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  • 1
    Electronic Resource
    Electronic Resource
    Sauvagine Regulates Ca2+ Oscillations and Electrical Membrane Activity of Melanotrope Cells of Xenopus laevis (2002)
    Oxford, UK : Blackwell Science, Ltd
    Journal of neuroendocrinology 14 (2002), S. 0 
    Details
    ISSN: 1365-2826
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Ca2+ oscillations regulate secretion of the hormone alpha-melanphore-stimulating hormone (α-MSH) by the neuroendocrine pituitary melanotrope cells of the amphibian Xenopus laevis. These Ca2+ oscillations are built up by discrete increments in the intracellular Ca2+ concentration, the Ca2+ steps, which are generated by electrical membrane bursting firing activity. It has been demonstrated that the patterns of Ca2+ oscillations and kinetics of the Ca2+ steps can be modulated by changing the degree of intracellular Ca2+ buffering. We hypothesized that neurotransmitters known to regulate α-MSH secretion also modulate the pattern of Ca2+ oscillations and related electrical membrane activity. In this study, we tested this hypothesis for the secretagogue sauvagine. Using high temporal-resolution Ca2+ imaging, we show that sauvagine modulated the pattern of Ca2+ signalling by increasing the frequency of Ca2+ oscillations and inducing a broadening of the oscillations through its effect on various Ca2+ step parameters. Second, we demonstrate that sauvagine caused a small but significant decrease in K+ currents measured in the whole-cell voltage-clamp, whereas Ca2+ currents remained unchanged. Third, in the cell-attached patch-clamp mode, a stimulatory effect of sauvagine on action current firing was observed. Moreover, sauvagine changed the shape of individual action currents. These results support the hypothesis that the secretagogue sauvagine stimulates the frequency of Ca2+ oscillations in Xenopus melanotropes by altering Ca2+ step parameters, an action that likely is evoked by an inhibition of K+ currents.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2826.2002.00838.x
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  • 2
    Electronic Resource
    Electronic Resource
    Voltage-activated ionic channels and conductances in embryonic chick osteoblast cultures (1988)
    Springer
    The journal of membrane biology 101 (1988), S. 141-150 
    Details
    ISSN: 1432-1424
    Keywords: ionic channel ; membrane conductance ; osteoblast ; patch-clamp ; voltage activated ; outward rectification
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Patch-clamp measurements were made on osteoblast-like cells isolated from embryonic chick calvaria. Cell-attachedpatch measurements revealed two types of high conductance (100–250 pS) channels, which rapidly activated upon 50–100 mV depolarization. One type showed sustained and the other transient activation over a 10-sec period of depolarization. The single-channel conductances of these channel types were about 100 or 250 pS, depending on whether the pipettes were filled with a low K+ (3mm) or high K+ (143mm) saline, respectively. The different reversal potentials under these conditions were consistent with at least K+ conduction. Whole-cell measurements revealed the existence of two types of outward rectifying conductances. The first type conducts K+ ions and activates within 20–200 msec (depending on the stimulus) upon depolarizing voltage steps from 〈−60 mV to 〉−30 mV. It inactivates almost completely with a time constant of 2–3 sec. Recovery from inactivation is biphasic with an initial rapid phase (1–2 sec) followed by a slow phase (〉20 sec). The second whole-cell conductance activates at positive membrane potentials of 〉+50 mV. It also rapidly turns on upon depolarizing voltage steps. Activation may partly disappear at the higher voltages. Its single channels of 140 pS conductance were identified in the whole cell and did conduct K+ ions but were not highly Cl− or Na+ selective. The results show that osteoblasts may express various types of voltage controlled ionic channels. We predict a role for such channels in mineral metabolism of bone tissue and its control by osteoblasts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01872829
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  • 3
    Electronic Resource
    Electronic Resource
    Differential depolarization-activated calcium responses in fetal and neonatal rat osteoblast-like cells (1994)
    Springer
    Calcified tissue international 54 (1994), S. 278-283 
    Details
    ISSN: 1432-0827
    Keywords: Calcium channels ; Development ; Osteoblastic cells ; Fura-2 ; Patch-clamp
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine , Physics
