II; ANP, atrial natriuretic peptide; DAG, diacylglycerol; ET-1, endothelin-1; G, G protein; IP3, inositol 1,4,5-trisphosphate; MLCK, myosin light chain kinase; MLCP, myosin light chain phosphatase; NE, norepinephrine; NO, nitric oxide; pGC, particular guanylyl cyclase; PIP2, phosphatidylinositol 4,5-bisphosphate; PKA, protein kinase A; PKC, protein kinase C; PKG, protein kinase G; PLC, phospholipase C; sGC, soluble guanylyl cyclase; SR, sarcoplasmic reticulum; VDCC, voltage-dependent calcium channel. (Modified from B Berk, in Vascular Medicine, 3rd ed. Philadelphia, Saunders, Elsevier, 2006, p. 23; with permission.)
Biochemical agonists also increase intracellular [Ca2+] by various mechanisms including receptor-dependent phospholipase C activation with hydrolysis of phosphatidylinositol 4,5-bisphosphate to generate diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). These membrane lipid derivatives in turn activate protein kinase C and increase intracellular [Ca2+]. In addition, IP3 binds specific sarcoplasmic reticulum (SR) receptors to increase calcium efflux from this storage pool into the cytoplasm.
Vascular smooth-muscle cell contraction depends on myosin light chain phosphorylation, which in the steady state reflects the balance between the actions of the relevant kinases and phosphatases. Calcium activates myosin light chain kinase via calmodulin, augmenting myosin ATPase activity enhancing contraction. Myosin light chain phosphatase conversely reduces myosin ATPase activity and contractile force. Other kinase/phosphorylase combinations result in a complex regulatory network that refines vascular tone and links it to physiologic requirements.
Control of Vascular Smooth-Muscle Cell Tone
The autonomic nervous system and endothelial cells modulate vascular smooth-muscle cells through similar convergent pathways. Autonomic neurons enter vessel media and modulate vascular smooth-muscle cell tone in response to baroreceptors and chemoreceptors within the aortic arch or carotid bodies and to thermoreceptors in the skin. Rapidly acting reflex arcs modulated by central inputs respond to multiple sensory inputs as well as emotional stimuli through three neuronal classes: sympathetic, whose principal neurotransmitters are epinephrine and norepinephrine; parasympathetic, whose principal neurotransmitter is acetylcholine; and nonadrenergic/noncholinergic, which include two subgroups—nitrergic, whose principal neurotransmitter is NO, and peptidergic, whose principal neurotransmitters are substance P, vasoactive intestinal peptide, calcitonin gene-related peptide, as well as a non-peptide, adenosine triphosphate (ATP).
Each of these neurotransmitters acts through specific receptors on the vascular smooth-muscle cell to modulate intracellular Ca2+ and consequently, contractile tone. Norepinephrine activates α adrenergic receptors, and epinephrine activates both α and β receptors; in most blood vessels, norepinephrine activates postjunctional α1 receptors in large arteries and α2 receptors in small arteries and arterioles, leading to vasoconstriction. Most blood vessels express β2-adrenergic receptors on their vascular smooth-muscle cells and respond to β agonists by cyclic AMP–dependent relaxation. Acetylcholine released from parasympathetic neurons binds to muscarinic receptors on vascular smooth-muscle cells causing vasorelaxation. Nitrergic neurons release NO, which relaxes vascular smooth-muscle cell via the cyclic GMP–dependent and –independent mechanisms outlined, and other peptidergic inputs that regulate vascular tone. For the detailed molecular physiology of the autonomic nervous system, see Chap. 432.
The release of endothelial effectors of vascular smooth-muscle cell tone (Figs. 232-2 and 232-3) integrates mechanical (shear stress, cyclic strain, etc.) and biochemical stimuli (purinergic agonists, muscarinic agonists, peptidergic agonists). In addition to these local paracrine modulators, a complex system of circulating modulators ranging from norepinephrine to the natriuretic peptides also modulate vascular smooth-muscle cell tone.
