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S Bevan

Publications and source records attributed to S Bevan.

At least 91 records · Page 5Linked to original sources
PubMed1991

The effect of tumour bearing on skeletal muscle glutamine metabolism.

1. The effects of tumour bearing on glutamine metabolism in rat skeletal muscle were examined using the Walker 256 carcinosarcoma. 2. There was a rapid and marked decrease in skeletal muscle glutamine content, which was correlated with the size of the tumour, and a decrease in plasma glutamine concentration. 3. The rate of release of glutamine from EDL muscle in vitro was increased in cachectic, tumour bearing animals, but was unaffected from the soleus muscle of the same animals. 4. It is hypothesized that the increase in the rate of muscle glutamine release during cachexia represents a response of this tissue in order to satisfy the demand for glutamine by the tumour or by cells of the immune system.

Animals↗
PubMed1991

Capsaicin causes prolonged inhibition of voltage-activated calcium currents in adult rat dorsal root ganglion neurons in culture.

The effect of capsaicin on voltage-activated calcium currents was investigated in voltage-clamped somata of cultured adult rat dorsal root ganglion neurons. About half the neurons studied were sensitive to capsaicin, which induced an inward current at negative membrane potentials accompanied by an increase in membrane conductance. In the sensitive neurons capsaicin inhibited voltage-activated calcium current to an extent that depended on the size and duration of the capsaicin-induced inward current. Calcium channels were protected from the long-lasting inhibitory action of capsaicin by substituting extracellular Ca with Mg or Ba when capsaicin was applied, which suggests that the inhibition was dependent on the presence of extracellular Ca. Substituting Ca with Co did not prevent the prolonged block of calcium channels. It is concluded that the inhibition of voltage-activated calcium currents by capsaicin is secondary to increased intracellular Ca levels due to calcium entry through capsaicin-activated cation-specific ion channels in the plasma membrane. Long-lasting inhibition of voltage-activated calcium channels may contribute to the mechanism of the analgesic and anti-inflammatory effects of capsaicin through inhibition of neurotransmitter release from central and peripheral terminals of primary afferent nociceptive neurons.

Animals↗
PubMed1991

Chemical activation of nociceptive peripheral neurones.

In inflammation, non-neuronal cells produce a variety of chemical mediators that act on nociceptive neurones. Ultimately, the discharge of these neurones is controlled by the activity of membrane ion channels. Some chemical mediators (e.g. ATP, protons, 5-hydroxytryptamine) act on receptors that are linked directly to ion channels. Other mediators (e.g. bradykinin) act indirectly through receptors linked to second messenger systems and in this way modulate the activity of ion channels and either activate or sensitize the neurones. The eicosanoids, which are produced by a variety of cell types, have important intra- and inter-cellular roles in nociception. The interactions between neurones and non-neuronal cells are likely to be complex as some types of non-neuronal cells express receptors for sensory neuropeptides (substance P). Recent studies also suggest that cytokines and growth factors can have long term effects on nociceptive neurone function.

Bradykinin↗
PubMed1991

Modes of hexamethonium action on acetylcholine receptor channels in frog skeletal muscle.

1. The antagonism between hexamethonium and cholinoceptor agonists was investigated in frog skeletal muscle fibres with voltage-clamp techniques. Hexamethonium caused a voltage-dependent reduction in the amplitude of endplate currents. For neurally evoked endplate currents, the reduction increased e-fold with a 38 mV membrane hyperpolarization. 2. The effect of hexamethonium on the time course of endplate currents was small, and was most apparent as a slight prolongation of the decay phase at hyperpolarized potentials (more negative than -100 mV). A similar small prolongation of single channel lifetime was detected with fluctuation analysis techniques. Hexamethonium produced a voltage-dependent reduction in apparent single channel conductance as the membrane was hyperpolarized. 3. Log (concentration-response) curves for acetylcholine (ACh)-induced currents, determined either from currents accompanying ramp changes in membrane potential or from steady state currents in voltage-jump experiments, were less steep for responses in the presence of hexamethonium. This reduction in slope became more pronounced at more negative membrane potentials. Observations at +50 mV suggested that the equilibrium constant for competitive antagonism was approximately 200 microM. 4. In voltage-jump experiments with a two-microelectrode voltage clamp, the current evoked by ACh in the presence of hexamethonium differed from that recorded with ACh alone. In the presence of hexamethonium, the expected 'instantaneous' ohmic increase in membrane current in response to a hyperpolarizing step was not detected; instead a decrease in current was observed. This problem was further investigated with a vaseline-gap voltage-clamp technique which provides improved temporal resolution. With this method a rapid decrease in the ACh-induced inward current was observed with step hyperpolarizations in the presence of hexamethonium. 5. When the membrane potential was stepped back to its resting level from a more hyperpolarized potential in the presence of hexamethonium, there was a surge of ACh-induced inward current that decayed with a time constant of less than 100 microseconds. 6. The slow relaxation in the ACh-induced current that followed a voltage step recorded in the presence of hexamethonium was slower than that recorded with ACh alone. In the presence of hexamethonium the time constant of this relaxation increased e-fold for a 67 mV hyperpolarization. 7. The results are consistent with a rapid voltage-dependent block of ACh-activated channels by hexamethonium with hyperpolarization, and voltage-dependent unblock with depolarization. The voltagedependent block is combined with competitive antagonism at the ACh receptors. However, not all observations appear to be compatible with a simple sequential block of open ion channels, but rather suggest that occupation of the channel by hexamethonium may not prevent channel closure.

