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S Bevan

Publications and source records attributed to S Bevan.

106 records · Page 6Linked to original sources
PubMed1984

A calcium-activated cation-selective channel in rat cultured Schwann cells.

Calcium-activated channels, in the plasma membrane of rat cultured Schwann cells were studied in isolated 'inside-out' membrane patches. With identical (150 mM NaCl) solutions on either side of the membrane, a single channel conductance of 32 pS was calculated for inward current; the conductance was somewhat less for outward current. The channel is about equally permeable to sodium and potassium ions, but is not detectably permeable to either chloride or calcium. Under our experimental conditions the channel is activated by high (more than 10(-4) M) concentrations of calcium and is sensitive to voltage, channel activity increasing with membrane depolarization.

Animals↗
PubMed1984

High conductance anion-selective channels in rat cultured Schwann cells.

Anion-selective channels, with very large single unit conductances, are present in the cell membrane of rat cultured Schwann cells measured with the patch-clamp technique. In inside-out membrane patches, channels with a conductance of about 450 pS (in symmetrical 150 mM NaCl) were observed. These channels did not become active until several minutes after the cytoplasmic surface had been exposed to the bathing medium, suggesting that these channels may normally be kept in an inactive state by some as yet unknown internal factor. The channel opened over a relatively small potential range (-10 mV to +20 mV) and closed rapidly at more positive and more negative potentials with voltage-dependent kinetics. Although the channels showed a slight permeability towards small cations the major permeability was to anions. The order of permeability was: I greater than Br greater than Cl greater than methyl SO4 greater than SO4 greater than acetate = isethionate. Aspartate and glutamate were not detectably permeant. The physiological role of these channels remains unknown.

Animals↗
PubMed1984

A study of the action of tetanus toxin at rat soleus neuromuscular junctions.

Tetanus toxin (TeTX) inhibits the evoked release of acetylcholine (ACh) at rat soleus end-plates. The effects of various procedures which evoke ACh release by raising the level of free intracellular calcium have been investigated at various stages of tetanus intoxication. At all stages studied TeTX has little or no effect on either the frequency or the amplitude of spontaneous miniature end-plate potentials (m.e.p.p.s). After TeTX poisoning, e.p.p. latency is more variable than normal and the slope of the relationship between ln m (quantal content) and ln [Ca]o is reduced from the control value of about 4. Plots of m-1/n against 1/[Ca]o, for n = 1-4, suggest that mmax, the maximum number of quanta releasable by nerve stimulation, is reduced at intoxicated end-plates. Blocking delayed rectification with 3-aminopyridine (1-5 mM) increases m, but has little or no effect on either the slope of ln m-ln [Ca]o plots or estimates of mmax. Several treatments which raise m.e.p.p. rate (high [Ca]o, hyperosmotic medium, addition of lanthanum) are less effective after TeTX poisoning. Some of the tested agents increase m.e.p.p. frequency by a mechanism which is thought to involve a mobilization of calcium from intracellular stores. The decline in m.e.p.p. rate after a period of high-frequency nerve stimulation is different at normal and TeTX-treated end-plates. At tetanus-intoxicated end-plates, the decline differs from that expected if TeTX acted simply to block calcium entry into the terminal. In addition, the increase in m.e.p.p. frequency observed with a high rate of nerve stimulation suggests that considerable amounts of calcium can enter the terminal with each action potential. It is concluded that TeTX blocks transmitter release by acting at a step between calcium influx to the terminal and transmitter release such that the mechanism for ACh release shows a reduced sensitivity to intracellular calcium. The possibility of an additional effect on the presynaptic calcium conductance cannot be excluded. Some differences between the properties of end-plates poisoned with TeTX and botulinum toxin are discussed.

Acetylcholine↗
PubMed1983

Denervation increases the degradation rate of acetylcholine receptors at end-plates in vivo and in vitro.

We have studied the effect of denervation on the degradation of the existing junctional acetylcholine (ACh) receptors at end-plates in rat muscles. ACh receptors were labelled by injecting animals with iodinated alpha-bungarotoxin (I-alpha BT); 1 day later the left hemidiaphragm was denervated. The degradation of bound I-alpha BT in normal and denervated muscles was examined in organ culture, beginning at various times after denervation in vivo. The original, pre-labelled end-plate ACh receptors are degraded more rapidly after denervation. The rate of degradation begins to increase shortly after the nerve is cut and reaches a maximum value at about 9 days of denervation. Muscles denervated only on transfer to organ culture also show an increase in the degradation rate of bound I-alpha BT with increasing time of denervation (time in culture). In normal diaphragm muscles, the initial rate of degradation of functional ACh receptors, after correcting for non-degradative loss of I-alpha BT, is 0.0018 h-1 (t1/2 = 383 h). The maximal rate at denervated end-plates is 0.0073 h-1 (t1/2 = 94 h). For soleus, sternomastoid, plantaris and intercostal innervated muscles the apparent rate of ACh receptor degradation either in vitro or in vivo ranged from 0.0005 h-1 to 0.002 h-1. The rate of loss of bound I-alpha BT in vivo is more rapid at denervated end-plates than at innervated end-plates. For diaphragm muscles, the rates of I-alpha BT degradation measured in organ culture are able to describe the relative rates of loss of I-alpha BT from innervated and denervated muscles in vivo. At short times after labelling, a fraction (10-20%) of the I-alpha BT bound to innervated muscles is degraded more rapidly than the remaining toxin. The possibility that these I-alpha BT binding sites are degraded at the rate characteristic of extrajunctional receptors on denervated muscle fibres is discussed.

Acetylcholine↗
PubMed1977

Modulation of acetylcholine receptor by antibody against the receptor.

