Ultrastructure of terminally differentiated adult rat cardiac muscle cells in culture

Increased stiffness and flow resistance of the inner wall of Schlemm's canal in glaucomatous human eyes

The cause of the elevated outflow resistance and consequent ocular hypertension characteristic of glaucoma is unknown. To investigate possible causes for this flow resistance, we used atomic force microscopy (AFM) with 10-µm spherical tips to probe the stiffness of the inner wall of Schlemm's canal as a function of distance from the tissue surface in normal and glaucomatous postmortem human eyes, and 1-µm spherical AFM tips to probe the region immediately below the tissue surface. To localize flow resistance, perfusion and imaging methods were used to characterize the pressure drop in the immediate vicinity of the inner wall using giant vacuoles that form in Schlemm's canal cells as micropressure sensors. Tissue stiffness increased with increasing AFM indentation depth. Tissues from glaucomatous eyes were stiffer compared with normal eyes, with greatly increased stiffness residing within ∼1 µm of the inner-wall surface. Giant vacuole size and density were similar in normal and glaucomatous eyes despite lower flow rate through the latter due to their higher flow resistance. This implied that the elevated flow resistance found in the glaucomatous eyes was localized to the same region as the increased tissue stiffness. Our findings implicate pathological changes to biophysical characteristics of Schlemm's canal endothelia and/or their immediate underlying extracellular matrix as cause for ocular hypertension in glaucoma.

Roles for the sarco-/endoplasmic reticulum in cardiac myocyte contraction, protein synthesis, and protein quality control

Although the function of the sarcoplasmic/endoplasmic reticulum (SR/ER) in cardiac contractile calcium handling is well established, its roles in protein synthesis, folding, and quality control in cardiac myocytes are not as clear. This review explores evidence suggesting that, in cardiac myocytes, protein synthesis and contractile calcium handling may be physically and functionally integrated.

Structural and functional plasticity in long-term cultures of adult ventricular myocytes

Cultured heart cells have long been valuable for characterizing biological mechanism and disease pathogenesis. However, these preparations have limitations, relating to immaturity in key properties like excitation-contraction coupling and β-adrenergic stimulation. Progressive attenuation of the latter is intimately related to pathogenesis and therapy in heart failure. Highly valuable would be a long-term culture system that emulates the structural and functional changes that accompany disease and development, while concurrently permitting ready access to underlying molecular events. Accordingly, we here produce functional monolayers of adult guinea-pig ventricular myocytes (aGPVMs) that can be maintained in long-term culture for several weeks. At baseline, these monolayers exhibit considerable myofibrillar organization and a significant contribution of sarcoplasmic reticular (SR) Ca(2+) release to global Ca(2+) transients. In terms of electrical signaling, these monolayers support propagated electrical activity and manifest monophasic restitution of action-potential duration and conduction velocity. Intriguingly, β-adrenergic stimulation increases chronotropy but not inotropy, indicating selective maintenance of β-adrenergic signaling. It is interesting that this overall phenotypic profile is not fixed, but can be readily enhanced by chronic electrical stimulation of cultures. This simple environmental cue significantly enhances myofibrillar organization as well as β-adrenergic sensitivity. In particular, the chronotropic response increases, and an inotropic effect now emerges, mimicking a reversal of the progression seen in heart failure. Thus, these aGPVM monolayer cultures offer a valuable platform for clarifying long elusive features of β-adrenergic signaling and its plasticity.

Chronic effects of ethanol on cultured myocardial cells: ultrastructural and morphometric studies

Ultrastructural alterations of the myocardium due to chronic ethanol exposure were investigated using an in vitro system-mouse ventricular myocardial cells in a monolayer culture, which were spontaneously and synchronously contracting-by chronic exposure to 12.5, 50, and 200 mM ethanol for up to 21 days. Morphometric analyses revealed that exposure to 12.5 mM ethanol for 14 days induced an increase in the number of residual bodies, which are lysosomes containing electron-dense, amorphous materials. Some cells exposed to 50 mM ethanol for 14 days contained an accumulation of glycogen granules, increasing in inverse proportion to the mitochondrial volume. The volumetric proportion of myofibrils on day 14 decreased as the ethanol dose became lower, and was in proportion to large and giant mitochondria within the limits of three ethanol groups. Dose-dependent increases in the size and volumetric proportion of mitochondria were observed after the 14-day exposure; at a low dose (12.5 mM) mitochondria of usual size tended to increase, whereas at a high dose (200 mM) giant mitochondria increased. Coincidentally with this mitochondrial increase or gigantism, all ethanol groups showed higher beat rates than the control. Consequently, it is most likely that chronic 14-day exposures to these three ethanol doses remodel the cellular function of the in vitro myocardium in different ways; the 200-mM dose induced mitochondrial hypertrophy, an adaptive response to switch myocardial energy metabolism over to some special one; the 50-mM dose was a boundary dose; and the 12.5-mM dose mostly mimicked the chronic in vivo administration of ethanol and induced slightly degenerative alterations-increased residual bodies and lysosomes, decreased myofibrils and lowered mitochondrial respiratory function.

Targeted delivery of DNA to the mitochondrial compartment via import sequence-conjugated peptide nucleic acid

We report that oligonucleotides can be introduced into the mitochondria of living mammalian cells by annealing them to peptide nucleic acids coupled to mitochondrial targeting peptides. These complexes are imported into the mitochondrial matrix through the outer and inner membrane import channels of isolated mitochondria. They are also imported into the mitochondria of cultured cells, provided that the cytosolic uptake of the complexes is facilitated by using synthetic polycations or membrane permeabilizing toxins. Our method now promises to provide a viable strategy for the genetic modification of the mitochondria in cultured cells, animals and patients.