Animals↗
PubMed1991

Properties of 5-hydroxytryptamine3 receptor-gated currents in adult rat dorsal root ganglion neurones.

1. Responses to 5-hydroxytryptamine (5-HT) were examined on rat dorsal root ganglion (DRG) neurones maintained in tissue cultures, by use of whole cell recording techniques. 2. 5-HT (usually 10 microM) evoked a depolarization associated with an increase in membrane conductance in 40% of DRG neurones. There was a considerable variation in the size and persistence of this response between different batches of cells. 3. The 5-HT response was mimicked by applying the agonists 2-methyl-5-HT (10 microM) and phenylbiguanide (10 microM). Responses were blocked by ICS 205-930 (100 nM), but not by methysergide (0.1-1.0 microM). 4. 5-HT currents could be carried by sodium and caesium ions, but not by choline ions. The amplitude and duration of the 5-HT responses were dependent on the concentration of divalent cations in the extracellular solution: both became greater when calcium and magnesium concentrations were decreased. 5. Staurosporine, a putative antagonist of protein kinases, inhibited responses to 5-HT.

Alkaloids↗
PubMed1991

Protons activate a cation conductance in a sub-population of rat dorsal root ganglion neurones.

1. The responses of adult and neonatal rat dorsal root ganglion (DRG) neurones to buffered acidic solutions were studied with both voltage clamp and radioactive ion flux techniques. Electrophysiological experiments were made on acutely isolated neurones and ion flux experiments were made on cells that had been in culture for 3-6 days. 2. Acid solutions of pH < 6.2 evoked a sustained, slowly inactivating inward current in neurones voltage clamped at negative holding potentials. The size of the current increased with increasing proton concentrations. This response was restricted to a sub-population (approximately 45%) of adult and neonatal rat DRG neurones and was distinct from a rapidly activating and inactivating proton-induced inward sodium current that was also found in DRG neurones. 3. The proton-activated sustained current was due to an increase in cation conductance that allowed K+, Cs+ and Na+ to pass with PK/PNa = 1.32 and PCs/PNa = 1.12. 4. Radioactive ion efflux experiments made on neonatal rat cultured DRG neurones showed that protons also increased the permeability to both [14C]guanidinium and 86Rb+ ions. The half-maximal increase in efflux rate for 86Rb+ occurred at pH 5.8. Acid solution also stimulated the efflux of 86Rb+ in cultures of adult rat neurones. 5. Cells that showed a late, sustained proton-activated current also responded to capsaicin. In addition, no proton-activated fluxes of either [14C]guanidinium or 86Rb+ ions were observed in cultures of DRG neurones that had been treated with high concentrations of capsaicin (10 microM) to kill the capsaicin-sensitive neurones. Thus this proton-activated current is restricted largely, if not exclusively, to capsaicin-sensitive peripheral sensory neurones.

Animals↗
PubMed1990

Cellular mechanism of action of resiniferatoxin: a potent sensory neuron excitotoxin.

