Immunofluorescent Visualization of 100 Å Filaments in Different Cultured Chick Embryo Cell Types

Antibody prepared against the 55,000 dalton subunit of reconstituted chick gizzard 100 A filaments (anti-G55K) bound to the 100 Å filaments of chick smooth muscle, cardiac muscle, and skeletal muscle cells, and to the 100 Å filaments of Schwann cells and satellite glial cells of the peripheral nervous system. Anti-G55K did not bind to replicating presumptive myoblasts, fibroblasts, chondroblasts, pigment cells, neurons, or to central nervous system glial cells. This contrasted with the wider range of binding of antibody to the 58,000 dalton subunit of chick fibroblast 100 A filaments (anti-F58K) which bound to the 100 Å filaments of all cell types examined except hepatocytes and skin epithelial cells. Anti-G55K staining revealed a morphologically distinct distribution of 100 A filaments in the three types of muscle cells. Spindle shaped smooth muscle cells exhibited dense fluorescent staining near the poles of the cells, and also exhibited unique patches of fluorescent material after cytochalasin B and Colcemid treatment. In myotubes, the fluorescence was limited to longitudinal bundles of filaments between the striated myofibrils. Cardiac cells contained uniformly distributed fine filaments. Lastly, smooth muscle cells in various phases of mitosis bound the anti-G55K, whereas replicating presumptive skeletal myoblasts failed to bind the anti-G55K.     

Selective binding of antibody against gizzard 10-nm filaments to different cell types in myogenic cultures.

Antibody against the intermediate-sized filaments from gizzard smooth muscle was used to determine the presence or absence of reacting 10-nm filaments in different cell types. The antibody against gizzard 10-nm filaments reacted with filaments in cultured smooth muscle cells, skeletal myotubes and postmitotic skeletal myoblasts. It did not bind to the 10-nm filaments present in replicating presumptive myoblasts and fibroblasts, or the 10-nm filaments in spinal ganglion cells.     

Biochemical and immunological heterogeneity of 100 A filament subunits from different chick cell types.

The 100 A filament subunit proteins of chick fibroblasts and gizzard smooth muscle were compared. These proteins are major cellular components in these cell types, constituting up to 98% of the cell's total protein. Co-electrophoresis of cytoskeletal fractions of fibroblasts and smooth muscle revealed that the subunit proteins differed in their molecular weights: 58,000 daltons in fibroblasts and 55,000 daltons in smooth muscle. Cytoskeletal fractions from other cell types were also examined: chondroblasts contained the 58,000 dalton subunit, and cytoskeletons of skeletal muscle and cardiac muscle contained both 55,000 and 58,000 dalton proteins. Chick skin and rat kangaroo Pt K2 cells had more complex subunit patterns which resemble prekeratin. The peptide patterns resulting from proteolytic digestion of the 58,000 dalton protein of fibroblasts, the 55,000 dalton proteins of smooth muscle and PT K2 cells, and chick brain tubulin differed from one another. Two-dimensional electrophoresis of reconstituted gizzard smooth muscle 100 A filaments showed the 55,000 dalton subunit to be composed of two major components, differing in their isoelectric points. Antibodies prepared against electrophoretically purified 55,000 dalton subunit protein reacted in immunodiffusion against the original smooth muscle antigen and cytoskeletal fractions from skeletal and cardiac muscle, but not from fibroblasts, brain, liver, or skin cells. A specific antigenic determinant common to subunit proteins in smooth, skeletal, and cardiac muscle, is therefore indicated. A previously described antibody against fibroblast subunit protein reacted weakly against smooth muscle filament protein in immunodiffusion revealing the presence of a common antigenic determinant between the two subunit proteins. These data demonstrate striking antigenic and primary structural differences in 100 A filament subunits from even such closely related cell types as fibroblasts on the one hand and muscle cells on the other.     

Redistribution of intermediate filament subunits during skeletal myogenesis and maturation in vitro

The distribution of intermediate filament (IF) subunits during maturation of skeletal myotubes in vitro was examined by immunofluorescence, using antibodies against two different types of chick IF subunits: (a) 58-kdalton subunits of fibroblasts (anti-58K), and (b) 55-kdalton subunits of smooth muscle (anti-55K). Anti-58K bound to a filament network in replicating presumptive myoblasts and fibroblasts, as well as in immature myotubes. The distribution in immature myotubes was in longitudinal filaments throughout the cytoplasm. With maturation, staining of myotubes by anti-58K diminished and eventually disappeared. Anti-55K selectively stained myotubes, and the fluorescence localization underwent a drastic change in distribution with maturation--from dense, longitudinal filaments in immature myotubes to a cross-striated distribution in mature myotubes that was associated with the I--Z region of myofibrils. However, the emergence of a cross-striated anti-55K pattern did not coincide temperally with the emergence of striated myofibrils, but occurred over a period of days thereafter.     