Thin filament protein dynamics in fully differentiated adult cardiac myocytes: toward a model of sarcomere maintenance

Sarcomere maintenance, the continual process of replacement of contractile proteins of the myofilament lattice with newly synthesized proteins, in fully differentiated contractile cells is not well understood. Adenoviral-mediated gene transfer of epitope-tagged tropomyosin (Tm) and troponin I (TnI) into adult cardiac myocytes in vitro along with confocal microscopy was used to examine the incorporation of these newly synthesized proteins into myofilaments of a fully differentiated contractile cell. The expression of epitope-tagged TnI resulted in greater replacement of the endogenous TnI than the replacement of the endogenous Tm with the expressed epitope-tagged Tm suggesting that the rates of myofilament replacement are limited by the turnover of the myofilament bound protein. Interestingly, while TnI was first detected in cardiac sarcomeres along the entire length of the thin filament, the epitope-tagged Tm preferentially replaced Tm at the pointed end of the thin filament. These results support a model for sarcomeric maintenance in fully differentiated cardiac myocytes where (a) as myofilament proteins turnover within the cell they are rapidly exchanged with newly synthesized proteins, and (b) the nature of replacement of myofilament proteins (ordered or stochastic) is protein specific, primarily affected by the structural properties of the myofilament proteins, and may have important functional consequences.

Generation of new intercellular junctions between cardiocytes. A possible mechanism compensating for mechanical overload in the hypertrophied human adult myocardium

Intercellular dehiscence is a common cardiocytic response to pathological conditions. However, little consideration has been given to the possibility of new intercellular junctions developing between cardiocytes within developed myocardium. To examine this possibility as it may relate to useful compensation for hemodynamic overloads, changes in cardiocytic connection were evaluated by scanning electron microscopy in hypertrophied myocardium of adult human hearts. Transmural myocardium of left ventricle was obtained at autopsy from five hearts with concentric hypertrophy, five hearts with eccentric hypertrophy, and five control hearts (noncardiac death). After formalin fixation, the number of cardiocytes connected to an individual cardiocyte was counted in tissues from the middle portion of the transmural samples by scanning electron microscopy. Cardiocytic diameter and connective tissue volume fraction were measured on the transmural sections by light microscopy. In concentrically hypertrophied hearts present both increased cardiocytic diameter and connective tissue volume fraction, the number of other cardiocytes connected to an individual cardiocyte (4.60 +/- 0.10 [mean +/- SE] was significantly increased (P < .05) compared with control hearts (4.19 +/- 0.12) or eccentrically hypertrophied hearts (4.11 +/- 0.10). The increase in junctions per cardiocyte in concentrically hypertrophied hearts suggests that new connections had been generated. More junctions developing during hypertrophy could add another structural advantage to those of cardiocytic hypertrophy and connective tissue proliferation as compensatory adjustments to hemodynamic overload in concentrically hypertrophied hearts.

Culture of rat renal medullary tissue in media made hyperosmotic with NaCl and urea

During antidiuresis, the rat kidney maintains a variable and steep osmotic gradient from the cortex (300 mOsm) to the inner medulla (at least 2,600 mOsm). Therefore, cells in the renal medulla must be able to adapt to a variably hyperosmotic environment. We have examined the ability of tissue fragments taken from various points on the cortical-medullary axis to survive and grow when cultured in media made hyperosmotic with urea and NaCl. Survival and growth were measured by the explants' ability to produce epithelial outgrowths. At osmotic concentrations of 1,100 and 1,200 mOsm, only explants from the inner medulla produced epithelial outgrowths. At 700 mOsm, all explants produced outgrowths but outgrowth size was a function of position on the cortical-medullary axis, with inner medullary fragments producing the largest outgrowths. Growth was most rapid at all osmolalities when the Na+:urea ratio was 1:1. These results are consistent with the hypothesis that renal medullary cells are adapted to elevated concentrations of Na+ and urea. Both explants and epithelial outgrowths were examined using light and electron microscopy. Physical continuities between the epithelial outgrowths and collecting duct epithelium in the explants, as well as the ultrastructural characteristics of the outgrowths at 700 mOsm, indicated that the outgrowths may have originated from collecting duct epithelium.

Cellular basis for the negative inotropic effects of tumor necrosis factor-alpha in the adult mammalian heart

To define the mechanism(s) responsible for the negative inotropic effects of tumor necrosis factor-alpha (TNF alpha) in the adult heart, we examined the functional effects of TNF alpha in the intact left ventricle and the isolated adult cardiac myocyte. Studies in both the ventricle and the isolated adult cardiac myocyte showed that TNF alpha exerted a concentration- and time-dependent negative inotropic effect that was fully reversible upon removal of this cytokine. Further, treatment with a neutralizing anti-TNF alpha antibody prevented the negative inotropic effects of TNF alpha in isolated myocytes. A cellular basis for the above findings was provided by studies which showed that treatment with TNF alpha resulted in decreased levels of peak intracellular calcium during the systolic contraction sequence; moreover, these findings did not appear to be secondary to alterations in the electrophysiological properties of the cardiac myocyte. Further studies showed that increased levels of nitric oxide, de novo protein synthesis, and metabolites of the arachidonic acid pathway were unlikely to be responsible for the TNF alpha-induced abnormalities in contractile function. Thus, these studies constitute the initial demonstration that the negative inotropic effects of TNF alpha are the direct result of alterations in intracellular calcium homeostasis in the adult cardiac myocyte.