The mechanism of activation of sensory neurons by the potent irritant resiniferatoxin (RTX) was compared with that of the pungent compound, capsaicin. RTX and capsaicin evoked an inward, depolarising current associated with an increase in membrane conductance in a subpopulation of dissociated cultured neurons from rat dorsal root ganglia. RTX also evoked an uptake of 45Ca into and an efflux of [14C]guanidinium and of 86Rb from these cells but was at least 100-fold more potent than capsaicin. The levels of cGMP, but not cAMP were elevated by RTX. Prolonged exposure to RTX damaged DRG neurons by a predominantly osmotic process. RTX-sensitive cells were identified by a cobalt-staining method; neurofilament-containing DRG neurons were RTX-insensitive as were all sympathetic neurons and non-neuronal cells. Cultured DRG neurons from chick embryos were also unaffected by RTX. In a neonatal rat spinal cord-tail preparation in vitro, RTX activated capsaicin-sensitive peripheral nociceptive fibres and caused a subsequent spinal cord depolarization measured in the ventral spinal roots. Neither prolonged exposure to a phorbol ester, to desensitize/down-regulate protein kinase C, nor inhibition of protein kinase C by staurosporine affected responses produced by RTX or capsaicin. The effects of capsaicin were abolished when preparations were exposed to desensitizing concentrations of RTX. RTX therefore acts as a highly potent capsaicin analogue to activate a subpopulation of rat sensory neurons.

Animals↗
PubMed1990

Sensory neuron-specific actions of capsaicin: mechanisms and applications.

Capsaicin acts specifically on a subset of primary afferent sensory neurons to open cation-selective ion channels, probably by interacting directly with a membrane receptor-ion channel complex. Another plant product--resiniferatoxin--has structural similarities to capsaicin and opens the same channels, but is up to 10,000 times as potent. Capsaicin-sensitive neurons are involved in nociception, are responsible for the neurogenic component of the inflammatory response and may also have efferent actions in the peripheral target tissues. In addition to its excitatory actions, capsaicin can have subsequent antinociceptive and anti-inflammatory effects. For these reasons Stuart Bevan and János Szolcsányi argue that drugs based on capsaicin and resiniferatoxin may have important clinical uses.

Animals↗
PubMed1988

The control of chick myoblast fusion by ion channels operated by prostaglandins and acetylcholine.

Chick myoblast fusion in culture was investigated using prostanoid synthesis inhibitors to delay spontaneous fusion. During this delay myoblast fusion could be induced by prostaglandin E1 (PGE1), by raising extracellular potassium and by addition of carbachol. Carbachol-induced fusion, but not PGE-induced fusion, was prevented by the acetylcholine receptor blocker alpha-bungarotoxin. Fusion induced by any of these agents was prevented by the Ca channel blockers lanthanum and D600. The threshold for potassium-induced fusion was 7-8 mM; maximal fusion occurred at 16-20 mM. Low extracellular potassium inhibited spontaneous fusion. Intracellular potassium in fusion competent myoblasts was 101 m-moles/l cell. Calcium flux measurements demonstrated that high potassium increased calcium permeability in fusion-competent myoblasts. A 30-s exposure to high potassium or PGE1 was sufficient to initiate myoblast fusion. Anion-exchange inhibitors (SITS and DIDS) delayed spontaneous myoblast fusion and blocked fusion induced by PGE1 but not carbachol. Blocking the acetylcholine receptor shifted the dose-response relation for PGE-induced fusion to higher concentrations. PGE1-induced fusion required chloride ions; carbachol-induced fusion required sodium ions. Provided calcium channels were available, potassium always induced fusion. We conclude that myoblasts possess at least three, independent pathways, each of which can initiate myoblast fusion and that the PGE-activated pathway and the acetylcholine receptor-activated pathway act synergistically. We suggest that fusion competent myoblasts have a high resting membrane potential and that fusion is controlled by depolarization initiated directly (potassium), by an increase in permeability to chloride ions (PGE), or by activation of the acetylcholine receptor (carbachol); depolarization triggers a rise in calcium permeability. The consequent increase in intracellular calcium initiates myoblast fusion.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1988

A role for acetylcholine receptors in the fusion of chick myoblasts.

