Ultrastructure of the myocardial sarcolemma

Mice lacking growth-associated protein 43 develop cardiac remodeling and hypertrophy

Growth-associated protein 43 (GAP43) is found in skeletal muscle, localized near the calcium release units. In interaction with calmodulin (CaM), it indirectly modulates the activity of dihydropyridine and ryanodine Ca²⁺ channels. GAP43–CaM interaction plays a key role in intracellular Ca²⁺ homeostasis and, consequently, in skeletal muscle activity. The control of intracellular Ca²⁺ signaling is also an important functional requisite in cardiac physiology. The aim of this study is to define the impact of GAP43 on cardiac tissue at macroscopic and cellular levels, using GAP43 knockout (GAP43^−/−) newborn C57/BL6 mice. Hearts from newborn GAP43^−/− mice were heavier than hearts from wild-type (WT) ones. In these GAP43^−/− hearts, histological section analyses revealed a thicker ventricular wall and interventricular septum with a reduced ventricular chamber area. In addition, increased collagen deposits between fibers and increased expression levels of myosin were observed in hearts from GAP43^−/− mice. Cardiac tropism and rhythm are controlled by multiple intrinsic and extrinsic factors, including cellular events such those linked to intracellular Ca²⁺ dynamics, in which GAP43 plays a role. Our data revealed that, in the absence of GAP43, there were cardiac morphological alterations and signs of hypertrophy, suggesting that GAP43 could play a role in the functional processes of the whole cardiac muscle. This paves the way for further studies investigating GAP43 involvement in signaling dynamics at the cellular level.

The Physiology and Pathophysiology of T-Tubules in the Heart

In cardiomyocytes, invaginations of the sarcolemmal membrane called t-tubules are critically important for triggering contraction by excitation-contraction (EC) coupling. These structures form functional junctions with the sarcoplasmic reticulum (SR), and thereby enable close contact between L-type Ca²⁺ channels (LTCCs) and Ryanodine Receptors (RyRs). This arrangement in turn ensures efficient triggering of Ca²⁺ release, and contraction. While new data indicate that t-tubules are capable of exhibiting compensatory remodeling, they are also widely reported to be structurally and functionally compromised during disease, resulting in disrupted Ca²⁺ homeostasis, impaired systolic and/or diastolic function, and arrhythmogenesis. This review summarizes these findings, while highlighting an emerging appreciation of the distinct roles of t-tubules in the pathophysiology of heart failure with reduced and preserved ejection fraction (HFrEF and HFpEF). In this context, we review current understanding of the processes underlying t-tubule growth, maintenance, and degradation, underscoring the involvement of a variety of regulatory proteins, including junctophilin-2 (JPH2), amphiphysin-2 (BIN1), caveolin-3 (Cav3), and newer candidate proteins. Upstream regulation of t-tubule structure/function by cardiac workload and specifically ventricular wall stress is also discussed, alongside perspectives for novel strategies which may therapeutically target these mechanisms.

Fine structure of the intercalated disc and cardiac junctions in the black widow spider Latrodectus mactans

Arthropods have an open circulatory system with a simple tubular heart, so it has been estimated that the contractile pumping structure of the cardiac muscle will be less efficient than that of vertebrates. Nevertheless, certain arthropods are known to have far superior properties and characteristics than vertebrates, so we investigated the fine structural features of intercalated discs and cardiac junctions of cardiac muscle cells in the black widow spider Latrodectus mactans. Characteristically, the spider cardiac muscle has typical striated features and represents a functional syncytium that supports multiple connections to adjacent cells by intercalated discs. Histologically, the boundary lamina of each sarcolemma connects to the basement membrane to form an elastic sheath, and the extracellular matrix allows the cells to be anchored to other tissues. Since the intercalated disc is also part of sarcolemma, it contains gap junctions for depolarization and desmosomes that keep the fibers together during cardiac muscle contraction. Furthermore, fascia adherens and macula adherens (desmosomes) were also identified as cell junctions in both sarcolemma and intercalated discs. To enable the coordinated heartbeat of the cardiac muscle, the muscle fibers have neuronal innervations by multiple axons from the motor ganglion.

The architecture and function of cardiac dyads

Cardiac excitation-contraction (EC) coupling, which links plasma membrane depolarization to activation of cardiomyocyte contraction, occurs at dyads, the nanoscopic microdomains formed by apposition of transverse (T)-tubules and junctional sarcoplasmic reticulum (jSR). In a dyadic junction, EC coupling occurs through Ca2+-induced Ca2+ release. Membrane depolarization opens voltage-gated L-type Ca2+ channels (LTCCs) in the T-tubule. The resulting influx of extracellular Ca2+ into the dyadic cleft opens Ca2+ release channels known as ryanodine receptors (RYRs) in the jSR, leading to the rapid increase in cytosolic Ca2+ that triggers sarcomere contraction. The efficacy of LTCC-RYR communication greatly affects a myriad of downstream intracellular signaling events, and it is controlled by many factors, including T-tubule and jSR structure, spatial distribution of ion channels, and regulatory proteins that closely regulate the activities of channels within dyads. Alterations in dyad architecture and/or channel activity are seen in many types of heart disease. This review will focus on the current knowledge regarding cardiac dyad structure and function, their alterations in heart failure, and new approaches to study the composition and function of dyads.

