Electromechanical coupling and regulation of force of cardiac contraction

Cardiac muscle fibres, like skeletal muscle fibres, are divided into sarcomeres, the basic unit of contraction. The contractile elements include actin, myosin, tropomyosin and troponin. The myosin molecules are arranged into thick filaments, while the actin molecules form the basis of the thin filaments. The troponin and tropomyosin are attached to the thin filaments as in skeletal muscle. In contrast to fast skeletal muscle fibres, which need to produce repetitive mechanical action only for short periods before resting, and hence can accrue an oxygen debt, cardiac muscle fibres need to perform repetitive activity for long periods (a lifetime) without rest. Consequently, cardiac muscle fibres are much more dependent on the utilization of oxygen and have an abundance of mitochondria, with rapid oxidation of substrates and formation of adenosine triphosphate, needed for mechanical contraction.     

Electromechanical coupling and regulation of force of cardiac contraction

Cardiac muscle fibres, like skeletal muscle fibres, are divided into sarcomeres, the basic unit of contraction. The contractile elements include actin, myosin, tropomyosin and troponin. The myosin molecules are arranged into thick filaments, while the actin molecules form the basis of the thin filaments. The troponin and tropomyosin are attached to the thin filaments as in skeletal muscle. In contrast to fast skeletal muscle fibres, which need to produce repetitive mechanical action only for short periods before resting, and hence can accrue an oxygen debt, cardiac muscle fibres need to perform repetitive activity for long periods (a lifetime) without rest. Consequently, cardiac muscle fibres are much more dependent on the utilization of oxygen and have an abundance of mitochondria, with rapid oxidation of substrates and formation of adenosine triphosphate, needed for mechanical contraction.     

Electromechanical coupling and regulation of force of cardiac contraction

Cardiac muscle fibres, like skeletal muscle fibres, are divided into sarcomeres, the basic unit of contraction. The contractile elements include actin, myosin, tropomyosin and troponin. The myosin molecules are arranged into thick filaments, while the actin molecules form the basis of the thin filaments. The troponin and tropomyosin are attached to the thin filaments as in skeletal muscle. In contrast to fast skeletal muscle fibres, which need to produce repetitive mechanical action only for short periods before resting, and hence can accrue an oxygen debt, cardiac muscle fibres need to perform repetitive activity for long periods (a lifetime) without rest. Consequently, cardiac muscle fibres are much more dependent on the utilization of oxygen and have an abundance of mitochondria, with rapid oxidation of substrates and formation of ATP, needed for mechanical contraction.     

Mechanism of cerebral vasospasm--contractile proteins in smooth muscle of bovine cerebral artery

Understanding of the pathogenesis of cerebral vasospasm requires basic study of the contraction mechanism of cerebral arteries. In this study, the morphological and biochemical properties of actomyosin from bovine cerebral arteries were examined. Arrays of thick and thin filaments were clearly seen on electron micrographs of a cross section of a muscle strip stretched to 1.5 times its normal length in Krebs-Ringer solution. The diameters of the thick and thin filaments were 110.0 +/- 19.9 A and 51.0 +/- 8.0 A, respectively. The average ratio of thick to thin filaments was about 1:15 to 1:16. The gel pattern of Ca2+-sensitive actomyosin extracted from bovine cerebral arteries revealed the presence of myosin, actin, and tropomyosin, but not skeletal troponin. The Mg-ATPase activity of actomyosin was greatly enhanced in the presence of Ca2+. Rapid superprecipitation occurred after the clearing phase, when 1 mM Mg-ATP was added to a suspension of actomyosin with 10(-5) M Ca2+. This superprecipitation did not occur with a lower Ca2+ concentration of 10(-8) M. Intracellular free Ca2+ appears to play a crucial role in the regulation of contraction and relaxation of cerebral arterial smooth muscle.     

Influence of the alpha1-adrenergic antagonist, doxazosin, on noradrenaline-induced modulation of cytoskeletal proteins in cultured hyperplastic prostatic stromal cells

