On the structural continuities of the transverse tubular system of rabbit and human myocardial cells

The Mechanics and Thermodynamics of Tubule Formation in Biological Membranes

Membrane tubulation is a ubiquitous process that occurs both at the plasma membrane and on the membranes of intracellular organelles. These tubulation events are known to be mediated by forces applied on the membrane either due to motor proteins, by polymerization of the cytoskeleton, or due to the interactions between membrane proteins binding onto the membrane. The numerous experimental observations of tube formation have been amply supported by mathematical modeling of the associated membrane mechanics and have provided insights into the force-displacement relationships of membrane tubes. Recent advances in quantitative biophysical measurements of membrane-protein interactions and tubule formation have necessitated the need for advances in modeling that will account for the interplay of multiple aspects of physics that occur simultaneously. Here, we present a comprehensive review of experimental observations of tubule formation and provide context from the framework of continuum modeling. Finally, we explore the scope for future research in this area with an emphasis on iterative modeling and experimental measurements that will enable us to expand our mechanistic understanding of tubulation processes in cells.

Remodeling of t-system and proteins underlying excitation-contraction coupling in aging versus failing human heart

It is well established that the aging heart progressively remodels towards a senescent phenotype, but alterations of cellular microstructure and their differences to chronic heart failure (HF) associated remodeling remain ill-defined. Here, we show that the transverse tubular system (t-system) and proteins underlying excitation-contraction coupling in cardiomyocytes are characteristically remodeled with age. We shed light on mechanisms of this remodeling and identified similarities and differences to chronic HF. Using left ventricular myocardium from donors and HF patients with ages between 19 and 75 years, we established a library of 3D reconstructions of the t-system as well as ryanodine receptor (RyR) and junctophilin 2 (JPH2) clusters. Aging was characterized by t-system alterations and sarcolemmal dissociation of RyR clusters. This remodeling was less pronounced than in HF and accompanied by major alterations of JPH2 arrangement. Our study indicates that targeting sarcolemmal association of JPH2 might ameliorate age-associated deficiencies of heart function.

The relationship between form and function throughout the history of excitation-contraction coupling

Franzini-Armstrong reviews the development of the excitation–contraction coupling field over time.

The concept of excitation–contraction coupling is almost as old as Journal of General Physiology. It was understood as early as the 1940s that a series of stereotyped events is responsible for the rapid contraction response of muscle fibers to an initial electrical event at the surface. These early developments, now lost in what seems to be the far past for most young investigators, have provided an endless source of experimental approaches. In this Milestone in Physiology, I describe in detail the experiments and concepts that introduced and established the field of excitation–contraction coupling in skeletal muscle. More recent advances are presented in an abbreviated form, as readers are likely to be familiar with recent work in the field.

Sub-microscopic analysis of t-tubule geometry in living cardiac ventricular myocytes using a shape-based analysis method

Transverse-axial tubules (TTs) are key structures involved in cardiac excitation-contraction coupling and can become deranged in disease. Although optical measurement of TTs is frequently employed to assess TT abundance and regularity, TT dimensions are generally below the diffraction limit of optical microscopy so determination of tubule size is problematic. TT diameter was measured by labeling both local surface membrane area and volume with fluorescent probes (FM4-64 and calcein, respectively), correcting image asymmetry by image processing and using the relationship between surface area and volume for a geometric primitive. This method shows that TTs have a mean (± SEM) diameter of 356 ± 18 nm in rabbit and 169 ± 15 nm in mouse (p < 0.001). Rabbit TT diameters were more variable than those of mouse (p < 0.01) and the smallest TT detected was 41 nm in mouse and the largest 695 nm in rabbit. These estimates are consistent with TT diameters derived from the more limited sampling of high-pressure frozen samples by electron tomography (which examines only a small fraction of the cell volume). Other measures of TT abundance and geometry (such as volume, membrane fractions and direction) were also derived. On the physiological time scale of E-C coupling (milliseconds), the average TT electrical space constant is ~ 175 μm in rabbit and ~ 120 μm in mouse and is ~ 50% of the steady-state space constant. This is sufficient to ensure reasonable electrical uniformity across normal cells. The image processing strategy and shape-based 3D approach to feature quantification is also generally applicable to other problems in quantification of sub-cellular anatomy.