The role of acetylcholine receptors in the control of chick myoblast fusion in culture has been explored. Spontaneous fusion of myoblasts was inhibited by the nicotinic acetylcholine receptor antagonists alpha-bungarotoxin, Naja naja toxin and monoclonal antibody mcAb 5.5. The muscarinic antagonists QNB and n-methyl scopolamine were without effect. Atropine had no effect below 1 microM, where it blocks muscarinic receptors; at higher concentrations, when it blocks nicotinic receptors also, atropine inhibited myoblast fusion. The inhibitions imposed by acetylcholine receptor antagonists lasted for approximately 12 h; fusion stimulated by other endogenous substances then took over. The inhibition was limited to myoblast fusion. The increases in cell number, DNA content, the level of creatine phosphokinase activity (both total and muscle-specific isozyme) and the appearance of heavy chain myosin, which accompany muscle differentiation, followed a normal time course. Pre-fusion myoblasts, fusing myoblasts, and young myotubes specifically bound labeled alpha-bungarotoxin, indicating the presence of acetylcholine receptors. The nicotinic acetylcholine receptor agonist, carbachol, induced uptake of [14C]Guanidinium through the acetylcholine receptor. Myoblasts, aligned myoblasts and young myotubes expressed the synthetic enzyme Choline acetyltransferase and stained positively with antibodies against acetylcholine. The appearance of ChAT activity in myogenic cultures was prevented by treatment with BUDR; nonmyogenic cells in the cultures expressed ChAT at a level which was too low to account for the activity in myogenic cultures. We conclude that activation of the nicotinic acetylcholine receptor is part of the mechanism controlling spontaneous myoblast fusion and that myoblasts synthesize an endogenous, fusion-inducing agent that activates the nicotinic ACh receptor.

Animals↗
PubMed1988

Capsaicin-induced ion fluxes in dorsal root ganglion cells in culture.

Capsaicin is a pungent pain-producing compound found in plants of the capsicum family; it exerts excitatory, desensitizing, and toxic effects on a subset of sensory neurons, including the polymodal nociceptor population. We have carried out a quantitative study of capsaicin-induced fluxes of sodium, guanidine, calcium, rubidium, and chloride ions in cultures of neonatal and adult rat DRG neurons, in conjunction with the use of a histochemical stain that identifies capsaicin-sensitive neurons by means of cobalt uptake. Those cells that take up cobalt in a capsaicin-dependent manner (EC50 = 0.2 microM) represent about 50% of the total neuronal population derived from neonatal DRGs on short-term culture. Overnight treatment of cultures with 2 microM capsaicin leads to the loss of the cobalt-staining subpopulation. The capsaicin-insensitive neurons contain immunoreactive neurofilament epitopes that are present in fewer than 10% of capsaicin-sensitive neurons. This observation provides indirect evidence that the sensitive cells correspond to the small, dark B-type neurons, which are negative for neurofilament immunoreactivity in vivo. A capsaicin-dependent calcium uptake (EC50 = 0.2 microM), as measured by 45Ca incorporation, is shown by a DRG neuronal subpopulation that, like the cobalt-staining population of DRG neurons, is lost after overnight capsaicin treatment (2 microM). Capsaicin application leads to the accumulation of millimolar levels of calcium within a few minutes. Cadmium and other divalent cations block capsaicin-induced calcium uptake, but little or no inhibition is seen with organic calcium channel antagonists. Mitochondria, rather than the endoplasmic reticulum, are the probable destination of the internalized calcium, because ruthenium red inhibits calcium uptake (IC50 = 0.05 microM), whereas methylxanthines are inactive. The subset of sensory neurons that takes up calcium also releases 86Rb when exposed to capsaicin (EC50 = 0.06 microM). No efflux of 36Cl ions could be induced by capsaicin. These cells also show a capsaicin-induced uptake of 22Na or 14C guanidine (EC50 = 0.06 microM). In contrast, chick DRG cells in culture showed no capsaicin-induced calcium or cobalt uptake. Primary cultures of rat superior cervical ganglion neurons and Schwann cells, and a number of neuronal cell lines, also failed to respond to capsaicin, as judged by the calcium, cobalt, or guanidine uptake assays.

Animals↗
PubMed1987

Voltage gated ionic channels in rat cultured astrocytes, reactive astrocytes and an astrocyte-oligodendrocyte progenitor cell.

