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  • 1
    Electronic Resource
    Electronic Resource
    Cellular and Subcellular Localization of Hexokinase, Glutamate Dehydrogenase, and Alanine Aminotransferase in the Honeybee Drone Retina (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 62 (1994), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Subcellular localization of hexokinase in the honeybee drone retina was examined following fractionation of cell homogenate using differential centrifugation. Nearly all hexokinase activity was found in the cytosolic fraction, following a similar distribution as the cytosolic enzymatic marker, phosphoglycerate kinase. The distribution of enzymatic markers of mitochondria (succinate dehydrogenase, rotenone-insensitive cytochrome c reductase, and adenylate kinase) indicated that the outer mitochondrial membrane was partly damaged, but their distributions were different from that of hexokinase. The activity of hexokinase in purified suspensions of cells was fivefold higher in glial cells than in photoreceptors. This result is consistent with the hypothesis based on quantitative 2-deoxy[3H]glucose autoradiography that only glial cells phosphorylate significant amounts of glucose to glucose-6-phosphate. The activities of alanine aminotransferase and to a lesser extent of glutamate dehydrogenase were higher in the cytosolic than in the mitochondrial fraction. This important cytosolic activity of glutamate dehydrogenase was consistent with the higher activity found in mitochondria-poor glial cells. In conclusion, this distribution of enzymes is consistent with the model of metabolic interactions between glial and photoreceptor cells in the intact bee retina.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1471-4159.1994.62051939.x
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  • 2
    Electronic Resource
    Electronic Resource
    Monovalent Cations Induce Microsporidian Spore Germination In Vitro (1994)
    Oxford, UK : Blackwell Publishing Ltd
    The @journal of eukaryotic microbiology 41 (1994), S. 0 
    Details
    ISSN: 1550-7408
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: The relative capacity of Na+, K+ and Cl- to stimulate germination of spores of the microsporidian Nosema algerae, a pathogen of mosquitoes, was examined by ion substitution experiments. Sodium at 0.1 M was ineffective to produce the high percentage of germination that typically occurs with 0.1 M NaCl (the normal stimulation solution) if Cl- was substituted with the usually impermeant anions SO42-, HPO42-, or the organic acids oxalate, cacodylate, EGTA, MES and HEPES. However, substantial concentration- and pH-dependent germination was seen with Na2SO4 in the 0.2-0.8 M Na+ range. Similar results were obtained with solutions of K+ accompanied by impermeant anions. In contrast, the chloride salts of usually impermeant cations, like choline and triethanolamine, failed to germinate spores even at 0.8 M unless Na+ or K+ was independently added. The presence of 0.5 M choline chloride in the medium reduced the levels of Na2SO4 required to produce germination down to equivalence with those of Na+ in the normal stimulation solution. Monensin, a Na+ ionophore, facilitated the germination induced by a medium-level stimulus (0.04 M NaCl) in sonicated samples. These findings indicate that N. algerae spores germinate in response to the alkali metal cations, while CI- plays a passive role by diffusing to maintain internal electroneutrality during cation influx. A possible mechanism of cation action in spore germination is suggested on the basis of these results and observations on other systems of intracellular motility.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1550-7408.1994.tb06043.x
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  • 3
    Electronic Resource
    Electronic Resource
    Germination of Nosema algerae (Microspora) Spores: Conditional Inhibition by D2O, Ethanol and Hg2+ Suggests Dependence of Water Influx upon Membrane Hydration and Specific Transmembrane Pathways (1997)
    Oxford, UK : Blackwell Publishing Ltd
    The @journal of eukaryotic microbiology 44 (1997), S. 0 
    Details
    ISSN: 1550-7408
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: . The germination of microsporidian spores under conditions expected to affect water flow across the plasma membrane-wall complex was studied by assessing their responses to in vitro stimulation with Na+ or K+. Partial or full substitution of common water with D2O, which more effectively coats ions and electrostatically-charged cell surfaces with relatively stable hydration layers, delayed and inhibited spore germination in a concentration-dependent manner; yet, preincubation in 100% D2O did not change the normal response to standard stimulation. Water structure-breaking conditions, such as an increase in temperature (within the 15° C to 40° C range) or in ionic strength (2- to 10-fold normal), opposed the inhibition by D2O and allowed significant stimulation by Li+, the monovalent cation with the largest hydration diameter and a usually weak stimulant action on the spores. Ethanol, known to reduce water permeation across cell membranes and phospholipid bilayers, also caused a powerful and dose-dependent (1% to 4% v/v) inhibition of spore germination, but pretreatment with ethanol did not affect the normal response. HgCl2, an inhibitor of specific water channels, blocked spore germination at just 250 μM in the normal stimulation solution irrespective of the temperature, and permitted only a delayed response in high salt stimulation solutions. However, the inhibition by Hg2+ was abolished by the simultaneous presence of 2-mercaptoethanol in the medium. These results suggest (1) that spore germination is keenly dependent upon the hydration states of both the plasma membrane-wall complex and the stimulant ions, and (2) that osmotic water flows into the spores through specific transmembrane pathways with critical sulfhydryl groups, i.e. analogous to the water channels that facilitate water movements across the plasma membranes of highly permeable cells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1550-7408.1997.tb05946.x
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  • 4
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    El impacto de los proyectos sectoriales sobre el ambiente en la República Mexicana (1992)
    Madrid : Periodicals Archive Online (PAO)
    Estudios geográficos. 53:206 (1992:enero/abr.) 199 
    Details
    ISSN: 0014-1496
    Topics: Geography
    URL: http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pao:&rft_dat=xri:pao:article:6094-1992-053-06-000014