Role of epicardial mesothelial cells in the modification of phenotype and function of adult rat ventricular myocytes in primary coculture

Adult rat ventricular myocytes undergo a well-documented sequence of phenotypic changes during adaptation to primary culture. However, we observed that coculture of myocytes with a specific subset of nonmyocyte cardiac cells could slow and even reverse the process of adaptation. These nonmyocyte cells were isolated and identified by immunohistochemical and ultrastructural criteria as being of epicardial mesothelial origin. When added to long-term primary cultures of adult ventricular myocytes, epicardial mesothelial cells appeared to induce myofibrillar arrays that were more organized than those seen in noncocultured myocytes; these changes that occurred were concurrent with the appearance of large amplitude contractions and multicellular synchronous beating that was facilitated by gap junctions between myocytes and epicardial mesothelial cells. The changes in morphology and function were accompanied by a marked increase in beta-myosin heavy chain isoform transcription in cocultured myocytes, a return to the ratio of cardiac to skeletal alpha-actin expected in adult rat myocardium, and a much reduced expression of smooth muscle alpha-actin. These changes in myocyte phenotype and function appeared to require epicardial cell-myocyte contact, or close apposition, because media conditioned by epicardial mesothelial cells alone or in coculture had no effect. Thus, these rapid and reversible changes in myocyte ultrastructure, function, and gene expression may provide a useful in vitro model with which to study the mechanism responsible for regulating the plasticity of ventricular myocyte phenotype and the role of specific cell-cell interactions.

Gene expression and atrial natriuretic factor processing and secretion in cultured AT-1 cardiac myocytes

BACKGROUND

Studies were carried out to characterize several biochemical features of cultured AT-1 cells.

METHODS AND RESULTS

These cells were obtained from a transplantable atrial cardiomyocyte tumor lineage. Reverse transcriptase-polymerase chain reaction-based analyses demonstrated that the pattern of gene expression of cultured AT-1 cells was similar to that of adult atrial myocytes. AT-1 cells expressed atrial natriuretic factor (ANF), alpha-cardiac myosin heavy chain, alpha-cardiac actin, and connexin43. Radioimmunoassays verified that the cells synthesized, stored, and secreted ANF. Through size-exclusion, reversed-phase, and carboxymethyl-ion-exchange high-performance liquid chromatography, it was shown that cultured AT-1 cells stored ANF as pro-ANF (ANF-[1-126]), which was cosecretionally processed quantitatively to ANF-(1-98) and the bioactive 28-amino-acid ANF-(99-126). In addition, cultured AT-1 cells secreted ANF at almost a sixfold greater rate in response to endothelin-1, a potent secretagogue of ANF. KCl, metenkephalinamide, isoproterenol, phenylephrine, and 12-O-tetradecanoyl-phorbol-13-acetate also stimulated ANF release.

CONCLUSIONS

These studies, in combination with previous findings, demonstrated that cultured AT-1 cells, while maintaining the ability to proliferate, have retained functional, biochemical, and ultrastructural features that are characteristic of adult atrial myocytes.

Incorporation of fluorescently labeled contractile proteins into freshly isolated living adult cardiac myocytes

When fluorescently labeled contractile proteins are injected into embryonic muscle cells, they become incorporated into the cells' myofibrils. In order to determine if this exchange of proteins is unique to the embryonic stage of development, we isolated adult cardiac myocytes and microinjected them with fluorescently labeled actin, myosin light chains, alpha-actinin, and vinculin. Each of these proteins was incorporated into the adult cardiomyocytes and was colocalized with the cells' native proteins, despite the fact that the labeled proteins were prepared from noncardiac tissues. Within 10 min of injection, alpha-actinin was incorporated into Z-bands surrounding the site of injection. Similarly, 30 sec after injection, actin was incorporated into the entire I-bands at the site of injection. Following a 3-h incubation, increased actin fluorescence was noted at the intercalated disc. Vinculin exchange was seen in the intercalated discs, as well as in the Z-bands throughout the cells. Myosin light chains required 4-6 h after injection to become incorporated into the A-bands of the adult muscle. Nonspecific proteins, such as fluorescent BSA, showed no association with the myofibrils or the former intercalated discs. When adult cells were maintained in culture for 10 days, they retain the ability to incorporate these contractile proteins into their myofibrils. T-tubules and the sarcoplasmic reticulum could be detected in periodic arrays in the freshly isolated cells using the membrane dye WW781 and DiOC6[3], respectively. In conclusion, the myofibrils in adult, as in embryonic, muscle cells are dynamic structures, permitting isoform transitions without dismantling of the myofibrils.

Morphological characterization of cardiomyocytes isolated from a transplantable cardiac tumor derived from transgenic mouse atria (AT-1 cells)

We have developed a transplantable tumor lineage derived from transgenic mouse atrial cardiomyocytes that express the SV40 large T oncogene and have named these cardiomyocytes AT-1 cells. In this study, the transplantable tumors, freshly isolated tumor cardiomyocytes, and cultured tumor cardiomyocytes were examined using phase-contrast microscopy, autoradiography, and electron microscopy. The vast majority of the subcutaneous tumor cells, greater than 90% of the cellular mass of the tumor, exhibited sarcomeric banding. Ultrastructural characteristics typical of in vivo atrial cardiac muscle cells, including well-organized myofibrils, gap junctions, and atrial-specific cytoplasmic granules, were observed in in situ and in freshly isolated AT-1 cells. Those cells that did not contain some form of organized myofibrils were primarily vascular elements, such as endothelial cells. Labeling with [3H]thymidine indicated that greater than 90% of cultured AT-1 cells were synthesizing DNA; furthermore, many cells could be seen undergoing cell division. Electron microscopy revealed that the cultured AT-1 cardiomyocytes contained all of the above-described characteristics, including a well-developed transverse tubular system.