Growth of new blood vessels can occur in response to conditions such as chronic hypoxemia and tissue ischemia. Growth factors, including vascular endothelial growth factor (VEGF) and forms of fibroblast growth factor (FGF), activate a signaling cascade that stimulates endothelial proliferation and tube formation, defined as angiogenesis. Guidance molecules, including members of the semaphorin family of secreted peptides, direct blood vessel patterning by attracting or repelling nascent endothelial tubes. The development of collateral vascular networks in the ischemic myocardium, an example of angiogenesis, can result from selective activation of local or circulating endothelial progenitor cells. True arteriogenesis, or the development of a new blood vessel that includes all three cell layers, normally does not occur in adult mammals, but recent scientific advances might help obviate such limitations (Chaps. 92 and 473).
CELLULAR BASIS OF CARDIAC CONTRACTION
Most of the ventricular mass is composed of cardiomyocytes, normally 60–140 μm in length and 17–25 μm in diameter (Fig. 232-5A). Each cell contains multiple myofibrils that run the length of the cell and are composed of series of repeating sarcomeres. The cytoplasm between the myofibrils contains other cell constituents, including a single centrally located nucleus, mitochondria, and the intracellular membrane system, the SR.
A shows the branching myocytes making up the cardiac myofibers. B illustrates the critical role played by the changing [Ca2+] in the myocardial cytosol. Ca2+ ions are schematically shown as entering through the calcium channel that opens in response to the wave of depolarization that travels along the sarcolemma. These Ca2+ ions “trigger” the release of more calcium from the sarcoplasmic reticulum (SR) and thereby initiate a contraction-relaxation cycle. Eventually the small quantity of Ca2+ that has entered the cell leaves predominantly through an Na+/Ca2+ exchanger, with a lesser role for the sarcolemmal Ca2+ pump. The varying actin-myosin overlap is shown for (B) systole, when [Ca2+] is maximal, and (C) diastole, when [Ca2+] is minimal. D. The myosin heads, attached to the thick filaments, interact with the thin actin filaments. (From LH Opie: Heart Physiology: From Cell to Circulation, 4th ed. Philadelphia, Lippincott, Williams & Wilkins, 2004. Reprinted with permission. Copyright LH Opie, 2004.)
The sarcomere, the structural and functional unit of contraction, lies between adjacent Z lines, which are dark repeating bands apparent on transmission electron microscopy. The distance between Z lines varies with the degree of contraction or stretch of the muscle and ranges between 1.6 and 2.2 μm. At the center of the sarcomere is a dark band of constant length (1.5 μm), the A band, which is flanked by two lighter bands, the I bands, which are of variable length. The sarcomere of heart muscle, like that of skeletal muscle, consists of interdigitating thick and thin myofilaments. Thicker filaments, composed principally of the protein myosin, traverse the A band; they are about 10 nm (100 Å) in diameter, with tapered ends. Thinner filaments, composed primarily of actin, course from the Z lines through the I band into the A band; they are ~5 nm (50 Å) in diameter and 1.0 μm in length. Thus, thick and thin filaments overlap only within the (dark) A band, whereas the (light) I band contains only thin filaments. On electron-microscopic examination, bridges extend between the thick and thin filaments within the A band; these are myosin heads (see below) bound to actin filaments.
The sliding filament model for muscle contraction rests on the central observation that both the thick and the thin filaments are constant in length during both contraction and relaxation. With activation, the actin filaments are propelled farther into the A band. In the process, the A band remains constant in length, whereas the I band shortens and the Z lines move toward one another.
The myosin molecule is a complex, asymmetric protein with a molecular mass of about 500,000 Da; it has a rod-like portion that is about 150 nm (1500 Å) in length with a globular portion (head) at its end. The globular portions of myosin form the bridges to actin and are the site of ATPase activity. In thick myofilaments, composed of ~300 longitudinally stacked myosin molecules, the rod-like segments of myosin assume an orderly, polarized manner, with outwardly projecting globular heads interacting with actin to generate force and shorten (Fig. 232-5B).