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Living cardiomyocytes were dual-labeled with fluorescent surface and volume probes.

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A novel 3D image processing strategy enabled calculation of t-tubule diameter.

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The method shows rabbit and mouse t-tubules have quite different morphologies.

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Mean diameters of rabbit and mouse t-tubules were 360 and 170 nm, respectively.

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Estimated electrical space constants are sufficient to ensure electrical uniformity.

Cardiac T-Tubule Microanatomy and Function

Unique to striated muscle cells, transverse tubules (t-tubules) are membrane organelles that consist of sarcolemma penetrating into the myocyte interior, forming a highly branched and interconnected network. Mature t-tubule networks are found in mammalian ventricular cardiomyocytes, with the transverse components of t-tubules occurring near sarcomeric z-discs. Cardiac t-tubules contain membrane microdomains enriched with ion channels and signaling molecules. The microdomains serve as key signaling hubs in regulation of cardiomyocyte function. Dyad microdomains formed at the junctional contact between t-tubule membrane and neighboring sarcoplasmic reticulum are critical in calcium signaling and excitation-contraction coupling necessary for beat-to-beat heart contraction. In this review, we provide an overview of the current knowledge in gross morphology and structure, membrane and protein composition, and function of the cardiac t-tubule network. We also review in detail current knowledge on the formation of functional membrane subdomains within t-tubules, with a particular focus on the cardiac dyad microdomain. Lastly, we discuss the dynamic nature of t-tubules including membrane turnover, trafficking of transmembrane proteins, and the life cycles of membrane subdomains such as the cardiac BIN1-microdomain, as well as t-tubule remodeling and alteration in diseased hearts. Understanding cardiac t-tubule biology in normal and failing hearts is providing novel diagnostic and therapeutic opportunities to better treat patients with failing hearts.

In situ single photon confocal imaging of cardiomyocyte T-tubule system from Langendorff-perfused hearts

Transverse tubules (T-tubules) are orderly invaginations of the sarcolemma in mammalian cardiomyocytes. The integrity of T-tubule architecture is critical for cardiac excitation–contraction coupling function. T-tubule remodeling is recognized as a key player in cardiac dysfunction. Early studies on T-tubule structure were based on electron microscopy, which uncovered important information about the T-tubule architecture. The advent of fluorescent membrane probes allowed the application of confocal microscopy to investigations of T-tubule structure. Studies have now been extended beyond single cardiomyocytes to examine the T-tubule network in intact hearts through in situ confocal imaging of Langendorff-perfused hearts. This technique has allowed visualization of T-tubule organization in their natural habitat, avoiding the damage induced by isolation of cardiomyocytes. Additionally, it is possible to obtain T-tubule images in different subepicardial regions in a single intact heart. We review how this state-of-the-art imaging technique has provided important mechanistic insights into maturation of T-tubules in developing hearts and defined the role of T-tubule remodeling in development and progression of heart failure.

Emerging mechanisms of T-tubule remodelling in heart failure

Cardiac excitation-contraction coupling occurs primarily at the sites of transverse (T)-tubule/sarcoplasmic reticulum junctions. The orderly T-tubule network guarantees the instantaneous excitation and synchronous activation of nearly all Ca(2+) release sites throughout the large ventricular myocyte. Because of the critical roles played by T-tubules and the array of channels and transporters localized to the T-tubule membrane network, T-tubule architecture has recently become an area of considerable research interest in the cardiovascular field. This review will focus on the current knowledge regarding normal T-tubule structure and function in the heart, T-tubule remodelling in the transition from compensated hypertrophy to heart failure, and the impact of T-tubule remodelling on myocyte Ca(2+) handling function. In the last section, we discuss the molecular mechanisms underlying T-tubule remodelling in heart disease.

Adding a new dimension to cardiac nano-architecture using electron microscopy: coupling membrane excitation to calcium signaling

Advances in microscopic imaging technologies and associated computational methods now allow descriptions of cellular anatomy to go beyond 2-dimensions, revealing new micro-domain dynamics at unprecedented resolutions. In cardiomyocytes, electron microscopy (EM) first described junctional membrane complexes between the sarcolemma and sarcoplasmic reticulum over a half-century ago. Since then, 3-dimensional EM technologies such as electron tomography have become successful in determining the realistic nano-geometry of membrane junctions (dyads and peripheral junctions) and associated structures such as transverse tubules (T-tubules, aka. T-system). Concomitantly, super-resolution light microscopy has gone beyond the diffraction-limit to determine the distribution of molecules, such as ryanodine receptors, with 10(-8) meter (10nm) order accuracy. This review provides the current structural perspective and functional interpretation of membrane junction complexes, which are the central machinery controlling cardiac excitation-contraction coupling via calcium signaling.

