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Biomedical subjects

A R Limbrick

Publications and source records attributed to A R Limbrick.

At least 19 recordsLinked to original sources
PubMed1988

GABA immunoreactivity and 3H-GABA uptake in mucosal epithelial cells of the rat stomach.

GABA, best known as a neurotransmitter in the central nervous system, is also present in various peripheral tissues including the gastrointestinal tract, where there is strong evidence that GABA acts as a transmitter in some intrinsic myenteric neurones. Several studies indicate that the gastric mucosa is one of the sites of action of GABA in the gut. Highly specific anti-GABA antibodies have been used to detect endogenous GABA in the mucosa of the rat gastrointestinal tract, and 3H-GABA uptake followed by autoradiography has been used to localise cells with uptake sites for exogenous GABA. It was found that although GABA immunoreactive nerve fibres are essentially absent from this site, some mucosal cells are strongly GABA-immunoreactive. These cells are common in the pyloric stomach and upper part of the small intestine. The autoradiographic experiments provide evidence that these cells also possess high-affinity GABA uptake sites. These observations raise the possibility that in the gastrointestinal tract GABA acts as a gut hormone in a subpopulation of mucosal endocrine cells, in addition to its role as an enteric neurotransmitter.

Animals↗
PubMed1984

Isolation of highly purified Golgi membranes from rat liver. Use of cycloheximide in vivo to remove Golgi contents.

Following administration of cycloheximide to rats in order to deplete the liver of secretory products, Golgi membranes have been isolated largely free of internal contents. These membranes have a high specific activity of galactosyltransferase (400 times that of the homogenate) and are thought to be derived from the trans Golgi. Their phospholipid and polypeptide composition resembles that of Golgi membranes prepared by other procedures but their triacylglycerol and cholesterol contents are greatly reduced. These results conflict with previous reports that trans Golgi membranes are rich in cholesterol.

Albumins↗
PubMed1983

Limited breakdown of cytoskeletal proteins by an endogenous protease controls Ca2+-induced membrane fusion events in chicken erythrocytes.

The profound morphological changes which follow the treatment of chicken erythrocytes with the ionophore A23187 and Ca2+ are associated with a concomitant breakdown of certain membrane-associated proteins including alpha-spectrin, goblin and microtubule-associated proteins (MAPS) which undergo a limited proteolysis to give large, well-defined fragments. The Ca2+-sensitive protease responsible for these changes appears to be present in the soluble fraction of the cells. Treatment with TLCK or iodoacetamide inhibits both the major morphological changes and the proteolytic events but these agents do not prevent the dissociation of microtubules or the activation of endogenous sphingomyelinase which occur in cells with raised levels of intracellular Ca2+. It is suggested that the sphingomyelinase is activated as a consequence of a Ca2+-induced loss of phospholipid asymmetry in the plasma membrane.

Animals↗
PubMed1982

Microvesiculation and sphingomyelinase activation in chicken erythrocytes treated with ionophore A23187 and Ca2+.

Treatment of chicken erythrocytes with ionophore A23187 and Ca2+ leads to the disappearance of the marginal band of microtubules and to a release of the constraints which normally maintain the nucleus in a central position in the cells. The consequent close apposition of the nucleus to the plasma membrane may allow nuclear-plasma membrane fusion to occur and subsequently results in the release of microvesicles from the hybrid surface membrane. The remnant cells are spherical, and have nuclei which appear to be partly exocytosed. Concomitant with these morphological changes, there is a breakdown of 20-30% of the total cell sphingomyelin by an endogenous sphingomyelinase which does not require Ca2+ and which releases phosphorylcholine only into the cell interior. It is suggested that the pool of sphingomyelin which is broken down as a consequence of Ca2+ entry into the cells is present in the nuclear membrane and that it becomes available to the plasma membrane sphingomyelinase as a result of the close apposition of nucleus and plasma membrane induced by Ca2+.

Animals↗
PubMed1982

Acetylcholine receptors at the rat neuromuscular junction as revealed by deep etching.

