Microprobe analysis of maturation-related elemental changes in rat parotid secretory granules

Electron probe X-ray microanalysis was use to quantitate the elemental and mass changes that take place during the secretory granule maturation process. A single injection of isoproterenol stimulated the depletion of secretory granules from rat parotid acinar cells. Granules at different stages of maturation were analyzed as they reaccumulated within the cells over time. Dry mass measurements revealed that secretory material becomes concentrated about twofold within maturing granules. Nearly all of the increase in mass concentration could be attributed to a reduction in water space. Data are presented that indicate that Na, K, Cl, and water all efflux from secretory granules during maturation. In contrast, granule S content is positively correlated with maturation. Hence, significant changes in granule elemental and water contents occur during the maturation process.

Panlobular emphysema in young intravenous Ritalin abusers

We studied a distinctive group of young intravenous Ritalin abusers with profound obstructive lung disease. Clinically, they seemed to have severe emphysema, but the pathologic basis of their symptoms had not been investigated previously. Seven patients have died and been autopsied: in four, the lungs were fixed, inflated, dried, and examined in detail radiologically, grossly, microscopically, and by electron probe X-ray microanalysis. All seven patients had severe panlobular (panacinar) emphysema that tended to be more severe in the lower lung zones and that was associated with microscopic talc granulomas. Vascular involvement by talc granulomas was variable, but significant interstitial fibrosis was not present. Five patients were tested for alpha-1-antitrypsin deficiency and found to be normal, as were six similar living patients. These findings indicate that some intravenous drug abusers develop emphysema that clinically, radiologically, and pathologically resembles that caused by alpha-1-antitrypsin deficiency but which must have a different pathogenesis. Talc from the Ritalin tablets may be important, but the mechanism remains to be elucidated.

Resting myosin cross-bridge configuration in frog muscle thick filaments

Clear images of myosin filaments have been seen in shadowed freeze-fracture replicas of single fibers of relaxed frog semitendinosus muscles rapidly frozen using a dual propane jet freezing device. These images have been analyzed by optical diffraction and computer averaging and have been modelled to reveal details of the myosin head configuration on the right-handed, three-stranded helix of cross-bridges. Both the characteristic 430-A and 140-150-A repeats of the myosin cross-bridge array could be seen. The measured filament backbone diameter was 140-160 A, and the outer diameter of the cross-bridge array was 300 A. Evidence is presented that suggests that the observed images are consistent with a model in which both of the heads of one myosin molecule tilt in the same direction at an angle of approximately 50-70 degrees to the normal to the filament long axis and are slewed so that they lie alongside each other and their radially projected density lies along the three right-handed helical tracks. Any perturbation of the myosin heads away from their ideal lattice sites needed to account for x-ray reflections not predicted for a perfect helix must be essentially along the three helical tracks of cross-bridges. Little trace of the presence of non-myosin proteins could be seen.

Beam induced mass loss in high resolution biological microanalysis

Electron beam induced loss of mass from the organic matrix and from higher Z constituents of biological samples was measured by monitoring bremsstrahlung and peak changes in EDS spectra. When any effects of contamination, extraneous X-rays, beam current drift, specimen drift, and specimen shrinkage were monitored and corrected for, the three types of samples gave consistent and similar results at 296 K. Bremsstrahlung losses averaged 45%, 46% and 50% respectively for muscle homogenate, salivary gland sections and albumin. Sulphur losses average 74%, 72% and 86% for the same three sample types. No other elements suffered significant losses. D1/e for bremsstrahlung averaged 0.14 C/cm2. Bremsstrahlung loss at 93 K began approximately one order of magnitude higher in dose, and the extent of loss varied. Sulphur losses, however, were greatly reduced at low temperatures.

Myosin rod-packing schemes in vertebrate muscle thick filaments

Muscle myosin filament backbones are known to be aggregates of long coiled-coil alpha-helical myosin rods, but the packing arrangement is not understood in detail. Here we present new data on fish muscle myosin filaments from low-angle X-ray diffraction and from freeze-fracture, deep-etch electron microscopy which put constraints on the kind of models that might explain all of the observations. In particular, it is known in the case of vertebrate striated muscle thick filaments that the myosin head array in resting muscle is not perfectly helical but contains periodic perturbations. We show by analysis of low-angle X-ray diffraction patterns from resting bony fish muscle that any radial, azimuthal, and axial perturbations of the myosin head origins on the filament surface (due to perturbed myosin rod packing) must all be rather small and that the main perturbations are in the myosin head configurations (i.e., tilts, slews, rotations) on those origins. We provide evidence that the likely arrangement of titin molecules on the myosin filament is with them aligned parallel to the filament long axis, rather than following helical tracks. We also show from freeze-fracture studies of fish muscle that the myosin filament backbone (including titin and other extra proteins) has a radius of about 65-75 A and appears to contain a small (approximately 15-20 A radius) hollow core. Together with previously published evidence showing that the myosin rods are nearly parallel to the thick filament long axis, these results are consistent with the curved crystalline layer model of Squire (J. M. Squire, 1973, J. Mol. Biol. 77, 291-323), and they suggest a general structure for the C-zone part of the thick filament