Morphometric evaluation of the contractile apparatus in primary cultures of rabbit cardiac myocytes

Rabbit cardiac myocytes remain quiescent for more than 1 month when cultured at low density. During this period, myofibrillar volume density declines sixfold as myofibrils are disassembled or degraded and are replaced by actin and alpha-actinin-positive, myosin-negative structures that resemble myofibrils but lack thick filaments. Such structures are termed minute myofibrils. The length of the sarcomeres in these altered myofibrils is significantly less than length values obtained from freshly isolated heart cells or from contracting myocytes. A number of high density cultures develop spontaneous, synchronous contraction during the second week of culture. Myofibrillar volume density is stabilized when beating begins, and no further decline is observed in the succeeding weeks of culture. Such contracting myocytes display myofibrils typical of normal heart with no visible evidence of minute myofibrils. The volume density of the transverse tubular system also declines significantly in both beating and nonbeating myocytes, and its reduction appears more closely correlated with cell spreading than with beating per se. No quantitative changes in volume density of mitochondria or sarcoplasmic reticulum could be documented, but the structural organization of the sarcoplasmic reticulum seems to be greatly influenced by the physiological state of the heart cell. The present observations document the importance of mechanical factors in regulating the integrity of the contractile apparatus in cardiac myocytes and emphasize the utility of the cultured heart cell to directly investigate structure-function relations in individual myocytes.

Adult cardiac muscle cells in long-term serum-free culture: myofibrillar organization and expression of myosin heavy chain isoforms

A culture system for adult rat cardiac muscle cells has been established without exposure of cells to serum at any step of the procedure. The methodology has been standardized and optimized to obtain better quality and high yield of cells and culture. Subsequent to enzyme perfusion, the release of myocytes from enzyme-perfused tissues was carried out in enzyme-free Joklik's medium instead of exposing cells to proteolytic enzyme(s) as done previously. Approximately 5 million cylindrical muscle cells per ventricle were obtained. The culture medium contained Eagle's minimum essential medium with Earle's salts, basic fibroblast growth factor, epidermal growth factor, insulin, transferrin, selenium, norepinephrine, triiodothyronine (T3), bovine serum albumin, nonessential amino acids, and ascorbic acid. The plating efficiency of the experimental cultures was comparable to that of the control cultures grown in the presence of serum. The cells in the serum-free medium contained myofibrillar and myosin isoforms characteristics of the adult myocytes. The cells underwent cellular reorganization comparable to that of the controls. The initial phase of reorganization involved the breakdown of myofibrils and extrusion of mitochondria, degraded myofibrils, and other cellular organelles. The latter phase of reorganization included myofibrillogenesis and organellogenesis resulting in the development of myofibrillar apparatus with cellular organelles. Myocytes were contractile throughout the culture period. Cardiac myocytes grown in serum-free medium expressed the predominant myosin isoform V1 similar to their counterparts in vivo. T3 is essential for the expression of isomyosin V1.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural morphometric analysis of cultured neonatal and adult rat ventricular cardiac muscle cells

Neonatal and adult rat ventricular cardiac muscle cells cultured on laminin differed from similar myocytes grown on plastic in the amount and distribution of their mitochondria and transverse tubules. Point-count morphometry was used at the electron microscopic level to quantify these differences. Adult myocytes grown on laminin contained more mitochondria per unit volume than adult myocytes grown on plastic. No significant differences were observed in the volume percent of myofibrils in either adult or neonatal ventricular myocytes when grown on laminin and compared to those grown on plastic. The transverse tubule system in neonatal and adult myocytes was reduced significantly when both groups were cultured on laminin. Furthermore, neonatal and adult myocytes cultured on laminin were flatter than those cultured on plastic. This may indicate a relationship between the surface/volume ratio and transverse tubule development in cultured myocytes. These studies establish that point-count morphometry can be used to quantify changes in the organelle volume densities of cultured cardiac muscle cells.

Culture and characterization of fetal human atrial and ventricular cardiac muscle cells

Atrial and ventricular cardiac muscle cells isolated from 14- to 18-wk old fetal human hearts were grown in culture and characterized. Once established in culture the flattened cells contracted spontaneously and possessed differentiated ultrastructural characteristics including organized sarcomeres, intercalated discs, and transverse tubules with couplings. Atrial granules were present in the cultured atrial cells. Some cultured ventricular myocytes also contained electron-dense granules associated with Golgi cisternae, which were similar in size and appearance to atrial granules. The cultured ventricular myocytes divided and expressed the genes for thymidine kinase, histone H4, myosin heavy chain, muscle-specific creatine kinase, atrial natriuretic factor, and insulin-like growth factor II. These results establish that differentiated fetal human heart muscle cells can be cultured in sufficient quantities for biochemical, molecular, and morphological analyses.