Mitosis and intermediate-sized filaments in developing skeletal muscle

A new class of filaments intermediate in diameter between actin and myosin filaments has been demonstrated in skeletal muscle cells cultured from chick embryos. These filaments, which account for the majority of free filaments, average 100 A in diameter. They may run for more than 2 µ in a single section and can be distinguished in size and appearance from the thick and thin filaments assembled into myofibrils. The 100-A filaments are seen scattered throughout the sarcoplasm at all stages of development and show no obvious association with the myofibrils. The 100-A filaments are particularly conspicuous in myotubes fragmented by the mitotic inhibitors, colchicine and Colcemid. In addition, filaments similar in size and appearance to those found in myotubes are present in fibroblasts, chondrocytes, and proliferating mononucleated myoblasts. The 100-A filaments are present in cells arrested in metaphase by mitotic inhibitors. Definitive thick (about 150 A) or thin (about 60 A) myofilaments are not found in skeletal myogenic cells arrested in metaphase. Myogenic cells arrested in metaphase do not bind fluorescein-labeled antibody directed against myosin or actin. For these reasons, it is concluded that not all "thin" filaments in myogenic cells are uniquely associated with myogenesis.     

Specificity of desmin to avian and mammalian muscle cells.

The extent of invariance and heterogeneity in desmin, the major component of the muscle form of 100 Å filaments, has been investigated in avian and mammalian muscle and nonmuscle cells with two-dimensional gel electrophoresis and indirect immunofluorescence. Desmin from chick, duck and quail, smooth, skeletal and cardiac muscle cells is resolved into two isoelectric variants, α and β, with each possessing the same charge and electrophoretic mobility in all three avian species irrespective of muscle type. Guinea pig and rat muscle desmin resolves into only one variant; it also possesses the same charge and electrophoretic mobility in the two mammalian species, but it is more acidic and slower in electrophoretic mobility than the two avian variants.In immunofluorescence, desmin is localized together with α-actinin along myofibril Z lines. Antibodies to chick smooth muscle desmin, prepared against the protein purified by preparative SDS gel electrophoresis prior to immunization, cross-react with myofibril Z lines in all three avian species. These antibodies do not cross-react with either rat or guinea pig myofibril Z lines. Similarly, they do not cross-react with avian or mammalian nonmuscle cells grown in tissue culture and known to contain cytoplasmic 100 Å filaments.These results demonstrate that desmin is highly conserved within avian muscle cells and within mammalian muscle cells. It is, however, both biochemically and immunologically distinguishable between avian and mammalian muscle cells, and between muscle and nonmuscle cells. We conclude that there are biochemically and immunologically specific forms of desmin for avian and mammalian muscle cells. Furthermore, within a particular vertebrate species, there are at least two separate classes of 100 Å filaments: the muscle class whose major component is desmin, and the nonmuscle class whose major component is distinct from desmin. Taking into consideration the immunological specificity reported by other laboratories for the 100 Å filaments in glial cells, for neurofilaments and for the epidermal 80 Å keratin filaments, we propose that a given vertebrate species contains at least four major distinguishable classes of 100 Å filaments: muscle 100 Å filaments (desmin filaments), glial filaments, neurofilaments and epidermal keratin filaments.     

The Localization of Skeletal Light Meromyosin in Cells of Myogenic Cultures

Fluorescent antibodies against skeletal light meromyosin were used to study the localization of this muscle-specific antigen in myotubes, myoblasts, presumptive myoblasts and fibroblasts found in six-day myogenic cultures. The labelled antibody bound only to the lateral edges of the A-bands in myofibrils. The antibody did not bind to antigens in the nucleus, cytoplasm or in the microfilaments beneath the plasmalemma in any of the cell types examined. Similarly, the external face of the cell surface of unfixed, living myotubes and mononucleated cells did not bind the antibody. Immunodiffusion tests confirm these results: high salt extracts of myotube-containing cultures reacted against anti-skeletal light meromyosin, whereas extracts of fibroblasts and presumptive myoblast cultures failed to precipitate the antibody. It is proposed that if myosin is present in the plasmalemma of these cells, as is suggested by the work of others, it is immunologically distinct from that present in the myofibrils of definitive muscle.     