Actin has a molecular mass of about 47,000 Da. Thin filaments consist of a double helix of two chains of actin molecules wound about each other on a larger molecule, tropomyosin. A group of regulatory proteins—troponins C, I, and T—localize at regular intervals on this filament (Fig. 232-6). In contrast to myosin, actin lacks intrinsic enzymatic activity, but combines reversibly with myosin in the presence of ATP and Ca2+. Calcium activates the myosin ATPase, which breaks down ATP to supply the energy for contraction (Fig. 232-6). The activity of myosin ATPase determines the rate of actomyosin cross-bridge formation and breakdown, and ultimately determines contraction velocity. In relaxed muscle, tropomyosin inhibits this interaction. Titin (Fig. 232-5D) an enormous, flexible, myofibrillar protein, connects myosin to the Z line; its stretching contributes to the elasticity of the heart. Dystrophin, a long cytoskeletal protein that binds to the dystroglycan complex at adherens junctions on the cell membrane, tethers the sarcomere to the cell membrane at regions tightly coupled to adjacent contracting myocytes. Mutations in multiple sarcomeric and cytoskeletal proteins cause different forms of inherited disease involving the heart and skeletal muscle.
Four steps in cardiac muscle contraction and relaxation. In relaxed muscle (upper left), ATP bound to the myosin cross-bridge dissociates the thick and thin filaments. Step 1: Hydrolysis of myosin-bound ATP by the ATPase site on the myosin head transfers the chemical energy of the nucleotide to the activated cross-bridge (upper right). When cytosolic Ca2+ concentration is low, as in relaxed muscle, the reaction cannot proceed because tropomyosin and the troponin complex on the thin filament do not allow the active sites on actin to interact with the cross-bridges. Therefore, even though the cross-bridges are energized, they cannot interact with actin. Step 2: When Ca2+ binding to troponin C has exposed active sites on the thin filament, actin interacts with the myosin cross-bridges to form an active complex (lower right) in which the energy derived from ATP is retained in the actin-bound cross-bridge, whose orientation has not yet shifted. Step 3: The muscle contracts when ADP dissociates from the cross-bridge. This step leads to the formation of the low-energy rigor complex (lower left) in which the chemical energy derived from ATP hydrolysis has been expended to perform mechanical work (the “rowing” motion of the cross-bridge). Step 4: The muscle returns to its resting state, and the cycle ends when a new molecule of ATP binds to the rigor complex and dissociates the cross-bridge from the thin filament. This cycle continues until calcium is dissociated from troponin C in the thin filament, which causes the contractile proteins to return to the resting state with the cross-bridge in the energized state. ADP, adenosine diphosphate; ATP, adenosine triphosphate; ATPase, adenosine triphosphatase. (From AM Katz: Heart failure: Cardiac function and dysfunction, in Atlas of Heart Diseases, 3rd ed, WS Colucci [ed]. Philadelphia, Current Medicine, 2002. Reprinted with permission.)
During activation of the cardiac myocyte, Ca2+ binds the heterotrimer troponin C, resulting in conformational changes in the regulatory protein tropomyosin and exposing actin cross-bridge interaction sites (Fig. 232-6). Repetitive interaction between myosin heads and actin filaments is termed cross-bridge cycling, and results in sliding of the actin along the myosin filaments, with muscle shortening and/or the development of tension. The splitting of ATP then dissociates the myosin cross-bridge from actin. In the presence of ATP (Fig. 232-6), actin and myosin filaments bind and dissociate cyclically if sufficient Ca2+ is present; these linkages cease when [Ca2+] falls below a critical level, and the troponin-tropomyosin complex once more inhibits actin-myosin interactions (Fig. 232-7).
Signal systems involved in positive inotropic and lusitropic (enhanced relaxation) effects of β-adrenergic stimulation. When the β-adrenergic agonist interacts with the β receptor, a series of G protein–mediated changes leads to activation of adenylyl cyclase and the formation of cyclic adenosine monophosphate (cAMP). The latter acts via protein kinase A to stimulate metabolism (left) and phosphorylate the Ca2+ channel protein (right). The result is an enhanced opening probability of the Ca2+ channel, thereby increasing the inward movement of Ca2+ ions through the sarcolemma (SL) of the T tubule. These Ca2+ ions release more calcium from the sarcoplasmic reticulum (SR) to increase cytosolic Ca2+ and activate troponin C. Ca2+ ions also increase the rate of breakdown of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and inorganic phosphate (Pi). Enhanced myosin ATPase activity explains the increased rate of contraction, with increased activation of troponin C explaining increased peak force development. An increased rate of relaxation results from the ability of cAMP to activate as well the protein phospholamban, situated on the membrane of the SR, that controls the rate of uptake of calcium into the SR. The latter effect explains enhanced relaxation (lusitropic effect). P, phosphorylation; PL, phospholamban; TnI, troponin I. (Modified from LH Opie: Heart Physiology: From Cell to Circulation, 4th ed. Philadelphia, Lippincott, Williams & Wilkins, 2004. Reprinted with permission. Copyright LH Opie, 2004.)
