The general picture of muscle contraction in the heart resembles that of skeletal muscle. Cardiac muscle, like skeletal muscle, is striated and uses the actin-myosin-tropomyosin-troponin system described above. Unlike skeletal muscle, cardiac muscle exhibits intrinsic rhythmicity, and individual myocytes communicate with each other because of its syncytial nature. The T-tubular system is more developed in cardiac muscle, whereas the SR is less extensive and consequently the intracellular supply of Ca2+ for contraction is less. Cardiac muscle thus relies on extracellular Ca2+ for contraction; if isolated cardiac muscle is deprived of Ca2+, it ceases to beat within approximately 1 minute, whereas skeletal muscle can continue to contract without an extracellular source of Ca2+ for a longer period. Cyclic AMP plays a more prominent role in cardiac than in skeletal muscle. It modulates intracellular levels of Ca2+ through the activation of the protein kinases that phosphorylate various transport proteins in the sarcolemma and SR. They also target the troponin-tropomyosin regulatory complex, affecting its responsiveness to intracellular Ca2+. There is a rough correlation between the phosphorylation of TpI and the increased contraction of cardiac muscle induced by catecholamines. This may account for the inotropic effects (increased contractility) of β-adrenergic compounds on the heart. Some differences among skeletal, cardiac, and smooth muscle are summarized in Table 51–3.
TABLE 51–3Some Differences among Skeletal, Cardiac, and Smooth Muscle ||Download (.pdf) TABLE 51–3 Some Differences among Skeletal, Cardiac, and Smooth Muscle
|Skeletal Muscle ||Cardiac Muscle ||Smooth Muscle |
Small T tubules
Sarcoplasmic reticulum well developed and Ca2+ pump acts rapidly.
Plasmalemma contains few hormone receptors.
Nerve impulse initiates contraction.
Extracellular fluid Ca2+ not important for contraction.
Troponin system present.
Caldesmon not involved.
Very rapid cycling of the cross-bridges.
Large T tubules
Sarcoplasmic reticulum present and Ca2+ pump acts relatively rapidly.
Plasmalemma contains a variety of receptors (eg, α- and β-adrenergic).
Has intrinsic rhythmicity.
Extracellular fluid Ca2+ important for contraction.
Troponin system present.
Caldesmon not involved.
Relatively rapid cycling of the cross-bridges.
Generally rudimentary T tubules
Sarcoplasmic reticulum often rudimentary and Ca2+ pump acts slowly.
Plasmalemma contains a variety of receptors (eg, α- and β-adrenergic).
Contraction initiated by nerve impulses, hormones, etc.
Extracellular fluid Ca2+ important for contraction.
Lacks troponin system; uses regulatory head of myosin.
Caldesmon is important regulatory protein.
Slow cycling of the cross-bridges permits slow, prolonged contraction and less utilization of ATP.
Ca2+ Enters Myocytes via Ca2+ Channels & Leaves via the Na+-Ca2+ Exchanger & the Ca2+ ATPase
As stated, extracellular Ca2+ plays an important role in contraction of cardiac muscle but not in skeletal muscle. This means that Ca2+ both enters and leaves myocytes in a regulated manner. We shall briefly consider three transmembrane proteins that play roles in this process.
Ca2+ enters myocytes via highly selective channels. The major portal of entry is the L-type (long-duration current, large conductance) or slow Ca2+ channel, which is voltage-gated, opening during depolarization induced by spread of the cardiac action potential and closing when the action potential declines. These channels are equivalent to the dihydropyridine receptors of skeletal muscle (Figure 51–8). Slow Ca2+ channels are regulated by cAMP-dependent protein kinases (stimulatory) and cGMP-dependent protein kinases (inhibitory) They are inhibited by so-called calcium channel blockers (eg, verapamil). Fast (or T, transient) Ca2+ channels are also present in the plasmalemma, though in much lower numbers; they probably contribute to the early phase of increase of myoplasmic Ca2+.