Targeting cardiomyocyte Ca2+ homeostasis in heart failure

Improved treatments for heart failure patients will require the development of novel therapeutic strategies that target basal disease mechanisms. Disrupted cardiomyocyte Ca²⁺ homeostasis is recognized as a major contributor to the heart failure phenotype, as it plays a key role in systolic and diastolic dysfunction, arrhythmogenesis, and hypertrophy and apoptosis signaling. In this review, we outline existing knowledge of the involvement of Ca²⁺ homeostasis in these deficits, and identify four promising targets for therapeutic intervention: the sarcoplasmic reticulum Ca²⁺ ATPase, the Na⁺-Ca²⁺ exchanger, the ryanodine receptor, and t-tubule structure. We discuss experimental data indicating the applicability of these targets that has led to recent and ongoing clinical trials, and suggest future therapeutic approaches.

AutoTT: automated detection and analysis of T-tubule architecture in cardiomyocytes

Cardiac transverse (T)-tubules provide a specialized structure for synchronization and stabilization of sarcoplasmic reticulum Ca(2+) release in healthy cardiomyocytes. The application of laser scanning confocal microscopy and the use of fluorescent lipophilic membrane dyes have boosted the discoveries that T-tubule remodeling is a significant factor contributing to cardiac contractile dysfunction. However, the analysis and quantification of the remodeling of T-tubules have been a challenge and remain inconsistent among different research laboratories. Fast Fourier transformation (FFT) is the major analysis method applied to calculate the spatial frequency spectrum, which is used to represent the regularity of T-tubule systems. However, this approach is flawed because the density of T-tubules as well as non-T-tubule signals in the images influence the spectrum power generated by FFT. Preprocessing of images and topological architecture extracting is necessary to remove non-T-tubule noise from the analysis. In addition, manual analysis of images is time consuming and prone to errors and investigator bias. Therefore, we developed AutoTT, an automated analysis program that incorporates image processing, morphological feature extraction, and FFT analysis of spectrum power. The underlying algorithm is implemented in MATLAB (The MathWorks, Natick, MA). The program outputs the densities of transversely oriented T-tubules and longitudinally oriented T-tubules, power spectrum of the overall T-tubule systems, and averaged spacing of T-tubules. We also combined the density and regularity of T-tubules to give an index of T-tubule integrity (TTint), which provides a global evaluation of T-tubule alterations. In summary, AutoTT provides a reliable, easy to use, and fast approach for analyzing myocyte T-tubules. This program can also be applied to measure the density and integrity of other cellular structures.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Cardiac aquaporins

Aquaporins are a group of proteins with high-selective permeability for water. A subgroup called aquaglyceroporins is also permeable to glycerol, urea and a few other solutes. Aquaporin function has mainly been studied in the brain, kidney, glands and skeletal muscle, while the information about aquaporins in the heart is still scarce. The current review explores the recent advances in this field, bringing aquaporins into focus in the context of myocardial ischemia, reperfusion, and blood osmolarity disturbances. Since the amount of data on aquaporins in the heart is still limited, examples and comparisons from better-studied areas of aquaporin biology have been used. The human heart expresses aquaporin-1, -3, -4 and -7 at the protein level. The potential roles of aquaporins in the heart are discussed, and some general phenomena that the myocardial aquaporins share with aquaporins in other organs are elaborated. Cardiac aquaporin-1 is mostly distributed in the microvasculature. Its main role is transcellular water flux across the endothelial membranes. Aquaporin-4 is expressed in myocytes, both in cardiac and in skeletal muscle. In addition to water flux, its function is connected to the calcium signaling machinery. It may play a role in ischemia-reperfusion injury. Aquaglyceroporins, especially aquaporin-7, may serve as a novel pathway for nutrient delivery into the heart. They also mediate toxicity of various poisons. Aquaporins cannot influence permeability by gating, therefore, their function is regulated by changes of expression-on the levels of transcription, translation (by microRNAs), post-translational modification, membrane trafficking, ubiquitination and subsequent degradation. Studies using mice genetically deficient for aquaporins have shown rather modest changes in the heart. However, they might still prove to be attractive targets for therapy directed to reduce myocardial edema and injury caused by ischemia and reperfusion.

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.

Aquaporin expression in normal and pathological skeletal muscles: a brief review with focus on AQP4

Freeze-fracture electron microscopy enabled us to observe the molecular architecture of the biological membranes. We were studying the myofiber plasma membranes of health and disease by using this technique and were interested in the special assembly called orthogonal arrays (OAs). OAs were present in normal myofiber plasma membranes and were especially numerous in fast twitch type 2 myofibers; while OAs were lost from sarcolemmal plasma membranes of severely affected muscles with dystrophinopathy and dysferlinopathy but not with caveolinopathy. In the mid nineties of the last century, the OAs turned out to be a water channel named aquaporin 4 (AQP4). Since this discovery, several groups of investigators have been studying AQP4 expression in diseased muscles. This review summarizes the papers which describe the expression of OAs, AQP4, and other AQPs at the sarcolemma of healthy and diseased muscle and discusses the possible role of AQPs, especially that of AQP4, in normal and pathological skeletal muscles.

