Formation Of Contraction Bands Research Articles

Pathobiology of Myocardial Ischemia and Reperfusion Injury: Models, Modes, Molecular Mechanisms, Modulation, and Clinical Applications.

This review presents an integrated approach to the analysis of myocardial ischemia and reperfusion injury and the modulating influence of myocardial conditioning during the evolution of acute myocardial infarction (AMI) and other clinical settings. Experimental studies have involved a spectrum of in vitro, ex vivo, and in vivo models, and guidelines have been developed for the conduct of rigorous preclinical studies and for the identification of various forms of cell injury and death in evolving AMI. AMI in vivo is dominated by oncosis (cell injury with swelling) leading to necroptosis and final necrosis of ischemic cardiomyocytes (CMCs), without or with contraction band formation. Early after coronary occlusion, reperfusion salvages a significant amount of ischemic myocardium in the subepicardium while reperfusion injury contributes up to 50% of the final subendocardial infarct. AMI progression is mediated by damage (or danger)-associated molecular patterns, also known as alarmins, which activate pattern recognition receptors and initiate the inflammatory response. In preclinical studies, lethal reperfusion injury can largely be prevented with preconditioning or postconditioning by pharmacologic or physical means due to effects on both the CMC and microvasculature. Conditioning involves triggers, cytosolic mediators, and intracellular effectors. Mitochondria have a central role in the maintenance and loss of viability of CMCs. Reperfusion of severely ischemic myocardium leads to sustained opening of the mitochondrial permeability transition pore (MPTP). Once the MPTP is opened, the mitochondrial membrane potential (ΔΨm) is rapidly lost and energy production ceases. Conditioning blocks the sustained opening of the MPTP. Translation of conditioning strategies to the clinical management of patients has been challenging. The status of translation of experimental findings to approaches to modulate and ameliorate ischemic and reperfusion injury is discussed for the clinical settings of acute coronary syndromes treated with percutaneous interventions and cardiac preservation during open heart surgery and cardiac transplantation.

Melatonin protects against myocardial ischemia-reperfusion injury by inhibiting contracture in isolated rat hearts

To investigate the protective effect of melatonin against myocardial ischemia reperfusion (IR) injury in isolated rat hearts and explore the underlying mechanisms. The isolated hearts from 40 male SD rats were randomly divided into 4 groups (n=10): the control group, where the hearts were perfused with KH solution for 175 min; IR group, where the hearts were subjected to global ischemia for 45 min followed by reperfusion for 120 min; IR+melatonin (Mel+IR) group, where melatonin (5 μmol/L) was administered to the hearts 1 min before ischemia and during the first 5 min of reperfusion, followed by 115 min of reperfusion; and IR+2, 3-butanedione monoxime (IR+BDM) group, where the hearts were treated with BDM (20 mmol/L) in the same manner as melatonin treatment. Myocardial injury in the isolated hearts was assessed based on myocardial injury area, caspase-3 activity, and expressions of cytochrome C and cleaved caspase-3 proteins. Cardiac contracture was assessed using HE staining and by detecting lactate dehydrogenase (LDH) activity and the content of cardiac troponin I (cTnI) in the coronary outflow, measurement of left ventricular end-diastolic pressure (LVEDP) and electron microscopy. The content of ATP in the cardiac tissue was also determined. Compared with those in the control group, the isolated hearts in IR group showed significantly larger myocardial injury area and higher caspase-3 activity and the protein expressions of cytochrome C and cleaved caspase-3 with significantly increased LDH activity and cTnI content in the coronary outflow and elevated LVEDP at the end of reperfusion; HE staining showed obvious fractures of the myocardial fibers and the content of ATP was significantly decreased in the cardiac tissue; electron microscopy revealed the development of contraction bands. In the isolated hearts with IR, treatment with Mel or BDM significantly reduced the myocardial injury area, caspase-3 activity, and protein expressions of cytochrome C and cleaved caspase-3, obviously inhibited LDH activity, lowered the content of cTnI and LVEDP, reduced myocardial fiber fracture, and increased ATP content in the cardiac tissue. Both Mel and BDM inhibited the formation of contraction bands in the isolated hearts with IR injury. Mel can alleviate myocardial IR injury in isolated rat hearts by inhibiting cardiac contracture, the mechanism of which may involve the upregulation of ATP in the cardiac myocytes to lessen the tear of membrane and reduce cell content leakage.

Cardiomyopathies and Adrenal Diseases.