The resultant increase of Ca2+ in the myoplasm acts on the Ca2+ release channel of the SR to open it. This is called Ca2+-induced Ca2+ release (CICR). It is estimated that approximately 10% of the Ca2+ involved in contraction enters the cytosol from the extracellular fluid and 90% from the SR. However, the former 10% is important, as the rate of increase of Ca2+ in the myoplasm is important, and entry via the Ca2+ channels contributes appreciably to this.
This is the principal route of exit of Ca2+ from myocytes. In resting myocytes, it helps to maintain a low level of free intracellular Ca2+ by exchanging one Ca2+ for three Na+. The energy for the uphill movement of Ca2+ out of the cell comes from the downhill movement of Na+ into the cell from the plasma. This exchange contributes to relaxation, but may run in the reverse direction during excitation. Because of the Ca2+-Na+ exchanger, anything that causes intracellular Na+ (Na+i) to rise will secondarily cause Ca2+i to rise, causing more forceful contraction. This is referred to as a positive inotropic effect. One example is when the drug digitalis is used to treat heart failure. Digitalis inhibits the sarcolemmal Na+-K+-ATPase, diminishing the exit of Na+ by this route, thus increasing Na+i. This promotes the inflow of Ca2+ via the Ca2+-Na+ exchanger. The increased Ca2+i results in increased force of cardiac contraction (Figure 51–12), of benefit in heart failure.
Scheme of how the drug digitalis (used in the treatment of certain cases of heart failure) increases cardiac contraction. Digitalis inhibits the Na+-K+ ATPase (see Chapter 40). This results in less Na+ being pumped out of the cardiac myocyte and leads to an increase of the intracellular concentration of Na+. In turn, this stimulates the Na+-Ca2+ exchanger so that more Na+ is exchanged outward, and more Ca2+ enters the myocyte. The resulting increased intracellular concentration of Ca2+ increases the force of muscular contraction.
While this Ca2+ pump, situated in the sarcolemma, also contributes to Ca2+ exit, it is believed to play a relatively minor role as compared with the Ca2+-Na+ exchanger.
It should be noted that a variety of ion channels (see Chapter 40) are present in most cells, for Na+, K+, Ca2+, etc. Many of them have been cloned and their dispositions in their respective membranes worked out (number of times each one crosses its membrane, location of the actual ion transport site in the protein, etc). They can be classified as indicated in Table 51–4. Cardiac muscle is rich in ion channels, and they are also important in skeletal muscle. Mutations in genes encoding ion channels have been shown to be responsible for a number of relatively rare conditions affecting muscle. These and other diseases due to mutations of ion channels have been termed channelopathies; some are listed in Table 51–5.
TABLE 51–4Major Types of Ion Channels Found in Cells ||Download (.pdf) TABLE 51–4 Major Types of Ion Channels Found in Cells
|Type ||Comment |
|External ligand gated ||Open in response to a specific extracellular molecule, for example, acetylcholine. |
|Internal ligand gated ||Open or close in response to a specific intracellular molecule, for example, a cyclic nucleotide. |
|Voltage gated ||Open in response to a change in membrane potential, for example, Na+, K+, and Ca2+ channels in heart. |
|Mechanically gated ||Open in response to change in mechanical pressure. |
TABLE 51–5Some Disorders (Channelopathies) due to Mutations in Genes Encoding Polypeptide Constituents of Ion Channels ||Download (.pdf) TABLE 51–5 Some Disorders (Channelopathies) due to Mutations in Genes Encoding Polypeptide Constituents of Ion Channels
|Disordera ||Ion Channel and Major Organs Involved |
|Central core disease (OMIM 117000) ||Ca2+ release channel (RYR1), skeletal muscle |
|Hyperkalemic periodic paralysis (OMIM 170500) ||Sodium channel, skeletal muscle |
|Hypokalemic periodic paralysis (OMIM 170400) ||Slow Ca2+ voltage channel (DHPR), skeletal muscle |
|Malignant hyperthermia (OMIM 145600) ||Ca2+ release channel (RYR1), skeletal muscle |
|Myotonia congenita (OMIM 160800) ||Chloride channel, skeletal muscle |
Inherited Cardiomyopathies Are due to Disorders of Cardiac Energy Metabolism or to Abnormal Myocardial Proteins
An inherited cardiomyopathy is any structural or functional abnormality of the ventricular myocardium due to an inherited cause. There are nonheritable types of cardiomyopathy, but these will not be described here. As shown in Table 51–6, the causes of inherited cardiomyopathies fall into two broad classes: (1) disorders of cardiac energy metabolism, mainly reflecting mutations in genes encoding enzymes or proteins involved in fatty acid oxidation (a major source of energy for the myocardium) and oxidative phosphorylation; (2) mutations in genes encoding proteins involved in or affecting myocardial contraction, such as myosin, tropomyosin, the troponins, and cardiac myosin-binding protein C. Mutations in the genes encoding these latter proteins cause familial hypertrophic cardiomyopathy, which will now be discussed.