BACKGROUND: Doxazosin, an alpha1-adrenergic antagonist, inhibits sympathetic contraction of prostatic stromal smooth muscle cells and is used in the relief of obstructive benign prostatic hyperplasia (BPH). In vitro application of noradrenaline stimulates expression of cytoskeletal filaments, particularly actin and myosin, by prostatic stromal cells, thus enhancing their differentiation towards smooth muscle cells. This study examined the possible role of doxazosin in reversing this phenotypic modulation as well as in inhibiting smooth muscle cell contraction. METHODS: Stromal cell tissue cultures derived from 10 human hyperplastic prostates were rendered quiescent by reduction of stripped fetal calf serum (FCS) to 1% (v/v) in the medium followed by treatment with 20 microM noradrenaline and/or 1 microM doxazosin for 10 days. Doxazosin, in 10-fold increments of concentration, was also added, separately, to two of these cell cultures, which were either quiescent or growing in 10% normal (unstripped) FCS. Harvested cells were labelled with fluorescein-labelled antisera to smooth muscle cytoskeletal filaments, and their individual fluorescence levels were analyzed flow-cytometrically. RESULTS: Noradrenaline increased expression of all cytoskeletal filaments studied. This effect was greatest for actin and myosin in proliferating cell cultures. Doxazosin largely reversed the increase in filament expression. This effect was most significant for actin and myosin and greatest in quiescent cultures. However, inhibition of the agonist effect of noradrenaline by doxazosin showed no clear dose-related response, in that expression of cytoskeletal filaments was differentially inhibited. CONCLUSIONS: The data suggest that doxazosin may inhibit not only stromal contraction of differentiated smooth muscle cells in BPH but also the phenotypic modulation of stromal smooth muscle cell differentiation induced by noradrenaline. These actions, together, may render prostatic stroma less contractile, and hence less able to respond to sympathetic stimulation, in patients with BPH. While effects on isolated stromal cells are of undoubted importance, failure to demonstrate a consistent dose-response relationship between expression of smooth muscle cell phenotype and inhibition by doxazosin suggests that additional influences, including humoral factors as well as the proximity of differentiated epithelium, are also likely to be involved in this interaction in the intact tissue.     

Muscle wasting in chronic kidney disease: the role of the ubiquitin proteasome system and its clinical impact

Muscle wasting in chronic kidney disease (CKD) and other catabolic diseases (e.g. sepsis, diabetes, cancer) can occur despite adequate nutritional intake. It is now known that complications of these various disorders, including acidosis, insulin resistance, inflammation, and increased glucocorticoid and angiotensin II production, all activate the ubiquitin–proteasome system (UPS) to degrade muscle proteins. The initial step in this process is activation of caspase-3 to cleave the myofibril into its components (actin, myosin, troponin, and tropomyosin). Caspase-3 is required because the UPS minimally degrades the myofibril but rapidly degrades its component proteins. Caspase-3 activity is easily detected because it leaves a characteristic 14kD actin fragment in muscle samples. Preliminary evidence from several experimental models of catabolic diseases, as well as from studies in patients, indicates that this fragment could be a useful biomarker because it correlates well with the degree of muscle degradation in dialysis patients and in other catabolic conditions.     

Muscle physiology and contraction

Skeletal muscle has important metabolic functions but the focus of this article is to examine its ability to generate mechanical force. Adult skeletal muscle fibres are each innervated by a single branch of the axon arising from an α-motoneuron in the spinal cord. The α-motoneuron and all the fibres it innervates constitute a motor unit, and this is the functional unit of the muscle. α-Motoneurons differ in size and excitability and it is the recruitment of these cell bodies in the spinal cord that determines which fibres within the muscle are active during a movement. Correct functioning of the neuromuscular junction is clearly critical for muscle action and it is a site at which many drugs affecting muscle have their action. Here we describe also the mechanism by which skeletal muscle generates force following activation, a process known as excitation–contraction coupling and examine the contractile properties of muscle as well as describing muscle weakness and fatigue and the assessment of muscle performance in health and disease.     

Muscle physiology and contraction

Skeletal muscle has important metabolic functions but the focus of this article is to examine its ability to generate mechanical force. Adult skeletal muscle fibres are each innervated by a single branch of the axon arising from an α-motoneuron in the spinal cord. The α-motoneuron and all the fibres it innervates constitute a motor unit, and this is the functional unit of the muscle. α-Motoneurons differ in size and excitability and it is the recruitment of these cell bodies in the spinal cord that determines which fibres within the muscle are active during a movement. Correct functioning of the neuromuscular junction is clearly critical for muscle action and it is a site at which many drugs affecting muscle have their action. Here we describe also the mechanism by which skeletal muscle generates force following activation, a process known as excitation–contraction coupling and examine the contractile properties of muscle as well as describing muscle weakness and fatigue and the assessment of muscle performance in health and disease.     