Cardiac-specific overexpression of caveolin-3 induces endogenous cardiac protection by mimicking ischemic preconditioning

BACKGROUND

Caveolae, lipid-rich microdomains of the sarcolemma, localize and enrich cardiac-protective signaling molecules. Caveolin-3 (Cav-3), the dominant isoform in cardiac myocytes, is a determinant of caveolar formation. We hypothesized that cardiac myocyte-specific overexpression of Cav-3 would enhance the formation of caveolae and augment cardiac protection in vivo.

METHODS AND RESULTS

Ischemic preconditioning in vivo increased the formation of caveolae. Adenovirus for Cav-3 increased caveolar formation and phosphorylation of survival kinases in cardiac myocytes. A transgenic mouse with cardiac myocyte-specific overexpression of Cav-3 (Cav-3 OE) showed enhanced formation of caveolae on the sarcolemma. Cav-3 OE mice subjected to ischemia/reperfusion injury had a significantly reduced infarct size relative to transgene-negative mice. Endogenous cardiac protection in Cav-3 OE mice was similar to wild-type mice undergoing ischemic preconditioning; no increased protection was observed in preconditioned Cav-3 OE mice. Cav-3 knockout mice did not show endogenous protection and showed no protection in response to ischemic preconditioning. Cav-3 OE mouse hearts had increased basal Akt and glycogen synthase kinase-3beta phosphorylation comparable to wild-type mice exposed to ischemic preconditioning. Wortmannin, a phosphoinositide 3-kinase inhibitor, attenuated basal phosphorylation of Akt and glycogen synthase kinase-3beta and blocked cardiac protection in Cav-3 OE mice. Cav-3 OE mice had improved functional recovery and reduced apoptosis at 24 hours of reperfusion.

CONCLUSIONS

Expression of caveolin-3 is both necessary and sufficient for cardiac protection, a conclusion that unites long-standing ultrastructural and molecular observations in the ischemic heart. The present results indicate that increased expression of caveolins, apparently via actions that depend on phosphoinositide 3-kinase, has the potential to protect hearts exposed to ischemia/reperfusion injury.

Upregulation of the water channel aquaporin-4 as a potential cause of postischemic cell swelling in a murine model of myocardial infarction

Ischemia of the myocardium is generally accepted to be characterized by swelling of myocytes resulting in cardiac dysfunction. However, data are limited concerning the molecular mechanisms of fast water fluxes across cell membranes in ischemic hearts. Since aquaporin-4 (AQP4) is a water channel with an enormous water flux capacity, we investigated in this study whether this water channel protein might play a role in myocyte swelling following myocardial infarction. For this purpose, we studied the expression of AQP4 mRNA at different time points of ischemia in a murine model of myocardial infarction. We observed a significant correlation between the upregulation of AQP4 mRNA and the size of the infarction. In situ hybridization experiments showed comparably higher expression levels of AQP4 mRNA in ischemic myocytes, and anti-AQP4 immunoreactivity was found to be stronger in the sarcolemma of ischemic myocytes. Our findings imply a role of AQP4 in the formation of myocardial edema and this might be important for future prevention and treatment strategies of this distressing situation in order to minimize cardiac dysfunction and mortality in a variety of cardiac diseases in which cell swelling is prevalent.

Orthogonal arrays of intramembrane particles in the supporting cells of the guinea-pig vestibular sensory epithelium

Membrane specializations of the contact region between afferent nerve endings and supporting cells of the sensory epithelia of guinea-pig vestibular endorgans were examined by thin-section and freeze-fracture electron microscopy. The calyx-type nerve endings (C-endings) are separated from supporting cells (SC) by a 25-30 nm space. At irregular intervals along the upper lateral surface of supporting cells, the intercellular space narrows markedly to form special close contacts between the C-ending and SC plasma membranes. Freeze-fracture replicas reveal membrane specializations--orthogonal arrays of particulate units--in the region where the close intercellular contacts were found in sections. Orthogonal arrays consisting of from 5 to 20 units were observed on the cytoplasmic (P) fracture face of the lateral SC plasma membrane. These particulate units from a 12 x 12-nm square, and each unit is composed of four 6-nm subunits. Possible roles of the orthogonal arrays are discussed.

Polarity and length of actin filaments at the fascia adherens of the cardiac intercalated disk

Digestion of canine and bovine intercalated disks with a calcium-activated protease (CAF) removes the electron-dense material similar to that found at the Z-line and presumably consisting primarily of alpha-actinin. The major filaments exposed by CAF are actin, and the polarity is away from the intercalated disk, as was confirmed by decoration with heavy meromyosin. The length of actin filaments associated with the fascia adherens region at the concave region is 1.2- to 2.2-fold that of actin filaments (I-filaments) in the sarcomere and varies depending on the interdigitation of the membrane at the cell junction. Actin filaments at the intercalated disk seem to be attached (or very close) to the membrane in a direct, rather than looping, manner.