TABLE 51–6Biochemical Causes of Inherited Cardiomyopathiesa ||Download (.pdf) TABLE 51–6 Biochemical Causes of Inherited Cardiomyopathiesa
|Cause ||Proteins or Process Affected |
|Inborn errors of fatty acid oxidation ||Carnitine entry into cells and mitochondria |
| ||Certain enzymes of fatty acid oxidation |
|Disorders of mitochondrial oxidative phosphorylation ||Proteins encoded by mitochondrial genes |
| ||Proteins encoded by nuclear genes |
|Abnormalities of myocardial contractile and structural proteins ||β-Myosin heavy chains, troponin, tropomyosin, dystrophin |
Mutations in the Cardiac β-Myosin Heavy Chain Gene Are One Cause of Familial Hypertrophic Cardiomyopathy
Familial hypertrophic cardiomyopathy is one of the most frequent hereditary cardiac diseases. Patients exhibit hypertrophy—often massive—of one or both ventricles, starting early in life, unrelated to any extrinsic cause such as hypertension. Most cases are transmitted in an autosomal dominant manner; the rest are sporadic. Until recently, its cause was obscure. However, this situation changed when studies of one affected family showed that a missense mutation (ie, substitution of one amino acid by another) in the β-myosin heavy chain gene was responsible for the condition. Subsequent studies have shown a number of missense mutations in this gene, all coding for highly conserved residues. Some individuals have shown other mutations, such as formation of an α/β-myosin heavy chain hybrid gene. Patients with familial hypertrophic cardiomyopathy can show great variation in clinical picture. This in part reflects genetic heterogeneity; that is, mutation in a number of other genes (eg, those encoding cardiac actin, tropomyosin, cardiac troponins I and T, essential and regulatory myosin light chains, cardiac myosin-binding protein C, titin, and mitochondrial tRNA-glycine and tRNA-isoleucine) may also cause familial hypertrophic cardiomyopathy. In addition, mutations at different sites in the gene for β-myosin heavy chain may affect the function of the protein to a greater or lesser extent. The missense mutations are clustered in the head and head-rod regions of the myosin heavy chain. One hypothesis is that the mutant polypeptides (“poison polypeptides”) cause formation of abnormal myofibrils, eventually resulting in compensatory hypertrophy. Some mutations alter the charge of an amino acid side chain (eg, substitution of arginine for glutamine) which presumably affects the conformation of the protein more markedly than other substitutions. Patients with these mutations have a significantly shorter life expectancy than patients in whom the mutation produced no alteration in charge. Thus, definition of the precise mutations involved in the genesis of FHC may prove to be of important prognostic value; it can be accomplished by appropriate use of the polymerase chain reaction on genomic DNA obtained from one sample of blood lymphocytes. Figure 51–13 is a simplified scheme of the events causing familial hypertrophic cardiomyopathy.
Simplified scheme of the causation of familial hypertrophic cardiomyopathy (OMIM 192600) due to mutations in the gene encoding β-myosin heavy chain. Mutations in genes encoding other proteins (see text) can also cause this condition.
Another type of cardiomyopathy is termed dilated cardiomyopathy. Mutations in the genes encoding dystrophin, muscle LIM protein (so-called because it was found to contain a cysteine-rich domain originally detected in three proteins: Lin-II, Isl-1, and Mec-3), the cyclic response-element binding protein (CREB), desmin, and lamin have been implicated in the causation of this condition. The first two proteins help organize the contractile apparatus of cardiac muscle cells, and CREB is involved in the regulation of a number of genes in these cells. Current research is not only elucidating the molecular causes of the cardiomyopathies but is also disclosing mutations that cause cardiac developmental disorders (eg, septal defects) and arrhythmias (eg, due to mutations affecting ion channels).
