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P L Donaldson

Publications and source records attributed to P L Donaldson.

11 recordsLinked to original sources
PubMed1983

Calcium accumulation precedes the degenerative effects of L-glutamate on locust muscle fibres.

Agonist-induced degeneration of locust muscle occurs only when desensitization of the excitatory glutamate receptors present on this tissue is inhibited. It has been suggested that an increase in intracellular Ca2+ is responsible for this degeneration. To test this proposal the accumulation of 45Ca by locusts muscle has been studied under various conditions, including those under which receptor desensitization was inhibited. Retractor unguis muscles from the metathoracic leg of locusts (Schistocerca gregaria) were used in these studies. All muscles exposed to L-glutamate exhibited an early increase in intracellular radioactivity but this was 2-3 times greater in muscle pretreated with concanavalin A (Con A) to block receptor desensitization. In the desensitizing system the increase in muscle radioactivity was not maintained, intracellular Ca2+-levels declining to control values after 30 min in 45Ca-saline-containing glutamate. In Con A-treated muscles intracellular Ca2+-levels plateaued well above control levels within 5 min of exposure to glutamate and were maintained at these levels throughout the period of glutamate treatment. These results support the contention that agonist-induced degeneration of locust muscle is triggered by entry of Ca2+ and a rise in intracellular concentration of this cation to a toxic level.

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PubMed1983

Investigations into the mechanism of excitant amino acid cytotoxicity using a well-characterized glutamatergic system.

The cytotoxicity of glutamate and several analogues was investigated using a well characterized glutamatergic system; the neuromuscular system of the locust leg. In the presence of Con A (10(-6) M) (which blocks glutamate receptor desensitization) bath application on L-glutamate to isolated nerve-muscle preparations induced degeneration of the muscle cells in a dose-dependent manner. The ability of glutamate analogues to cause similar damage corresponded to their pharmacological potency, i.e. L-quisqualate greater than L-glutamate greater than L-cysteine greater than L-aspartate and L-kainate. Glutamate and the more potent agonists initially caused muscle swelling. This was followed by an increase in opacity of the muscle due to vacuolation resulting from disruption of the sarcoplasmic reticulum. Ca2+-free saline slowed the cytotoxic action of these amino acids, whilst saline containing high concentrations of Ca2+ (20 mM; substituted for Na+) accelerated muscle destruction. Denervation induces supersensitivity of locust muscle to L-glutamate; in denervated muscles the cytotoxicity of L-glutamate was enhanced. Muscles swollen by exposure to high-potassium saline (100 mM; substituted for sodium) were not damaged. We conclude that in this insect glutamatergic system, when desensitization is prevented, activated glutamate receptors gate the influx of Ca2+ and Na2+ causing an ionic imbalance which results in cellular damage. This mechanism could also account for at least some of the neurotoxic effects of amino acids in the vertebrate central nervous system. The results of our studies also indicate that other transmitters which gate non-desensitizing cationic channels should, in principle, also be cytotoxic.

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PubMed1983

A quantitative study of potassium channel kinetics in rat skeletal muscle from 1 to 37 degrees C.

Potassium currents were measured using the three-microelectrode voltage-clamp technique in rat omohyoid muscle at temperatures from 1 to 37 degrees C. The currents were fitted according to the Hodgkin-Huxley equations as modified for K currents in frog skeletal muscle (Adrian et al., 1970a). The equations provided an approximate description of the time course of activation, the voltage dependence of the time constant of activation (tau n), and the voltage dependence of gK infinity. At higher temperatures the relationship between gK infinity and voltage was shifted in the hyperpolarizing direction. The effect of temperature on tau n was much greater in the cold than in the warm: tau n had a Q10 of nearly 6 at temperatures below 10 degrees C, but a Q10 of only approximately 2 over the range of 30-38 degrees C. The decreasing dependence of tau n on temperature was gradual and the Arrhenius plot of tau n revealed no obvious break-points. In addition to its quantitative effect on activation kinetics, temperature also had a qualitative effect. Near physiological temperatures (above approximately 25 degrees C), the current was well described by n4 kinetics. At intermediate temperatures (approximately 15-25 degrees C), the current was well described by n4 kinetics, but only if the n4 curve was translated rightward along the time axis (i.e., the current had a greater delay than could be accounted for by simple n4 kinetics). At low temperatures (below approximately 15 degrees C), n4 kinetics provided only an approximate fit whether or not the theoretical curve was translated along the time axis. In particular, currents in the cold displayed an initial rapid phase of activation followed by a much slower one. Thus, low temperatures appear to reveal steps in the gating process which are kinetically "hidden" at higher temperatures. Taken together, the effects of temperature on potassium currents in rat skeletal muscle demonstrate that the behavior of potassium channels at physiological temperatures cannot be extrapolated, either quantitatively or qualitatively, from experiments carried out in the cold.