Structural alterations in the membrane of the slow muscle fiber of Rana temporaria after denervation

Mixed fiber bundles as well as separated slow and fast fibers from normal and denervated muscles of Rana temporaria were freeze-fractured. The membranes of both fiber types are distinguished in this species by the presence of fairly regularly distributed particle aggregates or arrays of different shapes and sizes; the number per unit area of the membrane is six times higher in fast than in slow fibers. The intramembrane particle (IMP) density is higher in slow than in fast fibers. After denervation, the fast fiber membrane structure does not change whereas the slow fiber membrane acquires the characteristics of the fast fiber, i.e., an increase in the density of particle arrays and a decrease in IMP density. These changes in the slow fiber membrane are compared to the altered physiological properties of this fiber type after denervation.

Morphology of smooth muscle cells in the rat thoracic duct. A scanning and transmission electron-microscope study

The three-dimensional cytoarchitecture and ultrastructure of the smooth muscle cells in the wall of the rat thoracic duct were investigated by scanning and transmission electron microscopy. The muscle layer basically consists of a single layer of circularly arranged cells. The smooth muscle cell is fusiform or ribbon-like in shape, as in veins or venules with a similar or smaller diameter. Connections by spinous processes are observed between adjacent muscle cells along their length. Spot-like membrane contacts frequently occur in areas where facing membranes are closely apposed. These are thought to be gap junctions and may be responsible for electrical coupling and mechanical attachment. Large invaginations arranged regularly in rows on the surface of the smooth muscle cells can be observed. These invaginations are closely associated with a flattened sarcoplasmic reticulum, and caveolae tend to open into the invaginations.

Relationship between reestablishment of sarcolemma-glycocalyx ultrastructures and restoration of transmembrane potentials in cultured rat heart cells

Simultaneous studies on sarcolemma-glycocalyx ultrastructures and electrophysiological properties of the trypsin-released rat heart cells were carried out in order to define the relationship between sarcolemmal membrane repair and transmembrane potential recovery in the long-term cultured heart cells. Based on electron microscopic observations of the trypsinized heart cells maintained in the long-term culture, serial alterations of sarcolemma-glycocalyx complex could be divided into three successive stages. A defective stage of the sarcolemma-glycocalyx complex was present in the cultured cells between day 3 and day 6 of incubation. A repaired stage of the sarcolemma-glycocalyx complex was observed in the cells from day 7 to day 9 of incubation. A well-organized stage of the sarcolemma-glycocalyx complex was seen in the cells after ten days of incubation. Sequential measurements of electrophysiological parameters of the cultured heart cells showed a diphasic evolution of maximum diastolic potential and action potential amplitude, with an initial decrease from day 3 to day 9 of incubation and a later return to normal range after ten days of incubation. There seemed a good correlation between electrophysiological properties and sarcolemma-glycocalyx ultrastructures. Thus, we conclude that restoration of the electrophysiological properties of the cultured cells is closely related to the reorganization of the defective sarcolemmal membrane.

Intramembrane aspects of myotendinous and myomuscular junctions: a freeze-fracture study of the gill sac-muscle of the Atlantic hagfish, Myxine glutinosa

The intramembrane organization of the sarcolemma at the sites of myotendinous and myomuscular junctions was studied in the gill sac-muscle of the Atlantic hagfish, Myxine glutinosa, by using freeze-fracture replicas. At these sites rows of irregularly shaped particles (diameter approximately 6 nm) and short fibrils are present on the P face and a complementary pattern of grooves is present on the E face of the split plasma membrane. The center-to-center distance between adjacent rows of particles and grooves ranges from 12 to 20 nm. Rodlike projections being in register with the rows of particles and grooves, respectively, extend from the plasma membrane toward the extracellular space. These rodlike projections are also recognizable in thin sections, where they appear as spinelike projections (cross sections) or linear arrays (grazing sections) located in the lamina lucida of the basal lamina. The intramembrane particles are considered to be integral membrane proteins and to represent transmembrane links in a series of molecules by which intracellular actin filaments and extracellular collagen fibrils are connected across the plasma membrane. The rodlike projections are probably peripheral membrane proteins possibly connecting the plasma membrane with structural components of the basal lamina.