Intracytoplasmic [Ca2+] is a principal determinant of the inotropic state of the heart. Most agents that stimulate myocardial contractility (positive inotropic stimuli), including digitalis glycosides and β-adrenergic agonists, increase cytoplasmic [Ca2+], triggering cross-bridge cycling. Increased adrenergic neuronal activity stimulates myocardial contractility through norepinephrine release, activation of β adrenergic receptors and, via Gs-stimulated guanine nucleotide-binding proteins, activation of the adenylyl cyclase, which leads to the formation of the intracellular second messenger cyclic AMP from ATP (Fig. 232-7). Cyclic AMP in turn activates protein kinase A (PKA), which phosphorylates sarcolemmal Ca2+ channels, thereby enhancing the influx of Ca2+ into the myocyte.
The SR (Fig. 232-8), a complex network of anastomosing intracellular channels, invests the myofibrils. The transverse tubules, or T system, closely related to the SR, both structurally and functionally, arise from invaginations of the sarcolemma that extend into the myocardial fiber along the Z lines, i.e., the ends of the sarcomeres.
The Ca2+ fluxes and key structures involved in cardiac excitation-contraction coupling. The arrows denote the direction of Ca2+ fluxes. The thickness of each arrow indicates the magnitude of the calcium flux. Two Ca2+ cycles regulate excitation-contraction coupling and relaxation. The larger cycle is entirely intracellular and involves Ca2+ fluxes into and out of the sarcoplasmic reticulum, as well as Ca2+ binding to and release from troponin C. The smaller extracellular Ca2+ cycle occurs when this cation moves into and out of the cell. The action potential opens plasma membrane Ca2+ channels to allow passive entry of Ca2+ into the cell from the extracellular fluid (arrow A). Only a small portion of the Ca2+ that enters the cell directly activates the contractile proteins (arrow A1). The extracellular cycle is completed when Ca2+ is actively transported back out to the extracellular fluid by way of two plasma membrane fluxes mediated by the sodium-calcium exchanger (arrow B1) and the plasma membrane calcium pump (arrow B2). In the intracellular Ca2+ cycle, passive Ca2+ release occurs through channels in the cisternae (arrow C) and initiates contraction; active Ca2+ uptake by the Ca2+ pump of the sarcotubular network (arrow D) relaxes the heart. Diffusion of Ca2+ within the sarcoplasmic reticulum (arrow G) returns this activator cation to the cisternae, where it is stored in a complex with calsequestrin and other calcium-binding proteins. Ca2+ released from the sarcoplasmic reticulum initiates systole when it binds to troponin C (arrow E). Lowering of cytosolic [Ca2+] by the sarcoplasmic reticulum (SR) causes this ion to dissociate from troponin (arrow F) and relaxes the heart. Ca2+ also may move between mitochondria and cytoplasm (H). (Adapted from AM Katz: Physiology of the Heart, 4th ed. Philadelphia, Lippincott, Williams & Wilkins, 2005, with permission.)
In the inactive state, the cardiac cell is electrically polarized; i.e., the interior has a negative charge relative to the outside of the cell, with a transmembrane potential of –80 to –100 mV (Chap. 238). The sarcolemma, which in the resting state is largely impermeable to Na+, and a Na+- and K+- pump energized by ATP extrudes Na+ from the cell, and maintains the resting potential. In the resting state, intracellular [K+] is relatively high and [Na+] is far lower; conversely, extracellular [Na+] is high and [K+] is low. At the same time, in the resting state, extracellular [Ca2+] greatly exceeds free intracellular [Ca2+].
The action potential has four phases (see Fig. 238-1B). During the action potential plateau (phase 2), there is a slow inward current through sarcolemmal L-type Ca2+ channels (Fig. 232-8). Depolarizing current spreads across the cell membrane, penetrating deeply into the cell via the T tubular system. The absolute quantity of Ca2+ traversing sarcolemma and T tubules is modest and insufficient to fully activate contraction. However, this Ca2+ current, through Ca2+-induced Ca2+ release, triggers substantial Ca2+ release from the SR, inducing contraction.