Halothane and isoflurane do not directly interact with cardiac cross-bridge function

BACKGROUND: Halogenated anesthetics depress myocardial contractility by altering a number of specific mechanisms. These alterations include decreases in inward calcium current and sarcoplasmic reticulum function and reduced calcium myofilament sensitivity. However, the direct effects of volatile anesthetics on cross-bridge function have yet to be precisely determined. METHODS: Myosin monomers and actin filaments were isolated from fresh rat left ventricles and rabbit skeletal muscles, respectively. Halothane or isoflurane was added at concentrations equivalent to 1 and 2 minimum alveolar concentration (MAC). Motility of actin filaments over myosin was initiated by adding 2 mm adenosine triphosphate and was analyzed at 30 degrees C. Maximum actomyosin adenosine triphosphatase activity and the association constant of myosin for actin were determined from a double-reciprocal Lineweaver-Burk plot of the adenosine triphosphatase rate versus actin concentration. A known inhibitor of actomyosin function, 2,3-butanedione 2-monoxime (2 mm), was used in positive control experiments. Data are presented as mean +/- SD. RESULTS: Motility velocities driven by myosin were not significantly different between baseline and 1 and 2 MAC halothane (2.70 +/- 0.33, 2.72 +/- 0.36, and 2.70 +/- 0.40 microm/s, respectively). Similarly, motility velocities driven by myosin were not significantly different between baseline and 1 and 2 MAC isoflurane (2.73 +/- 0.33, 2.72 +/- 0.37, and 2.72 +/- 0.40 microm/s, respectively). Neither of the two halogenated anesthetics, at any concentration tested, significantly modified the maximum actomyosin adenosine triphosphatase activity or the association constant of myosin for actin as compared with baseline. 2,3-Butanedione 2-monoxime induced a drastic reduction in both motility velocity and maximum actomyosin adenosine triphosphatase activity. CONCLUSION: These results indicate that isoflurane and halothane do not directly depress the mechanical or enzymatic properties of cross-bridges in the heart.     

Perioperative ST-segment depression and troponin T release. Identification of patients with highest risk for myocardial damage

BACKGROUND: Patients undergoing major vascular surgery are at constant risk of developing perioperative myocardial complications, especially myocardial infarction. The following study was performed to answer the question whether ST segment changes, analysed by Holter monitoring and ST segment analysis, are accompanied by release of cardiac troponin T, a highly specific marker of myocardial damage. METHODS: Twenty patients undergoing elective aortic resection were studied by performing Holter ECG, including ST segment analysis, beginning on the evening before surgery until the third postoperative day. Within this period serum levels of cardiac troponin T were determined at 8 timepoints. RESULTS: A total of 8/20 of the patients (40%) showed significant ST depressions (range -0.17/-0.68 mV), without any clinical symptom, with a median of 9 episodes (range 2-24). In 3 of the 8 patients, each with repetitive periods of ST depression, elevated troponin T levels were found (0.45/0.52/1.69 micrograms/l). No troponin T release nor cardiac events were noticed in the remaining patients. No dependency could be found between troponin T release and the magnitude of ST depression or the number of ST depression episodes. CONCLUSION: Haemodynamic changes, oxygen imbalance and stress during major vascular surgery frequently lead to an ischaemic burden, which is indicated by ST segment changes during ECG ST analysis. Longlasting ST depression reaching an individual critical cut-off limit followed by structural myocardial damage may be verified by elevated levels of cardiac troponin T. Prolonged periods of ST depression should be followed by determination of cardiac troponin T.     

Regulation of actomyosin and contraction in smooth muscle

Summary Unlike striated muscle cells, smooth muscle cells do not have an organized sarcomeric structure. However, all smooth muscle cells contain the contractile proteins, myosin, actin, and tropomyosin. Polymorphism of the myosin heavy chain exists in smooth muscle cells. Two myosin heavy chain (MHC) isoforms, SM1 (204 kDa) and SM2 (200 kDa), are present in smooth muscle cells; however, their ratios vary in smooth muscles from different sources. The hypertrophy of the urinary bladder induced by partial outlet obstruction in rabbits is associated with an alteration of the SM1-to-SM2 ratio from 1:3 to 1:1. Both heavy chains react with polyclonal antibody against smooth muscle myosin; however, antibody prepared against a peptide from the C-terminal region of the SM2 heavy chain cross-reacts only with the SM2 heavy chain. Removal of the obstruction reverses the bladder to normal mass with a concomitant change in the SM1-to-SM2 ratio back to 1:3. The expression of the SM1 mRNA is increased in response to obstruction-induced hypertrophy, and it also returns to normal upon removal of the obstruction. Urinary bladder smooth muscle contains predominantly -actin. Obstruction-induced hypertrophy of the bladder smooth muscle is associated with an increase in the -actin at both protein and mRNA levels. The -non-muscle actin is decreased and the -smooth muscle actin is unchanged in response to obstruction-induced bladder hypertrophy.Contraction of all smooth muscles involves similar mechanisms. This review describes our current understanding of the mechanisms regulating contraction of the smooth muscle of the urinary bladder.This work was supported in part by NIH grants HL 22246, DK 47514 and DK 39740     

Morphological data correlated with the mechanism of striated muscle contraction

The classical theory of muscle contraction (Huxley, 1957) could not entirely explain a series of theoretical and experimental data. This theory sustains a longitudinal sliding of thin filaments on thick ones, by the tilting of myosin cross-bridges. Under different stimuli inducing contractions, the stereospecific conformations of cross-bridges appear to be different as it is shown using electron microscopy, fluorescence polarization, electron paramagnetic resonance (EPR) spectroscopy and equatorial X-ray diffraction. Our optic microscopy data also suggest the existence of such conformations, induced by different contractile stimuli.     