Ca2+ Also Regulates Contraction of Smooth Muscle
While all muscles contain actin, myosin, and tropomyosin, only vertebrate striated muscles contain the troponin system. Thus, the mechanisms that regulate contraction must differ in various contractile systems.
Smooth muscles have molecular structures similar to those in striated muscle, but the sarcomeres are not aligned so as to generate the striated appearance. Smooth muscles contain α-actinin and tropomyosin molecules, as do skeletal muscles. They do not have the troponin system, and the light chains of smooth muscle myosin molecules differ from those of striated muscle myosin. Regulation of smooth muscle contraction is myosin-based, unlike striated muscle, which is actin-based. However, like striated muscle, smooth muscle contraction is regulated by Ca2+.
Phosphorylation of Myosin Light Chains Initiates Contraction of Smooth Muscle
When smooth muscle myosin is bound to F-actin in the absence of other muscle proteins such as tropomyosin, there is no detectable ATPase activity. This absence of activity is quite unlike the situation described for striated muscle myosin and F-actin, which has abundant ATPase activity. Smooth muscle myosin contains light chains that prevent the binding of the myosin head to F-actin; they must be phosphorylated before they allow F-actin to activate myosin ATPase. The ATPase activity then attained hydrolyzes ATP about 10-fold more slowly than the corresponding activity in skeletal muscle. The phosphate on the myosin light chains may form a chelate with the Ca2+ bound to the tropomyosin-TpC-actin complex, leading to an increased rate of formation of cross bridges between the myosin heads and actin. The phosphorylation of light chains initiates the attachment-detachment contraction cycle of smooth muscle.
Myosin Light Chain Kinase Is Activated by Calmodulin-4Ca2+ & Then Phosphorylates the Light Chains
Smooth muscle sarcoplasm contains a myosin light chain kinase that is calcium dependent. The Ca2+ activation of myosin light chain kinase requires binding of calmodulin-4Ca2+ to its kinase subunit (Figure 51–14). The calmodulin-4Ca2+-activated light chain kinase phosphorylates the light chains, which then cease to inhibit the myosin–F-actin interaction. The contraction cycle then begins.
Regulation of smooth muscle contraction by Ca2+. The pL-myosin is the phosphorylated light chain of myosin and L-myosin is the dephosphorylated light chain. (Adapted, with permission, from Adelstein RS, Eisenberg R: Regulation and kinetics of actin–myosin ATP interaction. Annu Rev Biochem 1980;49:921. Copyright © 1980 by Annual Reviews, www.annualreviews.org.)
Another non-Ca2+-dependent pathway exists in smooth muscle for initiating contraction. This involves Rho kinase, which is activated by a variety of stimuli (not shown in Figure 51–14). This enzyme phosphorylates myosin light chain phosphatase, inhibiting it, and thus increasing the level of phosphorylated light chains. Rho kinase also directly phosphorylates the light chain of myosin. Both of these actions increase the contraction of smooth muscle.
Smooth Muscle Relaxes When the Concentration of Ca2+ Falls Below 10–7 Molar
Relaxation of smooth muscle occurs when sarcoplasmic Ca2+ falls below 10–7 mol/L. Ca2+ then dissociates from calmodulin, which in turn dissociates from the myosin light chain kinase, which renders the kinase inactive. No new phosphates are attached to the p-light chain, and light chain protein phosphatase, which is continually active and calcium independent, removes the existing phosphates from the light chains. Dephosphorylated myosin p-light chain then inhibits the binding of myosin heads to F-actin and their ATPase activity. The myosin head detaches from the F-actin in the presence of ATP, but it cannot reattach because of the presence of dephosphorylated p-light chain; hence, relaxation occurs.
Table 51–7 summarizes and compares the regulation of actin-myosin interactions (activation of myosin ATPase) in striated and smooth muscles.