Collagenase treatment of rat intercostal muscles yielded single muscle fibres in which the nerve terminals and basal lamina were removed allowing an unimpeded view of the external surface of the postsynaptic membrane. This was revealed by deep etching of freeze-fractured preparations and appeared as a maze of folds separated by deep troughs, showing on the crests of the folds a densely packed population of protrusions about 8.5 nm in diameter. These densely packed protrusions (ca. 9000 microns-2) are mainly confined to the postsynaptic regions of the sarcolemma and presumably represent the acetylcholine receptor molecules, which are highly concentrated in these areas. The protrusions are generally tightly packed without obvious regular arrangement, but in some areas, usually on the tops of the crests, they are arranged into irregular rows normal to the long axis of the folds.

Animals↗
PubMed1981

Microvesicles from sickle erythrocytes and their relation to irreversible sickling.

Incubation of sickle (HbS) erythrocytes for periods up to 96 h leads to the formation of irreversibly sickled cells (ISCs) and to the release of spectrin-free microvesicles similar to those derived from aged or Ca2+-ionophore-treated normal erythrocytes. The sickle microvesicles were somewhat larger than those from normal cells and showed minor differences in their membrane polypeptide composition. Sickle microvesicles were no different from their parent cells in their content of fetal haemoglobin. Neither microvesiculation nor formation of irreversibly sickled cells required the presence of Ca2+ in the medium but Ca2+ did accelerate both processes. Although in these prolonged incubations microvesiculation appeared to occur concomitantly with the formation of ISCs, it is not clear whether or not microvesiculation is a necessary prelude to irreversible sickling.

Anemia, Sickle Cell↗
PubMed1980

The isolation and characterization of 60 nm vesicles ('nanovesicles') produced during ionophore A23187-induced budding of human erythrocytes.

In addition to the microvesicles released during the treatment of human erythrocytes with Ca2+ and ionophore A23187, a new subpopulation of still smaller dense vesicles ('nanovesicles') has now been identified. Nanovesicles are about 60 nm in diameter, have an acetylcholinesterase activity higher than that of microvesicles and appear to be relatively enriched in sphingomyelin and correspondingly depleted of phosphatidylethanolamine. They have a polypeptide composition quite different from those of erythrocyte membranes or microvesicles, consisting largely of components of mol.wts. 60 000 and 26 000 in addition to haemoglobin. These two major polypeptides do not appear to represent contaminating cytoplasmic proteins or proteolytic subfragments of a larger protein.

Anti-Bacterial Agents↗
PubMed1978

Structure of occluding junctions in ileal epithelial cells of suckling rats.

Two kinds of occluding junctions are found between ileal epithelial cells of suckling rats: apical zonulae occludentes (ZO) and fasciae occludentes (FO) which are associated with the lateral plasma membranes of many epithelial cells. In unfixed preparations, glycerol treatment induces the further proliferation of extensive fasciae occludentes. Both kinds of junction have identical structural elements when visualized in freeze fracture replicas, although the arrangement of these elements differs. Zonulae occludentes consist of networks of branching and anastomosing linear ridges or rows of approximately 10 nm particles with 20--30 nm spaces between the rows which form narrow belt-like structures around the apical region of adjacent cells. Fasciae occludentes, on the other hand, consist of similar linear ridges or rows of particles but the junction strands are often discontinuous, open ended and only occasionally intersect with each other. Several different fracture planes through the plasma membrane in the region of the occluding junctions have been observed and these provide further evidence that two components, one from each membrane, fused at the level of the extracellular space, form the junction sealing element. Furthermore, we present evidence which indicates a staggered rather than an in-register arrangement of these two components.

Animals↗
PubMed1976

Regular structures in unit membranes. III. Further observations on the particulate component of the suckling rat ileum endocytic membrane complex.