Antibody against acetylcholine receptor induces an increase in the rate of degradation of acetylcholine receptors on a mouse cell line (BC(3)H-1) and cultured rat skeletal muscle. The increased rate of degradation results in a lowered density of acetylcholine receptors on muscle membrane and a lowered sensitivity to iontophoretically applied acetylcholine. The modulation of acetylcholine receptor is energy, temperature, and time dependent and may be related to antigenic modulation found in other systems. Acetylcholine noise analysis demonstrates that antibody against acetylcholine receptor reduces the channel mean conductance and mean open time slightly. It is concluded that antibody binds to the acetylcholine receptor, impairs its function, and induces receptor degradation. This results in a lowered density of acetylcholine receptor and a lowered sensitivity to acetylcholine. Patients with myasthenia gravis have antibodies to their acetylcholine receptor in their serum. Antigenic modulation of receptor in the muscle of patients with myasthenia gravis could contribute to the observed decrease in amplitudes of miniature endplate potentials and in muscle acetylcholine sensitivity, and the symptoms of muscular weakness.

Acetylcholine↗
PubMed1977

The distribution of alpha-bungarotoxin binding sites of mammalian skeletal muscle developing in vivo.

1. The distribution of alpha-bungarotoxin binding sites on embryonic and neonatal rat skeletal muscle fibres was determined by autoradiography. Most of the bungarotoxin binding could be inhibited by curare. This observation, together with the spatial distribution of toxin-binding sites, indicates that the distribution of bound toxin reflects that of acetylcholine (ACh) receptors on these developing muscle cells.2. At 15 days of embryogenesis, muscle fibres showed an essentially uniform distribution of receptors. By 16 days, many fibres showed an accumulation of receptors in their mid-region. This accumulation was at the same location as histochemically demonstrated cholinesterase activity.3. At 16 days ACh receptors were distributed over the entire length of the fibres, with a gradient of increasing density as the accumulation was appoached. The density of toxin binding sites in the accumulation was greater than the general level on 15 day cells, suggesting that the high junctional density does not develop solely by the loss of extrajunctional receptors.4. The accumulations of ACh receptors became more pronounced and circumscribed with embryonic development, and after birth the extent of the localizations appeared to follow the size of the neuromuscular junction. The extrajunctional receptor density decreased with development, and by 1 week after birth was undetectable by the methods used.5. The results suggest that the high junctional receptor density found on adult, innervated skeletal muscle fibres develops after the formation of the neuromuscular junction.

Age Factors↗
PubMed1976

Sub-miniature end-plate potentials at untreated frog neuromuscular junctions.

1. Some properties of unusually small spontaneous miniature end-plate potentials (sub-min.e.p.p.s) at untreated end-plates of adult frogs have been examined. 2. These small potentials, which show a fast time course, add a highly skewed component to the normal Gaussian distribution of min.e.p.p. amplitudes. 3. Sub-min.e.p.p.s were not evoked by single nerve stimuli, though their frequency could be raised by tetanic nerve stimulation. 4. When min.e.p.p. rate was raised by addition of lanthanum, submin.e.p.p. frequency was also increased, though by a smaller factor than normal min.e.p.p.s. In contrast, sub-min.e.p.p. frequency was unchanged when min.e.p.p. rates were raised by either ethanol or hypertonic solutions.

Action Potentials↗
PubMedDate not supplied

Retroviral gene transfer into porcine keratinocytes following improved methods of cultivation.

We embarked on a program examining the application of cultured epithelial sheets to skin wounds in pigs using retroviral gene transfer as a means to follow the grafted cells. In the past similar studies have been hampered by an inability to grow porcine keratinocytes without seeding at an extremely high density. In this study we found that excellent results could be achieved with Opti-MEM-1 (Gibco BRL Life Technologies) containing 1 per cent foetal calf serum, 0.5 mM Ca2+ and no other growth factors or stimulants. Keratinocytes were plated on gamma-irradiated 3T3 feeders on surfaces which had previously been coated with rat tail collagen I. Keratinocyte cultures were established at a seeding density of 5 x 10(4) cm-2. The yield of cells from 1 cm2 of skin was sufficient to set up a 75 cm2 flask. Cultures reached 80-90 per cent confluence in 7-10 days, after which they were passaged 1:3 multiple times, taking 3-4 days to reach the same confluency. Allowing cultures to remain confluent for 1 week was sufficient to allow Dispase removal of an intact sheet. Using these techniques porcine keratinocytes were transduced at an average frequency of 25.3 per cent (+/- 14.0 SEM) with the retroviral vector MFG lacZ nls by growth on the gamma-irradiated retroviral producer line GP + envAm12.

Animals↗
PubMedDate not supplied

Voltage-dependent potassium currents in cultured astrocytes.

Astrocytes are a major cell type in the mammalian central nervous system (CNS), yet their functions remain uncertain. There are two principal classes of these glial cells--protoplasmic astrocytes, found mainly in grey matter, and fibrous astrocytes, which occur mainly in white matter. Recently, these two types of astrocytes have been distinguished in cultures of developing CNS and have been shown to be biochemically distinct. Because both types contain large numbers of glial filaments in vitro and hence appear 'fibrous', we will refer to them as type 1 (protoplasmic) and type 2 (fibrous) astrocytes. Most type 2 astrocytes in culture share several properties with neurones; for example, they have a process-bearing morphology and bind tetanus toxin and the monoclonal antibody A2B5, both of which recognize specific gangliosides and were initially considered to be neurone-specific markers in the CNS. We have therefore investigated whether type 2 astrocytes also share electrophysiological properties with neurones. Using intracellular microelectrode and 'whole-cell' (patch-clamp) recording techniques, we have now found that both type 1 and type 2 astrocytes in culture have time- and voltage-dependent potassium ion conductances which, until recently, were considered to be confined largely to electrically excitable cells.

Animals↗