Load regulation of the properties of adult feline cardiocytes: growth induction by cellular deformation

Previous studies from this laboratory have demonstrated rapid and reversible changes in cardiac structure, composition, and function in response to load alterations in vivo. The purpose of the present in vitro study was to examine directly in the isolated, quiescent adult cardiocyte the potential growth-regulating effects of load changes through the use of an extremely simple and well-defined cell culture preparation. Freshly isolated cardiocytes were plated onto a deformable, laminin-coated substrate and maintained in serum-free culture medium for 3 days. On the third day in culture, the resting length of these quiescent cardiocytes, and thus their external load, was increased by linear deformation of the substrate to which these cells were firmly adhered. Cardiocyte loading resulted in increases of approximately 10% in cell length, approximately 8% in cell surface area, and approximately 7% in sarcomere length. Three markers of increased synthetic activity were then examined: 1) [3H]uridine incorporation into nuclear RNA, 2) [3H]phenylalanine incorporation into cytoplasmic protein, and 3) [3H]thymidine incorporation into DNA. Cardiocyte loading resulted in mean increases of 186% in nuclear RNA labeling and 89% in cytoplasmic protein labeling. The finding that the increase in [3H]phenylalanine incorporation could be blocked readily by cycloheximide showed that the increase in cytoplasmic labeling in response to cardiocyte loading was not simply the result of increased amino acid transport but instead resulted from the incorporation of label into newly synthesized protein. An absence of [3H]thymidine nuclear incorporation in the loaded cardiocytes indicated that DNA synthesis was not activated in these cells. These data constitute the initial demonstration that an increase in load is at least a sufficient stimulus for the induction of increased RNA and protein synthetic activity in the adult mammalian cardiocyte. This evidence for the role of load as an independent regulator of cardiac growth in the adult suggests that hemodynamic changes may lead directly to appropriate alterations in cardiac structure and composition through the transduction of this physical stimulus into one or more biochemical signals that modulate gene expression.

Primary cell culture and morphological characterization of ventricular myocytes from the adult newt, Notophthalmus viridescens

Previous work has demonstrated that adult newt cardiac myocytes possess a proliferative ability in response to an experimentally induced injury, in vivo. This study describes an in vitro model in which the proliferative events of the adult cardiac myocyte may be studied. Ventricles were minced and then enzymatically dissociated in a Ca++- and MG++-free salt solution containing 0.5% trypsin and 625 U/ml of CLS II collagenase for 8 to 10 hours at 25 degrees C. Enzyme digests were preplated and then cultured on bovine corneal endothelial-derived basement membrane "carpets" in either serum-free or serum-supplemented modified Leibovitz's medium for up to 30 days. Light and transmission electron microscopic characterization demonstrated that a majority of the myocytes underwent an initial period of disorganization characterized by a "rounding up" of the cell and a loss of myofibrillar organization. Once the myocytes had attached to the culture substratum they began to spread out, underwent a reassembly of their contractile elements, resumed spontaneous contractions, and demonstrated ultrastructural evidence of protein synthesis. Mitosis was observed in several myocytes 8 to 15 days following isolation. In 15-day serum-supplemented and serum-free cultures, 6.5% +/- 0.9% and 8.1% +/- 1.4% of the myocytes were binucleated, respectively. These results demonstrate that adult newt ventricular myocytes can be successfully placed into primary culture and are capable of undergoing mitosis. This work may be considered as a foundation for future investigations which will focus on the mechanisms which control cardiac myocyte proliferation.

Light, scanning, and transmission electron microscopy of a single population of detergent-extracted cardiac myocytes in vitro

A technique for performing light, scanning, and transverse transmission electron microscopy on cultured cells grown within a single tissue culture flask is described. Permanent light microscopy slides are obtained by removing selected portions of the plastic tissue culture vessel and mounting them on glass slides with an aqueous mounting solution. The images obtained from these slides are superior to viewing through the bottom of the flask with an inverted stage microscope. For scanning electron microscopy, selected areas are also cut from the remainder of the vessel and prepared for viewing. The final portion of the culture container is transferred and attached to a new tissue culture vessel and prepared for transmission electron microscopy using alcohol instead of acetone and propylene oxide during dehydration, infiltration, and embedding.

Differentiation of adult rat cardiac myocytes in cell culture

Cardiac myocytes isolated from adult rat hearts were grown on laminin coated culture dishes for more than a month. During this time, the cells underwent a morphological transformation which has also been referred to by others as cell remodeling (Guo J-X, Jacobson SL, Brown DL: Cell Mot Cytoskeleton 1986;6:291-304). This results in a change in myocyte morphology from its typical in vivo cylindrical shape to one which is more pleiomorphic. Despite the long-term change in morphology, myocytes expressed for differing lengths of time several aspects of the adult phenotype as evidenced by the following: 1) maintenance of cylindrical shape and/or evident cross-striations for the first 24-48 hours in culture, 2) reappearance of cross-striations during the second week in culture, 3) little or no spontaneous contractility for the first 4 days in culture, 4) expression of only the V1 isoform of myosin for at least 7 days, and 5) altered myosin isoform expression in response to changes in environmental conditions. These factors taken together suggest that in culture the adult cardiac myocyte remains a highly differentiated cell (as opposed to possible dedifferentiation) and maintains many of its previous in vivo characteristics. Such highly differentiated adult cells should be suitable as an in vitro system for studying the direct cellular effects of factors which regulate growth and differentiation of the in vivo heart.

Immunocytochemical analysis of the regeneration of myofibrils in long-term cultures of adult cardiomyocytes of the rat