Subsarcomeric distribution of calcium in demembranated fibers of rabbit psoas muscle

Direct measurements were made of the Ca distribution within sarcomeres of glycerinated rabbit psoas muscle fibers in rigor using electron probe x-ray microanalysis. Both analogue raster analysis and digital x-ray imaging were used to quantitate the Ca distribution along thick and thin filaments as a function of the concentration of free Ca2+. Even when corrected for the estimated contribution of Ca bound to thick filaments, the Ca measured in the region of overlap between thick and thin filaments significantly exceeded the Ca in the I-band at subsaturating concentrations of free Ca2+. At saturating levels of free Ca2+, the excess Ca in the overlap region was diminished but still statistically significant. The data thus suggest that the formation of rigor linkages exerts multiple effects on the binding of Ca2+ to thin filaments in the overlap region by increasing the affinity of troponin C for Ca2+ and possibly by unmasking additional Ca2+ binding sites. The data also show that the cooperativity invested in the thin filaments is insufficient to permit the effects of rigor cross-bridge formation on Ca2+ binding to propagate far along the thin filaments into the I-band.

Changes in subcellular elemental distributions accompanying the acrosome reaction in sea urchin sperm

Energy-dispersive x-ray microanalysis was used to analyze changes in the subcellular distributions of Na, Mg, P, S, Cl, K, and Ca associated with the acrosome reaction of sea urchin sperm. Within 5 sec after induction of the acrosome reaction, nuclear Na and mitochondrial Ca increased and nuclear and mitochondrial K decreased. Uptake of mitochondrial P was detected after several minutes, and increases in nuclear Mg were detected only after 5-10 min of incubation following induction of the reaction. The results suggest that sudden permeability changes in the sperm plasma membrane are associated with the acrosome reaction, but that complete breakdown of membrane and cell function does not occur for several minutes.

High spatial resolution spectroscopy in the elemental microanalysis and imaging of biological systems

The application of analytical electron microscopy to the high spatial resolution study of biological systems is reviewed. Specimen preparation, quantitative analysis, capabilities and limitations are all discussed, principally in the context of energy-dispersive X-ray analysis. Results are presented using both current techniques and the developing quantitative image analysis. Finally the role of new instrumental approaches, including electron energy loss spectrometry, is discussed.

Effects of 4-hydroxy-2-nonenal on beef heart mitochondrial ultrastructure, oxygen consumption, and metmyoglobin reduction

The effects of 4-hydroxy-2-nonenal (HNE) on mitochondria isolated from bovine hearts (n=5) were assessed using ultrastructure, oxygen consumption, membrane permeability, HNE binding, and metmyoglobin reduction in vitro. Pre-incubation (pH 5.6 and 7.4 at 25°C) of mitochondria with HNE decreased oxygen consumption compared with samples without HNE (P<0.05). Electron microscopy revealed that HNE-treated mitochondria were swollen and had increased membrane permeability at pH 7.4, compared with ethanol controls. Conversely, mitochondria incubated with HNE at pH 5.6 had decreased volume and permeability. Fluorescence studies indicate that HNE binds to the membrane of mitochondria isolated from bovine cardiac muscle (at pH 5.6 and 7.4). HNE-treated mitochondria at both pH 5.6 and 7.4 had lower metmyoglobin reduction and NADH dependent metmyoglobin reductase activity compared with control mitochondria without HNE (P<0.05). In addition to covalent binding with myoglobin, HNE may influence beef color stability by interacting with mitochondria.

Structure and nucleotide-dependent changes of thick filaments in relaxed and rigor plaice fin muscle

The myosin crossbridge array, positions of non-crossbridge densities on the backbone, and the A-band "end filaments" have been compared in chemically skinned, unfixed, uncryoprotected relaxed, and rigor plaice fin muscles using the freeze-fracture, deep-etch, rotary-shadowing technique. The images provide a direct demonstration of the helical packing of the myosin heads in situ in relaxed muscle and show rearrangements of the myosin heads, and possibly of other myosin filament proteins, when the heads lose ATP on going into rigor. In the H-zone these changes are consistent with crossbridge changes previously shown by others using freeze-substitution. In addition, new evidence is presented of protein rearrangements in the M-region (bare zone), associated with the transition from the relaxed to the rigor state, including a 27-nm increase in the apparent width of the M-region. This is interpreted as being mostly due to loss or rearrangement of a nonmyosin (M9) protein component at the M-region edge. The structure and titin periodicity of the end-filaments are described, as are suggestions of titin structure on the myosin filament backbone.

