Sarcoplasmic reticulum in the cross-striated adductor muscle of the bay scallop, Aequipecten irridians

Dispositions of junctional feet in muscles of invertebrates

The structure and disposition of the feet occupying the junctions between sarcoplasmic reticulum (SR) and surface membrane/transverse tubules were studied in muscles from a variety of invertebrates. Feet were imaged by rotary shadowing of isolated junctional SR vesicles and by filtering of micrographs from grazing views of the junction in thin sections. The overall size and shape of invertebrate feet is the same as that of feet in skeletal and cardiac muscle of vertebrates. However, the arrangement of feet in invertebrate muscles differs from that in vertebrates. These findings are discussed in terms of known variations in properties of excitation-contraction coupling of the two phyla.

Effect of phosphorylation on scallop sarcoplasmic reticulum

Fragmented sarcoplasmic reticulum prepared from the cross-striated adductor muscle of the deep sea scallop (Placopecten magellanicus) was phosphorylated with inorganic phosphate to the E2P (ADP-insensitive) form. Negative staining of these preparations showed that the Ca-ATPase was organized into a quasi-crystalline array, which differed from the 'dimer ribbon' structure previously reported for the membrane under relaxing conditions (Castellani & Hardwicke, J. cell. Biol. 97 (1983) 557-61; Castellani et al., J. molec. Biol. 185 (1985) 579-94). In this new form there was only a single Ca-ATPase per unit cell. Dephosphorylation of the E2P membranes and incubation with substrate or substrate analogues in the absence of Ca2+ caused the 'dimer ribbon' structure to appear. These results imply that rotation of at least half of the Ca-ATPase subunits in the scallop sarcoplasmic reticulum may occur about an axis perpendicular to the plane of the membrane on conversion from the E2P state to the state corresponding to that existing in the relaxed muscle.

Effect of Ca2+ on the dimeric structure of scallop sarcoplasmic reticulum

Scallop sarcoplasmic reticulum (SR), visualized in situ by freeze-fracture and deep-etching, is characterized by long tubes displaying crystalline arrays of Ca2+-ATPase dimer ribbons, resembling those observed in isolated SR vesicles. The orderly arrangement of the Ca2+-ATPase molecules is well preserved in muscle bundles permeabilized with saponin. Treatment with saponin, however, is not needed to isolate SR vesicles displaying a crystalline surface structure. Omission of ATP from the isolation procedure of SR vesicles does not alter the dimeric organization of the Ca2+-ATPase, although the overall appearance of the tubes seems to be affected: the edges of the vesicles are scalloped and the individual Ca2+-ATPase molecules are not clearly defined. The effect of Ca2+ on isolated scallop SR vesicles was investigated by correlating the enzymatic activity and calcium-binding properties of the Ca2+-ATPase with the surface structure of the vesicles, as revealed by electron microscopy. The dimeric organization of the membrane is preserved at Ca2+ concentrations where the Ca2+ binds to the high affinity sites (half-maximum saturation at pCa approximately 7.0 with a Hill coefficient of 2.1) and the Ca2+-ATPase is activated (half-maximum activation at pCa approximately 6.8 with a Hill coefficient of 1.84). Higher Ca2+ concentrations disrupt the crystalline surface array of the SR tubes, both in the presence and absence of ATP. We discuss here whether the Ca2+-ATPase dimer identified as a structural unit of the SR membrane represents the Ca2+ pump in the membrane.

The effect of low ATP concentrations on relaxation in the myosin regulated myofibrils from scallop

Troponin-tropomyosin-regulated myofibrils show a significant increase in ATPase activity and contract in the absence of calcium when the ATP concentration falls significantly below the saturation level. By contrast, the ATPase of the myosin-regulated myofibrils of scallop striated muscle was not activated in the absence of calcium when the ATP concentration was lowered to 10mM. Nevertheless, a very small fraction of crossbridges were active at 10mM ATP resulting in very slow myofibrillar shortening. In contrast to the behaviour of rabbit contractile proteins there was no correlation between myofibrillar shortening and ATP induced turbidity changes of actomyosin taken from scallop.

