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C Franzini-Armstrong

Publications and source records attributed to C Franzini-Armstrong.

At least 73 records · Page 4Linked to original sources
PubMed1988

Discrimination between fast- and slow-twitch fibres of guinea pig skeletal muscle using the relative surface density of junctional transverse tubule membrane.

The black reaction of Golgi was used to infiltrate transverse (T) tubules in fast-twitch glycolytic (FW), fast-twitch oxidative-glycolytic (FR) and slow-twitch (S) type fibres in muscles of guinea pigs. Non-junctional (fT) and junctional (jT) segments of the T-tubule network are clearly demarcated by this technique. Digitized planimetry and direct measurements were used to determine the proportion of T-tubule network forming junctions with the sarcoplasmic reticulum (%LjT) and to estimate the surface density (surface area per fibre volume) of total and junctional T membrane. From these data, the volume density (number per fibre volume) of junctional feet was calculated. All three types of fibres have approximately equal surface density of T tubules, but the FW and FR fibres have a much higher proportion of jT. The calculated volume density of feet is twice as high in fast-twitch as in slow-twitch fibres.

Animals↗
PubMed1988

The structure of the membrane systems in a novel muscle cell modified for heat production.

A thermogenic organ, modified from an eye muscle, warms the brain and eyes of several oceanic fish. The extraocular muscles associated with thermogenesis are composed of modified muscle cells that are structurally distinct from all other types of muscle previously described. In "heater" cells, contractile filaments are virtually absent and the cell volume is packed with mitochondria and smooth membranes. Freeze-fracture studies and negative staining of microsomal fractions treated with vanadate indicate that most of the membrane system of heater cells has a high Ca2+-ATPase density and is equivalent to skeletal muscle sarcoplasmic reticulum (SR). High voltage electron micrographs of heater cells infiltrated with the Golgi stain demonstrate that the cells also have an extensive transverse tubule system with a complicated three-dimensional structure. Junctional regions between transverse tubules and SR occur in the heater cell and contain feet protein. Activation of thermogenesis in heater cells may occur through the same protein components involved in excitation-contraction coupling and appears to be associated with the ATP-dependent cycling of calcium at the SR.

Animals↗
PubMed1988

Structural evidence for direct interaction between the molecular components of the transverse tubule/sarcoplasmic reticulum junction in skeletal muscle.

The architecture of the junctional sarcoplasmic reticulum (SR) and transverse tubule (T tubule) membranes and the morphology of the two major proteins isolated from these membranes, the ryanodine receptor (or foot protein) and the dihydropyridine receptor, have been examined in detail. Evidence for a direct interaction between the foot protein and a protein component of the junctional T tubule membrane is presented. Comparisons between freeze-fracture images of the junctional SR and rotary-shadowed images of isolated triads and of the isolated foot protein, show that the foot protein has two domains. One is the large hydrophilic foot which spans the junctional gap and is composed of four subunits. The other is a hydrophobic domain which presumably forms the SR Ca2+-release channel and which also has a fourfold symmetry. Freeze-fracture images of the junctional T tubule membranes demonstrate the presence of diamond-shaped clusters of particles that correspond exactly in position to the subunits of the feet protein. These results suggest the presence of a large junctional complex spanning the two junctional membranes and intervening gap. This junctional complex is an ideal candidate for a mechanical coupling hypothesis of excitation-contraction coupling at the triadic junction.

Animals↗
PubMed1987

Golgi stain identifies three types of fibres in fish muscle.

Using Golgi infiltration we have studied the structure and disposition of transverse tubules in muscle fibres from the sand dab fin musculature. Three types of fibres differ significantly from each other in the extent and disposition of junctions between transverse tubules and the sarcoplasmic reticulum. These correlate with the three groups of fibres having different relaxation times shown in the accompanying paper (Gilly & Aladjem, 1987). Fibres with very slow relaxation (tonic fibres) correspond to those which have an unusual disposition of T tubules and very rare T-SR junctions. In the fast twitch fibres the peripheral T tubules segments converge into tangentially arranged tubules before joining the plasmalemma.

Animals↗
PubMed1987

The structure of calsequestrin in triads of vertebrate skeletal muscle: a deep-etch study.

We have examined the structure of calsequestrin in three-dimensional images from deep-etched rotary-replicated freeze fractures of skeletal muscle fibers. We selected a fast-acting muscle because the sarcoplasmic reticulum has an orderly disposition and is rich in internal membranes. Calsequestrin forms a network in the center of the terminal cisternae and is anchored to the sarcoplasmic reticulum membrane, with preference for the junctional portion. The anchorage is responsible for maintaining calsequestrin in the region of the sarcoplasmic reticulum close to the calcium-release channels, and it corroborates the finding that calsequestrin and the spanning protein of the junctional feet may interact with each other in the junctional membrane. Anchoring filaments may be composed of a protein other than calsequestrin.

Animals↗
PubMed1987

Rigor crossbridges are double-headed in fast muscle from crayfish.