Ca2+ is released from the SR through a Ca2+ release channel, a cardiac isoform of the ryanodine receptor (RyR2). Several regulatory proteins, including calstabin 2, inhibit RyR2 and thus SR Ca2+ release. Inherited disorders or exogenous factors affecting the efficiency or stability of SR Ca2+ handling can impair contraction, leading to heart failure, or to ventricular arrhythmias.
The Ca2+ released from the SR diffuses to interact with myofibrillar troponin C (Fig. 232-7), repressing this protein’s inhibition of contraction, and so activating myofilaments to shorten. During repolarization, the activity of the SR Ca2+ ATPase (SERCA2A) leads to Ca2+ uptake against a concentration gradient into the SR where complexes with another specialized protein, calsequestrin. The uptake of Ca2+ is ATP (energy)-dependent and lowers cytoplasmic [Ca2+] to a level where actomyosin interaction is inhibited and myocardial relaxation occurs. There is also a sarcolemmal exchange of Ca2+ for Na+ (Fig. 232-8), reducing the cytoplasmic [Ca2+]. Additional control of calcium compartmentalizaion results from cyclic AMP–dependent PKA phosphorylation of the SR protein phospholamban, permitting SERCA2A activation, increasing SR Ca2+ uptake, and so accelerating the relaxation rates, loading the SR with Ca2+ for subsequent release, and stimulating contraction.
Thus, the combination of the cell membrane, transverse tubules, and SR, with their ability to transmit the action potential and release and then reaccumulate Ca2+, controls the cyclic contraction and relaxation of heart muscle. Genetic or pharmacologic alterations of any component, whatever its etiology, can disturb any of the functions of this finely tuned system.
CONTROL OF CARDIAC PERFORMANCE AND OUTPUT
The extent of shortening of heart muscle and, therefore, ventricular stroke volume in the intact heart, depends on three major influences: (1) the length of the muscle at the onset of contraction, i.e., the preload; (2) the tension that the muscle must develop during contraction, i.e., the afterload; and (3) muscle contractility, i.e., the extent and velocity of shortening at any given preload and afterload. Table 232-2 lists the major determinants of preload, afterload, and contractility.
TABLE 232-2Determinants of Stroke Volume ||Download (.pdf) TABLE 232-2 Determinants of Stroke Volume
I. Ventricular Preload
Distribution of blood volume
Pumping action of skeletal muscles
II. Ventricular Afterload
Systemic vascular resistance
Elasticity of arterial tree
Arterial blood volume
Ventricular wall tension
Ventricular wall thickness
III. Myocardial Contractilitya
Intramyocardial [Ca2+] ↑↓
Cardiac adrenergic nerve activity ↑↓b
Circulating catecholamines ↑↓b
Cardiac rate ↑↓b
Exogenous inotropic agents ↑
Myocardial ischemia ↓
Myocardial cell death (necrosis, apoptosis, autophagy) ↓
Alterations of sarcomeric and cytoskeletal proteins ↓
Myocardial fibrosis ↓
Chronic overexpression of neurohormones ↓
Ventricular remodeling ↓
Chronic and/or excessive myocardial hypertrophy ↓
THE ROLE OF MUSCLE LENGTH (PRELOAD)
Preload determines sarcomere length at the onset of contraction. Contractile force is optimal at specific sarcomere lengths (~2.2 μm) where myofilament Ca2+ sensitivity is maximal, and where myofilament interactions and activation of contraction are most efficient. The relationship between initial muscle fiber length and the developed force is the basis of Starling’s law of the heart, which states that within limits, the ventricular contraction force depends on the end-diastolic length of the cardiac muscle; which in vivo relates closely to the ventricular end-diastolic volume.
Ventricular end-diastolic or “filling” pressure can serve as a surrogate for end-diastolic volume. In isolated heart and heart-lung preparations, stroke volume varies directly with the end-diastolic fiber length (preload) and inversely with the arterial resistance (afterload), and as the heart fails—i.e., as its contractility declines—it delivers a progressively smaller stroke volume from a normal or even elevated end-diastolic volume. The relation between ventricular end-diastolic pressure and the stroke work of the ventricle (the ventricular function curve) provides a working definition of cardiac contractility in the intact organism. An increase in contractility is accompanied by a shift of the ventricular function curve upward and to the left (greater stroke work at any level of ventricular end-diastolic pressure, or lower end-diastolic volume at any level of stroke work), whereas a shift downward and to the right characterizes depression of contractility (Fig. 232-9).