Cardiomyopathies are myocardial disorders in which heart muscle is structurally and/or functionally abnormal. Previously, structural cardiomyocyte disorders due to adrenal diseases, such as hyperaldosteronism, hypercortisolism, and hypercatecholaminism, were misunderstood, and endomyocardial biopsy (EMB) was not performed because was considered dangerous and too invasive. Recent data confirm that, if performed in experienced centers, EMB is a safe technique and gives precious information about physiopathological processes implied in clinical abnormalities in patients with different systemic disturbances. In this review, we illustrate the most important features in patients affected by primary aldosteronism (PA), Cushing’s syndrome (CS), and pheochromocytoma (PHEO). Then, we critically describe microscopic and ultrastructural aspects that have emerged from the newest EMB studies. In PA, the autonomous hypersecretion of aldosterone induces the alteration of ion and water homeostasis, intracellular vacuolization, and swelling; interstitial oedema could be a peculiar feature of myocardial toxicity. In CS, cardiomyocyte hypertrophy and myofibrillolysis could be related to higher expression of atrogin-1. Finally, in PHEO, the hypercontraction of myofilaments with the formation of contraction bands and occasional cellular necrosis has been observed. We expect to clear the role of EMB in patients with cardiomyopathies and adrenal disease, and we believe EMB is a valid tool to implement new management and therapies.

Contraction Bands Are Induced by Cardiac Massage Itself.

Pathological contraction bands (CB) are recognized as a type of necrosis pattern found in the myocardium. It is well known that CB are induced by cardiopulmonary resuscitation (CPR) with cardiac massage. However, it is not known whether the reperfusion or massage itself causes the formation of CB. We studied 10 cardiac tissues taken from forensic autopsy cases that had not received CPR. We excluded the cases where the cause of death was a cardiac event. After making sections for forensic research, we massaged the left ventricles for 10 minutes. We found CB in all cases with massage performed within 12 hours after death, which is the time frame for supravital reactions. We did not find CB in any cases where more than 24 hours had elapsed since the time of death. Contraction bands were not observed in any sections that were taken before massage. We suggested here that CB induced by CPR were caused by the cardiac massage itself, and that most CPR-induced CB are not contraction band necrosis but rather artifactual CB.

Apolipoprotein A-IMilano/POPC complex attenuates post-ischemic ventricular dysfunction in the isolated rabbit heart

Irreversible myocardial injury is a potential consequence of coronary artery revascularization. Reperfusion leads to the production of oxidized products that can damage myocardium. High-density lipoproteins (HDL) are effective at removing oxidized lipids. We hypothesized that a synthetic HDL preparation, comprising recombinant apolipoprotein A-IMilano (apoA-IM) complexed with 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) (apoA-IM/POPC) would protect the heart from reperfusion injury. The ex vivo model consisted of rabbit hearts perfused by the Langendorff method. Hearts were equilibrated with Krebs–Henseleit buffer (10min), pretreated with either apoA-IM/POPC (0.45mg/mL) or vehicle (10min), subjected to global ischemia (30min) and reperfused for 60min. ApoA-IM/POPC (n=7) prevented the left ventricular end-diastolic pressure elevation observed in the vehicle group (n=6) at the end of reperfusion (p<0.05). During reperfusion, coronary artery perfusion pressure increased in the controls (p<0.001), but not with apoA-IM/POPC. ApoA-IM/POPC reduced the release of creatine kinase at the end of the ischemic period (p<0.001). It also reduced cardiac left ventricle muscle lipid hydroperoxides by 46% (p<0.05). Direct comparison of the antioxidant potential indicated that recombinant apoA-IM was much more potent than apoA-I in attenuating low-density lipoprotein oxidation. Electron microscopy showed that apoA-IM/POPC prevented mitochondrial granulation, disorganization and sarcomere contraction band formation indicative of reperfusion injury.The apoA-IM/POPC complex thus appears to reduce reperfusion injury under global ischemic conditions, and may therefore have therapeutic application in the reduction of myocardial ischemia.

Apolipoprotein A-IMilano and 1-palmitoyl-2-oleoyl phosphatidylcholine complex (ETC-216) protects the in vivo rabbit heart from regional ischemia-reperfusion injury.