Subcellular structures and function of myocytes impaired during heart failure are restored by cardiac resynchronization therapy

RATIONALE

Cardiac resynchronization therapy (CRT) is an established treatment for patients with chronic heart failure. However, CRT-associated structural and functional remodeling at cellular and subcellular levels is only partly understood.

OBJECTIVE

To investigate the effects of CRT on subcellular structures and protein distributions associated with excitation-contraction coupling of ventricular cardiomyocytes.

METHODS AND RESULTS

Our studies revealed remodeling of the transverse tubular system (t-system) and the spatial association of ryanodine receptor (RyR) clusters in a canine model of dyssynchronous heart failure (DHF). We did not find this remodeling in a synchronous heart failure model based on atrial tachypacing. Remodeling in DHF ranged from minor alterations in anterior left ventricular myocytes to nearly complete loss of the t-system and dissociation of RyRs from sarcolemmal structures in lateral cells. After CRT, we found a remarkable and almost complete reverse remodeling of these structures despite persistent left ventricular dysfunction. Studies of whole-cell Ca(2+) transients showed that the structural remodeling and restoration were accompanied with remodeling and restoration of Ca(2+) signaling.

CONCLUSIONS

DHF is associated with regional remodeling of the t-system. Myocytes undergo substantial structural and functional restoration after only 3 weeks of CRT. The finding suggests that t-system status can provide an early marker of the success of this therapy. The results could also guide us to an understanding of the loss and remodeling of proteins associated with the t-system. The steep relationship between free Ca(2+) and contraction suggests that some restoration of Ca(2+) release units will have a disproportionately large effect on contractility.

Cardiac autoantibodies from patients affected by a new variant of endemic pemphigus foliaceus in Colombia, South America

Several patients affected by a new variant of endemic pemphigus foliaceus in El Bagre, Colombia (El Bagre-EPF) have experienced a sudden death syndrome, including persons below the age of 50. El Bagre-EPF patients share several autoantigens with paraneoplastic pemphigus patients, such as reactivity to plakins. Further, paraneoplastic pemphigus patients have autoantibodies to the heart. Therefore, we tested 15 El Bagre-EPF patients and 15 controls from the endemic area for autoreactivity to heart tissue using direct and indirect immunofluorescence, confocal microscopy, immunohistochemistry, immunoblotting, and immunoelectron microscopy utilizing heart extracts as antigens. We found that 7 of 15 El Bagre patients exhibited a polyclonal immune response to several cell junctions of the heart, often colocalizing with known markers. These colocalizing markers included those for the area composita of the heart, such as anti-desmoplakins I and II; markers for gap junctions, such as connexin 43; markers for tight junctions, such as ezrin and junctional adhesion molecule A; and adherens junctions, such pan-cadherin. We also detected colocalization of the patient antibodies within blood vessels, Purkinje fibers, and cardiac sarcomeres. We conclude that El Bagre-EPF patients display autoreactivity to multiple cardiac epitopes, that this disease may resemble what is found in patients with rheumatic carditis, and further, that the cardiac pathophysiology of this disorder warrants further evaluation.

Two-photon laser scanning microscopy of the transverse-axial tubule system in ventricular cardiomyocytes from failing and non-failing human hearts

Objective. The transverse-axial tubule system (TATS) of cardiomyocytes allows a spatially coordinated conversion of electrical excitation into an intracellular Ca(2+) signal and consequently contraction. Previous reports have indicated alterations of structure and/or volume of the TATS in cardiac hypertrophy and failure, suggesting a contribution to the impairment of excitation contraction coupling. To test whether structural alterations are present in human heart failure, the TATS was visualized in myocytes from failing and non-failing human hearts. Methods and Results. In freshly isolated myocytes, the plasmalemmal membranes were labeled with Di-8-ANEPPS and imaged using two-photon excitation at 780 nm. Optical sections were taken every 300 nm through the cells. After deconvolution, the TATS was determined within the 3D data sets, revealing no significant difference in normalized surface area or volume. To rule out possible inhomogeneity in the arrangement of the TATS, Euclidian distance maps were plotted for every section, allowing to measure the closest distance between any cytosolic and any membrane point. There was a trend towards greater spacing in cells from failing hearts, without statistical significance. Conclusion. Only small changes, but no significant changes in the geometrical dimensions of the TATS were observed in cardiomyocytes from failing compared to non-failing human myocardium.