The structure of rigor crossbridges was examined by comparing rigor crossbridges in fast muscle fibers from glycerol-extracted abdominal flexor muscle of crayfish with those in "natively decorated" thin filaments from the same muscle. Natively decorated thin filaments were obtained by dissociating the backbone of the myosin filaments of rigor myofibrils in 0.6 M KCl. Intact fibers were freeze-fractured, deep-etched, and rotary shadowed; isolated filaments were either negatively stained or freeze dried and rotary shadowed. The crossbridges on the natively decorated actin maintain the original spacing and the disposition in chevrons and double chevrons for several hours, indicating that no rearrangement of the actomyosin interactions occurs. Thus the crossbridges of the natively decorated filaments were formed within the geometrical constraints of the intact myofibril. The majority of crossbridges in the intact muscle have a triangular shape indicative of double-headed crossbridge. The triangular shape is maintained in the isolated filaments and negative staining resolves two heads in a single crossbridge. In the isolated filaments, crossbridges are attached at uniform acute angles. Unlike those in insect flight muscle (Taylor et al., 1984), lead and rear elements of the double chevron may be both double-headed. Deep-etched images reveal a twisted arrangement of subfilaments in the backbone of the thick filament.

Actin Cytoskeleton↗
PubMed1986

Different calcium dependence of the capping and cutting activities of villin.

The concentration of ionized calcium required for the capping of barbed filament ends by villin is about 4 orders of magnitude lower than that required for the cutting activity of villin. Capping was 50% complete at about 10-30 nM Ca2+, a level expected in resting cells, whereas the cutting rate was half-maximal at about 200 microM, making it possible to completely separate filament capping from filament cutting. Analysis of capping in terms of coupled equilibria between calcium binding to villin and calcium-villin binding to the barbed ends of actin filaments gives a value of 10(16)-10(17) M-2 for the product of the two binding constants. By comparison the binding constant reported for the rapidly exchanging calcium sites on villin is 2 X 10(5) M-1 and that for binding of calcium-saturated villin to barbed ends has a minimum value of 10(11) M-1 giving a product of 2 X 10(16) M-1. The close similarity of the two sets of values suggests that capping is regulated by the rapidly exchanging calcium sites on villin. In terms of coupled equilibria the calcium requirement for filament capping decreases with increasing concentrations of free villin. The scant information on the mechanism of cutting allows only an estimate of the maximal value for the calcium-binding constant of the site regulating cutting which is about 2-5 X 10(3) M-1. Cutting is followed by rapid capping of the newly released barbed ends.

Actins↗
PubMed1986

Shape and disposition of clefts, tubules, and sarcoplasmic reticulum in long and short sarcomere fibers of crab and crayfish.

The disposition of surface invaginations (clefts, Z and T tubules) and of the sarcoplasmic reticulum has been examined by electron microscopy at three accelerating voltages (100, 200 and 1000 kV) and by phase-contrast light microscopy in crustacean muscles infiltrated by the "Golgi stain." In long-sarcomere, tonic type fibers, an extensive system of invaginating clefts has been observed, along with both Z and T tubules. Z and T tubules form interconnections with each other, but only T tubules form specific contacts with the sarcoplasmic reticulum, which in these fibers forms an extended and continuously fenestrated network. In short-sarcomere, phasic type fibers, a ladder-like disposition of an abundant T network is found. Z tubules are absent in these fibers. The sarcoplasmic reticulum forms more frequent junctions with flattened areas of T tubules and with clefts, but has less extensive free surfaces than in the long-sarcomere fibers.

Animals↗
PubMed1985

New views of smooth muscle structure using freezing, deep-etching and rotary shadowing.

Freezing, deep-etching and rotary shadowing techniques have been applied to study smooth muscle ultrastructure. The results show some new aspects of intracellular and extracellular connections, interior views of the sarcoplasmic reticulum showing a luminal content, coated pits and vesicles, contractile filaments and other organelles in smooth muscle.

Animals↗
PubMed1985

Ordered arrays of Ca2+-ATPase on the cytoplasmic surface of isolated sarcoplasmic reticulum.

Isolated sarcoplasmic reticulum (SR) vesicles with polymerized calcium pump protein were freeze-dried and rotary shadowed following uranyl acetate stabilization. This technique allows direct observation of a single side of the vesicle without requiring optical filtering. The heads of individual ATPase molecules, projecting above the cytoplasmic surface, are clearly resolved in the replicas. Ca ATPase molecules form extensive arrays in vanadate-treated, rabbit SR vesicles and in gently isolated, native SR vesicles from scallop. Gentle isolation results in limited areas of orderly structure in native SR isolated from vertebrate muscles. Special attention is given to the effect of various shadow thicknesses on the appearance of the heads. This information is essential to the interpretation of images in the accompanying paper (Franzini-Armstrong, C., and D.J. Ferguson, 1985, Biophys. J., 48:607-615).

Acetates↗
PubMed1985

Density and disposition of Ca2+-ATPase in sarcoplasmic reticulum membrane as determined by shadowing techniques.