Ex vivo studies demonstrated that a synthetic high-density lipoprotein (HDL) comprised of a complex of recombinant apolipoprotein A-IMilano and 1-palmitoyl-2-oleoyl phosphatidylcholine protects the isolated rabbit heart from reperfusion injury. Therefore, we sought to determine whether a pharmaceutical preparation of this complex, ETC-216, was cardioprotective in an in vivo model of left anterior descending artery (LAD) occlusion and reperfusion. Initially, ETC-216 (100 mg/kg) was tested in acute (one-treatment) and chronic (two-treatment) i.v. administrations. ETC-216-treated rabbits developed smaller infarcts expressed as percentage of area at risk (p <0.01) compared with vehicle treatments. No differences were noted between chronic and acute administration. Therefore, ETC-216 (10, 3, or 1 mg/kg) or equivalent vehicle volumes were acutely infused. Compared with vehicle, ETC-216 reduced infarct size as a percentage of the area at risk at 10 (p <0.0005) and 3 mg/kg (p <0.05). No significant differences occurred at 1 mg/kg. To determine whether ETC-216 could protect the heart after initiation of ischemia, the synthetic HDL (10 mg/kg) was infused intravenously beginning 5 min before the end of 30 min of LAD occlusion. Infarct size as percentage of the area at risk was 31.6 +/- 3.0 (ETC-216) versus 49.5 +/- 2.5 (vehicle) (p <0.001), and as percentage of left ventricle was 19.7 +/- 1.6 (ETC-216) versus 34.1 +/- 2.3 (vehicle) (p <0.0005). Electron microscopy demonstrated that ETC-216 prevented irreversible cardiac damage as assessed by mitochondrial granulation and sarcomere contraction band formation. These findings suggest ETC-216 reduces reperfusion injury and may have utility for coronary artery revascularization procedures.

Early myocardial lesions induced by cardiotoxic compounds in Sprague-Dawley rats.

Early focal myocardial lesions in rats induced by five cardiotoxic compounds were histopathologically observed 1 hr and 4 hr after a single intravenous injection with 1/10 LD50 and LD50. The lesions were observed 1 hr and 4 hr after the treatment with LD50 of isoproterenol (ISP), 4 hr with 1/10 LD50 of ISP, 4 hr with LD50 of hydralazine (HYD), caffeine (CAF) and cyclophosphamide (CYC), but not with adriamycin (ADR). The lesions consisted of homogeneously intensely eosinophilic staining, contraction band formation and fragmentation of cardiac muscle fibers. The lesions were interspersed in the inner one third of the left ventricular walls including the papillary muscles with ISP, HYD and CAF, and were all over the ventricular myocardium with CYC.

Structure-function relationship in recombinant A-I M/A-I M

Irreversible myocardial injury is a potential consequence of coronary artery revascularization. Reperfusion leads to the production of oxidized products that can damage myocardium. High-density lipoproteins (HDL) are effective at removing oxidized lipids. We hypothesized that a synthetic HDL preparation, comprising recombinant apolipoprotein A-IMilano (apoA-IM) complexed with 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) (apoA-IM/POPC) would protect the heart from reperfusion injury. The ex vivo model consisted of rabbit hearts perfused by the Langendorff method. Hearts were equilibrated with Krebs–Henseleit buffer (10 min), pretreated with either apoA-IM/POPC (0.45 mg/mL) or vehicle (10 min), subjected to global ischemia (30 min) and reperfused for 60 min. ApoA-IM/POPC (n = 7) prevented the left ventricular end-diastolic pressure elevation observed in the vehicle group (n = 6) at the end of reperfusion (p < 0.05). During reperfusion, coronary artery perfusion pressure increased in the controls (p < 0.001), but not with apoA-IM/POPC. ApoA-IM/POPC reduced the release of creatine kinase at the end of the ischemic period (p < 0.001). It also reduced cardiac left ventricle muscle lipid hydroperoxides by 46% (p < 0.05). Direct comparison of the antioxidant potential indicated that recombinant apoA-IM was much more potent than apoA-I in attenuating low-density lipoprotein oxidation. Electron microscopy showed that apoA-IM/POPC prevented mitochondrial granulation, disorganization and sarcomere contraction band formation indicative of reperfusion injury.The apoA-IM/POPC complex thus appears to reduce reperfusion injury under global ischemic conditions, and may therefore have therapeutic application in the reduction of myocardial ischemia.