Novel features of the rabbit transverse tubular system revealed by quantitative analysis of three-dimensional reconstructions from confocal images

With scanning confocal microscopy we obtained three-dimensional (3D) reconstructions of the transverse tubular system (t-system) of rabbit ventricular cells. We accomplished this by labeling the t-system with dextran linked to fluorescein or, alternatively, wheat-germ agglutinin conjugated to an Alexa fluor dye. Image processing and visualization techniques allowed us to reconstruct the t-system in three dimensions. In a myocyte lying flat on a coverslip, t-tubules typically progressed from its upper and lower surfaces. 3D reconstructions of the t-tubules also suggested that some of them progressed from the sides of the cell. The analysis of single t-tubules revealed novel morphological features. The average diameter of single t-tubules from six cells was estimated to 448 +/- 172 nm (mean +/- SD, number of t-tubules 348, number of cross sections 5323). From reconstructions we were able to identify constrictions occurring every 1.87 +/- 1.09 microm along the principal axis of the tubule. The cross-sectional area of these constrictions was reduced to an average of 57.7 +/- 27.5% (number of constrictions 170) of the adjacent local maximal areas. Principal component analysis revealed flattening of t-tubular cross sections, confirming findings that we obtained from electron micrographs. Dextran- and wheat-germ agglutinin-associated signals were correlated in the t-system and are therefore equally good markers. The 3D structure of the t-system in rabbit ventricular myocytes seems to be less complex than that found in rat. Moreover, we found that t-tubules in rabbit have approximately twice the diameter of those in rat. We speculate that the constrictions (or regions between them) are sites of dyadic clefts and therefore can provide geometric markers for colocalizing dyadic proteins. In consideration of the resolution of the imaging system, we suggest that our methods permit us to obtain spatially resolved 3D reconstructions of the t-system in rabbit cells. We also propose that our methods allow us to characterize pathological defects of the t-system, e.g., its remodeling as a result of heart failure.

NUCLEOSIDE PHOSPHATASE ACTIVITIES IN RAT CARDIAC MUSCLE

Localizations of aldehyde-resistant nucleoside phosphatase activities in frozen sections of rat cardiac muscle have been studied by electron microscopy. Activities are higher after fixation with formaldehyde than with glutaraldehyde. After incubation with adenosine triphosphate or inosine diphosphate at pH 7.2, reaction product is found in the "terminal cisternae" or "transverse sacs" of the sarcoplasmic reticulum, which, together with the "intermediary vesicles" (T system), constitute the "dyads" or "triads". Reaction product is also present at the membranes of micropinocytotic vacuoles which apparently form from the plasma membrane of capillary endothelial cells and from the sarcolemma. In certain regions of the intercalated discs, reaction product is found within the narrow spaces between sarcolemmas of adjacent cells and within micropinocytotic vacuoles that seem to form from the sarcolemma. With inosine diphosphate, reaction product is also found in other parts of the sarcoplasmic reticulum. After incubation with cytidine monophosphate at pH 5, reaction product is present in the transverse sacs of sarcoplasmic reticulum, in micropinocytotic vacuoles in capillary endothelium, and in lysosomes of muscle fibers and capillaries. The possible significance of the sarcoplasmic reticulum phosphatases is discussed in relation to the role the reticulum probably plays in moving calcium ions and thereby controlling contraction and relaxation of the muscle fiber.