Differential expression and distribution of chicken skeletal- and smooth-muscle-type alpha-actinins during myogenesis in culture

Antibodies to chicken fast skeletal muscle (pectoralis) alpha-actinin and to smooth muscle (gizzard) alpha-actinin were absorbed with opposite antigens by affinity chromatography, and four antibody fractions were thus obtained: common antibodies reactive with both pectoralis and gizzard alpha-actinins ([C]anti-P alpha-An and [C]anti-G alpha-An), antibody specifically reactive with pectoralis alpha-actinin ([S]anti-P alpha-An), and antibody specifically reactive with gizzard alpha-actinin ([S]anti-G alpha-An). In indirect immunofluorescence microscopy, (C)anti-P alpha-An, (S)anti-P alpha-An, and (C)anti-G alpha- An stained Z bands of skeletal muscle myofibrils, whereas (S)anti-G alpha-An did not. Although (S)anti-G alpha-An and two common antibodies stained smooth muscle cells, (S)anti-P alpha-An did not. We used (S)anti-P alpha-An and (S)anti-G alpha-An for immunofluorescence microscopy to investigate the expression and distribution of skeletal- and smooth-muscle-type alpha-actinins during myogenesis of cultured skeletal muscle cells. Skeletal-muscle-type alpha-actinin was found to be absent from myogenic cells before fusion but present in them after fusion, restricted to Z bodies or Z bands. Smooth-muscle-type alpha- actinin was present diffusely in the cytoplasm and on membrane- associated structures of mononucleated and fused myoblasts, and then confined to membrane-associated structures of myotubes. Immunoblotting and peptide mapping by limited proteolysis support the above results that skeletal-muscle-type alpha-actinin appears at the onset of fusion and that smooth-muscle-type alpha-actinin persists throughout the myogenesis. These results indicate (a) that the timing of expression of skeletal-muscle-type alpha-actinin is under regulation coordination with other major skeletal muscle proteins; (b) that, with respect to expression and distribution, skeletal-muscle-type alpha-actinin is closely related to alpha-actin, whereas smooth-muscle-type alpha- actinin is to gamma- and beta-actins; and (c) that skeletal- and smooth- muscle-type alpha-actinins have complementary distribution and do not co-exist in situ.     

Immunocytochemical evidence for the presence of an albumin-like substance in the rat pineal gland

Summary Two rabbits and two guinea pigs were immunized with arginine vasotocin (AVT) conjugated to bovine albumin with glutaraldehyde. Only one preparation of antiserum (anti-G 7) was of value. Anti-G 7 immunochemically defined the same rat pineal cells previously reported as presumptive AVT cells. However, absorption of anti-G 7 with bovine albumin inhibited the staining of the pineal cells demonstrating that they contained an albumin-like substance. Positive immunochemical staining of the rat pars nervosa suggested that anti-G 7 contained antibodies able to react with arginine vasopressin (AVP). Loss of a positive reaction in the posterior lobe on absorption of anti-G 7 with AVT or AVP confirmed this. However, the addition of AVT to anti-G 7 failed to inhibit the immunochemical staining of the pineal cells. This study reports the presence of an albumin-like substance in pineal cells previously described as presumptive AVT cells, and discusses possible explanations for the inability of anti-G 7 to recognize immunocytochemically the native AVT molecule. Confirmation of AVT in the pineal gland by immunocytochemistry must await the availability of more specific antisera.     

Myosin accumulation in mononucleated cells of chick muscle cultures.

The biosynthesis and accumulation of the myosin heavy chain (MHC) peptide has been examined in embryonic chick skeletal muscle cultures under conditions of normal or arrested cell fusion. When compared with primary chick fibroblasts, the myogenic cells accumulated significantly more MHC, even while mononucleated. Electron microscopy of the fusion-blocked cultures revealed the presence of myosinlike thick filaments in the myoblasts. It is concluded that cell fusion is not a prerequisite for myosin accumulation or myofilament assembly during embryonic chick muscle differentiation.     