The interrelations among influences on ventricular end-diastolic volume (EDV) through stretching of the myocardium and the contractile state of the myocardium. Levels of ventricular EDV associated with filling pressures that result in dyspnea and pulmonary edema are shown on the abscissa. Levels of ventricular performance required when the subject is at rest, while walking, and during maximal activity are designated on the ordinate. The broken lines are the descending limbs of the ventricular-performance curves, which are rarely seen during life but show the level of ventricular performance if end-diastolic volume could be elevated to very high levels. For further explanation, see text. (Modified from WS Colucci and E Braunwald: Pathophysiology of heart failure, in Braunwald’s Heart Disease, 7th ed, DP Zipes et al [eds]. Philadelphia: Elsevier, 2005, pp 509–538.)
In the intact heart, as ex vivo, the extent and velocity of shortening of ventricular muscle fibers at any level of preload and of myocardial contractility relate inversely to the afterload, i.e., the instantaneous load opposing shortening. In the intact heart, the afterload may be defined as the tension developed in the ventricular wall during ejection. Afterload is determined by the aortic pressure as well as by the volume and thickness of the ventricular cavity. Laplace’s law specifies that the tension of the myocardial fiber is the product of the intra-cavitary ventricular pressure and ventricular radius divided by wall thickness. Therefore, at any given aortic pressure, the afterload on a dilated left ventricle exceeds that on a normal-sized ventricle. Conversely, at the same aortic pressure and ventricular diastolic volume, the afterload on a hypertrophied ventricle is lower than that on a normal chamber. Aortic pressure in turn depends on the peripheral vascular resistance, the biomechanics of the arterial tree, and the volume of blood it contains at the onset of ejection.
Ventricular afterload finely regulates cardiovascular performance (Fig. 232-10). As noted, elevations in both preload and contractility increase myocardial fiber shortening, whereas increases in afterload reduce it. The extent of myocardial fiber shortening and left ventricular size determine stroke volume. An increase in arterial pressure induced by vasoconstriction, for example, augments afterload, which opposes myocardial fiber shortening, reducing stroke volume.
Interactions in the intact circulation of preload, contractility, and afterload in producing stroke volume. Stroke volume combined with heart rate determines cardiac output, which, when combined with peripheral vascular resistance, determines arterial pressure for tissue perfusion. The characteristics of the arterial system also contribute to afterload, an increase that reduces stroke volume. The interaction of these components with carotid and aortic arch baroreceptors provides a feedback mechanism to higher medullary and vasomotor cardiac centers and to higher levels in the central nervous system to effect a modulating influence on heart rate, peripheral vascular resistance, venous return, and contractility. (From MR Starling: Physiology of myocardial contraction, in Atlas of Heart Failure: Cardiac Function and Dysfunction, 3rd ed, WS Colucci and E Braunwald [eds]. Philadelphia: Current Medicine, 2002, pp 19–35.)
When myocardial contractility is impaired and the ventricle dilates, afterload rises (Laplace’s law) and limits cardiac output. Increased afterload also may result from neural and humoral stimuli that occur in response to a fall in cardiac output. This increased afterload may reduce cardiac output further, thereby increasing ventricular volume and initiating a vicious circle, especially in patients with ischemic heart disease and limited myocardial O2 supply. Treatment with vasodilators has the opposite effect; when afterload falls, cardiac output rises (Chap. 252).
Under normal circumstances, the various influences acting on cardiac performance interact in a complex fashion to maintain cardiac output at a level responsive to the requirements of tissue metabolic demands (Fig. 232-10). Interference with a single mechanism may not influence the cardiac output due to homeostatic adjustments. For example, a moderate reduction of blood volume or the loss of the atrial contribution to ventricular contraction can be tolerated without a reduction in resting cardiac output. Under these circumstances, other factors, such as adrenergic neuronal impulses to the heart, heart rate, and venous tone, will serve as compensatory mechanisms and sustain cardiac output in a normal individual. Ultimately, understanding the complex interactions between these different variables requires rigorous models to predict relevant outcomes, and led to the early application of systems engineering principles in medicine.