CAT HEART MUSCLE IN VITRO. V. DIFFUSION THROUGH A SHEET OF RIGHT VENTRICLE

The rate of transfer of labeled molecules across a sheet of quiescent cat right ventricle separating two chambers containing chemically identical solutions was followed at 23°C. For the diffusion of sucrose, SO₄, and Na the experimental points fit the entire time course of the plot of the diffusion equation for a plane sheet. The tortuosity factor of the extracellular diffusion channel, λ was 1.44 ± 0.05 (mean ± SEM) for sucrose and similar for SO₄ and Na. The fractional area available for extracellular diffusion, calculated from λ and the slope of the linear asymptote approached during steady state diffusion, was 0.17–0.23 for both impermeant species (sucrose, SO₄, Na) and permeant species (water, urea, glycerol). Permeant molecules showed a characteristic prolongation of the approach to the steady state, with an unexplained "hump" in the curve for water. The observed time courses for diffusion of permeant molecules are interpreted in terms of a model proposed by Fatt et al. for diffusion through linear porous media containing dead-end pore volume. Large molecules like inulin and dialyzed dextran (diameter 150 to 180 A) diffuse through the sheet. These molecules may have a reflection coefficient σ > 0. The fraction of muscle water occupied by the sucrose diffusion channel is significantly smaller than the 3 hr. mannitol, sucrose, and inulin spaces.

SOME OBSERVATIONS ON THE FINE STRUCTURE AND METABOLIC ACTIVITY OF NORMAL AND GLYCERINATED VENTRICULAR MUSCLE OF TOAD

Fine structure, enzyme activity, and transmembrane potentials of normal and glycerinated ventricular muscle of the toad were studied. For electron microscopy, osmium tetroxide and Araldite were used. Plasma membranes are firmly attached to Z bands. Both the T system and sarcoplasmic reticulum are poorly developed. Small bodies of medium density may be lysosomes derived from the Golgi zone. Denser bodies may be catecholamine granules. Fine tubules of unknown significance, about 200 A in diameter and of considerable length, lie in conspicuous, although infrequent bundles. Glycogen and mitochondria are abundant. After weeks of extraction in 50 per cent buffered glycerol, most organelles were still present, and much of the gross damage was probably due to osmotic destruction of membranes weakened by extraction. Many mitochondria were well preserved. Plasma and nuclear membranes had diffuse outlines and tended to be broken. Considerable activity remained of the enzymes succinic dehydrogenase, cytochrome oxidase, and phosphorylase after the extraction, but decreased with prolonged soaking. The normal transmembrane potential was about 95 mv; in extracted muscle after 6 weeks it was about 35 mv. The view that glycerinated muscle is a simple system of actin and myosin is clearly wrong. The activity of other organelles still present must affect the actions of many drugs and ions experimentally added.

SARCOLEMMAL INVAGINATIONS CONSTITUTING THE T SYSTEM IN FISH MUSCLE FIBERS

Striated muscle fibers from the body and tail myotomes of a fish, the black Mollie, have been examined with particular attention to the sarcoplasmic reticulum (SR) and transverse tubular (or T) system. The material was fixed in osmium tetroxide and in glutaraldehyde, and the images provided by the two kinds of fixatives were compared. Glutaraldehyde fixes a fine structure that is broadly comparable with that preserved by osmium tetroxide alone but differs in some significant details. Especially significant improvements were obtained in the preservation of the T system, that is, the system of small tubules that pervades the fiber at every Z line or A-I junction level. As a result of this improved glutaraldehyde fixation, the T system is now clearly defined as an entity of fine structure distinct from the SR but uniquely associated with the SR and myofibrils. Glutaraldehyde fixation also reveals that the T system is a sarcolemmal derivative that retains its continuity with the sarcolemma and limits a space that is in direct communication with the extracellular environment. These structural features favor the conclusion that the T system plays a prominent role in the fast intracellular conduction of the excitatory impulse. The preservation of other elements of muscle fine structure, including the myofibrils, seems for reasons discussed, to be substantially improved by glutaraldehyde.

