Mechanisms of Cardiac Contraction and Relaxation

 Microanatomy of Contractile Cells and Proteins

Ultrastructure of Contractile Cells

The major function of myocardial muscle cells (cardiomyocytes or myocytes) is to execute the cardiac contraction-relaxation cycle. The contractile proteins of the heart lie within these myocytes, which constitute about 75% of the total volume of the myocardium although only about one third of all the cells in number. About half of each ventricular cell is occupied by the myofibrils of the myofibers (Fig. 1) and about one quarter to one third by mitochondria (Table 24-1). A myofiber is a group of myocytes (see Fig.1) held together by surrounding collagen connective tissue, the major component of the extracellular matrix. Further strands of collagen connect myofibers to each other. Excess collagen, one cause of left ventricular (LV) diastolic dysfunction, accumulates as part of the growth response to LV pressure overload.

FIGURE 1  The crux of the contractile process lies in the changing concentrations of Ca²⁺ ions in the myocardial cytosol. The upper panel shows the difference between the myocardial cell or myocyte and the myofiber, composed of many myocytes. In the middle and lower panels, Ca²⁺ ions are schematically shown as entering via 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. The small amount of calcium that has entered the cell will eventually leave predominantly by a Na⁺/Ca²⁺ exchanger, with a lesser role for the sarcolemmal calcium pump. The varying actin-myosin overlap is shown for systole, when calcium ions arrive, and for diastole, when calcium ions leave. The myosin heads, attached to the thick filaments, interact with the thin actin filaments, as shown in Figure 5. For the role of titin, see Figure 4.

The individual contractile myocytes that account for more than half of the heart's weight are roughly cylindrical (Fig.2). Those in the atrium are quite small, being less than 10 ?m in diameter and about 20 ?m in length. Relative to atrial cells, human ventricular myocytes are large, measuring about 17 to 25 ?m in diameter and 60 to 140 ?m in length (see Table 24-1).

FIGURE 2  The sarcomere is the distance between the two Z-lines. Note the presence of numerous mitochondria (mit) sandwiched between the myofibrils and the presence of T tubules (T), which penetrate into the muscle at the level of the Z-lines. This two-dimensional picture should not disguise the fact that the Z-line is really a disc, as is the M-line (M), also shown in Figure 1. H = central clear zone containing only myosin filament bodies and the M-line; A = band of actin-myosin overlap; I = band of actin filaments, titin, and Z-line; g = glycogen granules. (?32,000, rat papillary muscle.)

When they are examined under the light microscope, the atrial and ventricular myocytes have cross striations and are branched. Each myocyte is bounded by a complex cell membrane, the sarcolemma (sarco, flesh; lemma, thin husk), and is filled with rodlike bundles of myofibrils, the contractile elements (see Fig. 1). The sarcolemma of the myocyte invaginates to form an extensive tubular network (the T tubules) that extends the extracellular space into the interior of the cell (see Figs. 1 and 2). The nucleus, which contains almost all of the cell's genetic information, is often centrally located. Some myocytes have several nuclei. Interspersed between the myofibrils and immediately beneath the sarcolemma are many mitochondria, the main function of which is to generate the energy in the form of adenosine triphosphate (ATP) needed to maintain the heart's contractile function and the associated ion gradients. Of the other organelles, the sarcoplasmic reticulum (SR) is most important (see Fig. 1). When the wave of electrical excitation reaches the closely approximated T tubules, the tubular calcium channels open to a relatively small amount of calcium to trigger the release of much more calcium from the calcium release channels of the SR. This is the calcium that initiates myocardial contraction. When the calcium is once again taken up into the SR, relaxation ensues.