TABLE 51–7Actin-Myosin Interactions in Striated and Smooth Muscle ||Download (.pdf) TABLE 51–7 Actin-Myosin Interactions in Striated and Smooth Muscle
| ||Striated Muscle ||Smooth Muscle (and Nonmuscle Cells) |
|Proteins of muscle filaments || |
Troponin (Tpl, TpT, TpC)
|Spontaneous interaction of F-actin and myosin alone (spontaneous activation of myosin ATPase by F-actin) ||Yes ||No |
|Inhibitor of F-actin–myosin interaction (inhibitor of F-actin-dependentactivation of ATPase) ||Troponin system (Tpl) ||Unphosphorylated myosin light chain |
|Contraction activated by ||Ca2+ ||Ca2+ |
|Direct effect of Ca2+ ||4Ca2+ bind to TpC ||4Ca2+ bind to calmodulin |
|Effect of protein-bound Ca2+ ||TpC·4Ca2+ antagonizes Tpl inhibition of F-actin-myosin interaction (allows F-actin activation of ATPase) ||Calmodulin·4Ca2+ activates myosin light chain kinase that phosphorylates myosin p-light chain. The phosphorylated p-light chain no longer inhibits F-actin–myosin interaction (allows F-actin activation of ATPase) |
The myosin light chain kinase is not directly affected or activated by cAMP. However, cAMP-activated protein kinase can phosphorylate myosin light chain kinase (not the chains themselves). Phosphorylated myosin light chain kinase exhibits a significantly lower affinity for calmodulin-4Ca2+ and thus is less sensitive to activation. Accordingly, an increase in cAMP dampens the contraction response of smooth muscle to a given elevation of sarcoplasmic Ca2+. This molecular mechanism can explain the relaxing effect of β-adrenergic stimulation on smooth muscle.
Another protein that appears to play a Ca2+-dependent role in the regulation of smooth muscle contraction is caldesmon (87 kDa). This protein is ubiquitous in smooth muscle and is also found in nonmuscle tissue. At low concentrations of Ca2+, it binds to tropomyosin and actin. This prevents interaction of actin with myosin, keeping muscle in a relaxed state. At higher concentrations of Ca2+, calmodulin-4Ca2+ binds caldesmon, releasing it from actin. The latter is then free to bind to myosin, and contraction can occur. Caldesmon is also subject to phosphorylation-dephosphorylation; when phosphorylated, it cannot bind actin, again freeing the latter to interact with myosin. Caldesmon may also participate in organizing the structure of the contractile apparatus in smooth muscle. Many of its effects have been demonstrated in vitro, and its physiologic significance is still under investigation.
As noted in Table 51–3, slow cycling of the cross-bridges permits slow prolonged contraction of smooth muscle (eg, in viscera and blood vessels) with less utilization of ATP compared with striated muscle. The ability of smooth muscle to maintain force at reduced velocities of contraction is referred to as the latch state; this is an important feature of smooth muscle, and its precise molecular bases are under study.
Nitric Oxide (NO) Relaxes the Smooth Muscle of Blood Vessels & Also Has Many Other Important Biologic Functions
Acetylcholine is a vasodilator that acts by causing relaxation of the smooth muscle of blood vessels. However, it does not act directly on smooth muscle. A key observation was that if endothelial cells were stripped away from underlying smooth muscle cells, acetylcholine no longer exerted its vasodilator effect. This finding indicated that vasodilators such as acetylcholine initially interact with the endothelial cells of small blood vessels via receptors. The receptors are coupled to the phosphoinositide cycle, leading to the intracellular release of Ca2+ through the action of inositol trisphosphate. In turn, the elevation of Ca2+ leads to the liberation of endothelium-derived relaxing factor (EDRF), which diffuses into the adjacent smooth muscle. There, it reacts with the heme moiety of a soluble guanylyl cyclase, resulting in activation of the latter, with a consequent elevation of intracellular levels of cGMP (Figure 51–15). This in turn stimulates the activities of certain cGMP-dependent protein kinases, which probably phosphorylate specific muscle proteins, causing relaxation; however, the details are still being clarified. The important coronary artery vasodilator nitroglycerin, widely used to relieve angina pectoris, acts to increase intracellular release of EDRF and thus of cGMP.