Further morphological observations on the particulate components decorating the lumenal surfaces of membranes of the endocytic complex of the epithelial cells of the suckling rat ileum are presented. The particles each measure approximately 7.5 nm across and give the appearance of the capital letter H in frontal view. They consist of the enzyme n-acetyl-beta-glucosaminidase (NAG). They are arranged in rows called "decorated strips" with the symmetrical lateral bars in register and spaced approximately 14.5 nm apart. Decorated strips lie side-by-side in the external (lumenal) surface of the membrane. They are parallel and sometimes spaced approximately 14.5 nm apart making an orthogonal lattice. The lateral spacing between the decorated strips under certain conditions is reduced and sometimes there is shear between the adjacent ones. Occasionally, shear is present within the decorated strips themselves, with slight displacement of the two sides of each H-shaped particle. A purified preparation of these membranes has been studied by electron microscopy using thin sectioning, negative stain, Markham translation and optical diffraction computer image reconstruction methods. The individual particles comprising the array can be seen in the membrane surface in profile view when dried in a pool of negative stain. They appear either triangular or diamond-shaped in such views. If triangular, they appear to consist of three domains at the corners of an equilateral triangle. One side of each triangular figure is parallel to the membrane surface but separated from it by a dense band of negative stain approximately 2 nm thick that runs along the surface of the membrane. Sometimes a fourth symmetrical domain is visible within this dense band, giving a diamond-shaped figure. This fourth domain connects the particle to the membrane. Thus, each H-shaped particle is a double structure, with each half in profile view appearing as a diamond figure of four symmetrical domains. Each H-shaped particle is believed to consist of either two or four molecules of NAG.

Animals↗
PubMed1975

The perturbation of the human erythrocyte membrane by phospholipase C.

A study has been made of freeze-fractured preparations of erythrocyte ghosts modified by phospholipase C (Clostridium welchii). Such membranes show a decrease in surface area of up to about 47% and lipid droplets appear on their external surface but there is no loss of protein. Freeze-fracture of maximally hydrolysed membranes exposes only very small areas of A faces and these appear particle-free. Most of the membranes are simply cross-fractured. At lower levels of hydrolysis there is more extensive exposure of A fracture faces but the particle density is less than in control preparations. If such exposed faces were representative of the whole membrane then the particle density would have been expected to increase. It is suggested either that areas of membrane with increased particle density do not fracture or that the particles revealed by freeze-fracture involve phospholipid as well as protein and are not revealed in the absence of phospholipid.

Blood Proteins↗
PubMed1974

Regular structures in membranes. I. Membranes in the endocytic complex of ileal epithelial cells.

An "apical endocytic complex" in the ileal lining cells of suckling rats is described. The complex consists of a continuous network of membrane-limited tubules which originate as invaginations of the apical plasma membrane at the base of the microvilli, some associated vesicles, and a giant vacuole. The lumenal surface of this tubular network of membranes and associated vesicles is covered with a regular repeating particulate structure. The repeating unit is an approximately 7.5-nm diameter particle which has a distinct subunit structure composed of possibly nine smaller particles each approximately 3 nm in diameter. The approximately 7.5-nm diameter particles are joined together with a center-to-center separation of approximately 15 nm to form long rows. These linear aggregates, when arranged laterally, give rise to several square and oblique two-dimensional lattice arrangements of the particles which cover the surface of the membrane. Whether a square or oblique lattice is generated depends on the center-to-center separation of the rows and on the relative displacement of the particles in adjacent rows. Four membrane faces are revealed by fracturing frozen membranes of the apical tubules and vesicles: two complementary inner membrane faces exposed by the fracturing process and the lumenal and cytoplasmic membrane surfaces revealed by etching. The outer membrane face reveals a distinct array of membrane particles. This array also sometimes can be seen on the outer (B) fracture face and is sometimes faintly visible on the inner (A) fracture face. Combined data from sectioned, negatively stained, and freeze-etched preparations indicate that this regular particulate structure is a specialization that is primarily localized in the outer half of the membrane mainly in the outer leaflet.

Animals↗
PubMed1971

The molecular organization of lipids in the membrane of Escherichia coli: phase transitions.

X-ray diffraction techniques have been used to investigate the physical state of the lipids in the membrane of an unsaturated fatty acid auxotroph of Escherichia coli. Thermotropic phase transitions have been detected in membranes prepared from cells grown on various fatty acid supplements. Below the transition temperature, the X-ray diffraction pattern features a sharp ring at 420 pm (4.2 A) due to the close hexagonal packing of the apolar groups of the lipids; above the transition temperature the lipids are in a less organized liquid-crystalline state that gives rise to a diffuse band centered at a Bragg spacing of 460 pm (4.6 A). This transition occurs at or below the temperature at which the cells were grown. Disparities between this transition temperature and the temperature of discontinuities in the Arrhenius plots for proline transport and succinic dehydrogenase activity lead us to conclude that the distribution of lipids within the membrane is heterogeneous.

Cell Membrane↗