We have studied the disposition of calcium ATPase in the native sarcoplasmic reticulum (SR) membrane of vertebrate muscles by rotary shadowing of freeze-dried isolated vesicles and of freeze-fractured in situ membranes. The predominant disposition of the ATPase molecules is disorderly, but small oligomers (dimers, tetramers, and occasionally larger aggregates) are seen. In vesicles from white hind legs of rabbits, the density of ATPase over nonjunctional SR is 31-34,000/microns2. ATPase density is always quite high, but small protein-free lipid patches may be interspersed with it.

Animals↗
PubMed1984

The structure and disposition of crossbridges in deep-etched fish muscle.

Deep-etching and rotary-shadowing techniques were used to describe crossbridges in fish (Chanda ranga) muscle, relaxed and in iodoacetate rigor conditions. Three major fracture planes from rigor muscle were studied using stereomicroscopy and Fourier image analysis. The 1,0 plane reveals alternating thick and thin filaments with the thick filaments frontmost in the fracture and the thin filaments in the recessed plane. All crossbridges coming from the frontmost thick filaments are visible on actin filaments in the 1,0 plane. Fourier transforms of digitized images from these fracture planes exhibit axial periodicities of 14 and 36 nm. The actin layer, a fracture plane just below the myosin filaments in the 1,0 plane, shows end-on views of crossbridges projecting out of the fracture plane and limited transverse alignment of crossbridges. Actin layer Fourier transforms demonstrate a 14 nm reflection associated with the attachment of crossbridges with a mean axial periodicity determined by their myosin origins. The 1,1 lattice direction shows pairs of thin filaments alternated with single thick filaments. In this view, all crossbridges coming from three adjacent myosins are visible. In all fracture planes, decoration of individual thin filaments by crossbridges is variable, but usually one (singlet) or two (doublet) closely spaced crossbridges mark each actin target zone, at intervals of 35-38 nm. Counts of crossbridges decorating actin filaments give an average of four every three target zones. The anticipated stagger of target zones for crossbridges from two adjacent myosin filaments is observed. Alignment of actin target zones across the sarcomere is good. We can distinguish two distinct shapes for rigor crossbridges: a narrow, straight bridge and a wider bridge with a triangular shape. We interpret these as being the appearance of crossbridges with one or two S1 subfragments (single and double headed) respectively. Comparison between rigor and relaxed structures indicates attachment of all crossbridges in rigor.

Actins↗
PubMed1984

Biochemical and morphological characterization of a plasma membrane-enriched fraction from bovine parathyroid cells.

A fraction of enriched plasma membranes from bovine parathyroid cells has been prepared by differential centrifugation. Biochemical characterization shows that this fraction has a specific activity enrichment of 7.2-fold in ouabain-sensitive Na+-K+ ATPase, and 3.5-fold in 5'-nucleotidase. Less than 4% of the total mitochondria and lysosomes are present within the plasma membranes, while microsomal contamination accounts for 14% of total specific activity. Parathyroid hormone radioimmunoassay also reveals the presence of some secretory granules within the plasma membrane fraction. The characteristic morphological aspect of the unusual surface membrane is shown by freeze-fracture electron microscopy. In the enriched pellets, vesicles identified as having a plasma membrane origin have variable sizes, and 50% show an inside-out conformation. Even though the plasma membrane fraction described herein is not absolutely free from contamination by other subcellular components, this protocol represents the first attempt to purify surface membrane from parathyroid tissue and provide the starting material for understanding, at a molecular level, the properties of extracellular Ca2+ regulation and its coupling with secretion of parathyroid hormone.

5'-Nucleotidase↗
PubMed1984

Freeze-fracture of frog slow tonic fibers. Structure of surface and internal membranes.

Slow frog fibers from frog cruralis were examined by freeze-fracture and compared with twitch fibers. Major differences in membrane architecture between slow and twitch fibers are in the shape and disposition of T tubules, the shape of junctional regions, the frequency of peripheral couplings and of Ca pump particles. Aggregates of ACh receptors on the surface membrane are distinguishable from peripheral couplings on the basis of size and shape of intramembranous particles.

Animals↗
PubMed1984

Subunit structure of junctional feet in triads of skeletal muscle: a freeze-drying, rotary-shadowing study.

Isolated heavy sarcoplasmic reticulum vesicles retain junctional specializations (feet) on their outer surface. We have obtained en face three-dimensional views of the feet by shadowing and replicating the surfaces of freeze-dried isolated vesicles. Feet are clearly visible as large structures located on raised platforms. New details of foot structure include a four subunit structure and the fact that adjacent feet do not abut directly corner to corner but are offset by half a subunit. Feet aligned within rows were observed to be rotated at a slight angle off the long axis of the row creating a center-to-center spacing (32.5 nm) slightly less than the average diagonal of the feet (35.3 nm). Comparison with previous information from thin sections and freeze-fracture showed that this approach to the study of membranes faithfully preserves structure and allows better visualization of surface details than either thin-sectioning or negative-staining.

Animals↗
PubMed1983

Junctional feet and particles in the triads of a fast-twitch muscle fibre.

Structural details of junctional feet in triads of fish muscle are described. These feet have a less dense central core and contact both sarcoplasmic reticulum and T-tubule membranes at tetragonally disposed sites. The distribution of intramembraneous particles differs at the junctional T-membrane, and the junction is asymmetric.

Air Sacs↗