A-band architecture in vertebrate skeletal muscle: polarity of the myosin head array

Despite extensive knowledge of many muscle A-band proteins (myosin molecules, titin, C-protein (MyBP-C)), details of the organization of these molecules to form myosin filaments remain unclear. Recently the myosin head (crossbridge) configuration in a relaxed vertebrate muscle was determined from low-angle X-ray diffraction (Hudson et al. (1997), J Mol Biol 273: 440-455). This showed that, even without C-protein, the myosin head array displays a characteristic polar pattern with every third 143 A-spaced crossbridge level particularly prominent. However, X-ray diffraction cannot determine the polarity of the crossbridge array relative to the neighbouring actin filaments; information crucial to a proper understanding of the contractile event. Here, electron micrographs of negatively-stained goldfish A-segments and of fast-frozen, freeze-fractured plaice A-bands have been used to determine the resting myosin head polarity relative to the M-band. In agreement with the X-ray data, the prominent 429 A-spaced striations are seen outside the C-zone, where no non-myosin proteins apart from titin are thought to be located. The head orientation is with the concave side of the curved myosin heads (containing the entrance to the ATP-binding site) facing towards the M-band and the convex surface (containing the actin-binding region at one end) facing away from the M-band.

A review of electron probe X-ray microanalysis studies of salivary gland cells

Electron probe X-ray microanalysis (EPXMA) has now been successfully applied to several salivary gland preparations. This paper briefly reviews the principles underlying this technique and the specific sample preparation procedures which permit accurate measurement of elemental concentrations in the various intracellular spaces. Findings from salivary gland studies indicate that cytoplasmic and nuclear spaces of nonstimulated acinar cells have high concentrations of K and P, and low concentrations of Mg, Ca, and S; and that mature secretory granules have high concentrations of Ca and S, and relatively low concentrations of K and P. No consistent differences have been found between the elemental concentrations of mucous and serous secretory granules. In vivo and in vitro EPXMA studies of the elemental changes associated with secretory granule maturation indicate there are at least two stages in this process: an early stage during which granule S concentration increases in parallel with mass density as condensing vacuoles mature into secretory granules, and a late stage during which granule mass density and protein content increase with no further elemental concentration changes. Findings from other in vivo and in vitro studies indicate that secretory granule membranes are permeable to Na, K, and Cl ions because the granular concentrations of these elements are altered by electrochemical gradients. Recent EPXMA results indicate that cells stimulated with parasympathomimetic agonists have decreased K and Cl concentrations, and increased Na concentrations. Furthermore, the magnitude of these changes are quantitatively consistent with changes measured using radio-isotope equilibration and other techniques. In contrast, cells stimulated with the beta-adrenergic agonist, isoproterenol, have increased concentrations of Na and Cl, but unchanged K concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Maturation-related changes in mass and elemental contents of secretory granules as measured by electron-microprobe

The relationship between granule density, protein content, and Ca and S contents were studied in two secretory granule fractions, from parotid glands of the rat, previously shown to constitute different stages in granule maturation. The density of the lighter fraction was between 1.133 and 1.142 g/ml, while that of the heavier fraction was greater than 1.142 g/ml. The mean protein content of the denser granules was 12% greater than that of the lighter granules (P less than 0.03), while the dry-mass elemental concentrations in the two granule fractions were unchanged. These results indicate that protein is added to granules during the maturation process (presumably by vesicular traffic), and that the resulting increase in granule density is not driven simply by decrease in water content and/or increased concentrations of inorganic Ca or S in the granules. The elemental concentration values also indicate that the diffusible elements permeate the granule membrane during the fractionation procedures.

Comparison of elemental concentrations in the acinar cells of the human labial salivary gland

Two types of acinar cells were observed in human labial glands by conventional and analytical electron microscopic and light microscopic techniques. The predominant type contained large and prominent secretory granules that were strongly mucicarmine and PAS (with and without diastase) positive. The second type contained small, lacy, secretory granules, and these cells were faintly positive with these stains. The elemental contents of the two types of granules were measured by analytical electron microscopy using digital mapping and spot analysis applied to freeze-dried cryosections prepared from gland slices incubated in vitro under non-stimulated conditions. The large secretory granules had significantly higher Ca, S and Mg concentrations and significantly lower Cl and K concentrations than the small granules. The difference in elemental contents probably reflects differences in the content of secretory macromolecules. Specifically, the S content is thought to reflect the anionic properties of the secretory macromolecules, while the levels of divalent cations are thought to be determined by electroneutrality requirements for macromolecular folding and storage. No differences were found in nuclear or cytoplasmic elemental concentrations between the two cell types.