The Contraction State of Myofibrils During Global Ischemia and After Reperfusion Following Different Forms of Cardiac Arrest: Correlation with Metabolic Parameters in the Canine Heart

This study was undertaken in order to obtain information on the mode of reaction of the contractile apparatus after different forms of cardiac arrest, global ischemia and reperfusion, as well as on possible correlations between the contraction state of myofibrils and biochemical parameters. During the survival time, before the level of 3 mumol/gww creatine phosphate (CP) is reached, the contraction state shows only minor changes. During the revival time in which ATP tissue concentrations decay to 4 mumol/gww, the contribution of ATP, lactate, anorganic phosphate (Pa) and acidosis to the degree of relaxation depends on the method of cardiac arrest. At defined biochemical values, the degree of relaxation is comparable after aortic cross clamping (ACC) and St. Thomas perfusion, but significantly different compared to HTK perfusion. Thus, during the revival time, the relaxation of sarcomeres depends predominantly on the composition of the solutions used for cardiac arrest. The re-entry of contraction below 3 mumol/gww ATP is correlated with the ATP concentration, independent of the form of cardiac arrest. Reperfusion after HTK or St. Thomas cardioplegia and reversible ischemia leads to the focal formation of contraction bands, which do not occur during ischemia. This contraction state is significantly more pronounced after reperfusion of St. Thomas arrested hearts. Thus, the contraction state of myofibrils is influenced not only by alterations in metabolite concentrations, but also by the composition of cardioplegic solutions and by the characteristic conditions (sufficient energy, oxygen and Calcium) during reperfusion.

Cardiac lesions in sudden infant death syndrome

Sequential morphological changes as found in the hearts of 250 sudden infant death syndrome (SIDS) infants are described. Detailed examination of macroscopic and microscopic lesions reveal that all SIDS infants had identifiable lesions at the time of their death. The lesions can best be described as selective focal anoxic muscle fibre necrosis at chronologically different developmental stages. The extent of these lesions vary markedly from case to case, from a minimal muscle fibre eosinophilia through contraction band formation, myocytolysis, stromal condensation to scar formation. The morphological variations in the lesions amongst individual cases can be interpreted as relating to the time interval of the development of the lesions. The intramural and coronary arteries in some cases are also affected showing intimal hyperplasia. Although these sequential morphological aberrations are not specific and typical to SIDS infants only, they were present in all SIDS infants in this series.

Ultrastructural alterations in cardiac muscle of diabetic BB Wistar rats

The cardiac muscle of BB Wistar rats suffering from diabetes for 8 and 16 weeks (8-Wk and 16-Wk of DM) were examined by light and electron microscopy. The diabetic rats were kept alive by injections of small doses of insulin and exhibited severe hyperglycaemia, glycosuria and weight loss. The heart/body weight ratio of all diabetic groups was greater than that of age matched controls. Over the experimental period, the left ventricular myocardium of the diabetic BB rats sustained damage that was progressively more serious with the duration of the diabetic state. In BB rats after 8-wk of diabetes the myocardium contained large numbers of lipid droplets and glycogen granules around mitochondria which showed patchy swelling, and slight loss of myofilaments. Disruption of mitochondrial membranes and extensive loss of myofilaments were seen in rats diabetic for 16 wk. In addition, dilatation of the sarcoplasmic reticulum-transverse tubular system, formation of a contraction band and myelin bodies and widening of the intercellular space at the fasciae adherens of the intercalated disc were characteristically observed in BB rats after 16-wk of diabetes. However, there were no evident alterations in the capillaries of any diabetic BB rats. Morphometric analyses showed the volume percentage of myofibrils in diabetic rats to be significantly decreased when compared with controls. The loss of myofibrillar elements may be a primary damage induced by insulin deficiency. The formation of contraction bands suggests Ca2+ overload caused by diabetic metabolic disturbances.

Effects of ouabain on calcium paradox in rat hearts.

The effects of ouabain (10(-7) to 10(-5) M) on the interrelationship between cell-cell contacts, resting tension, and creatine phosphokinase (CK) leakage owing to myocardial cell injury during Ca2+ paradox were studied in isolated perfused rat heart preparations. After perfusing for 15 min with Ca2+ -containing medium, hearts were perfused for 5 min with Ca2+ -free medium followed by a reperfusion with Ca2+ -containing medium for 5 min. This resulted in a transient increase in resting tension and a substantial release of CK into the perfusate during the calcium reperfusion period. These changes were accompanied by extensive structural damage in the myocardial cell, including formation of contraction bands, swelling of the mitochondria, and cell-cell separation. Inclusion of 10(-5) M ouabain for 5 min in the Ca2+ -containing perfusion medium prior to the start of Ca2+ -free perfusion resulted in a higher and sustained resting tension that was accompanied by a reduced loss of CK from the heart during Ca2+ reperfusion. In a histological examination of these ouabain exposed hearts, most of the structural changes owing to calcium paradox were apparent, but the cell-cell contacts were maintained. The results are consistent with the hypothesis that the loss of cell-cell contacts in the intercalated disc during the occurrence of Ca2+ paradox may be the cause of the delayed decline in the resting tension and is only partially responsible for the loss of CK. These differences in myocardial changes during Ca2+ paradox with or without ouabain may be due to the retention of calcium at certain crucial sites under the influence of ouabain.