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PubMed1983

Slow components of potassium tail currents in rat skeletal muscle.

The kinetics of potassium tail currents have been studied in the omohyoid muscle of the rat using the three-microelectrode voltage-clamp technique. The currents were elicited by a two-pulse protocol in which a conditioning pulse to open channels was followed by a test step to varying levels. The tail currents reversed at a single well-defined potential (VK). At hyperpolarized test potentials (-100 mV and below), tail currents were inward and exhibited two clearly distinguishable phases of decay, a fast tail with a time constant of 2-3 ms and a slow tail with a time constant of approximately 150 ms. At depolarized potentials (-60 mV and above), tail currents were outward and did not show two such easily separable phases of decay, although a slow kinetic component was present. The slow kinetic phase of outward tail currents appeared to be functionally distinct from the slow inward tail since the channels responsible for the latter did not allow significant outward current. Substitution of Rb for extracellular K abolished current through the anomalous (inward-going) rectifier and at the same time eliminated the slow inward tail, which suggests that the slow inward tail current flows through anomalous rectifier channels. The amplitude of the slow inward tail was increased and VK was shifted in the depolarizing direction by longer conditioning pulses. The shift in VK implies that during outward currents potassium accumulates in a restricted extracellular space, and it is suggested that this excess K causes the slow inward tail by increasing the inward current through the anomalous rectifier. By this hypothesis, the tail current slowly decays as K diffuses from the restricted space. Consistent with such a hypothesis, the decay of the slow inward tail was not strongly affected by changing temperature. It is concluded that a single delayed K channel is present in the omohyoid. Substitution of Rb for K has little effect on the magnitude or time course of outward current tails, but reduces the magnitude and slows the decay of the fast component of inward tails. Both effects are consistent with a mechanism proposed for squid giant axon (Swenson and Armstrong, 1981): that (a) the delayed potassium channel cannot close while Rb is inside it, and (b) that Rb remains in the channel longer than K.

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PubMed1983

Calcium currents in a fast-twitch skeletal muscle of the rat.

Slow ionic currents were measured in the rat omohyoid muscle with the three-microelectrode voltage-clamp technique. Sodium and delayed rectifier potassium currents were blocked pharmacologically. Under these conditions, depolarizing test pulses elicited an early outward current, followed by a transient slow inward current, followed in turn by a late outward current. The early outward current appeared to be a residual delayed rectifier current. The slow inward current was identified as a calcium current on the basis that (a) its magnitude depended on extracellular calcium concentration, (b) it was blocked by the addition of the divalent cations cadmium or nickel, and reduced in magnitude by the addition of manganese or cobalt, and (c) barium was able to replace calcium as an inward current carrier. The threshold potential for inward calcium current was around -20 mV in 10mM extracellular calcium and about -35 mV in 2 mM calcium. Currents were net inward over part of their time course for potentials up to at least +30 mV. At temperatures of 20-26 degrees C, the peak inward current (at approximately 0 mV) was 139 +/- 14 microA/cm2 (mean +/- SD), increasing to 226 +/- 28 microA/cm2 at temperatures of 27-37 degrees C. The late outward current exhibited considerable fiber-to-fiber variability. In some fibers it was primarily a time-independent, nonlinear leakage current. In other fibers it was primarily a time-independent, nonlinear leakage current. In other fibers it appeared to be the sum of both leak and a slowly activated outward current. The rate of activation of inward calcium current was strongly temperature dependent. For example, in a representative fiber, the time-to-peak inward current for a +10-mV test pulse decreased from approximately 250 ms at 20 degrees C to 100 ms at 30 degrees C. At 37 degrees C, the time-to-peak current was typically approximately 25 ms. The earliest phase of activation was difficult to quantify because the ionic current was partially obscured by nonlinear charge movement. Nonetheless, at physiological temperatures, the rate of calcium channel activation in rat skeletal muscle is about five times faster than activation of calcium channels in frog muscle. This pathway may be an important source of calcium entry in mammalian muscle.