Distributions of calcium in A and I bands of skinned vertebrate muscle fibers stretched to beyond filament overlap

Measurements were made of the distributions of total calcium along the length of A and I bands in skinned frog semitendinosus muscles using electron probe x-ray microanalysis. Since calcium in the water space was kept below the detection limit of the technique, the signal was assumed to reflect the distribution of calcium bound to myofilament proteins. Data from sarcomeres with overlap between thick and thin filaments showed enhancement of calcium in this region, as previously demonstrated in rabbit psoas muscle fibers in rigor (Cantino, M. E., T. S. Allen, and A. M. Gordon. 1993. Subsarcomeric distribution of calcium in demembranated fibers of rabbit psoas muscle. Biophys. J. 64:211-222). Such enhancement could arise from intrinsic non-uniformities in calcium binding to either thick or thin filaments or from enhancement of calcium binding to either filament by rigor cross-bridge attachment. To test for intrinsic variations in calcium binding, calcium distributions were determined in fibers stretched to beyond filament overlap. Calcium binding was found to be relatively uniform along both thick and thin filaments, and therefore cannot account for the increased calcium observed in the overlap region. From these results it can be concluded that the observed enhancement of calcium is due to an increase in calcium binding to myofilaments as a result of rigor attachment of cross-bridges to actin. The source of the enhancement is most likely an increase in calcium binding to troponin, although enhancement of calcium binding to myosin light chains cannot be ruled out.

Nonuniform distribution of myosin light chains within the thick filaments of lobster slow muscle: Immunocytochemical study

The in situ distribution of the alpha and beta myosin light chains was investigated at the subsarcomeric and subfilament levels in individual fibers of the superficial flexor muscle (SFM) of the lobster, Homarus americanus. Polyclonal antibodies were produced against the two classes of myosin light chains and used for subsequent immunolocalization on thin sections of sarcomeres and on isolated filaments from both the medial and lateral fiber bundles of the SFM. The beta myosin light chains were uniformly distributed within the crossbridge region of sarcomeres of both medial and lateral bundles. The alpha myosin light chains were uniformly distributed within the crossbridge region of sarcomeres from the medial bundle, but were nonuniformly distributed over the crossbridge region of lateral bundle sarcomeres. In the latter, the number of alpha myosin light chains was highest toward the center of the thick filaments, diminishing towards the ends. Similar distributions of alpha light chains were found in isolated myosin filaments. These data demonstrate that heterogeneity in protein composition extends to the level of the myosin filament and suggest that the myosin filament substructure in lobster may be different than that found in vertebrate skeletal muscle.

Myosin crossbridge configurations in equilibrium states of vertebrate skeletal muscle. Heads swing axially or turn upside-down between resting and rigor

The positions and orientations of the myosin heads in relaxed, active, rigor and S1-labelled fish muscle are being determined by analysis both of electron micrographs and of low-angle X-ray diffraction patterns. The X-ray analysis of resting muscle makes use of the head shape defined from the study of S1 crystals, with variable head configurational parameters being used on each of the three different 3-fold symmetric 14.3 nm-spaced 'crowns' of myosin heads within the 42.9 nm axial repeat of the myosin filaments. Diffraction patterns were stripped using CCP13 fibre diffraction software. Searches and optimisation were carried out using simulated annealing and local refinement procedures to give a 'best fit' relaxed structure with a crystallographic R-factor of about 4%. It had heads oriented all the same way up (i.e. with similar rotations around their own long axes) on the myosin filament, but with a small range of axial tilts. Head configuration in rigor fish muscle is being determined by X-ray diffraction and electron microscopy of normal rigor muscle and of skinned muscle soaked with extrinsic myosin S1. Computed 3-D reconstructions of acto-S1 using X-ray amplitudes and phases from electron microscopy are informative and help to analyse the X-ray diffraction data that extend axially to about 1 nm resolution. An ambiguity is the axial direction of the observed resting myosin head array relative to the known polarity of the actin filaments. One polarity would give little axial displacement (2-3 nm) between opposite ends of the resting and rigor heads, and in this case the heads would need to rotate around their own long axes by about 115 degrees to make a rigor attachment. The other (preferred) filament polarity would provide considerable axial swinging (14-15 nm) between the two states. We are attempting to define the absolute polarity of the resting muscle myosin head array using electron microscopy and image processing either of cryo-sections or of replicas from shadowed, freeze-fractured, rapidly frozen fish muscle fibres.