Structural and functional properties of adult rat heart myocytes lysed with digitonin.

Low concentrations of digitonin disrupt the sarcolemma of adult rat heart myocytes selectively and completely. When the digitonin lysis is carried out in the presence of 10 mM Mg-ATP, the permeabilized cells retain the rod-cell morphology typical of heart cells in situ and show spontaneous phasic contractions. The rate of contraction is a function of the free Ca2+ concentration from a pCa of 7.2 to 5.2. Higher levels of free Ca2+ result in hypercontracture of the myocytes into round cells with characteristically distorted morphology. The sarcoplasmic reticulum of digitonin-lysed myocytes takes up Ca2+ in an ATP-dependent reaction that is inhibited and reversed by caffeine and strongly enhanced by procaine or ruthenium red. The Ca2+ accumulation has a Km of 0.6 microM Ca2+, depends on Pi (Km of 13 mM), and is strongly inhibited by bicarbonate ion. The hypercontracture of digitonin-lysed myocytes is a function of both the pCa and the Mg-ATP concentration of the suspending medium. Hypercontracture requires ATP. Hypercontracture due to Ca2+ overload occurs at lower Ca2+ concentrations when Mg-ATP is decreased from 10 to 1 mM. However, at low concentrations of Mg-ATP (in the range from 1 to 10 microM), hypercontracture also occurs and is essentially Ca2+-independent. Since hypercontracture of heart myocytes appears analogous to the formation of contraction bands in situ, these observations may be relevant to the phenomena of oxygen paradox and of Ca2+ paradox in intact myocardial tissue.

Contraction bands: differences between physiologically vs. maximally activated single heart muscle cells.

High resolution interference and phase microscopy were used to inspect the striations' appearance in shortening rat heart cells. Isolated cells were treated with detergent so that shortening could be graded by addition of calcium. Upon activation sarcomeres shortened to form (a) contraction densities in the middle of the A band at 1.7 micrometer (b) disappearance of the I bands and (c) phase brightening of the A bands at 1.6 micrometer and (d) dense Cz contraction bands at shorter lengths. These changes are totally consistent with the uniform sliding of myofilaments of previously accepted fixed dimensions. However, the striated patterns differed significantly in intact cells which were electrically stimulated to shorten. Here individual A bands remained distinct, without phase brightening or contraction band formation despite sarcomere shortening to less than the length of the A band as measured in the unstimulated cell. Maximal activation of intact cells by barium contracture elicited the full sequence of striation changes (a-d) seen in the chemically skinned cells. Light diffraction analysis gave comparable interpretation, i.e., the protein within the shortened sarcomere in the physiologically activated cardiac cell is more narrowly distributed than expected for thick filaments of fixed dimensions. These optical differences may reflect the restricted presence of the globular myosin heads at the ends of the cardiac sarcomere. This situation would explain the narrow range of the cardiac length-tension relation.

Effects of pressure treatment on the ultrastructure of striated muscle

Pre- and post-rigor sheep semimembranosus muscles were subjected to a hydrostatic pressure of 100 meganewtons/m 2 at 25°C. The ultrastructure of the muscle fibres was compared with that of non pressure-treated samples. A conspicuous feature of pressure-treated post-rigor samples was the absence of the M-band in the central region of the A-band. Therefore it appeared that some, if not all, of the proteins in the M-line were very susceptible to disaggregation under high pressure. Another feature of the post-rigor pressure-treated sample was the loss of integrity and aggregation of I-band filaments which presumably involved an F-G transformation of actin. Pressure treatment of pre-rigor muscle resulted in extensive structural disruption with contraction band formation. It is suggested that a weakening of thin filaments and M-line bridges, when combined with a pressure-induced contraction, facilitates disruption of pre-rigor pressure-treated muscle.