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  • 5
    Electronic Resource
    Electronic Resource
    Calcium uptake by smooth endoplasmic reticulum of peeled retinal photoreceptors of the crayfish (1988)
    Springer
    Journal of comparative physiology 162 (1988), S. 91-100 
    Details
    ISSN: 1432-1351
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary The localization and basic properties of Ca2+-accumulating sites in crayfish photoreceptors were studied with a novel preparation of peeled retinula cells in suspension. Peeled photoreceptors were obtained by gentle mechanical disruption of the retina, and incubated in media based on a Ca2+-EGTA buffer with ATP and oxalate. Electron microscopy of photoreceptors so treated showed the appearance of peculiar dense deposits inside vesicles of smooth endoplasmic reticulum (SER). EGTA-extraction and energy-dispersive X-ray microanalysis identified Ca as a major constituent of such deposits.45Ca2+ uptake experiments with peeled photoreceptors or with the crude particulate fraction of retinal homogenates revealed a rapid binding of radioactivity over the first 8 min, followed by a slower continued accumulation, which did not occur in the absence of ATP.45Ca2+ uptake is stimulated by an increase in the concentration of free Ca over the range 4×10−7 to 5×10−6 M, and becomes inhibited at higher levels.45Ca2+ uptake is depressed when K+ is replaced by Na+ or Li+ as the main monovalent cation in the medium, but it is not affected by illumination nor by the presence of caffeine or ruthenium red. These findings attest that the SER has a Na+-sensitive capacity for regulating the intracellular concentration of Ca2+ in these photoreceptors, and support the hypothesis of its probable role in the control of pigment granule transport and other structural changes involved in light/dark adaptation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01342706
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  • 6
    Electronic Resource
    Electronic Resource
    Contraction of epithelial (MDCK) cells in response to low extracellular calcium is dependent on extracellular sodium (1999)
    Springer
    Journal of muscle research and cell motility 20 (1999), S. 761-770 
    Details
    ISSN: 1573-2657
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract Like other cells of epithelial origin, MDCK cells respond with a reversible structural transformation to a diminution in the concentration of extracellular Ca2+. Upon deprivation of Ca2+ in the medium the cells undergo an active contraction mediated by the actin-myosin cytoskeleton, in parallel to detachment of the intercellular contacts and appearance of free spaces in the epithelium or monolayer (Castillo et al., 1998). We now present results indicating that the decrease of external Ca2+ plays an indirect and non-specific role in activating contraction, probably by allowing an influx of Na+. The omission of external Ca2+ had no effect when it was replaced by Mg2+, Ba2+ or Hg2+, and the addition of any of these divalent cations induced relaxation of cells previously contracted by exposure to low Ca2+. A null or weak response was observed also when Ca2+ was lowered in a solution where Na+ was replaced by choline or in the presence of amiloride (30 μM), which reduces the permeability of the plasma membrane to Na+. Restitution of Na+ or removal of amiloride were followed by contraction in the same cultures. Li+ proved an able substitute of Na+ as requisite for cell contraction in response to Ca2+ depletion. Monensin (0.1 mM) –an ionophore selective for Na+– and to a lesser extent ouabain (0.1 mM) –an inhibitor of Na+ extrusion across the plasma membrane– , both stimulated contraction in the presence of the normal level of external Ca2+. Decreasing by half the normal concentration of external K+ facilitated cell contraction, but typical responses were observed when K+ was increased to 40 mM by partial substitution for Na+. These findings attest that cell contraction in response to low Ca2+ is likely due to an increase in the permeability of the plasma membrane to Na+, though not to membrane depolarization as such. Evidences from other motile systems suggest that Na+ influx might in turn cause an elevation of cytoplasmic Ca2+, which activates the actin-myosin cytoskeleton.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005580425932
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  • 7
    Electronic Resource
    Electronic Resource
    Dynamics of polar filament discharge and sporoplasm expulsion by microsporidian spores (1992)
    New York, NY : Wiley-Blackwell
    Cell Motility and the Cytoskeleton 22 (1992), S. 38-50 
    Details
    ISSN: 0886-1544
    Keywords: cytomechanics ; invasion mechanisms ; kinematic analysis ; parasites ; protoplasm flow ; video microscopy ; Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: Spores of the microsporidium Nosema algerae were stimulated to germinate in vitro while observed with video-enhanced contrast microscopy. Field-by-field playback of tape-recorded sequences yielded the first serial illustrations and kinematic analysis of the explosive discharge of the polar filament and the sporoplasm. The filament emerges from the anterior pole of the spore in a regularly pitched helicoidal course along a nearly straight axis, with a mean maximum instant velocity of 105 μm/s. Just before elongation is completed the filament tip follows a tortuous path that often results in a curved or spiralling terminal configuration. Then elongation stops and, after a lag that may vary from less than 15 to over 500 ms, the sporoplasm pours out at the filament tip forming a globule that quickly grows up to a size larger than its original volume within the spore. Concomitantly, the helical filament becomes straightened and frequently the spore body is pulled forward. Thereafter a relaxed filament, usually 5-10% shorter than when maximally extended, remains connecting the empty spore case and the sporoplasmic droplet. Experiments with hyperosmolar media produced a considerable slowdown of filament extrusion and often precluded sporoplasm discharge. The present results are fully consistent with the hypothesis of a hydrostatic pressure-triggered mechanism of spore germination, and revealed that the process is composed of two discrete phases separated by a variable lag: (1) complete eversion of the polar filament, and (2) passage of the main sporoplasm mass along the tube. The data provide a preliminary basis toward the conception of a quantitative physical model of microsporidian spore germination. © 1992 Wiley-Liss, Inc.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cm.970220105
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