THE LOCALIZATION OF CHOLINESTERASE ACTIVITY IN RAT CARDIAC MUSCLE BY ELECTRON MICROSCOPY

A method has been developed for localizing sites of cholinesterase activity in rat cardiac muscle by electron microscopy. The method utilizes thiocholine esters as substrates, and is believed to be dependent on the reduction of ferricyanide to ferrocyanide by thiocholine released by enzymatic activity. The ferrocyanide thus formed is captured by copper to form fine, electron-opaque deposits of copper ferrocyanide, which sharply delineate sites of enzymatic activity at the ultrastructural level. Cholinesterase activity in formalin-fixed heart muscle was localized: (a) in longitudinal elements of the sarcoplasmic reticulum, but not in the T, or transverse, elements; and (b) in the A band, with virtually no activity noted in the M band, or in the H zone. The I band was also negative. No activity was detected in the sarcolemma, or in invaginations of the sarcolemma at the level of the Z band. The perinuclear element of the sarcoplasmic (endoplasmic) reticulum was frequently strongly positive. Activity at all sites was completely abolished by omitting the substrates, or by inhibition with eserine 10(-4)M and diisopropylfluorophosphate 10(-5)M. Eserine 10(-5)M completely inhibited reaction in the sarcoplasmic reticulum, and virtually abolished that in the A band. These observations, together with the use of the relatively specific substrates and suitable controls to eliminate non-enzymatic staining, indicate that cholinesterase activity was being demonstrated. The activity in rat heart against different substrates was that of non-specific cholinesterases, in accordance with biochemical data. The activity in the A band was considered to be probably due to myosincholinesterase. It is proposed that the localization of cholinesterases in myocardium at the ultrastructural level should be taken into account in considering the possible functions of these myocardial enzymes, and it is hoped that knowledge of their localization will open up new avenues of approach in considering their physiological role in myocardium, which at present is not definitely known.

SARCOPLASMIC RETICULUM: ULTRASTRUCTURE OF THE TRIADIC JUNCTION

The appositional region between the intermediate element and adjacent cisternae in the triads of the sarcoplasmic reticulum of striated muscle from humans, copepods, ostracods, and barnacles shows a five-layered construction similar to that of a tight junction. Known functions of tight junctions and cisternal elements suggest that a membrane depolarization, conducted by the intermediate element, is transmitted to the cisternae by way of the triadic junction to cause a release of calcium ions from the cisternae.

CAT HEART MUSCLE IN VITRO. VI. POTASSIUM EXCHANGE IN PAPILLARY MUSCLES

The exchange of cell K with K⁴², J_K, has been measured in cat right ventricular papillary muscle under conditions of a steady state with respect to intracellular K concentration. Within the limits of the measurement, all of cell K exchanged at a single rate. Cells from small cats are smaller and have larger surface/volume ratios than cells from large cats. The larger surface/volume ratio results in larger flux values. J_K increases in an approximately linear manner as the external K concentration is increased twentyfold, from 2.5 to 50 mM, at constant intracellular K concentration. The permeability for K ions, P_K, calculated from the influx and membrane potential, remains very nearly constant over this range of external K concentrations. J_K is not affected by replacement of O₂ by N₂, or by stimulated contractions at 60 per minute, but K influx decreases markedly in 10^-5M and 10^-8M ouabain.

Immunocytochemical studies of spectrin in hamster cardiac tissue

The spectrins are a family of cytoskeletal-membrane proteins that have a wide tissue distribution. In the present study, we employed polyclonal antibodies made against mammalian and avian erythroid spectrins as well as mammalian brain spectrin to assess their presence and distributions in the mammalian heart. Western blot analyses revealed that all three antibodies were specific for a 240,000 molecular weight alpha-spectrin subunit found in hamster erythrocyte ghost homogenates, whole hamster heart, and isolated hamster cardiac myofibril homogenates. Spectrin staining was absent from the Triton X-100-extracted supernatant fraction of myofibril preparations, suggesting that the protein is linked to the myofibril precipitate after exposure to the detergent. Frozen, unfixed, 2-microns-thick; sections of adult. Syrian golden hamster cardiac tissue exhibited strong immunofluorescent staining of intercalated discs and Z-bands using all three antibodies. In addition, the mammalian erythroid spectrin antibodies showed staining of the sarcolemma, and in cross section, revealed a delicate internal network of staining that appears to surround individual myofibrils. This may be T-tubule-associated staining. Myofibrils isolated from cardiac myocytes using Triton X-100 show positive Z-band staining using all three antibodies. Double staining with Texas Red-labeled monoclonal desmin and FITC-labeled polyclonal spectrin antibodies revealed that both stained the myofibrillar Z-line regions. These results demonstrate that spectrin is closely associated with the membranes, myofibrils, and intermediate filaments in the mammalian heart.