Dissociated adult rat ventricular cardiomyocytes obtained from hearts by retrograde perfusion with collagenase were investigated in long-term cultures. Myofibril regeneration, isoprotein transition of alpha- and beta-myosin heavy chain (MHC), and M-band localization of M-creatine kinase in the reconstituting heart cells were studied. Myofibril formation was demonstrated by the use of antibodies against either cardiac C-protein or myomesin as early differentiation markers. Four days after plating, small myofibrils could be identified in attached cells in a perinuclear fashion; later in culture the cells displayed various shapes and myofibril distribution. Frequently a patchy distribution of myofibrils within the extending peripheral processes could be observed. Colocalization of sarcomeres and phalloidin-stained F-actin filament bundles was demonstrated by double fluorescence staining and by the use of high intensifying video microscopy and computerized image processing. The immunofluorescence distribution of alpha- and beta-MHC isoproteins in newly isolated and cultured cardiomyocytes changed from 100% alpha-MHC and 70% beta-MHC in rod-shaped cells to about 100% beta-MHC and 70% alpha-MHC in spread out cultured cells. This shift was corroborated by a relative gradual decline in alpha-MHC at the expense of increasing amounts of beta-MHC with time in culture as assessed by sodium dodecyl sulfate gel electrophoresis of total cell homogenates. In addition, whereas rod-shaped newly isolated cardiomyocytes showed a clear M-band association of M-creatine kinase as found in adult heart tissue, adult cultivated spread out cells did not show a cross-striated pattern after incubation with antibody. Taken together, these observations suggest that adult cardiomyocytes not only undergo extensive morphological transitions in long-term cultures, but also generate new myofibrillar structures lacking M-creatine kinase and containing the beta-MHC, thus fitting the characteristics of fetal myofibrils. These results indicate a change from the adult terminally differentiated to a less differentiated state of the cardiac cells in culture.

Attachment and maintenance of adult rabbit cardiac myocytes in primary cell culture

The present observations demonstrate that quiescent calcium-tolerant adult rabbit cardiac myocytes can be isolated by collagenase-hyaluronidase perfusion and maintained in primary culture for at least 2 wk. Culturing large numbers of myocytes requires that the freshly isolated cells be attached to a suitable substratum such as laminin, type IV collagen, or fetal bovine serum. The cultured myocytes retain their rod-like morphology for approximately 7 days before gradually spreading into a flattened conformation by 14 days. During the 1st wk of culture, contaminating interstitial cells rapidly proliferate, making cultures unsuitable for long-term study. Pure myocyte populations can be established and maintained if freshly isolated cells are cultured in the presence of cytosine arabinoside (Ara-C, 10 microM). This antimetabolite does not appear to adversely affect high-energy phosphates, since ATP and creatine phosphate (CrP) content of the myocytes is maintained at levels normally found in biopsy samples of rabbit myocardium. These results illustrate that an energetically stable population of adult cardiac myocytes can be maintained in primary culture in sufficient numbers to make them useful for future investigations of myocyte function.

A controlled silver impregnation method to characterize cultured cardiomyocytes

A morphological characterization of cultured cardiomyocytes was attempted using a modification of a silver impregnation technique originally described for connective tissue. Cardiac cells, obtained from newborn rats and grown as dissociated cultures on plastic surfaces, were fixed in methanol plus 5% glacial acetic acid, treated with potassium permanganate, decolorized in oxalic acid, sensitized with potassium bichromate, impregnated with a silver-ammonium complex, reduced in gelatin-formalin preparation, toned with gold chloride and fixed in sodium thiosulfate. The cultured cardiac cells tended to form a monolayer, although many myocytes remained isolated. Spherical nuclei, sharply stained with silver, were centrally located and surrounded by relatively plentiful cytoplasm packed with well delineated myofibrils. Contaminating fibroblasts were readily distinguished by their spindle-shaped nuclei and the presence of overstained collagen fibers, as well as the absence of myofibrils. In the absence of specific antibody for immunocytochemical identification of cardiomyocytes, morphological characterization of cell type and degree of differentiation by the controlled silver impregnation procedure described here provides a viable alternative, both in short- and long-term studies.

Growth factors and TPA stimulate DNA synthesis and alter the morphology of cultured terminally differentiated adult rat cardiac muscle cells

Previous studies have established that the terminally differentiated ventricular cardiac muscle cell of the adult rat reinitiates semiconservative DNA replication when grown in culture (W. C. Claycomb and H. D. Bradshaw, Jr., 1983, Dev. Biol. 90, 331-337). Work reported here shows that several growth factors and chemicals will stimulate this DNA synthetic activity in a concentration-dependent manner. Autoradiographic experiments establish that this stimulated DNA synthesis is due to cells not previously synthesizing DNA being induced to enter the S phase of the cell cycle. By far the greatest stimulation (250%) is observed with the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Fifty ng/ml is the optimal concentration, and the maximal effect is observed 5 days after adding TPA. TPA also substantially increases the protein content of the cultured myocytes. Diacylyglcerols (DAG) induce these same changes, indicating that the effect of TPA is mediated by protein kinase C. The morphology of the cultured cardiac muscle cells is profoundly altered by TPA and DAG. TPA- and DAG-treated myocytes spread more thinly on the surface of the culture flask, acquire multiple nuclei, and undergo nucleolar fragmentation. The myofibrillar ultrastructure of the treated cells becomes almost totally disorganized, and intermediate filaments and rough endoplasmic reticulum accumulate in the cytoplasm. These TPA results suggest a possible relationship between the degree of ultrastructural differentiation of the ventricular cardiac muscle cell and DNA synthetic activity. This easily altered cellular plasticity should be very useful for studies of the regulation of cardiac muscle cell proliferation and cell differentiation.

Modulation of beta-receptors as adult and neonatal cardiac myocytes progress into culture

Modulation of beta-adrenergic receptors and their ability to respond to beta-receptor stimulation was studied in cultures of adult and neonatal rat cardiac myocytes. The radioligand iodocyanopindolol (125I-CYP) was used to identify beta-adrenoceptors on the intact cells. 125I-CYP was found to bind to the receptors in a stereospecific and saturable manner. Freshly isolated neonatal and adult myocytes both had a receptor density of approximately 50 fmol/mg protein. The number of beta-receptors per milligram protein was similar during a 10-d culture period for adult myocytes but increased after a 5-d culture period for neonatal myocytes. Both cell types responded to beta-receptor stimulation with isoproterenol by a twofold increase in the concentration of cAMP and this response increased with time in culture. The number of receptors as well as the response to isoproterenol was similar for neonatal myocytes cultured on laminin, collagen type I, or on uncoated culture dishes. From these data we conclude that cultured cardiac myocytes maintain functional beta-receptors as they progress into culture, and the expression of beta-receptors is not influenced by culture substrates.