The SR is a fine network spreading throughout the myocytes, demarcated by its lipid bilayer, which is similar to that of the sarcolemma. The calcium release channels (also called the ryanodine receptors) are found in the expanded parts of the SR that lie in close apposition to the T tubules. These are called subsarcolemmal cisternae (Latin, boxes or baskets) or the junctional SR. The second part of the SR, the longitudinal or network SR, consists of ramifying tubules (see Fig. 1) and is concerned with the uptake of calcium that initiates relaxation. This uptake is achieved by the ATP-requiring calcium pump, also called SERCA (sarcoendoplasmic reticulum Ca²⁺-ATPase), that increases its activity in response to beta-adrenergic stimulation. Calcium taken up into the SR is then stored at high concentration in a number of storage proteins, including calsequestrin, before being released again in response to the next wave of depolarization.

The cytoplasm is the intracellular fluid and the proteins therein contained within the sarcolemma but excluding the contents of organelles, such as mitochondria and the SR. The fluid component of the cytoplasm minus the proteins is called the cytosol. It is in the cytosol that the concentrations of calcium ions rise and fall to cause cardiac contraction and relaxation. The proteins of the sarcoplasm include myriad specialized molecules, the enzymes that accelerate the conversion of one chemical form to another, thereby stimulating crucial metabolic or signaling paths and thereby eventually promoting energy production.

Subcellular Microarchitecture

The molecular signal systems that convey messages from surface receptors to intracellular organelles may be directed to specific sites by molecules that “anchor” components of the internal messenger chain to specific organelles, as when the beta-adrenergic chain must link up with the calcium pump of the SR (see later). Scaffolding proteins bring interacting molecules close together, as in the case of the signaling chain, leading to myocyte growth. An example of physiologic subcellular compartmentalization is the local unloading of ATP where it is needed, by the exact location of the enzyme creatine kinase that converts creatine phosphate to ATP.

Contractile Proteins

The major molecules involved in the contraction-relaxation cycle are the two chief contractile proteins, the thin actin filament and the thick myosin filament (see Fig. 1). Calcium ions initiate the contraction cycle by interacting with troponin C to relieve the inhibition otherwise exerted by troponin I (Fig. 3). Titin is a newly discovered large elastic molecule that supports myosin (Fig. 4). During contraction, the filaments slide over each other without the individual molecules of actin or myosin actually shortening. As they slide, they pull together the two ends of the fundamental contractile unit called the sarcomere. On electron microscopy, the sarcomere is limited on either side by the Z-line (Z, abbreviation for German Zuckung, contraction) to which the actin filaments are attached (see Fig. 2). Conversely, the myosin filaments extend from the center of the sarcomere in either direction toward but not actually reaching the Z-lines (see Fig. 1).

FIGURE 3  The major molecules of the contractile system. The thin actin filament (A) interacts with the myosin head (B) when Ca²⁺ ions arrive at troponin C (C). A complex interaction between TnC and the other troponins moves tropomyosin to “uncover” an actin site to which a myosin head can attach (see Fig. 5). The molecular aspects are as follows. A, The thin actin filament contains troponin C (TnC) and its Ca²⁺ binding sites. When TnC is not activated by Ca²⁺, troponin I (TnI) inhibits the actin-myosin interaction. Troponin T (TnT) is an elongated protein that interacts with all the other components of the thin filament, thereby participating in the activation cycle (D). B, Myosin head molecular structure (based on Rayment and associates) is composed of heavy and light chains. The heavy head chain in turn has two major domains: one of 70 kDa (i.e., 70,000 molecular weight) that interacts with actin at the actin cleft and has an ATP-binding pocket. The neck domain of 20 kDa, also called the lever, is an elongated alpha helix that extends and bends and has two light chains surrounding it as a collar. The essential light chain is part of the structure. The other regulatory light chain may respond to phosphorylation to influence the extent of the actin-myosin interaction. C, Troponin C with sites in the regulatory domain for activation by calcium and for interaction with troponin I. D, Calcium binding to TnC induces a conformational change in TnC that elongates (compare systole with diastole). TnI closes up to TnC, and the normal inhibition of TnI on actin-tropomyosin is lessened. There is a strengthening of the interaction between TnC and TnT. These changes allow repositioning of tropomyosin in relation to actin, with lessening of its normal inhibitory effects, as shown in the bottom panel. Now the contractile cycle can start.