The base-exchange enzyme activities of sarcolemma and sarcoplasmic reticulum from rat heart

The Ca2+ dependent incorporation of [14C]ethanolamine, L-[14C]serine and [14C]choline into phosphatidylethanolamine, phosphatidylserine and phosphatidylcholine, respectively, were investigated in membrane preparations from rat heart. The ethanolamine and serine base-exchange enzyme-catalyzed reactions were associated with the sarcolemma and sarcoplasmic reticulum. There was a 17.2-fold and 6.8-fold enrichment, respectively, of the serine and the ethanolamine base-exchange enzyme activities in the sarcolemma compared to the starting whole homogenate. The sarcoplasmic reticulum was enriched in the ethanolamine and serine base-exchange enzyme activities. The choline base-exchange enzyme activity of all membranes fractions was negligible compared to the ethanolamine or serine base-exchange enzyme activities. The apparent Km for the ethanolamine and serine base-exchange enzyme in sarcolemma was 14 microM and 25 microM, respectively. The pH optimum for these base-exchange activities was 7.5-8.0. There was a dependence upon Ca2+ for these reactions with a 1 or 4 mM concentration required for maximal activity. The properties of the sarcoplasmic reticulum base-exchange enzymes were similar to the sarcolemmal base-exchange enzymes.

Structure-function studies of canine cardiac sarcolemmal membranes. II. Structural organization of the sarcolemmal membrane as determined by electron microscopy and lamellar X-ray diffraction

The morphological and ultrastructural properties of highly purified canine cardiac sarcolemmal vesicles, prepared by a modification (Colvin, R.A., Ashavaid, T.F. and Herbette, L.G. (1985) Biochim. Biophys. Acta 812, 601-608) of the method of Jones et al. (Jones L.R., Madlock, S.W. and Besch, H.R. (1980) J. Biol. Chem. 255, 9971-9980), were examined by several techniques. Thin-section electron microscopy showed predominantly intact unilamellar vesicles with little staining beyond the lipid bilayer boundaries. Freeze-fracture electron microscopy demonstrated that the majority of particles are approx. 90 A diameter and present at a density of 780 +/- 190 micrometers-2 (+/- S.D.). If it is assumed that some of these particles represent the (Na+ + K+)-ATPase, the finding that they are largely confined to the convex fracture face suggests a predominant right-side-out orientation of these sarcolemmal vesicles that is consistent with biochemical assays. The sarcolemmal membrane width measured by electron microscopy (unhydrated membrane width of 50-70 A) is consistent with the unit cell dimensions of 56-77 A determined by lamellar X-ray diffraction (hydrated membrane width). A unit cell dimension of 56-62 A was also found by X-ray diffraction for sarcolemmal lipids extracted from these preparations, indicating that the isolated sarcolemmal preparations do not contain a significant surface coat (glycocalyx). As both cardiac and skeletal sarcoplasmic reticulum membranes have a 80-100 A membrane width, these findings demonstrate that the purified sarcolemmal membrane is structurally distinct from both cardiac and skeletal sarcoplasmic reticulum. In contrast to the protein-rich skeletal sarcoplasmic reticulum membrane, which contains a single essential protein responsible for the regulation of cytosolic Ca2+ concentration, the sarcolemma is a lipid-rich membrane that contains a variety of proteins associated with many regulatory functions served by this membrane in cardiac muscle.

Intercalated discs of mammalian heart: a review of structure and function

Intercalated discs are exceptionally complex entities, and possess considerable functional significance in terms of the workings of the myocardium. Examination of different species and heart regions indicates that the original histological term has become out-moded; it is likely, however, that all such complexes will continue to fall under the generic heading of 'intercalated discs'. The membranes of the intercalated discs establish specific associations with a variety of intracellular and extracellular structures, as well as with numerous types of proteins and glycoproteins. Characterization of discs and their components has already brought together a large number of research disciplines, including microscopy, cytochemistry, morphometry, cell isolation and culture, cell fractionation, cryogenics, immunology, biochemistry, and electrophysiology. The continued dissection of substance and function of intercalated discs will depend on such interdisciplinary approaches. The intercalated disc component which continues to attract the greatest amount of interest is the so-called gap junction. All indications thus far point to a great deal of inherent lability in the architecture of the gap junction. There is thus considerable potential for the creation of artefact while preserving and observing gap junctions, and this problem will doubtless continue to hamper the understanding of their functions. A question of special interest concerns whether the gap junctions of intercalated discs are required for transfer of electrical excitation between cells, or maintain cell-to-cell adhesion, or in fact subserve both electrical and structural phenomena. Two schools of thought exist with respect to cell-to-cell coupling in the heart. One proposes that low-resistance junctions in the discs mediate electrical coupling, whereas the other supports the possibility of coupling across ordinary high-resistance membranes. Thus the intercalated discs continue to be a source of controversy, just as they have been since they were originally discovered in heart muscle over a century ago.

Alterations in the morphology of rabbit skeletal muscle plasma membrane during membrane isolation

This study describes changes in morphology of plasmalemma from fast skeletal muscle in the course of tissue disruption and isolation. We find that conditions used to solubilize muscle contractile elements, in the isolation of plasmalemma, including the use of 0.6 M KCl or 0.4 M LiBr in the cold (0-4 degrees C), lead to altered plasmalemma morphology. The intramembrane particles, as revealed by freeze-fracture electron microscopy, become aggregated, leaving large domains devoid of particles. The square arrays in the P face and the complementary "pits" in the E face also become aggregated, sometimes forming sizeable aggregates of square arrays. Thin-section electron microscopy using tannic acid enhancement reveals plasma membrane associated components, on both cytoplasmic and extracellular faces, are largely reduced by the salt treatment. Pyrophosphate and magnesium at lower concentrations, sometimes used instead of high salt, also resulted in particle aggregation, although less pronounced than with concentrated salt solutions. The plasma membrane-associated proteins on both plasma membrane surfaces were likewise decreased by this treatment. Pyrophosphate treatment also separated the basal lamina from the plasma membrane. Incubation of muscle in isoosmotic sucrose does not alter the morphology of the plasmalemma with regard to particle aggregation, diminution of membrane associated components, or separation of the basal lamina. Our observations suggest that membrane-associated protein and/or cytoskeleton constrains the mobility of components in the plane of the membrane and that removal of this constraint leads to aggregation of intramembrane particles.