Rearrangement of tubulin, actin, and myosin in cultured ventricular cardiomyocytes of the adult rat

Antitubulin, phalloidin, and antimyosin were used to study the distribution of microtubules, microfilaments, and myofibrils in cultured adult cardiomyocytes. These cells undergo a stereotypic sequence of morphological change in which myotypic features are lost and then reconstructed during a period of polymorphic growth. Microtubules, though rearranged during these events in culture, are always present in an organized network. Myosin and actin structures, on the other hand, initially degenerate. This initial degeneration is reversed when a cell attaches to the culture substratum. Upon attachment, new microtubules are laid down as a cortical network adjacent to the sarcolemma and, subsequently, as a network in the basal part of the cell. Actin and then myosin filament bundles appear next, in a pattern corresponding to the pattern of the microtubules. Finally, striated myofibrils are formed, first in the central part of the cell, and subsequently in the outgrowing processes of the cell. A mechanism is suggested by which the eventual polymorphic shape of a cell is related to the shape of its initial area of contact with the culture substratum. Finally, a model of myofibrillogenesis is proposed in which microtubules participate in the insertion of myosin among previously formed actin filament bundles to produce myofibrils.

Culture of atrial and ventricular cardiac muscle cells from the adult squirrel monkey Saimiri sciureus

Atrial and ventricular cardiac muscle cells isolated from the adult squirrel monkey Saimiri sciureus were cultured and characterized by light and transmission electron microscopy (TEM). Freshly isolated cells were striated and cylindrical in shape and, when placed in culture, rounded up and lost their highly organized morphology. With prolonged time in culture, they spread out on the surface of the culture flask and reacquired many of the internal ultrastructural characteristics of their in vivo atrial and ventricular cardiac muscle cell counterparts. Autoradiographic experiments indicated that both atrial and ventricular myocytes synthesized DNA when grown in culture. In some binucleated atrial cells only one nucleus became labelled. These studies show that it is now possible to culture cardiac muscle cells isolated from an adult primate.

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.

Secretory patterns of atrial natriuretic factor (ANF) by cultured cardiocytes of right and left atrium from newborn and adult rats

Atrial cardiocytes from newborn (2-5 day old) and adult rats were cultured and the secretory patterns of atrial natriuretic factor (ANF) from isolated right and left atrial cells were investigated by radioimmunoassay. Newborn atrial cardiocytes from the left atrium consistently secreted larger amounts of ANF than those from the right with a peak on the 6th day and a decrease up to the 12th day. In contrast, adult atrial cardiocytes secreted much less ANF and this decreased to very low levels from the 3rd day up to the 12th day in culture although ANF was present in measurable amounts in these cells.

Ultrastructure of cultured atrial cardiac muscle cells from adult rats

Atrial cardiac muscle cells enzymatically isolated from adult rats were maintained in culture for 0-17 days and examined by transmission electron microscopy (TEM). Cells were stained with conventional TEM stains as well as with osmium ferrocyanide and tannic acid. Our results show that cultured adult atrial cells are capable of in vitro ultrastructural reorganization and possess differentiated ultrastructural characteristics including specific atrial granules, sarcomerically arranged myofilaments, appropriately organized sarcoplasmic reticulum (both junctional and nonjunctional), and intercalated disc components. In addition, the cultured atrial cells also possess rare, but ultrastructurally typical, elements of the transverse tubular system. These can be identified on the basis of size, location, association with internal junctional sarcoplasmic reticulum, and accumulation of extracellular tracer. Atrial muscle cells are capable of reestablishing a myotypic ultrastructure, although they have a considerably less complex and organized in vitro ultrastructure than similarly cultured adult ventricular myocytes. This lessened in vitro ultrastructural specialization is in accord with the in vivo comparative ultrastructure of atrial vs. ventricular myocytes.

Isolation and culture of the terminally differentiated adult mammalian ventricular cardiac muscle cell

We have carried out systematic studies to optimize and standardize methodology to isolate and culture the adult rat ventricular cardiac muscle cell. Four hearts were perfused simultaneously with a calcium-free medium containing collagenase. The ventricular tissue was then minced and further digested to liberate individual cells. Approximately 16 million rod-shaped muscle cells were obtained. The plating efficiency has been greatly improved by culturing the cells in a conditioned medium prepared from a rabbit corneal cell line. This medium also contained added fetal bovine serum, essential and nonessential amino acids, vitamins, insulin, transferrin, and 25 trace minerals. The culture flasks were precoated with rat-tail collagen. Fibroblast contamination was virtually eliminated by including cytosine arabinoside in the medium during the first 7 d of culture. After this time the cells could be cultured in the absence of serum in a chemically defined medium composed of MEM, vitamins, nonessential amino acids, and trace minerals. They continued to contract spontaneously and do well in this medium for at least 3 d thereafter. This improved methodology resulted in a reproducible culture system with improved plating efficiency. It provided a new and unique system to study the structure and function of the adult mammalian ventricular cardiac muscle cell.