Muscle

Two important types of muscle are skeletal and smooth muscle. Their similarities and differences are described with regard to general morphology and ultrastructure (arrangement of filaments and T tubules), linking structure to physiological functions. The sliding-filament theory of muscle contraction is explained. Finally, the processes by which nerves excite muscle contraction (excitation–contraction coupling) are outlined, again relating variations in these processes in the two types of muscle to differences between them in their role in the body.     

Halothane and Isoflurane Do Not Directly Interact with Cardiac Cross-bridge Function

Background: Halogenated anesthetics depress myocardial contractility by altering a number of specific mechanisms. These alterations include decreases in inward calcium current and sarcoplasmic reticulum function and reduced calcium myofilament sensitivity. However, the direct effects of volatile anesthetics on cross-bridge function have yet to be precisely determined.

Methods: Myosin monomers and actin filaments were isolated from fresh rat left ventricles and rabbit skeletal muscles, respectively. Halothane or isoflurane was added at concentrations equivalent to 1 and 2 minimum alveolar concentration (MAC). Motility of actin filaments over myosin was initiated by adding 2 mm adenosine triphosphate and was analyzed at 30[degrees]C. Maximum actomyosin adenosine triphosphatase activity and the association constant of myosin for actin were determined from a double-reciprocal Lineweaver-Burk plot of the adenosine triphosphatase rate versus actin concentration. A known inhibitor of actomyosin function, 2,3-butanedione 2-monoxime (2 mm), was used in positive control experiments. Data are presented as mean +/- SD.

Results: Motility velocities driven by myosin were not significantly different between baseline and 1 and 2 MAC halothane (2.70 +/- 0.33, 2.72 +/- 0.36, and 2.70 +/- 0.40 [mu]m/s, respectively). Similarly, motility velocities driven by myosin were not significantly different between baseline and 1 and 2 MAC isoflurane (2.73 +/- 0.33, 2.72 +/- 0.37, and 2.72 +/- 0.40 [mu]m/s, respectively). Neither of the two halogenated anesthetics, at any concentration tested, significantly modified the maximum actomyosin adenosine triphosphatase activity or the association constant of myosin for actin as compared with baseline. 2,3-Butanedione 2-monoxime induced a drastic reduction in both motility velocity and maximum actomyosin adenosine triphosphatase activity.     

Skeletal muscle contractions uncoupled from gravitational loading directly increase cortical bone blood flow rates in vivo

The direct and indirect effects of muscle contraction on bone microcirculation and fluid flow are neither well documented nor explained. However, skeletal muscle contractions may affect the acquisition and maintenance of bone via stimulation of bone circulatory and interstitial fluid flow parameters. The purposes of this study were to assess the effects of transcutaneous electrical neuromuscular stimulation (TENS)‐induced muscle contractions on cortical bone blood flow and bone mineral content, and to demonstrate that alterations in blood flow could occur independently of mechanical loading and systemic circulatory mechanisms. Bone chamber implants were used in a rabbit model to observe real‐time blood flow rates and TENS‐induced muscle contractions. Video recording of fluorescent microspheres injected into the blood circulation was used to calculate changes in cortical blood flow rates. TENS‐induced repetitive muscle contractions uncoupled from mechanical loading instantaneously increased cortical microcirculatory flow, directly increased bone blood flow rates by 130%, and significantly increased bone mineral content over 7 weeks. Heart rates and blood pressure did not significantly increase due to TENS treatment. Our findings suggest that muscle contraction therapies have potential clinical applications for improving blood flow to cortical bone in the appendicular skeleton. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27: 651–656, 2009     

Mechanical events and the pressure–volume relationships

Depolarization of cardiac muscle fibres spreads from fibre to fibre throughout the myocardium. In a single fibre, contraction starts just after depolarization and lasts until just after repolarization is complete. The atria contract, completing the filling of the ventricles and thus enhancing their action. In the absence of effective atrial contraction (e.g. atrial fibrillation) cardiac output is decreased on average by 15%. During diastole, when cardiac muscle is relaxed, blood returns to the heart and passes through the atrioventricular (AV) valves into the ventricles. The semilunar valves, between the ventricles and the arteries, are closed as arterial pressure exceeds ventricular pressure. Under normal circumstances, 70% of ventricular filling occurs by late diastole.