    Notes: Abstract The present study evaluates differential occurrence of voltage-dependent calcium channels (VDCC) in the membranes of fetal (FROB) and neonatal (NROB) calvarian rat osteoblastic cells in primary culture. The intracellular calcium concentration ([Ca2+]i) was monitored upon depolarization of the cell membrane with the use of high K+ containing extracellular solutions. [Ca2+]i was measured in populations of cells as well as in individual cells using Fura-2, whereas the membrane potential (Em) was recorded in parallel experiments using patch-clamp techniques. Increasing the extracellular K+ concentration resulted in an instantaneous depolarization of Em of both FROB and NROB. This depolarization of Em did not significantly affect [Ca2+]i of populations of FROB and neonatal osteoblast precursors (NpROB). In contrast to FROB and NpROB, NROB populations responded to depolarization with significant transient [Ca2+]i increases that could be blocked by the calcium antagonist verapamil and were absent if extracellular Na+ was replaced for choline instead of K+. In individual cell measurements, response frequencies as well as the magnitude of [Ca2+]i responses upon depolarization of NROB were much higher than those of FROB, suggesting that more NROB than FROB possess VDCC. This phenomenon might point to a development-related expression of VDCC in the membranes of osteoblast-like cells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00295951
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  • 4
    Electronic Resource
    Electronic Resource
    Modelling action potentials and membrane currents of mammalian skeletal muscle fibres in coherence with potassium concentration changes in the T-tubular system (1999)
    Springer
    European biophysics journal 28 (1999), S. 317-329 
    Details
    ISSN: 1432-1017
    Keywords: Key words Action potential model ; Inward rectifier ; Sodium-potassium pump ; Tubular potassium concentration ; Muscle fatigue
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract During prolonged activity the action potentials of skeletal muscle fibres change their shape. A model study was made as to whether potassium accumulation and removal in the tubular space is important with respect to those variations. Classical Hodgkin-Huxley type sodium and (potassium) delayed rectifier currents were used to determine the sarcolemmal and tubular action potentials. The resting membrane potential was described with a chloride conductance, a potassium conductance (inward rather than outward rectifier) and a sodium conductance (minor influence) in both sarcolemmal and tubular membranes. The two potassium conductances, the Na-K pump and the potassium diffusion between tubular compartments and to the external medium contributed to the settlement of the potassium concentration in the tubular space. This space was divided into 20 coupled concentric compartments. In the longitudinal direction the fibre was a cable series of 56 short segments. All the results are concerned with one of the middle segments. During action potentials, potassium accumulates in the tubular space by outward current through both the delayed and inward rectifier potassium conductances. In between the action potentials the potassium concentration decreases in all compartments owing to potassium removal processes. In the outer tubular compartment the diffusion-driven potassium export to the bathing solution is the main process. In the inner tubular compartment, potassium removal is mainly effected by re-uptake into the sarcoplasm by means of the inward rectifier and the Na-K pump. This inward transport of potassium strongly reduces the positive shift of the tubular resting membrane potential and the consequent decrease of the action potential amplitude caused by inactivation of the sodium channels. Therefore, both potassium removal processes maintain excitability of the tubular membrane in the centre of the fibre, promote excitation-contraction coupling and contribute to the prevention of fatigue.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002490050214
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  • 5
    Electronic Resource
    Electronic Resource
    K+ conductance of mechanically dissociated rat sensory neurones in long lerm cultures in a defined medium (1989)
    Springer
    Pflügers Archiv 414 (1989), S. S129 
    Details
    ISSN: 1432-2013
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00582265
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  • 6
    Electronic Resource
    Electronic Resource
    Feedback of isotonic muscle contraction on neuromuscular impulse transmission in the frog (1975)
    Springer
    Pflügers Archiv 355 (1975), S. 291-306 
    Details
    ISSN: 1432-2013
    Keywords: Neuromuscular Transmission ; Isotonic Contraction ; Muscle Length ; Feedback