The integrated response to exercise illustrates the interactions among the three determinants of stroke volume: preload, afterload, and contractility (Fig. 232-9). Hyperventilation, the pumping action of the exercising muscles, and venoconstriction during exercise all augment venous return and hence ventricular filling and preload (Table 232-2). Simultaneously, the increase in the adrenergic neuronal stimulation of the myocardium, the increased concentration of circulating catecholamines, and the tachycardia that occur during exercise combine to augment the myocardial contractility (Fig. 232-9, curves 1 and 2), together elevating stroke volume and stroke work, without a change in or even a reduction of end-diastolic pressure and volume (Fig. 232-9, points A and B). Vasodilation occurs in the exercising muscles, thus limiting the increase in afterload that otherwise would occur as cardiac output rises to levels as high as five times greater than basal levels during maximal exercise. This vasodilation ultimately allows the achievement of elevated cardiac outputs during exercise at arterial pressures only moderately higher than the resting state.
ASSESSMENT OF CARDIAC FUNCTION
Several techniques can define impaired cardiac function in clinical practice. Cardiac output and stroke volume may decline in the presence of heart failure, but these variables are often within normal limits, especially at rest. A more sensitive index of cardiac function is the ejection fraction, i.e., the ratio of stroke volume to end-diastolic volume (normal value = 67 ± 8%), which is frequently depressed in systolic heart failure even when stroke volume is normal. Alternatively, abnormally elevated ventricular end-diastolic volume (normal value = 75 ± 20 mL/m2) or end-systolic volume (normal value = 25 ± 7 mL/m2) signifies left ventricular systolic impairment.
Noninvasive techniques, particularly echocardiography, radionuclide scintigraphy and cardiac magnetic resonance imaging (MRI) (Chap. 236) have great value in the clinical assessment of myocardial function. They provide measurements of end-diastolic and end-systolic volumes, ejection fraction, and systolic shortening rate, and they allow assessment of ventricular filling (see below) as well as regional contraction and relaxation. The latter measurements have particular importance in ischemic heart disease, as myocardial infarction causes regional myocardial damage.
Strong dependence on ventricular loading conditions influence the of measurements of cardiac output, ejection fraction, and ventricular volumes as indices of cardiac function. Thus, a depressed ejection fraction and lowered cardiac output may occur in patients with normal ventricular function but reduced preload, as occurs in hypovolemia, or with increased afterload, as occurs in acutely elevated arterial pressure.
The end-systolic left ventricular pressure-volume relationship has particular value as an index of ventricular performance as it does not depend on preload and afterload (Fig. 232-11). At any level of myocardial contractility, left ventricular end-systolic volume varies inversely with end-systolic pressure; as contractility declines, end-systolic volume (at any level of end-systolic pressure) rises. Measurement of end-systolic left ventricular pressure-volume loops adds rigor to research studies of left ventricular function, but has not replaced the more readily assessed indices, such as ventricular volumes and ejection fraction, in clinical practice.
The responses of the left ventricle to increased afterload, increased preload, and increased and reduced contractility are shown in the pressure-volume plane. Left. Effects of increases in preload and afterload on the pressure-volume loop. Because there has been no change in contractility, the end-systolic pressure-volume relationship (ESPVR) is unchanged. With an increase in afterload, stroke volume falls (1 → 2); with an increase in preload, stroke volume rises (1 → 3). Right. With increased myocardial contractility and constant left ventricular end-diastolic volume, the ESPVR moves to the left of the normal line (lower end-systolic volume at any end-systolic pressure) and stroke volume rises (1 → 3). With reduced myocardial contractility, the ESPVR moves to the right; end-systolic volume is increased, and stroke volume falls (1 → 2).
Ventricular filling is influenced by the extent and speed of myocardial relaxation, a function of the rate of uptake of Ca2+ by the SR; the latter may be enhanced by adrenergic activation and reduced by ischemia, which reduces the ATP available for pumping Ca2+ into the SR (see above). The passive stiffness of the ventricular wall also may impede filling. Ventricular stiffness increases with hypertrophy and conditions that infiltrate the ventricle, such as amyloid, or can result from an extrinsic constraint (e.g., pericardial compression) (Fig. 232-12).
