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  • 1
    ISSN: 0167-8760
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Medicine , Psychology
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-1017
    Keywords: Polyene antibiotics ; Cation conductance ; Cation effluxes ; Muscle fibre membrane
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract The effects of the polyene antibiotics nystatin (2 × 10−5−10−4 mol/l), mycoheptin (1.3 × 10−6−10−5 mol/l) and levorin (10−8−5 × 10−5 mol/l)on isolated frog skeletal muscle fibres and whole sartorius muscles of the frog have been investigated. Cation conductance was measured under current clamp conditions using a double sucrosegap technique. Cation effluxes were studied by means of flame emission photometry. All three antibiotics increased the cation conductance and efflux rates; however, differences between the polyenes were found in the steady state values of induced cation transport at a given concentration. The values of both induced conductance gA ∞ and efflux rate constants KA formed the following sequence: levorin 〉 mycoheptin 〉 nystatin, demonstrating a correlation with the order of antifungal activities. The dose-response curves of lg polyene-induced cation transport against lg of antibiotic concentration in our experiments had slope values which were much lower than those in bilayers: 1.7 and 1.3 for nystatin and mycoheptin, respectively, whereas the aromatic heptaene levorin had an even smaller concentration dependence. The decline in the equilibrium conductance caused by nystatin- and mycoheptin removal was very fast (during the first minute τ = 0.74 and 2.39 min, respectively). In contrast, levorin-induced conductance was irreversible. It is proposed that the processes which limit the rate of channel formation are different in biological and model membranes.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 0022-2828
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Medicine
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1432-2013
    Keywords: Free fatty acids ; Skeletal muscle ; Membrane conductance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract 1. Na-octanoate (in Ringer's solution) affects the membrane resistance of isolated frog skeletal muscle fibres in a biphasic way. Initially there is an increase followed by a more slowly developing decrease. The effect depends on both the concentration of the fatty acid (1–20 mM) and the time of exposure (2–20 min). 2. In muscles exposed either to isotonic K2SO4 or to Ringer's solution with Cl− substituted by methylsulphate, the octanoate-induced resistance drop is the same as in normal Ringer's solution whereas an initial increase is not observed. 3. In Na-free (Tris-) Ringer's solution, only an increase in membrane resistance is caused by octanoate. 4. The results suggest that Na-octanoate decreases the conductance of the resting muscle cell membrane for anions (Cl−) whereas the permeability for cations (Na+, K+) is increased.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1439-6327
    Keywords: Isometric contraction ; Fibre type ; Fatigue ; Position ; Hydrostatic blood pressure component
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Nine healthy men, aged between 25 and 35 years, performed sustained maximal voluntary contractions (MVC) of foot plantar, foot dorsal, and finger flexor muscles. Contractions lasted 10 min and were followed by short test contractions at 30% MVC during recovery. Two positions of the working extremity high or low were established by different body postures (supine or sitting). Under these conditions, studies of force, integrated electromyogram (iEMG), blood pressure, and heart rate showed firstly that force decreased throughout the first few minutes of maximal contraction but reached a near steady-state value after 5 to 6 min. Secondly, force decay and steady-state level depended on muscle group and body position. When sitting (low leg), muscles with a high incidence of slow twitch fibres (plantar flexors) showed a slower force decay and a higher relative steady-state force than fast dorsal flexor muscles. When supine (high leg), plantar and dorsal flexor muscles reached about the same low level of relative steady-state force. Changes in iEMG, blood pressure, and heart rate did not differ in the two positions. Thirdly, during recovery, plantar flexor muscles showed higher iEMG values as well as higher values of blood pressure and heart rate when supine than when sitting. Recovery of dorsal flexor muscles was little affected by body posture. Fourthly, force development and recovery of predominantly fast finger flexor muscles were almost independent of arm position. It was concluded that muscle fibre composition was the main factor in determining endurance capacity. However, endurance was influenced by changes in the hydrostatic blood pressure component. Because of their strong dependence on blood supply, slow plantar flexor muscles may have been more sensitive to hydrostatically induced changes of blood flow than fast dorsal and finger flexor muscles.
    Type of Medium: Electronic Resource
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