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The effect of isotonic contraction on neuromuscular impulse transmission was studied in thein vivo M. gastrocnemius preparation of the frog. The N. ischiadicus was stimulated regularly with single pulses at different frequencies in the range of 1/8 to 8 Hz. When steady state conditions were reached, each half-minute a second stimulus was added to one of these pulses. The interval between the pulses of the pair was varied within the contraction cycle of the first stimulus. Compound extracellular or single intracellular action potentials were recorded from the muscle. At frequencies of 1/2–2 Hz a depression of the amplitude of the second compound muscle action potential of up to 50% of the first response was found when the muscle contracted isotonically. However, the second response was facilitated during an isometric contraction. The time course of the depression was equal to that of muscle shortening during the twitch, while the time course of facilitation corresponded roughly to that of facilitation of transmitter release. At lower frequencies the “isotonic depression” or “isometric facilitation” was not or only slightly present. However, at 1/8 Hz the depression could be evoked or increased by curarization. At frequencies higher than 2 Hz facilitation dominated over depression under isotonic conditions. With flexible intracellular micro-electrodes it was shown that the depression of the amplitude of the compound muscle action potential observed during the isotonic twitch was due to a reduction in neuromuscular impulse transmission. It is concluded that the isotonic depression is a negative feedback effect of the change of length of the contracting muscle on synaptic impulse transmission, probably due to an effect of length on transmitter release.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00579851
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  • 7
    Electronic Resource
    Electronic Resource
    Effects of potassium channel blockers on differentiation ofDictyostelium discoideum (1989)
    Springer
    Pflügers Archiv 414 (1989), S. S148 
    Details
    ISSN: 1432-2013
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00582276
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  • 8
    Electronic Resource
    Electronic Resource
    Three voltage-activated K+ conductances and an ATP-activated conductance in freshly isolated embryonic chick osteoclasts (1989)
    Springer
    Pflügers Archiv 414 (1989), S. S166 
    Details
    ISSN: 1432-2013
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00582285
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  • 9
    Electronic Resource
    Electronic Resource
    A topographical study of the distribution of end-plates in the cutaneus pectoris, sartorius, and gastrocnemius muscles of the frog (1978)
    New York, NY : Wiley-Blackwell
    Journal of Morphology 155 (1978), S. 327-348 
    Details
    ISSN: 0362-2525
    Keywords: Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: End-plate distributions have been determined for three frog muscles of different morphology in order to relate end-plate topography to spatial muscle structure and nerve branching. Koelle's cholinesterase technique was applied, both on whole muscles and frozen sections. The end-plates of the short parallel-fibered cutaneus pectoris muscle appeared to be located in short bands along the nerve branches. The nerve tree is restricted to a zonal area across the middle part of the muscle. Depending on the way the nerve branches, the end-plate bands form innervation patterns, varying from one single continuous band to multiple distributed bands. In the latter case one frequently observes that different end-plate bands do not run across the same longitudinal muscle fiber area, although the respective nerve branches run parallel across this area. The long parallel-fibered sartorius muscle has a wider nerve tree and exhibits the same phenomenon for close parallel nerve branches, but end-plate bands along parallel nerve branches far apart cover the same muscle fiber area. The end-plate distribution in the bipennate, short-fibered gastrocnemius is zonal throughout the muscle except in certain compartments containing tonic fibers. The end-plate zone centers around the inner aponeurosis about half-way between the muscle tendon junctions of the fibers and is visible only at the muscle surface where muscle fibers run over their entire length at that surface. The results are of general use in the electrophysiology of neuromuscular transmission because they illustrate how in certain twitch muscles neuromuscular morphology may help to localize end-plates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jmor.1051550306
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