FIGURE 4  Titin, a very large elongated protein with elasticity, binds myosin to the Z-line. It may act as a bidirectional spring that develops passive forces in stretched sarcomeres and resting forces in shortened sarcomeres. As the sarcomere is stretched to its maximum physiologic diastolic length of 2.2 ?m , titin first undergoes straightening (up to 2 ?m) and then elongation, the latter rapidly increasing the passive forces generated. At low sarcomere lengths, when sarcomeres are slack at about the diastolic limit of 1.85 ?m , the mechanically active elastic domain is folded on top of itself. At even shorter lengths, which may not be physiologic in the intact heart, substantial restoring forces are generated.

The interaction of the myosin heads with actin filaments when sufficient calcium arrives from the SR (see Fig. 1) is called cross-bridge cycling. As the actin filaments move inward toward the center of the sarcomere, they draw the Z-lines closer together, so that the sarcomere shortens. The energy for this shortening is provided by the breakdown of ATP, chiefly made in the mitochondria.

Titin and Length Sensing

Titin is a giant molecule, the largest protein yet described. It is an extraordinarily long, flexible, and slender myofibrillar protein (see Fig. 4). Titin acts as a third filament to provide elasticity. Being between 0.6 and 1.2 ?m in length, the titin molecule extends from the Z-line, stopping just short of the M-line (see Fig. 1). It has two distinct segments: an inextensible anchoring segment and an extensible elastic segment that stretches as sarcomere length increases. Titin has multiple functions. First, it tethers the myosin molecule to the Z-line, thereby stabilizing the contractile proteins. Second, as it stretches and relaxes, its elasticity explains the stress-strain relation of cardiac and skeletal muscle. At short sarcomere lengths, the elastic domain is folded on itself to generate restoring forces (see Fig. 4). These changes in titin help explain the series elastic element, which postulates that there is elasticity in series between the contractile elements and the ends of the muscle. Third, increased diastolic stretch of titin as the sarcomere length of cardiac muscle is increased causes the enfolded part of the titin molecule to straighten. This stretched molecular spring then contracts more vigorously in systole. Such enhanced systolic contraction helps explain the Frank-Starling mechanism (see later). Fourth, titin may transduce mechanical stretch into growth signals. In sustained diastolic stretch as in volume overload, the elastic segment of titin is constantly under strain and transmits this mechanical signal to the muscle LIM protein (MLP) attached to the terminal part of titin that forms part of the Z-disc complex. The MLP protein is proposed as the stretch sensor that transmits the signals resulting in the myocyte growth pattern characteristic of volume overload. This signal system may be defective in a subset of human dilated cardiomyopathy.