Astrocytes (both type 1 and type 2), cultured from the central nervous system of newborn or 7 day old rats show voltage gated sodium and potassium channels that are activated when the membrane is depolarized to greater than -40 mV. The sodium channels in these cells have an h-infinity curve similar to that of nodal membranes but the activation (peak current-voltage) curves are shifted along the voltage axis by about +30 mV. These sodium currents are blocked only by high concentrations of tetrodotoxin. The voltage activated potassium currents in both types of astrocyte show at least two components; an inactivating component that is suppressed at holding potentials of greater than -40 mV and a persistent, non-inactivating current. Several types of single channel currents were observed in outside-out membrane patches from type 2 astrocytes. One type of potassium channel showed inactivation on depolarization and may contribute to the whole-cell inactivating current. In contrast, oligodendrocytes showed no obvious voltage gated membrane channels. The properties of the type 2 astrocyte-oligodendrocyte progenitor cell were investigated in two ways: 1) by examination of cells just beginning to differentiate along the "electrically silent" oligodendrocyte pathway or 2) by recording from progenitor cells cultured for 24 hours in the presence of cycloheximide to block the appearance of new membrane channels. In both cases, voltage gated inward (sodium) and outward (potassium) currents were noted. The outward current response showed both an inactivating and a non-inactivating component. Similar voltage activated inward and outward membrane currents were noted in reactive astrocytes freshly isolated (3-6 hours) from lesioned areas of adult rat brains.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Ion channels in rabbit cultured fibroblasts.

Large outward currents are recorded with the whole-cell patch-clamp technique on depolarization of rabbit cultured fibroblasts. Our findings suggest that these outward currents consist of two voltage-dependent components, one of which also depends on cytoplasmic calcium concentration. Total replacement of external Cl- by the large anion ascorbate does not affect the amplitude of the currents, indicating that both components must be carried by K+. Consistent with these findings with whole-cell currents, in single channel recordings from fibroblasts we found that most patches contain high-conductance potassium-selective channels whose activation depends on both membrane potential and the calcium concentration at the cytoplasmic surface of the membrane. In a smaller number of patches, a second population of high-conductance calcium-independent potassium channels is observed having different voltage-dependence. The calcium- and voltage-dependence suggest that these two channels correspond with the two components of outward current seen in the whole-cell recordings. The single channel conductance of both channels in symmetrical KCl (150 mM) is 260-270 pS. Both channels are highly selective for K+ over both Na+ and Cl-. The conductance of the channels when outward current is carried by Rb+ is considerably smaller than when it is carried by K+. Some evidence is adduced to support the hypothesis that these potassium channel populations may be involved in the control of cell proliferation.

4-Aminopyridine↗
PubMed1985

The presence of voltage-gated sodium, potassium and chloride channels in rat cultured astrocytes.

Patch-clamp recording from the plasmalemma of rat cultured astrocytes reveals the presence of both voltage-dependent sodium and voltage-dependent potassium conductances. These conductances are similar but not identical to the corresponding conductances in the axolemma. Whereas the h infinity relation of the sodium channels has the same voltage dependence as in the nodal axolemma, the peak current-voltage relation is shifted by about 30 mV along the voltage axis in the depolarizing direction. It is speculated that the glial cells synthesize sodium and potassium channels for later insertion into the axolemma of neighbouring axons. The astrocytes also express a plasmalemmal voltage-dependent anion conductance that is turned on at about -40 mV (that is, near the resting potential of the cultured astrocytes). The channels involved are large enough to be just permeable to glutamate but not to ascorbate. It is suggested that the conductance of this channel for chloride plays a physiological role in the spatial buffering of potassium by glial cells.

Animals↗
PubMed1985

Some properties of acetylcholine receptors in human cultured myotubes.

The distribution and single channel properties of acetylcholine (ACh) receptors in human myotubes grown in tissue culture have been examined. Radioautography of myotubes labelled with [125I]alpha-bungarotoxin showed that ACh receptors are distributed uniformly over the myotube surface at a density of 3.9 +/- 0.5 receptors per square micrometre. Accumulations of ACh receptors (hot spots) were found rarely. The conductance and kinetics of ACh-activated channels were investigated with the patch-clamp technique. Cell-attached membrane patches were used in all experiments. A single channel conductance in the range 40-45 pS was calculated. No sublevels of conductance (substates) of the activated channel were observed. The distribution of channel open-times varied with ACh concentration. With 100 nM ACh, the distribution was best fitted by the sum of two exponentials, whereas with 1 microM ACh a single exponential could be fitted. The mean channel open-time at the myotube resting potential (ca. -70 mV, 22 degrees C) was 8.2 ms. The distribution of channel closed-times was complex at all concentrations of ACh studied (100 nM to 10 microM). With desensitizing doses of ACh (10 microM), channel openings occurred in obvious bursts; each burst usually appeared as part of a 'cluster' of bursts. Both burst duration and mean interval between bursts increased with membrane hyperpolarization. Individual channel open-times and burst durations showed similar voltage dependence (e-fold increase per 80 mV hyperpolarization), whereas both the channel closed-times within a burst and the number of openings per burst were independent of membrane potential.

Acetylcholine↗