Vinculin is a component of an extensive network of myofibril-sarcolemma attachment regions in cardiac muscle fibers

Immunofluorescent staining of bovine and avian cardiac tissue with affinity-purified antibody to chicken gizzard vinculin reveals two new sites of vinculin reactivity. First, vinculin is organized at the sarcolemma in a striking array of rib-like bands, or costameres. The costameres encircle the cardiac muscle cell perpendicular to the long axis of the fiber and overlie the I bands of the immediately subjacent sarcomeres. The second new site of vinculin reactivity is found in bovine cardiocytes at tubular invaginations of the plasma membrane. The frequency and location of these invaginations correspond to the known frequency and distribution of the transverse tubular system in bovine atrial, ventricular, and Purkinje fibers. We do not detect tubular invaginations that stain with antivinculin in avian cardiocytes and, in fact, a transverse tubular system has not been found in avian cardiac fibers. Apparent lateral Z-line attachments to the sarcolemma and its invaginations have been observed in cardiac muscle by electron microscopy in the same regions where we find vinculin. On the basis of these previous ultrastructural findings and our published evidence for a physical connection between costameres and the underlying myofibrils in skeletal muscle, we interpret the immunofluorescence data of this study to mean that, in cardiac muscle, vinculin is a component of an extensive system of lateral attachment of myofibrils to the plasma membrane and its invaginations.

Three dimensional arrangement of mitochondria and endoplasmic reticulum in the heart muscle fiber of the rat

The three-dimensional arrangement of mitochondria and endoplasmic reticulum was studied in thick sections of the heart left ventricle fixed in glutaraldehyde and impregnated with the Ur-Pb-Cu technique and in thin sections of glutaraldehyde-fixed tissue post-fixed in potassium ferrocyanide-reduced osmium. Squarish flattened mitochondria, approximately the size of a sarcomere, were arranged in longitudinal columns in the clefts between the myofibrils. At the periphery of the fiber, the endoplasmic reticulum took the appearance of a subsarcolemmal network of plate-like and tubular cisternae running parallel to the cell surface. Between the myofibrils, the ER network formed longitudinally oriented repetitive units whose structure varied according to their position in relation to the A- or I- bands of the myofibrils. In front of the A-band, the endoplasmic reticulum appeared as a single layered network of anastomotic tubules compressed between the adjacent myofibrils. In front of the I-band, it formed a multilayered network the three-dimensional arrangement of which was dependent upon the presence or absence of the T-tubule. In the absence of the T-tubule, the ER cisternae were loosely anastomosed and occasionally displayed bulbous terminal swellings. In the presence of T-tubules, tubular ER cisternae were seen running parallel on both sides of the T-tubules and were continuous with sheet-like cisternae sandwiched between the distended T-tubule and adjacent extremities of longitudinally arranged mitochondria. These tubular or flattened cisternae were connected to each other by numerous bridging cisternae around the T-tubules.

The T-tubule system in the specialized and general myocardium of the rat

The T-tubule system in cardiac muscle cells has been investigated with the electron microscope in 10 adult rats after infiltration with horseradish peroxidase. All cardiac muscle cells possess a T-system, but its complexity varies according to the region of the heart. It is most extensive in the general ventricular myocardium where there are primary, secondary and tertiary transverse tubules as well as longitudinal elements, and there are numerous couplings between the T-system and the sarcoplasmic reticulum (SR). The T-system and associated SR couplings are less extensive in the atrium and in the atrioventricular conducting system. It is least well developed in the nodes, particularly in the sinuatrial node, where it is restricted to primary T-tubules. There are numerous SR couplings with the sarcolemma in all types of cardiac muscle cells. Where intercalated discs occur, SR couplings are associated with non-specialized parts of the disc. The possible significance of these ultrastructural features in regard to the speed of conduction by myocardial tissues, and in the excitation-contraction sequence is discussed.