Strong and Weak Binding States

Although the events underlying the cross-bridge cycle are exceedingly complex at a molecular level, one simple current hypothesis is that the cross bridges exist in either a strong or a weak binding state (Fig. 5). The arrival of calcium ions at the contractile proteins is a crucial link in the series of events known as excitation-contraction coupling. The ensuing interaction of calcium with troponin C and the deinhibition of troponin I put the cross bridges in the strong binding state. As long as enough calcium ions are present, the strong binding state potentially dominates (see Fig. 5). If, however, the strong binding state were continuously present, the contractile proteins could never relax. Thus, the proposal is that the binding of ATP to the myosin head puts the cross bridges into a weak binding state even when calcium is high. Conversely, when ATP is hydrolyzed to ADP and Pi, the strong binding state again predominates (see Fig. 5). Thus, the ATP-induced changes in the molecular configuration of the myosin head result in corresponding variations in the physical properties (a similar concept is common in metabolic regulation). Length activation also promotes the strong binding state (see section on length-dependent activation). Conversely, the weak binding state predominates when cytosolic calcium levels fall at the start of diastole. As the calcium ions leave troponin C, a master switch is turned off, and tropomyosin again assumes the inhibitory configuration. FIGURE 5  Cross-bridge cycling molecular model updated from the original Rayment five-step model for interaction between the myosin head and the actin filament, taking into account other models. The cross bridge (only one myosin head depicted) is pear shaped and consists of the catalytic motor domain that interacts with the actin molecule and an extended alpha-helical neck region acting as a lever arm. The nucleotide pocket receiving and binding ATP is a depression near the center of the catalytic domain. The actin-binding cleft bisects the catalytic motor domain. During the cross-bridge cycle, the width of the actin-binding cleft changes in size, although details remain controversial. Starting with the rigor state (A), the binding of ATP to the pocket (B) is followed by ATP hydrolysis (C) that partly closes the actin-binding cleft. The cleft opens when phosphate is released (through the cleft rather than through the pocket), and the myosin head strongly attaches to actin to induce the power stroke (D, E). During the power stroke, the myosin head rotates about a fulcrum in the region where the helix terminates within the catalytic motor domain. As the head flexes, the actin filament is displaced by about 10 nm (E). In the process, ADP is also released so that the binding pocket becomes vacant. Finally, the rigor state is reached again (A), when the myosin head is again ready to receive ATP to reinitiate the cross-bridge cycle. Throughout, the actin monomer with which the myosin head is interacting is speckled with dots. Actin and Troponin Complex Although calcium ions provide the essential switch-on signal to the cross-bridge cycle by binding to troponin C, current evidence suggests more than an on-off signaling process. Rather, the arrival of calcium initiates a series of interactions between the troponin components of the thin filament to allow movement of the tropomyosin molecule, which in turn promotes the strong binding state (see Fig. 5) so that contraction takes place. To understand the role of calcium first requires a brief description of the molecular structure of actin and the troponin complex. Thin filaments are composed of two actin units, which intertwine in a helical pattern, both being carried on a heavier tropomyosin molecule that functions as a backbone . At regular intervals of 38.5 nm along this twisting structure is a closely bound group of three regulatory proteins called the troponin complex. Of these three, it is troponin C that responds to the calcium ions that are released in large amounts from the SR to start the cross-bridge cycle. When the cytosolic calcium level is low, the tropomyosin molecule is twisted in such a way that the myosin heads cannot interact with actin (see Fig. 3). Thereby, most cross bridges are in the “blocked position,” although some are still in the weakly binding state. As calcium ions increasingly arrive at the start of the contractile cycle and interact with troponin C, the activated troponin C binds tightly to the inhibitory molecule troponin I, which moves to a new position on the thin filament, thereby weakening the interaction between troponin T and tropomyosin (see Fig. 3). Ultimately, tropomyosin is repositioned on the thin filament, thereby removing most of the inhibition exerted by tropomyosin on the actin-myosin interaction. Thus, weakly bound or blocked cross bridges enter the strongly bound state, and