Involvement of transglutaminase in myofibril assembly of chick embryonic myoblasts in culture

Involvement of transglutaminase in myofibrillogenesis of chick embryonic myoblasts has been investigated in vitro. Both the activity and protein level of transglutaminase initially decreased to a minimal level at the time of burst of myoblast fusion but gradually increased thereafter. The localization of transglutaminase underwent a dramatic change from the whole cytoplasm in a diffuse pattern to the cross- striated sarcomeric A band, being strictly colocalized with the myosin thick filaments. For a brief period prior to the appearance of cross- striation, transglutaminase was localized in nonstriated filamental structures that coincided with the stress fiber-like structures. When 12-o-tetradecanoyl phorbol acetate was added to muscle cell cultures to induce the sequential disassembly of thin and thick filaments, transglutaminase was strictly colocalized with the myosin thick filaments even in the myosacs, of which most of the thin filaments were disrupted. Moreover, monodansylcadaverine, a competitive inhibitor of transglutaminase, reversibly inhibited the myofibril maturation. In addition, myosin heavy chain behaved as one of the potential intracellular substrates for transglutaminase. The cross-linked myosin complex constituted approximately 5% of the total Triton X-100- insoluble pool of myosin molecules in developing muscle cells, and its level was reduced to below 1% upon treatment with monodansylcadaverine. These results suggest that transglutaminase plays a crucial role in myofibrillogenesis of developing chick skeletal muscle.     

Induction of muscle genes in neural cells

The regulation of skeletal muscle genes was examined in heterokaryons formed by fusing differentiated chick skeletal myocytes to four different rat neural cell lines. Highly enriched populations of heterokaryons isolated using irreversible biochemical inhibitors were labeled with [35S]methionine and analyzed on two-dimensional gels. Rat skeletal myosin light chains were induced in three of the four cell combinations. The one exception, the S-20 cholinergic cell line, not only failed to synthesize rat muscle proteins but also suppressed chick myogenic functions. Experiments with heterokaryons between chick myocytes and cells from whole embryonic rat brain cultures demonstrated that rat skeletal myosin light chains are inducible in normal diploid neural cells as well as in established neural cell lines. In contrast, dividing cell hybrids between rat myoblasts and rat glial cells were nonmyogenic. These results demonstrate that although neural cells may contain factors that prevent the decision to differentiate along myogenic lines in cell hybrids, most neural cell lines do not dominantly suppress the expression of muscle structural genes in heterokaryons. Furthermore, the skeletal myosin light chain genes in most neural cell lines are regulated by a mechanism that permits them to respond to putative chick skeletal myocyte-inducing factors. The "open" state of these myogenic genes may explain many of the reports of apparent "transdifferentiation" to muscle in neural cultures and neural tumors.     

Synthesis of rat myosin light chains in heterokaryons formed between undifferentiated rat myoblasts and chick skeletal myocytes

The control of gene expression during terminal myogenesis was explored in heterokaryons between differentiated and undifferentiated myogenic cells by analyzing the formation of species specific myosin light chains of chick and rat skeletal muscle. Dividing L6 rat myoblasts served as the biochemically undifferentiated parent. The differentiated parental cells were mononucleated muscle cells (myocytes) that were obtained from primary cultures of embryonic chick thigh muscle by blocking myotube formation with EGTA and later incubating the postimitotic cells in cytochalasin B. Heterokaryons were isolated by the selective rescue of fusion products between cells previously treated with lethal doses of different cell poisons. 95-99% pure populations of heterokaryons formed between undifferentiated rat myoblasts and differentiated chick myocytes were obtained. The cells were labeled with [35S]methionine, and whole cell extracts were analyzed on two-dimensional polyacrylamide gels. These heterokaryons synthesize the light chain of chick myosin and both embryonic and adult light chains of rat skeletal myosin. Control homokaryons formed by fusing undifferentiated cells to themselves did not synthesize skeletal myosin light chains. Control heterokaryons formed between undifferentiated rat myoblasts and chick fibroblasts also failed to synthesize myosin light chains. These results indicate that differentiated chick muscle cells provide some factor that induces L6 myoblasts to synthesize rat myosin light chains. This system provides a model for investigating the processes by which differentiated cell functions are induced.     