Mechanisms that cause diastolic dysfunction reflected in the pressure-volume relation. The bottom half of the pressure-volume loop is depicted. Solid lines represent normal subjects; broken lines represent patients with diastolic dysfunction. (From JD Carroll et al: The differential effects of positive inotropic and vasodilator therapy on diastolic properties in patients with congestive cardiomyopathy. Circulation 74:815, 1986; with permission.)
Ventricular filling can be assessed by measuring flow velocity across the mitral valve using Doppler ultrasound. Normally, inflow velocity is more rapid in early diastole than during atrial systole; with mild to moderately impaired relaxation, the rate of early diastolic filling declines, as presystolic filling rates rise. With further stiffening, flow is “pseudo-normalized,” as early ventricular filling becomes more rapid with rising left atrial pressure upstream of the left ventricle.
The heart requires a continuous supply of energy (ATP) not only to drive mechanical contraction, but also to maintain ionic and biochemical homeostasis. The development of tension, the frequency of contraction, and myocardial contractility levels are the principal determinants of the heart’s energy needs, rendering its O2 requirements ~15% of that of the entire organism.
Most ATP production depends on oxidation of the substrates glucose and free fatty acids (FFAs). FFAs used by the myocardium derive from circulating FFAs, principally from lipolysis in adipose tissue, whereas the myocyte’s glucose derives from plasma as well as from the cell’s breakdown of its glycogen stores (glycogenolysis). These two principal sources of acetyl coenzyme A in cardiac muscle vary reciprocally. Glucose is broken down in the cytoplasm into a three-carbon product, pyruvate, which passes into mitochondria, where it is metabolized to the two-carbon fragment, acetyl-CoA, and undergoes oxidation. FFAs are converted to acyl-CoA in the cytoplasm and acetyl-CoA in the mitochondria. Acetyl-CoA enters the citric acid (Krebs) cycle to produce ATP by oxidative phosphorylation; ATP then enters the cytoplasm from the mitochondrial compartment. Intracellular adenosine diphosphate (ADP), resulting from ATP breakdown, enhances ATP production.
In the fasted, resting state, circulating FFA concentrations and their myocardial uptake are high, and they furnish most of the heart’s acetyl-CoA (~70%). In the fed state, with elevations of blood glucose and insulin, glucose oxidation increases and FFA oxidation subsides. Increased cardiac work, inotropic agents, hypoxia, and mild ischemia all enhance myocardial glucose uptake, glucose production resulting from glycogenolysis, and glucose metabolism to pyruvate (glycolysis). By contrast, β-adrenergic stimulation, possibly due to stress, raises the circulating levels and metabolism of FFAs in favor of glucose. Severe ischemia inhibits the cytoplasmic pyruvate dehydrogenase, and despite both glycogen and glucose breakdown, glucose undergoes incomplete metabolism to lactic acid (anaerobic glycolysis), which does not enter the citric acid cycle. Anaerobic glycolysis produces much less ATP than does aerobic glucose metabolism. High concentrations of circulating FFAs, which can occur when adrenergic stimulation is superimposed on severe ischemia, reduce oxidative phosphorylation and cause ATP wastage; the myocardial content of ATP declines impairing contraction. In addition, FFA breakdown products may exert toxic or arrhythmogenic effects on cardiac cell membranes.
Myocardial energy is stored as creatine phosphate (CP), which is in equilibrium with ATP, the immediate energy source. In states of reduced energy availability, the CP stores decline first. Cardiac hypertrophy, fibrosis, tachycardia, increased wall tension due to ventricular dilation, and increased intracytoplasmic [Ca2+] all contribute to increased myocardial energy needs. When coupled with reduced coronary flow reserve, as occurs with obstruction of coronary arteries or abnormalities of the coronary microcirculation, an imbalance in myocardial ATP production relative to demand may occur, and the resulting ischemia can worsen or cause heart failure.
REGENERATING CARDIAC TISSUE
Until very recently, adult mammalian myocardial cells were viewed as fully differentiated and without regenerative potential. Evidence currently supports the existence of limited regenerative potential of the mature heart. Considerable current effort is being devoted to evaluating the utility of various putative stem cell populations and regenerative approaches to enhance cardiac repair after injury. The success of such approaches would offer the exciting possibility of reconstructing an infarcted or failing ventricle (Chap. 473).
The authors wish to thank Jonathan Epstein for his contribution to the prior version of this chapter.
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