The musculature of squid arms and tentacles: Ultrastructural evidence for functional differences

The transverse muscle mass of the arm and the transverse and circular muscle masses of the tentacle of squid (Loligo pealei and Illex illecebrosus) were examined by transmission electron microscopy. Previous work had indicated that although similar in gross arrangement, the transverse muscle mass of the tentacle creates rapid elongation during prey capture while the transverse muscle mass of the arm is involved in creating bending movements. The difference in function between the transverse muscle masses of the arms and tentacles is reflected in differences in ultrastructure. The transverse muscle mass of the arm is made up of regular, obliquely striated muscle fibers 1-6 μm in diameter. The transverse and associated circular muscle masses of the tentacle are made up of cross-striated muscle fibers 1-3 μm in diameter. The cross-striated muscle fibers have A bands approximately 0.5 μm (I. illecebrosus) and 0.9 μm (L. pealei) in length and a resting sarcomere length of 0.9 μm (I. illecebrosus) and 1.6 μm (L. pealei), suggesting a relatively high shortening speed for this muscle type. The cross-striated cells are not divided up into myofibrils, and the sarcoplasmic reticulum is located beneath the sarcolemma. Vernier displacements of the sarcomeres were observed. The myofilaments of the obliquely striated muscle fibers of the arm surround a central core containing mitochondria and the cell nucleus. The sarcoplasmic reticulum of the obliquely striated cells is located beneath the sarcolemma, in the plane of the Z elements, and surrounding the mitochondrial core.

A mechanical study of regulation in the striated adductor muscle of the scallop

Chemically skinned fibre bundles were prepared from the striated adductor muscle of the sea scallop, Placopecten magellanicus. The relation between tension and calcium concentration was determined in activating solutions containing 5 mM-MgATP, ionic strength 0.2, pH 7.1 at 20 degrees C. The isometric tension rose from zero to its maximum value between pCa 6.0 and 5.2. The steepness of the relation cannot be accounted for in terms of the binding of calcium to the two known sites on myosin and suggests that there must be an additional, co-operative mechanism. The regulatory light chain content of the fibre bundles was determined by urea gel electrophoresis and was found to be approximately 2 light chains per myosin molecule. The regulatory light chains were removed completely by treatment with EDTA at 25-30 degrees C. Fibre bundles then showed a total loss of control over contraction; a high tension was generated whether or not calcium was present in the bathing solution. Complete removal of the regulatory light chains did not greatly affect the tension generated or the stiffness in the rigor state. Control of contraction could be restored completely by the addition of regulatory light chains from scallop muscle. Treatment with EDTA at 0-12 degrees C resulted in the removal of 0.76-2.0 regulatory light chains per myosin molecule. Fibre bundles for which removal was less than complete were partially sensitive to calcium, i.e. tension was higher in the presence of calcium than in its absence. The results indicate that the normal mechanism of tension generation in scallop muscle is mediated primarily through myosin and not thin filament control. This finding is consistent with previous studies of the ATPase activity of myofibrils from scallop muscle.

Crystalline structure of sarcoplasmic reticulum from scallop

Negatively stained sarcoplasmic reticulum from the scallop Placopecten magellanicus presented a variety of crystalline forms, the most common being tubular structures. These were characterized by paired rows of morphological units, spaced at approximately 120 A, running diagonally across the tubules. The orthogonal unit cell (120 X 55 A) contained two units, related by a twofold axis, which probably represented the part of the Ca2+-ATPase molecule projecting from the outer surface of the membrane.

Mechanics and energetics of contraction in striated muscle of the sea scallop, Placopecten magellanicus

1. Mechanics and energetics of contraction in the striated adductor muscle of the sea scallop, Placopecten magellanicus, were determined and compared to a more conventional muscle preparation, the semitendinosus muscle of the frog, Rana pipiens. Experiments were conducted at 10 degrees C. 2. Scallop and frog striated muscle exhibited similar isometric twitch kinetics, heat coefficients (ratio of force produced to energy liberated), and twitch to tetanus ratios. 3. There were significant differences between scallop and frog muscle in that scallop muscle demonstrated a 30% lower isometric force, an inability to maintain force during a tetanus, and a transient decrease in economy of force development in a twitch after a tetanus, i.e. a decrease in the isometric heat coefficient.

Molluscan visceral muscle fine structure. General structure and sarcolemmal organization in the smooth muscle of the intestinal wall of Buccinum undatum L

Fine structure of intestinal muscle in the gastropod Buccinum undatum is described. Myofibrillar organization is typical of non-pseudostriated molluscan muscles. The dense body system is poorly developed but there are extensive attachments plaques. The sarcolemma is elaborately modified. Deep infoldings of the membrane give the cells an irregular outline. Such infoldings enclose extracellular matrix and are associated with attachment plaques. Arising from these and from the general sarcolemma are numerous tubular membranous invaginations ending blindly at varying depth in the sarcoplasm. These structures have a helical coat of particles on the cytoplasmic face. Associated with both types of invagination are subsarcolemmal vesicles. The possibility that the tubular invaginations are analogues of vertebrate smooth muscle caveolae or striated muscle T-tubules and that the vesicles are the corresponding sarcoplasmic reticulum is discussed. The occurrence of such structures in molluscan muscle and elsewhere is reviewed.