Ultrastructure of sarcoplasmic reticulum in atrial myocardium of patients with mitral valvular disease

Alterations observed in the sarcoplasmic reticulum of muscle cells in left and right atrial myocardium from 10 patients with mitral valvular disease consisted of: a) proliferation of rough-surfaced endoplasmic reticulum, which formed large cisterns in perinuclear areas of hypertrophied cells and was considered indicative of increased protein synthesis; b) proliferation of free sarcoplasmic reticulum, a change that occurred in degenerated cells and appeared to be related to loss of contractile elements; c) two types of aggregates of tubules of free SR--one type was associated wtih abnormal Z-band material and was found only in cells showing loss of myofibrils and proliferation of free SR, whereas the other was not associated with either of these changes and occurred in less severely altered cells; and d) proliferation and enlargement of cisterns of extended junctional sarcoplasmic reticulum, which formed two distinct types of complexes: the first of these consisted of large, convoluted (Type A) cisterns that were wide (550 to 650 A in thickness) and did not have a central dense lamina; the second was composed of stacks of concentric or parallel (Type B) cisterns that were narrower (220 to 300 A in thickness), had a central dense lamina, and were separated from one another by layers of glycogen granules. The formation of these complexes of cisterns was regarded as an extreme form of overdevelopment of extended junctional sarcoplasmic reticulum in atrial muscle cells.

Formation of myocardial zonal lesions

Right ventricular papillary muscles from control cats and from cats subjected to hemorrhagic shock were studied by electron microscopy. Half of the muscles were fixed at the apex of their active length-tension curves following stimulation in a papillary muscle bath. The other half were also fixed under tension. The ultrastructure of each muscle was examined in detail. A series of stages in the formation of myocardial zonal lesions, ranging from minimal changes in the intercalated discs to severe lesions, were identified and interpreted as representing the sequential stages in the formation of zonal lesions. One of the earliest (least severe) changes in the formation of zonal lesions, the separations of actin filaments from the intercalated disc, may be critical to the subsequent development of cardiac failure in hypovolemic shock. Mitochondrial displacement was a late event in the formation of the lesions, occurring only after major alterations had taken place in the sarcomeres and intercalated discs. It was noted that the ultrastructure of mitochondria remained essentially normal, and that there was no cell swelling associated with zonal lesions. This serves as confirmatory evidence that myocardial zonal lesions are a unique form of myocyte injury, are potentially reversible, and are not caused by ischemia.

Disorder in excitation-contraction coupling of cardiac muscle from cats with experimentally produced right ventricular hypertrophy

The contractile and electrical activity of papillary muscles from hypertrophied right ventricles of cats with artificial stenosis of the pulmonary artery was investigated. Contractility was considerably decreased along the entire force-velocity relationship, whereas no measurable alterations could be detected in the electrical activities as recorded by intracellular microelectrodes. By supramaximal Ca 2+ activation, the contractility of both the hypertrophied and the normal control preparations was increased to about the same final value. These findings are consistent with the concept that a disorder in the mechanism of excitation-contraction coupling underlies the depressed contractile state of hypertrophied cardiac muscle. In addition, it could be shown that the increase in volume of each cellular unit is clearly related to the decrease in contractility. This can tentatively be explained by the following assumptions. If the amount of Ca 2+ entering the cell per unit area is not changed in hypertrophy, then in a cell of increased diameter, the amount of Ca 2+ distributed per unit cell volume will be diminished. Since the excitation-contraction coupling of the heart is very sensitive to Ca 2+ , this Ca 2+ deficit should be reflected in a depression of contractility.

A study of the T system in rat heart

The technique of extracellular space tracing with horseradish peroxidase is adapted for labeling the transverse tubular system (T system) in rat heart. In rat ventricular muscle the T system shows extensive branching and remarkable tortuosity. The T system can only be defined operationally, since it does not display specific morphological features throughout its entire structure. Owing to branching of the T system, a sizable proportion of the apposition between the T system and L system (or closed system) occurs at the level of longitudinal branches of the T system and is not restricted to the Z line region. The regions of apposition between the T system and L system are analyzed in rat ventricular muscle and skeletal muscle (diaphragm) and compared with the intercellular tight junctions (nexuses) of heart muscle by the use of a photometric method. The over-all thickness of the nexus is significantly smaller than that of T-L junctions in both cardiac and skeletal muscles. The thickness of the membranes of the T and L systems are not significantly different in the two muscles, but the gap between both membranes is larger in the heart. In atrial muscle the following two types of cells are found: (a) those cells with a well-developed T system in which the tubular diameter is quite uniform and the orientation predominantly longitudinal and, (b) cells with no T system, but with a well-developed L system. Atrial cells possessing a T system are richly provided with specific granules and show little micropinocytotic activity, whereas cells devoid of T system show intense micropinocytotic activity and few specific granules. The possible functional implications of these findings are discussed.