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PubMed1982

Block of locust muscle glutamate receptors by delta-philanthotoxin occurs after receptor activations.

One component (delta-philanthotoxin (delta-PTX) of the venom from the wasp Philanthus triangulum blocks transmission postsynaptically at excitatory synapses on locust muscle. delta-PTX depresses both the iontophoretic glutamate potential and the excitatory junctional current (e.j.c.) in a glutamate receptor activation-dependent manner. The rate of recovery from the effects of the toxin is reduced following either prolonged application of L-glutamate or repetitive iontophoretic application of this amino acid or high frequency neural stimulation of the muscle in the presence of delta-PTX. The decay phase of the e.j.c. is shortened by delta-PTX. The effects of delta-PTX on the e.j.c. are not voltage dependent. The open-close kinetics of glutamate channels in extrajunctional muscle membrane are modified by delta-PTX as shown by patch clamp analysis. The mean life time of the glutamate channel is reduced, whilst the mean interval between single opening events is increased with the events often occurring in bursts. These data are consistent with glutamate channel blocking by this toxin. It is proposed that the toxin blocks open channels gated by both junctional and extrajunctional glutamate receptors on locust muscle. It is further proposed that delta-PTX enters a compartment of the muscle through the glutamate open channels and that it can also block the open channels from this site.

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PubMed1981

Collateral sprouting of insect motorneurons.

The metathoracic extensor tibiae muscle of the cricket, Teleogryllus oceanicus, is innervated by two excitatory axons: a fast axon, which initiates large twitches to single stimuli, and a slow axon, which evokes minute twitches to single stimuli, but which, through facilitation and summation, evokes readily measurable tension to repetitive stimulation. The fast axon and the slow axon leave the metathoracic ganglia in different nerve roots, the fast axon through nerve 5 and the slow axon through nerve 3. The fast axon innervates muscle fibers in the middle of the extensor tibiae, and the slow axon innervates muscle fibers at the proximal and distal ends of the muscle. A central region of muscle fibers is innervated by only the fast axon. This region is flanked on either side by dually innervated fibers, fibers that receive both the fast and the slow axons. Fibers with only slow axon innervation are restricted to a wedge-shaped patch in the proximal extensor tibiae and a larger region in the most distal portion of the muscle. Sectioning nerve 5 containing the fast axon, or nerve 3 containing the slow axon, partially denervates the extensor tibiae. Functional transmission by the fast axon fails 7-10 days after nerve section. The innervation field of the intact motorneuron expands in a partially denervated muscle. The linear expansion rate of the slow axon field is about 20-40 micrometer per day. The enlarged slow field does not regress when axons regenerate to the muscle through nerve 5. The progressive expansion of the slow innervation field suggests that the expansion is due to collateral sprouting of slow axon terminals.

Animals↗
PubMed1981

Increased effectiveness of a motorneuron after partial denervation of its target muscle in the cricket Telegryllus oceanicus.

Fibers of the metathoracic extensor tibia muscle of the cricket Teleogryllus oceanicus are innervated by a slow excitatory axon (slow fibers), a fast excitatory axon (fast fibers), or by both slow and fast axons (dual fibers). Sectioning metathoracic nerve 5 removes the fast axon input to the muscle but not that of the slow axon. Following such partial denervation, the mechanical responses initiated by the slow axon increase progressively for at least 30 days; twitch tensions reach 5-10 times those of control muscles and tetanic tensions 10-30 times control values. After sectioning nerve 5, resting membrane potentials decrease in those fibers which originally received fast axon input and the input resistance of all fiber types increases, including that of slow fibers which are not innervated through nerve 5. Excitatory junctional potentials (EJPs) initiated by the slow axon become larger following partial denervation, accounting in part for the larger contraction amplitudes. The increased input resistance is adequate to account for the larger EJPs in slow fibers but not for the proportionally greater increase in EJP amplitude in fibers which were formerly dually innervated. The change in EJP amplitude is abrupt in slow fibers and gradual in formerly dual fibers.

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