Membrane structure in cultured astrocytes

Astrocytes cultured from the brains of neonatal rat pups acquire at least two specializations of intramembrane particle distribution: 'assemblies' and gap junctions. The number and appearance of assemblies in the cultured astrocytes is not markedly influenced by the presence or absence of a collagen substrate, and the range of concentrations of assemblies in astrocytic membranes is fairly stable from 7 through 28 days in culture. The assemblies are not concentrated in apposition to the substrate, even though the astrocytic membranes containing the highest concentration of assemblies in vivo are apposed to basal lamina. Quantitative analysis shows that assemblies are not uniformly distributed over the plasmalemma of a single cell, raising the possibility that the nature of cells around an astrocytic process may influence its membrane composition in vitro.

III. Three-dimensional electron microscopy of mammalian cardiac sarcoplasmic reticulum at 80 kV

The Golgi black reaction method was combined with stereoscopic techniques to obtain three-dimensional views of cardiac sarcoplasmic reticulum (SR) using a conventional electron microscope operating at 80 kV. We have previously described the SR in avian and mammalian skeletal muscles with similar techniques. It was necessary to modify these earlier techniques for cardiac muscle. Two regions of mammalian heart were explored: trabecular and papillary muscles. These muscles presented striking differences with regard to relative volume of mitochondria and myofibrils, but both muscles presented similar dispositions of the inner tubules of SR. The SR near myofibrils appeared heterogeneous and consisted of fenestrated collar, bulbous extensions at the Z line (corbular SR), and flat extended regions (cisternal SR). The SR near mitochondria, however, always formed a simple rete with occasional cisternal SR. Specific "staining" of the inner tubules of cardiac SR by the Golgi method offers new views of cardiac fibers that suggest a more extensively developed SR than previously acknowledged.

Regional organization of astrocytic membranes in cerebellar cortex

The internal organization of plasmalemmal membrane, as revealed by freeze-fracture techniques, varies dramatically and predictably over the surface of astrocytes in mouse cerebellar cortex. Assemblies of uniform, small intramembrane particles packed in orthogonal order into square or rectangular aggregates are specialized distribution of intramembrane particles which, in the cerebellar cortex, are found only in astrocytes. The concentration of assemblies is greatest in astrocytic membrane juxtaposed to vascular structures or facing the cerebrospinal fluid at the glial limitans. Many fewer are present in regions of astrocytic membrane apposed to neural structures and virtually none are present on the astrocyte cell body. Corresponding structures have not yet been found in thin-sectioned preparations. While the distribution of assemblies in membranes facing blood and cerebrospinal fluid compartments suggests that they may have a role in transport of some material into or out of those compartments, their function is unknown. A second, distinct specialization of intramembrane structure appears to represent a junction between apposed astrocytic processes. We have provisionally described this as a 'polygonal particle junction', since it appears as large, irregular particles densely packed without obvious order in co-extensive regions of two astrocytic membranes. This junction is regularly present just below the cerebellar surface in the processes of the glial limitans as well as between large, more proximal radial Bergmann fibers, and also occurs occasionally throughout the molecular layer. With tannic acid mordant after aldehyde-osmium fixation or rapid freezing and freeze-substitution, it is possible to demonstrate subtle electron-dense specializations of the astrocytic membranes and extracellular matrix in thin-sections which correspond to the sites of polygonal particle junctions. The function of this astrocytic specialization is also unknown. Cerebellar astrocytes manifest numerous gap junctions as well, whose structure in freeze-fractured and thin-sectioned preparations is quite distinct from that of assemblies or of polygonal particle junctions.

Freeze fracture studies of muscle plasma membrane in human muscular dystrophy

Freeze fracture analysis of intramembranous particle density in skeletal muscle plasma dystrophy from 7 patients with Duchenne muscular dystrophy (DMD), 5 patients with facioscapulohumeral muscular dystrophy (FSH) and 5 patients with myotonic dystrophy (MyD) were carried out. Marked depletion of intramembranous particles including orthogonal arrays was noted in DMD while only orthogonal arrays were significantly decreased in FSH. No abnormalities were noted in MyD.