the cross-bridge cycle is initiated. As the strong cross bridges form, they activate near neighbors and thereby spread the activation process. They also promote further tropomyosin movement to cause more forceful cross-bridge interaction. Myosin and Molecular Basis of Muscle Contraction Each myosin head is the terminal part of a heavy chain. The bodies of two of these chains intertwine and each terminates in a short neck that carries the elongated myosin head (see Fig. 3). According to the Rayment model, it is the base of the head, also sometimes called the neck, that changes configuration in the contractile cycle. Together with the bodies of all the other heads, the myosin thick filament is formed. Each lobe of the bilobed head has an ATP-binding pocket (also called nucleotide pocket) and a narrow cleft that extends from the base of this pocket to the actin-binding face. ATP and its breakdown products ADP and Pi bind to the nucleotide pocket close to the myosin ATPase activity that breaks down ATP to its products (see Fig. 3). Currently, there is controversy about the role in the contractile cycle of the narrow actin-binding cleft that splits the central 50-kDa segment of the myosin head. According to the revised Rayment model, this cleft responds to the binding of ATP or its breakdown products to the nucleotide pocket in such a way that the conformational changes necessary for movement of the head are produced. According to Dominguez and coworkers, the cleft is closed in the weakly attached states before the power stroke (Fig. 6) but opens when inorganic phosphate is released through the cleft, whereupon the myosin head attaches strongly to actin to induce the power stroke . FIGURE 6  Calcium fluxes in the myocardium. Crucial features are (1) entry of Ca2+ ions via the voltage-sensitive L-type Ca2+ channels, acting as a trigger to the release of Ca2+ ions from the sarcoplasmic reticulum (SR); (2) the effect of beta-adrenergic stimulation with adenylyl cyclase forming cAMP, the latter helping both to open the Ca2+ channel and to increase the rate of uptake of Ca2+ into the SR; and (3) exit of Ca2+ ions chiefly via the Na+/Ca2+ exchanger, with the sodium pump thereafter extruding the Na+ ions thus gained. The latter process requires ATP. Note the much higher extracellular (10−3 M) than intracellular cytosolic Ca2+ values, with much higher calcium values in the SR because of its storage function. Mitochondria can act as a buffer against excessive changes in the free cytosolic calcium concentration. Starting with the rigor state (see Fig. 5A), the binding of ATP to its pocket changes the molecular configuration of the myosin head so that the head detaches from actin to terminate the rigor state (see Fig. 5B). Next, the ATPase activity of the myosin head splits ATP into ADP and Pi, and the head flexes (see Fig. 5C). As ATP is hydrolyzed, the myosin head binds to an adjacent actin unit. Then Pi is released from the head through the cleft, and there is strong binding of the myosin head to actin (see Fig. 5D). Next, the head extends (i.e., straightens). A power stroke takes place, the actin molecule moves by about 10 nm, and the myosin head is now in the rigor state. The pocket then releases ADP, ready for acceptance of ATP and repetition of the cycle. The Rayment model postulates straightening and not flexion of the light chain region of the head (i.e., the neck) that produces the power stroke. The lever arm model is more applicable to many cells and organelles that depend on movement of myosin rather than of actin, for example, for intracellular transport. By contrast, in contracting cardiomyocytes, myosin is fixed and tethered by titin and myosin-binding protein C. The lever arm model proposes that movements of the neck, which is the lever arm, produce large displacements that translate into movement of the whole myosin molecule. This model provides evolutionary data that reinforce the crucial nature of movements of the myosin neck (shown as the flexible domain in Fig. 5C). Myosin ATPase activity normally responds to calcium in such a way that increases in calcium concentrations associated with the contraction cycle in the whole heart increase the myosin ATPase activity several-fold in addition to increasing calcium binding to troponin C and force development (see Fig. 6). Myosin heavy chain isoforms help regulate myosin ATPase activity. Each myosin filament consists of two heavy chains (the bodies of which are intertwined, each ending in one head) and four light chains (two in apposition to each head). The heavy chains, containing the myosin ATPase activity on the heads, occur in two isoforms, β and α, of the same molecular weight but with substantially different ATPase activities. The beta–heavy chain (β-MHC) isoform has lower ATPase activity and is the predominant form in the adult human. In small animals, the faster α-MHC form changes to a predominant