Chick brain actin and myosin. Isolation and characterization

Brain actin extracted from an acetone powder of chick brains was purified by a cycle of polymerization-depolymerization followed by molecular sieve chromatography. The brain actin had a subunit molecular weight of 42,000 daltons as determined by co-electrophoresis with muscle actin. It underwent salt-dependent g to f transformation to form double helical actin filaments which could be "decorated" by muscle myosin subfragment 1. A critical concentration for polymerization of 1.3 microM was determined by measuring either the change in viscosity or absorbance at 232 nm. Brain actin was also capable of stimulating the ATPase activity of muscle myosin. Brain myosin was isolated from whole chick brain by a procedure involving high salt extraction, ammonium sulfate fractionation and molecular sieve chromatography. The purified myosin was composed of a 200,000-dalton heavy chain and three lower molecular weight light chains. In 0.6 M KCl the brain myosin had ATPase activity which was inhibited by Mg++, stimulated by Ca++, and maximally activated by EDTA. When dialyzed against 0.1 M KCl, the brain myosin self-assembled into short bipolar filaments. The bipolar filaments associated with each other to form long concatamers, and this association was enhanced by high concentrations of Mg++ ion. The brain myosin did not interact with chicken skeletal muscle myosin to form hybrid filaments. Furthermore, antibody recognition studies demonstrated that myosins from chicken brain, skeletal muscle, and smooth muscle were unique.     

Isoproterenol induces an increase in muscle fiber size by the proliferation of Pax7‐positive cells and in a mTOR‐independent mechanism

β‐Adrenergic signaling regulates many physiological processes in skeletal muscles. A wealth of evidence has shown that β‐agonists can increase skeletal muscle mass in vertebrates. Nevertheless, to date, the specific role of β‐adrenergic receptors in different cell phenotypes (myoblasts, fibroblasts, and myotubes) and during the different steps of embryonic skeletal muscle differentiation has not been studied. Therefore, here we address this question through the analysis of embryonic chick primary cultures of skeletal muscle cells during the formation of multinucleated myotubes. We used isoproterenol (ISO), a β‐adrenergic receptor agonist, to activate the β‐adrenergic signaling and quantified several aspects of muscle differentiation. ISO induced an increase in myoblast proliferation, in the percentage of Pax7‐positive myoblasts and in the size of skeletal muscle fibers, suggesting that ISO activates a hyperplasic and hypertrophic muscle response. Interestingly, treatment with ISO did not alter the number of fibroblast cells, suggesting that ISO effects are specific to muscle cells in the case of chick myogenic cell culture. We also show that rapamycin, an inhibitor of the mammalian target of rapamycin signaling pathway, did not prevent the effects of ISO on chick muscle fiber size. The collection of these results provides new insights into the role of β‐adrenergic signaling during skeletal muscle proliferation and differentiation and specifically in the regulation of skeletal muscle hyperplasia and hypertrophy.     

Na(+)/K(+) pump expression in the L8 rat myogenic cell line: effects of heterologous alpha subunit transfection

We have characterized the physiological and biochemical properties of the Na(+)/K(+) pump and its molecular expression in L8 rat muscle cells. Pump properties were measured by [(3)H]ouabain binding and (86)Rb uptake. Scatchard plot analysis of specific ouabain binding indicated the presence of a single family of binding sites with a B(max) of approximately 135 fmol/ mg P and a K(D) of 3.3 x 10(-8). (86)Rb uptake due to specific pump activity was found to be 20% of the total in L8 cells. The results indicated lower affinity of L8 cells for ouabain and lower activity of the pump than that reported for chick or rat skeletal muscle in primary culture. Both the alpha(1) and beta(1) protein and mRNA isoforms were expressed in myoblasts and in myotubes, while the alpha(2), alpha(3), and beta(2) isoforms were not detectable. We attempted to overcome low physiological expression of the Na(+)/K(+) pump by employing a vector expressing an avian high affinity alpha subunit. This allowed identification of the transfected subunit separate from that endogenously expressed in L8 cells. Successful transfection into L8 myoblasts and myotubes was recognized by anti-avian alpha subunit monoclonal antibodies. Fusion index, Na(+)/K(+) pump activity, and the level of the transmembrane resting potential were all significantly greater in transfected L8 (tL8) cells than in non-tL8. The total amount of alpha subunit (avian and rat) in tL8 cells was greater than that (only rat) in non-tL8 cells. This relatively high abundance of the Na(+)/K(+) pump in transfected cells may indicate that avian and rat alpha subunits hybridize to form functional pump complexes.     