A freeze-fracture study of the anterior and posterior latissimus dorsi muscle of the chicken

The sarcolemma, sarcoplasmic reticulum (SR), and T system of the anterior (tonic) and posterior (fast twitch) latissimus dorsi muscles of the chicken have been examined by the freeze-fracture technique, and quantitative data on the P and E fracture faces have been obtained. The fractured plasma membranes reveal (a) profiles of surface caveolae, (b) randomly distributed intramembranous particles ranging in size from 40-100 A in diameter, and (c) orthogonal assemblies composed of groups of 60 A particles in close association, and differences with respect to all three structures are present between the tonic (ALD) and fast twitch (PLD) muscles. In the ALD muscle, the surface caveolae are more uniformly distributed and have smaller openings than in the PLD muscle; the former muscle also has a two-fold higher caveolae density than the latter muscle. The intramembranous particles are more numerous in the ALD than in the PLD muscle in both fracture faces, but the orthogonal assemblies are fewer. The functional significance of these differences in the two fiber types are discussed. The fractured membranes of the SR have intramembranous particles (IMPs) approximately 80 A in diameter, with a two-fold higher packing density in the PLD than in the ALD muscle. This difference is present in both the longitudinal and cisternal components of the SR. In addition, there are collar-like expansions (CLE's) in the SR of the ALD muscle which are particularly poor in intramembranous particles. These particles are considered to represent Ca2+ transport ATP-ase, and the reduced density of IMP's could be a significant factor in the low calcium uptake and and slow relaxation characteristics of the ALD muscle.

Structures located at the levels of the Z bands in mouse ventricular myocardial cells

Within ventricular myocardial cells of the mouse, the myoplasmic regions located immediately adjacent to the Z lines of the sarcomeres contain a variety of structures. These include: (1) transversely oriented 10 nm ('intermediate') filaments that apparently contribute to the cytoskeleton of the myocardial cell; (2) the majority of the transverse elements of the T-axial tubular system; (3) specialized segments of the sarcoplasmic reticulum (SR) that are closely apposed to the sarcolemma or T-axial tubules (junctional SR); (4) 'extended junctional SR' ('corbular SR') that exists free of association with the cell membrane; (5) 'Z tubules' of SR that are intimately apposed to the Z line substance; and (6) leptofibrils. In addition, fasciae adherentes supplant Z lines where myofibrils insert into the transverse borders (intercalated discs) of the cells. The concentration of these myocardial components at the level of the Z lines suggests that a particular specialization of structural and physiological activities exists in the Z-level regions of the myoplasm. In particular, it appears that the combination of intermediate filaments, T tubules, and Z-level SR elements forms a series of parallel planar bodies that extend across each myocardial cell to impart transverse rigidity. The movement and compartmentation of calcium ion (Ca2+) would seem especially active near the Z lines of the myofibrils, in view of the preferential location there of Ca2+-sequestering myocardial structures such as T tubules, junctional SR, extended junctional SR and Z tubules.

A freeze-fracture study of fiber types in normal human muscle

The sarcoplasmic reticulum (SR) and plasma membranes of Type 1 and Type 2 fibers of normal human muscle were examined by the freeze-fracture technique. Total particle counts in the SR appeared much lower than in other mammals and a packing density of about 1200 particles/micrometer 2 was found in both longitudinal and cisternal components of SR. There was no difference in particle density of Type 1 and Type 2 fibers. In freeze-fracture replicas of plasma membranes several fiber type differences were seen. The surface caveolae were uniformly distributed in Type 1 fibers whereas in Type 2 they were clustered preferentially at the I-band levels. Total density of intramembranous particles was greater in Type 1 fibers (347 +/- 68/micrometer 2 in P-face, 58 +/- 11/micrometer 2 in E face) than in Type 2 fibers (207 +/- 30/micrometer 2 in P-face, 80 +/- 9/micrometer 2 in E-face). There was a striking difference in respect to rectilinear arrays which were virtually absent in Type 1 fibers (0--2/micrometer 2) and numberous (up to 50--70/micrometer 2) in Type 2 fibers.

Mass isolation of cell surface membrane fragments from pigeon heart

Cell surface membrane fragments were isolated and purified by successive rate zonal and isopycnic centrifugation of calcium oxalate-loaded pigeon heart microsomes in sucrose density gradients. The most highly purified cell membrane fraction sediments at a buoyant density of 1.105 g/ml. Some of the membrane pieces are present as open fragments and leaky vesicles, while others form tightly sealed vesicles of both inside-in and inside-out membrane orientation. The pigeon heart cell membrane preparation exhibits high (Na+ + K+ + Mg2+)-ATPase and adenylate cyclase activities. Additional activity of these enzymes is uncovered by sodium dodecyl sulfate and alamethicin, respectively. Electron microscopic inspection of the cell surface membrane preparation revealed (a) a predominance of thick-walled vesicles with smooth surfaces on negative staining and (b) binding of concanavalin A to the bulk of isolated membrane pieces following their incubation with the lectin.