The incidence and significance of contraction bands in endomyocardial biopsies from normal human hearts

With increasing use of myocardial biopsy as a diagnostic tool, different criteria of pathology have evolved from tissues obtained from biopsy and postmorten specimens. We reported that contraction bands may be produced artifactually by the biopsy procedure in the fresh, but not the postmorten or perfused fixed normal rat and dog hearts. Contraction bands have been used as a morphological index of pathology. From 12 normal human transplant donors endomyocardial biopsy samples were obtained from the right ventricle, were fixed immediately and then processed for light and electron microscopy. Contraction bands were quantitated by assigning 0, 1, 2, and 3 to represent 0, 1 to 5, 6 to 10, and 11 or more cells per square millimeter with contraction bands. A large number of contraction bands were present in 11 out of 12 hearts; 3rd, 2nd, 1st, and zero degrees of contraction bands were present in 11, 0, 1, and 0 samples, respectively. We conclude that contraction bands may be produced artifactually in normally human hearts, and based upon the rat and dog study, that the determinants of contraction band formation are activation of the contractile machinery by the biopsy procedure, together with sarcomeres capable of hypercontraction because their shortening is unopposed. Furthermore, contraction bands may be considered a morphological index of pathology in tissue obtained postmortem and after perfusion fixation, but not in tissue obtained from biopsies.

The Significance of Contraction Bands in the Normal Heart

Contraction bands are a cardiac histological alteration found in many experimental and pathological conditions--such as ischemia, cardiomyopathies, electric shock, hemorrhagic shock, catecholamine infusion, and open heart surgery. Of importance, contraction bands have also been considered by some investigators as an artifact of preparation. We have frequently observed contraction bands in normal dog heart tissue obtained by endocardial biopsy. In order to study the determinants for the production of contraction bands, we utilized the rat, a less expensive animal in order to more fully investigate the various possible mechanisms operational. The aim of the present study was to investigate the determinants of contraction band formation in normal hearts.

Encephalogenic cardiomyopathy after stimulation of the brain stem in monkeys

Repeated electrical stimulation of the brain stem including the hypothalamus, pons and medulla in monkeys produced an increase in arterial blood pressure, cardiac arrhythmias, various degrees of S-T segment or T wave changes and structural damage to the myocardium. The following ultrastructural changes in the myocardium occurred: distension of sarcoplasmic reticulum, swelling of mitochondria, disarrangement of filamentous arrays, formation of contraction bands, dissociation of intercalated discs and focal necrosis. These changes are similar to those found in ischemic myocardial tissue and myocardial necrosis induced by catecholamines. The myocardial lesions are probably related to activation of the sympathetic mechanism in the brain stem.

Ultrastructure studies of the contraction-relaxation cycle of glycerinated rabbit psoas muscle: I. The ultrastructure of glycerinated fibers contracted by treatment with ATP

Studies of ultrastructural changes of glycerinated rabbit psoas muscle during contraction induced by ATP have shown that the sarcomere patterns of striated muscle seen in electron micrographs can be directly correlated with the physiological state of the tissue at the time of fixation. The relationships of band and sarcomere lengths of rigid control fibers agree well with those upon which the postulated sliding model is based. This is not true for tissue fixed during contraction. Sarcomeres fixed during ATP-induced contraction are characterized by the appearance of contraction bands at the A—I-boundary of sarcomeres with patent I-bands. The contraction band formation usually is accompanied by a slight decrease in thick filament length with no increase in filament diameter. In fibers which were held at extended length during glycerination, the ATP-contracted sarcomeres often have thin filaments which are shorter than those of rigid controls; and the contraction bands usually are extremely dense. Some of these sarcomeres which are rather short still have wide H-bands. Thus, although filament sliding usually accompanies contraction when the fibers contract from rest length, the contraction can occur with no sliding in connection with shortening of stretched muscle fibers with a narrow zone of filament overlap. Contraction band formation, on the other hand, is a feature which clearly distinguishes fibers fixed during contraction from those which have not been contracted.

CONTRACTION-BAND FORMATION IN BARNACLE MYOFIBRILS.

Contractions induced by adenosine triphosphate, in myofibrils isolated from the barnacle, Balanus aquilia, were observed with a phase microscope. The formation of contraction bands was well under way before the A-band came into contact with the Z-membrane. This is in disagreement with the hypothesis that contraction bands are formed when the A-band pushes against the Z-membrane.