Intracellular filament bundles in whole mounts of chick and human myoblasts extracted with Triton X-100

The method of Triton X-100 extraction and critical point drying of whole mounts of cultured chick and human myoblasts was used to study the presence of intracellular bundles of filaments within these cells. Observation by means of transmission and scanning electron microscopy demonstrated a complex system of filament bundles which appeared morphologically and spatially heterogeneous. Most obvious were long dense bundles or cables traversing along the ventral surface of developing myoblasts, presumably the ‘stress fibers’ seen in light microscopy. Other bundle types occurred which were composed of loose aggregates of filaments coursing through the remnant cell body. A prominent accumulation of filaments was also seen at the lateral edges of these myoblasts. These lateral edge cables were thicker and denser than any other type of filament bundle observed in the myoblasts. Reaction of unextracted myoblasts directly to human antiplatelet myosin conjugated to rhodamine demonstrated that the most intense reaction also occurred along the lateral edges of both human and chick myoblasts. During development of chick myoblasts the filament bundles became oriented parallel to the cell axis giving the cell a fusiform morphology. It is possible that the various filament bundle structures and their differing structural and spatial dispositions could be related to functional differences among the diverse population of intracellular bundles of filaments.     

In situ reconstitution of myosin filaments within the myosin-extracted myofibril in cultured skeletal muscle cells

We studied the in situ reconstitution of myosin filaments within the myosin-extracted myofibrils in cultured chick embryo skeletal muscle cells using the electron microscope and polarization microscope. Myosin was first extracted from the myofibrils in glycerinated muscle cells with a high-salt solution containing 0.6 M KCl. When rabbit skeletal muscle myosin was added to the myosin-extracted cells in the high-salt solution, thin filaments in the ghost myofibrils were bound with myosin to form arrowhead complexes. Subsequent dilution of KCl in the myosin solution to 0.1 M resulted in the formation of thick myosin filaments within the myofibrils, increasing the birefringence of the myofibrils. When Mg-ATP was added such myosin-reassembled myofibrils were induced either to form supercontraction bands or to restore the sarcomeric arrangement of thick and thin filaments. Under the polarization microscope, vibrational movement of the myofibrils was seen transiently upon addition of Mg-ATP, often resulting in a regular arrangement of myofibrils in register. These myofibrils, with reconstituted myosin filaments, structurally and functionally resembled the native myofibrils. The findings are discussed with special reference to the myofibril formation in developing muscle cells.     

Properties of a clonal muscle cell line from rat heart

Two clonal cell lines have been derived from the thoracic aorta of embryonic rats. Both of these cell lines, at some stage of their development possess membranes capable of generating overshooting action potentials spontaneously. Contiguous cells of each of these lines are electrically coupled. Ultrastructural analysis consistently reveals the presence of rows of pinocytotic vesicles, a well-developed rough endoplasmic reticulum, massive tracts of thin filaments oriented parallel to the longitudinal axis of the cell and randomly dispersed intermediate sized filaments. The specific activities of the enzymes myokinase and creatine phosphokinase (CPK) increase 3- to 5-fold after growth has ceased. These two cell lines synthesize a muscle type CPK isoenzyme after the cessation of cell division. It is concluded that these cell lines proliferate as myoblasts and develop into cells which phenotypically resemble smooth muscle. A third clonal cell line, from fetal rat aorta, with properties of both smooth and skeletal muscle, is also described.     

Photoaffinity labelling of a 33-35,000 dalton protein in cardiac, skeletal and smooth muscle membranes using a new 125I-labelled 1,4-dihydropyridine calcium channel antagonist

The binding sites for Ca2+ channel antagonists were probed using Bay P 8857 [2-iodoethyl isopropyl 1,4-dihydropyridine-2,6-dimethyl-4-(3-nitrophenyl)-pyridine-3,5-dicarbox ylate] that has been radiolabelled with 125I. This drug was shown to bind with high affinity to cardiac, smooth, and skeletal muscle membranes, with a KD approximately equal to 0.3 nM. A protein of molecular weight 33-35,000 daltons was specifically and irreversibly radiolabelled after irradiation of cardiac, skeletal and aortic smooth muscle membranes, incubated with the [125I]-Bay P 8857. The peptide labelled by 1,4-dihydropyridine binding therefore appears similar in size for cardiac, skeletal, and smooth muscle. This data suggests that of the three peptide subunits which reportedly comprise the skeletal and cardiac muscle 1,4-dihydropyridine receptor complex, the 33-35,000 dalton peptide contains the dihydropyridine binding site.