A thin-section and freeze-fracture study of microfilament-membrane attachments in choroid plexus and intestinal microvilli

Choroid plexus and intestinal microvilli in thin sections have microfilaments in the cytoplasm adjacent to the membranes, and in replicas have broken strands of filaments in both cytoplasm and on E faces of plasm membranes. The microfilaments contain actin as indicated by their binding of heavy meromyosin (HMM). In sections of choroid plexus, the microfilaments are 7-8 nm in diameter and form a loose meshwork which lies parallel to the membrane and which is connected to the membranes both by short, connecting filaments (8 times 30 nm) and dense globules (approximately 15-20 nm). The filamentous strands seen in replicas are approximately 8 nm in diameter. Because they are similar in diameter and are connected to the membrane, these filamentous strands seen in replicas apparently represent the connecting structures, portions of the microfilaments, or both. The filamentous strands attached to the membrane are usually associated with the E face and appear to be pulled through the P half-membrane. In replicas of intestinal brush border microvilli, the connecting strands attaching core microfilaments to the membrane are readily visualized. In contrast, regions of attachment of core microfilaments to dense material at the tips of microvilli are associated with few particles on P faces and with few filamentous strands on the E faces of the membranes. Freeze-fracture replicas suggest a morphologically similar type of connecting strand attachment for microfilament-membrane binding in both choroid plexus and intestinal microvilli, despite the lack of a prominent core bundle of microfilaments in choroid plexus microvilli.

Quantitative ultrastructural analysis in cardiac membrane physiology

Quantitative measurements on electron micrographs of heart muscle can yield information useful for cellular physiologists and at present not obtainable in other ways. These methods are subject to preparative artifact, sampling problems, and problems inherent in the mathematical description of ultrastructure. Nevertheless they provide the best available data for membrane areas of the plasmalemma and its components, as well as for membrane areas of the sarcoplasmic reticulum and mitochondria. Morphometric methods can be used to study growth of membranes. Changes in the volumes of intracellular membrane-limited subcompartments can also be measured. Quantitative analysis of freeze-fractured membrane replicas can be carried out either by a statistical approach or by optical diffraction. In this way, physiological perturbations or developmental events leading to changes in membrane permeability can be studied for correlated changes in membrane structure.

Arrangement of smooth muscle cells and intramuscular septa in the taenia coli

Bands of electron-dense material beneath the cell membrane of smooth muscle cells of the guinea-pig taenia coli provide attachment to thin myofilaments and to intermediate (10nm) filaments; about 50% of the cell membrane is occupied by dense bands in muscle cells transversely sectioned at the level of their nucleus, and between 50 and 100% in small cell profiles nearer the cell's ends. In addition to the known cell-to-cell junctions (intermediate contacts), more complex apparatuses anchor muscle cells together, either end-to-end or end-to-side of side-to-side. They consist of elaborate folds, invaginations and protrusions accompanied by large amounts of basal lamina material. In the end-to-end anchoring apparatuses numerous finger-like and laminar processes from the two cells interdigitate. Other muscle cells have a star-shaped profile in the last few microns of their length, or show longitudinal invaginations occupied by a thickened basal lamina and occasionally by collagen fibrils. The septa of connective tissue extend only for a few hundred microns along the length of the taenia. In taeniae fixed in condition of mild stretch the muscle cells form an angle of about 5 degrees with the septa. In muscles fixed during isotonic contraction the angle increases to about 29--22 degrees, and in longitudinal sections the muscle cells appear arranged in a herring-bone pattern. The collagen concentration in the taenia coli is 4--6 times greater than in skeletal and cardiac muscles. These various structures are discussed in terms of their possible role in the mechanism of force transmission.

Studies of excitable membranes. II. A comparison of specializations at neuromuscular junctions and nonjunctional sarcolemmas of mammalian fast and slow twitch muscle fibers

Mammalian fast and slow twitch skeletal muscles are compared by freeze-fracture, thick and thin sectioning, and histochemical techniques using conventional and high voltage electron microscopy. Despite gross morphological differences in endplate structure visualized at relatively low magnifications in this sections, rat extensor digitorum longus (EDL) (fast twitch) and soleus (slow twitch) fibers cannot be distinguished on the basis of size, number, or distribution of molecular specializations of the pre- and postsynaptic junctional membranes exposed by freeze fracturing. Specializations in the cortex of the juxtaneuronal portions of the junctional folds are revealed by high voltage electron stereomicroscopy as a branching, ladder-like filamentous network associated with the putative acetylcholline receptor complexes. These filaments are considered to be involved in restricting the mobility of receptor proteins to the perineuronal aspects of the postynaptic membrane. Although the junctional membranes of both EDL and soleus appear similar, a differential specialization of the secondary synaptic cleft was noted. The extracellular matrix in the bottom of soleus clefts was observed as an ordered system of filamentous "combs," These filamentous arrays have not been detected in EDL junctions. Examination of the extrajunctional sarcolemmas of EDL and soleus reveal additional differences which may be correlated with variations in electrical and contractile properties. For example, particle aggregates termed "square arrays" previously described in the sarcolemmas of some fibers of the rat diaphragm were observed in large numbers in sarcolemmas of EDL fibers but were seldom encountered in soleus fibers. These gross compositional differences in the membranes are discussed in the light of functional differences between fiber types.