β-MHC pattern in heart failure. Myosin Heads: Two Are Better than One The double-headed structure is required to produce the full displacement of actin, about 10 nm, versus only 6 nm with single-headed myosin. The myosin neck is chiefly formed by a long alpha helix (see Fig. 3B), surrounded by two light chains (four per bilobed head) that act as a cervical collar. The light chain that is more proximal to the myosin head, the essential light chain (MLC-1), may inhibit the contractile process by interaction with actin. The other regulatory light chain (MLC-2) is a potential site for phosphorylation, for example, in response to beta-adrenergic stimulation. Such phosphorylation (i.e., the gaining of a phosphate grouping) may promote cross-bridge cycling by increasing the affinity of myosin for actin. Mutation of this light chain in one type of human cardiomyopathy impairs the contractile response to tachycardia. In vascular smooth muscle, the phosphorylation that occurs under the influence of the enzyme myosin light chain kinase (MLCK) is an obligatory step in the initiation of the contractile process. Myosin-binding protein C runs at approximately right angles to the myosin molecules to tether myosin molecules by linking the structures that lie around subfragments of the myosin heads. This binding protein, which stabilizes the myosin head, itself flexes and extends at the level of the light chains. Defects in the binding protein C may be involved in some types of hypertrophic cardiomyopathy. Graded Effects of Increased Cytosolic Calcium Levels on Cross-Bridge Cycle Calcium ions play a crucial role in linking external neurohumoral control of the heart to stimulation of the contractile process by acting at multiple control sites. Calcium interaction with troponin C is essential for cross-bridge cycling. Does calcium act as an on-off switch to regulate the total number of cycling cross bridges? According to this proposal, the enhanced force development in response to a greater calcium ion concentration must be due to recruitment of additional cross bridges. Alternatively, to explain the graded model, there may be (1) a graded response of troponin C to calcium ions, including altered rates of calcium binding and release; (2) a graded response of myosin ATPase to calcium; (3) near-neighbor self-activation, whereby actin-myosin interaction activates additional cross bridges even in the absence of increased binding of calcium to the troponin C of those cross bridges; or (4) alterations in the extent of myosin light chain phosphorylation. Of specific interest is the proposal that tightly bound cross bridges act to spread activation on the thin filament to near-neighbor units to achieve full activation. By such mechanisms, one calcium-troponin complex could turn on as many as 14 actin molecules. Length-Dependent Activation In addition to the cytosolic calcium concentration, the other major factor influencing the strength of contraction is the length of the muscle fiber at the end of diastole, just before the onset of systole. Starling observed that the greater the volume of the heart in diastole, the more forceful the contraction. The increased heart volume translates into an increased muscle length, which acts by a length-sensing mechanism. This relation was previously ascribed to a more optimal overlap between actin and myosin. The current view is that an increased sarcomere length leads to greater sensitivity of the contractile apparatus to the prevailing cytosolic calcium ion concentration. A plausible mechanism for this regulatory change may reside in the decreasing interfilament spacing as the heart muscle is stretched. This satisfactory lattice-dependent explanation for the Frank-Starling relationship has been dealt a setback by the careful x-ray diffraction studies of de Tombe's group. Reducing sarcomere lattice spacing by osmotic compression failed to influence calcium sensitivity. Alternatively, sarcomere stretch increases the passive forces built up by titin, which could in turn hypothetically influence the position of myosin heads. Another proposal, that troponin C is the length sensor, is currently less favored. Probably several mechanisms are at work. Cross-Bridge Cycling Differs from Cardiac Contraction-Relaxation Cycle The cardiac cycle of Wiggers (see later) must be distinguished from the cross-bridge cycle. The Wiggers cycle reflects the overall pressure changes in the left ventricle, whereas the cross-bridge cycle is the repetitive interaction between myosin heads and actin. According to the Rayment model, the binding of ATP or ADP regulates in part whether the cross bridges would be weak or strong in nature (see Fig. 24-5). So long as enough calcium ions are bound to troponin C, many repetitive cycles of this nature occur. Thus, at any given moment, some myosin heads will be flexing or flexed, some will be extending or extended, some will be