Diagram showing formation in an endothelial cell of nitric oxide (NO) from arginine in a reaction catalyzed by NO synthase. Interaction of an agonist (eg, acetylcholine) with a receptor (R) probably leads to intracellular release of Ca2+ via inositol trisphosphate generated by the phosphoinositide pathway, resulting in activation of NO synthase. The NO subsequently diffuses into adjacent smooth muscle, where it leads to activation of guanylyl cyclase, formation of cGMP, stimulation of cGMP protein kinases, and subsequent relaxation. The vasodilator nitroglycerin is shown entering the smooth muscle cell, where its metabolism also leads to formation of NO.
Quite unexpectedly, EDRF was found to be the gas NO. NO is formed by the action of the enzyme NO synthase, which is cytosolic. The endothelial and neuronal forms of NO synthase are activated by Ca2+ (Table 51–8). The substrate is arginine, and the products are citrulline and NO.
TABLE 51–8Summary of the Nomenclature of the NO Synthases and of the Effects of Knockout of Their Genes in Mice ||Download (.pdf) TABLE 51–8 Summary of the Nomenclature of the NO Synthases and of the Effects of Knockout of Their Genes in Mice
|Subtype ||Namea ||Comments ||Result of Gene Knockout in Miceb |
|1 ||nNOS ||Activity depends on elevated Ca2+; first identified in neurons; calmodulin-activated ||Pyloric stenosis, resistant to vascular stroke, aggressive sexual behavior (males) |
|2 ||iNOSc ||Independent of elevated Ca2+; prominent in macrophages ||More susceptible to certain types of infection |
|3 ||eNOS ||Activity depends on elevated Ca2+; first identified in endothelial cells ||Elevated mean blood pressure |
NO synthase catalyzes a five-electron oxidation of an amidine nitrogen of arginine. L-hydroxyarginine is an intermediate that remains tightly bound to the enzyme. NO synthase is a very complex enzyme, employing five redox cofactors: NADPH, FAD, FMN, heme, and tetrahydrobiopterin. NO can also be formed from nitrite, derived from vasodilators such as glyceryl trinitrate during their metabolism. NO has a very short half-life (approximately 3-4 seconds) in tissues because it reacts with oxygen and superoxide. The product of the reaction with superoxide is peroxynitrite (ONOO−), which yields the highly reactive OH · radical when it decomposes. NO binds tightly to hemoglobin and other heme proteins. Chemical inhibitors of NO synthase are available that can markedly decrease formation of NO. Administration of such inhibitors leads to vasoconstriction and a marked elevation of blood pressure, indicating that NO is of major importance in the maintenance of blood pressure in vivo. Another important cardiovascular effect is the inhibiton of platelet aggregation, a consequence of the increased synthesis of cGMP (see Chapter 51).
Since the discovery of the role of NO as a vasodilator, there has been intense experimental interest in this molecule. It has turned out to have a variety of physiologic roles, involving virtually every tissue of the body (Table 51–9). Three major isoforms of NO synthase have been identified, each of which has been cloned, and the chromosomal locations of their genes in humans have been determined. Gene knockout experiments have been performed on each of the three isoforms and have helped establish some of the postulated functions of NO.
TABLE 51–9Some Physiologic Functions and Pathologic Involvements of Nitric Oxide (NO) ||Download (.pdf) TABLE 51–9 Some Physiologic Functions and Pathologic Involvements of Nitric Oxide (NO)
Vasodilator, important in regulation of blood pressure.
Involved in penile erection; sildenafil citrate (Viagra) affects this process by inhibiting a cGMP phosphodiesterase.
Neurotransmitter in the brain and the peripheral autonomic nervous system.
Role in long-term potentiation.
Role in neurotoxicity.
Low level of NO involved in causation of pylorospasm in infantile hypertrophic pyloric stenosis.
May have role in relaxation of skeletal muscle.
May constitute part of a primitive immune system.
Inhibits adhesion, activation, and aggregation of platelets.
To summarize, research in the past decade has shown that NO plays an important role in many physiologic and pathologic processes.