Culture of the terminally differentiated ventricular cardiac muscle cell. Characterization of exogenous substrate oxidation and the adenylate cyclase system

Oxidation of several exogenous substrates by cultured adult rat ventricular cardiac muscle cells has been assessed. Unlike freshly isolated cardiac muscle cells which oxidize glucose preferentially, the cultured cells more closely resemble metabolically the in situ heart and the isolated perfused heart, in that their preference for exogenous substrates is in the order of fatty acid greater than glucose. This switch in metabolic preference from glucose to fatty acid is complete within 12 h after placing freshly isolated cells in culture. Glucose oxidation is stimulated by insulin and isoproterenol and inhibited by beta-hydroxybutyrate and octanoate. The adenylate cyclase system has also been examined in these cultured cells. Isoproterenol, norepinephrine and epinephrine stimulate the accumulation of cyclic adenosine 3':5'-monophosphate (cyclic AMP) in a concentration-dependent manner. The order of potency is isoproterenol greater than norepinephrine approximately equal to epinephrine. This stimulation is potentiated by 1-isobutyl-3-methylxanthine and inhibited by 1-propranolol.

Acquisition of multiple nuclei and the activity of DNA polymerase alpha and reinitiation of DNA replication in terminally differentiated adult cardiac muscle cells in culture

Terminally differentiated ventricular cardiac muscle cells isolated from the adult rat and maintained in cell culture have been observed to acquire multiple nuclei. In one cultured myocyte as many as 10 nuclei have been counted. Apparently, these multiple nuclei are formed by DNA replication followed by karyokinesis; the cells must then fail to complete mitosis and divide. To investigate whether DNA synthesis was occurring, the cells were cultured in the presence of [3H]thymidine and then processed for autoradiography. Mononucleated, binucleated, and multinucleated cells incorporate [3H]thymidine into DNA as evidenced by the high concentration of silver grains over their nuclei. Peak periods of incorporation were observed to occur at 10- to 12-day intervals; at 11, 23, and 33 days after initially placing the cells in culture. When the cells were maintained in the presence of [3H]thymidine continuously from Day 7 to Day 17 of culture, 23% of the cells became labeled. If the cells were cultured continuously for 30 days in the presence of [3H]thymidine, from Day 10 to Day 40, 56% of the cells were labeled. Isopycnic gradient analysis indicates that this thymidine incorporation was into DNA that was being replicated semiconservatively; these experiments did not eliminate the possibility, however, that this incorporation was due to amplification of specific genes, such as those coding for the contractile proteins. The activity of DNA polymerase alpha also returns to these cells. These studies demonstrate that the terminally differentiated mammalian ventricular cardiac muscle cell, previously thought to have permanently lost the capacity to replicate DNA during early development, is able to reinitiate semiconservative DNA replication when grown in culture.

Specific attachment of desmin filaments to desmosomal plaques in cardiac myocytes

Intercellular junctions which are similar in ultrastructure and protein composition to typical desmosomes have so far only been found in epithelial cells and in heart tissue, specifically in the intercalated disks of cardiac myocytes and at cell boundaries between Purkinje fiber cells. In epithelial cells the cytoplasmic side of desmosomes, the 'desmosomal plaque', represents a specific attachment structure for the anchorage of intermediate filaments (IF) of the cytokeratin type. Cardiac myocytes do not contain cytokeratin filaments. In primary cultures of rat cardiac myocytes, we have examined by immunofluorescence and electron microscopy, using single and double label techniques, whether other types of IF are attached to the desmosomal plaques of the heart. Antibodies to desmoplakin, the major protein of the desmosomal plaque, have been used to label specifically the desmosomal plaques. It is shown that the desmoplakin-containing structures are often associated with IF stained by antibodies to desmin, i.e., the characteristic type of IF present in these cells. Like cytokeratin filaments in epithelial cells, desmin filaments attach laterally to the desmosomal plaque. They also remain attached to these plaques after endocytotic internalization of desmosomal domains by treatment of the cells with EGTA. These desmin filaments do not appear to attach to junctions of the fascia adherens type and to nexuses (gap junctions). These observations show that anchorage at desmosomal plaques is not restricted to IF of the cytokeratin type and that IF composed of either cytokeratin or desmin, specifically attach, in a lateral fashion, to desmoplakin-containing regions of the plasma membrane. We conclude that special domains exist in these two IF proteins that are involved in binding to the desmosomal plaque.

Desmoplakins of epithelial and myocardial desmosomes are immunologically and biochemically related

Guinea pig antibodies against desmoplakins from bovine muzzle epidermis showed specific reaction in several epithelial tissues with desmoplakin I (Mr 250,000) and desmoplakin II (Mr 215,000). By immunofluorescence microscopy, prominent punctate staining was observed in various lines of cultured epithelial cells, revealing desmosomal junctions at sites of established cell-to-cell contacts as well as hemidesmosomes and internalized desmosome-derived membrane domains. On frozen tissue sections punctate staining was observed along plasma membranes of epithelial cells, and electron microscopy using the immunoperoxidase technique revealed that the antibodies were specifically localized at the plaques associated with desmosomes and hemidesmosomes. Of a large number of non-epithelial cells examined positive staining was only observed on desmosome-like junctions of myocardial cells and Purkinje fiber cells. In both epithelial and myocardial tissues the antibodies showed a broad range of cross-reactivity between diverse vertebrate species such as man, cow, rodent, and chicken, indicating that desmoplakins contain determinants strongly conserved during evolution. When binding of these antibodies to cytoskeletal polypeptides separated by gel electrophoresis and blotted on nitrocellulose paper sheets was examined, specific reaction was noted with desmoplakin I and, to a variable degree, also desmoplakin II from various epithelial cells. Reaction was also observed with a myocardial polypeptide from bovine and human hearts which had a similar Mr value (250,000) and isoelectric pH range as desmoplakin I. We conclude that desmoplakins are the major proteins present in the desmosomal plaques of both epithelial and myocardial cells and that the desmoplakin polypeptides present in these two different cell types are very similar, if not identical.