attached to actin, and some will be detached from actin. Numerous such cross-bridge cycles, each lasting only a few microseconds, actively move the thin actin filaments toward the central bare area of the thick myosin filaments, thereby shortening the sarcomere. The sum total of all the shortening sarcomeres leads to systole, which is the contraction phase of the cardiac cycle. When calcium ions depart from their binding sites on troponin C, cross-bridge cycling cannot occur, and the diastolic phase of the cardiac cycle sets in. Myofilament Response to Hemodynamic Demands Myofilament activity is coupled to the prevailing hemodynamic demands of the circulation. In addition to length-dependent activation, there are two other chief mechanisms. First, there may be variable rates of calcium binding and release from troponin C (as discussed in a previous section). Second, phosphorylation and dephosphorylation of the contractile proteins may help control the extent of activation of the myofilaments. Thus, increased beta-adrenergic–dependent phosphorylation of troponin I reduces the myofilament sensitivity to calcium and thereby leads to an increased rate of relaxation during beta-adrenergic stimulation. Hypothetically, this mechanism enhances the relaxant (lusitropic) effect of increased uptake rates of calcium into the SR. The effects of phosphorylation of other proteins, such as the myosin essential light chains and C protein, still imperfectly understood, may also be important. Force Transmission Volume and pressure overload may owe their different effects on myocardial growth to different patterns of force transmission. Whereas increased diastolic forces are transmitted longitudinally via titin to reach the postulated sensor, the MLP protein (see section on titin), increased systolic forces may be transmitted laterally (i.e., at right angles) via the Z-disc and cytoplasmic actin to reach the proteins of the cell-to-matrix junctions, such as the focal adhesion complex. How these mechanical forces become translated into signals that activate the growth pathways, such as those leading to MAP (mitogen-activated protein) kinase, still remains to be discovered. Current proposals for the onward transmission of mechanical forces from sarcomere show different patterns for systole and diastole, which could explain why greater end-systolic stress leads to thicker myocytes and greater end-diastolic stress to longer myocytes. During systole, the sarcomere shortens as a result of myosin head flexion and movement of actin filaments toward the M-line. Increased horizontal force is exerted on the Z-line, which in turn transmits lateral forces to the sarcolemma by the thin and intermediate filaments, cytoplasmic actin and desmin. Further transmission is to the integrins of the costameres and associated cytoskeletal proteins that connect the stress generated in the Z-lines of the sarcomeres to the extracellular matrix, from where the intracellular hypertrophy program is activated. During diastole, as the sarcomere stretches, the elastic segment of titin expands and transmits force via MLP, the muscle LIM protein that acts as a mechanical sensor. When long-continued, as in sustained volume overload, MLP-induced stimuli may induce myocyte lengthening as in the dilated failing heart. Further signaling is not clear. Pressure and volume overload may induce distinctly different signal transduction paths; pressure load rapidly activates Akt and the expected downstream kinases, whereas volume loading gives delayed Akt activation not involving the same downstream signals. This clear distinction is consistent with the observation in isolated muscle strip preparations that only afterload (pressure overload) but not preload (volume overload) activates ventricular expression of brain natriuretic peptide (BNP). Contractile Proteins and Cardiomyopathy The concept is that genetic-based hypertrophic and dilated cardiomyopathies produce hearts that not only look and behave differently but have diverse molecular causes. Hypertrophic cardiomyopathy is, in general, linked to mutant genes that cause abnormalities in the force-generating system, such as β-MHC. Less commonly, there may be defects in the genes encoding troponin T, myosin light chain isoforms, troponin I and C isoforms, myosin-binding protein C, and alpha-tropomyosin . The current hypothesis is that the mutations increase the contractile performance or the energy demand. How such defects translate into hypertrophic cardiomyopathy is still obscure. In contrast, dilated cardiomyopathy can be related to mutations in non–force-generating cytoskeletal proteins, such as dystrophin, nuclear lamin, cytoplasmic actin, and titin, as well as to defects in the enzymes that control the integrity of the matrix . This distinction between the two types of cardiomyopathy remains useful but is oversimplified, with several examples of overlapping mechanisms.