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Biomedical subjects

B R Ransom

Publications and source records attributed to B R Ransom.

At least 109 records · Page 6Linked to original sources
PubMed1987

Tip size of ion-exchanger based K+-selective microelectrodes. I. Effects on selectivity.

Double-barreled ion-exchanger based K+-selective microelectrodes (K+ ISMs) of a variety of tip diameters were used to study the dependency of ion selectivity upon tip size. The selectivity of K+ ISMs depended on tip size and barrel configuration. Within the range of tip diameters tested (approximately 0.5-6 micron) all K+ ISMs constructed of two barrels glued side by side ("figure-eight glass") exhibited sensitivity to K+ and NH4+. Figure-eight K+ ISMs with tip diameters less than 1.5 micron were not sensitive to tetramethylammonium, tetraethylammonium, or choline, whereas K+ ISMs with tip diameters greater than or equal to 1.5 micron sensed all of the quaternary amines. Tip size dependent selectivity was not present in K+ ISMs made from thick septum theta glass. The explanation for tip size dependent changes in ion selectivity is unknown but a discussion of theoretical possibilities is given.

Amines↗
PubMed1987

Tip size of ion-exchanger based K+-selective microelectrodes. II. Effects on measurement of evoked [K+]0 transients.

Double-barreled ion-exchanger based K+-selective microelectrodes (K+ ISMs) of a variety of tip diameters were used to provide an experimental test of predictions that microelectrode tip induced tissue damage influences the magnitude of measured [K+]0 increases. The measured magnitude of stimulation induced [K+]0 rises in the rat optic nerve depended on the tip diameter of the measuring K+ ISM; smaller tipped K+ ISMs tended to measure larger rises of [K+]0 than bigger tipped K+ ISMs. When stimulated [K+]0 increases were simultaneously recorded with 2 K+ ISMs of different tip size, the smaller tipped K+ ISM recorded [K+]0 increases that were, on average, 14% larger than the increases measured by the bigger tipped K+ ISM. In view of these results, and those presented in the previous paper, investigators should standardize the tip sizes of the ISMs used in their experiments.

Action Potentials↗
PubMed1986

A quantitative study of developing axons and glia following altered gliogenesis in rat optic nerve.

Axonal and glial cell development within rat optic nerve in which gliogenesis was altered by systemic injection of 5-azacytidine (5-AZ) was examined by quantitative electron microscopy. In neonatal (0-2 days) rat optic nerves, all fibers are premyelinated, and they exhibit a fairly uniform diameter (approximately 0.22 micron). These fibers occupy approximately 55% of the optic nerve volume. At this early age, glia within the optic nerve consist only of cells of astrocytic lineage and progenitor cells. These glia occupy approximately 28% of the optic nerve volume, and there are approximately 80 glial cells/optic nerve cross section. In 14-day-old normal optic nerves, myelinated and ensheathed fibers comprise approximately 17% and 9%, respectively, of the total number of axons. Mean axonal diameter of myelinated fibers is approximately 0.75 micron, while mean diameter for ensheathed axons is approximately 0.50 micron. By volume, these fibers occupy approximately 25% of the nerve, which is similar to the volume occupied by premyelinated axons in these nerves. At 14 days of age, there are approximately 300 glial cells/optic nerve transverse section, and these glia occupy approximately 37% of the volume in normal optic nerve. Oligodendroglia represent approximately 40% of total glial cells present, while astroglia and progenitor cell each comprise approximately 30% of the cells. In optic nerves from 14-day-old rats treated with 5-AZ, few myelinated fibers are present and the number of oligodendroglia is markedly reduced. Axonal diameter of premyelinated fibers is similar to that of age-matched controls. Myelinated and ensheathed fibers comprise approximately 2% of the total fibers present in 5-AZ-treated optic nerves, with the remaining fibers being premyelinated. The few myelinated and ensheathed fibers present in 5-AZ-treated optic nerves display similar axonal diameters to corresponding fibers from age-matched control tissue. Glial cells occupy approximately 40% of the nerve volume, and there are approximately 200 glia/nerve cross section in 5-AZ-treated rats. Astroglia comprise approximately 63% of the total glial cells, while approximately 12% of the cells are oligodendroglia. These results demonstrate that 5-AZ is a potent inhibitor of oligodendrogliogenesis, with a concomitant marked reduction in the number of myelinated fibers.

Age Factors↗
PubMed1986

Ion-sensitive microelectrode measurements of free intracellular chloride and potassium concentrations in hyperthermia-treated neuroblastoma cells.

Murine NG108-15 neuroblastoma cells were heated for times of 5-40 min at 45.5 degrees C, and survival ranged from 0.7-0.0015, respectively. Ion-sensitive microelectrodes (ISM) were used to measure the free intracellular concentrations of Cl- and K+ immediately after heating and up to 30 hr later. The free intracellular Cl- and K+ concentrations, [Cl-]i and [K+]i respectively, of the heated cells remained identical to those of the controls for the first 10 hr after heating. At later times, some cells had increased [Cl-]i values and decreased [K+]i values identical to those of the extracellular medium. These cells had a mottled morphology, no longer excluded the vital stain trypan blue, and had no membrane potential. The number of these dye-including, physiologically dead cells increased with time, and was always greater following longer heating times. No changes in mean cellular volume were observed until 25 hr after heating. All trypan-blue-excluding, physiologically live cells had the same [Cl-]i and [K+]i as the control cells, even when the majority of them were destined for clonogenic death.

Animals↗
PubMed1986

Brain extracellular space: developmental studies in rat optic nerve.

Analysis of neural activity-dependent fluctuations in K+, H+, and ECS dimensions in the developing RON has revealed major changes during the first two to three postnatal weeks. The emergence of the adult ceiling level for evoked extracellular K+ (10 to 12 mM) and significant ECS shrinkage are roughly correlated in time with the proliferation and maturation of glial cells in this structure. This observation and others have led to the hypothesis that ECS shrinkage depends upon electrolyte and water transport into glial cells with subsequent swelling. Development of the adult K+ ceiling level may also depend upon glial cells, but it is likely that other factors contribute to this homeostatic mechanism. Marked alterations in activity-dependent pHo shifts were seen with development and may be related to changes in the activity of carbonic anhydrase in this structure. The technological means are at hand to pursue these questions vigorously in an effort to provide further insight into the mechanisms of ionic and fluid homeostasis of brain ECS, and the developing RON appears to be a useful model system in this regard.

Aging↗
PubMed1985

Rat optic nerve: disruption of gliogenesis with 5-azacytidine during early postnatal development.

The normal sequence of gliogenesis in the rat optic nerve was disrupted by neonatal treatment with the mitotic inhibitor 5-azacytidine (5-AZ). This protocol caused a marked reduction in the number of glial cells, especially oligodendrocytes, seen in 11- and 14-day-old animals. Myelin formation was also greatly reduced in animals of this age compared to controls, but optic nerve axons appeared to be well preserved. Electrophysiological studies demonstrated similar excitability properties and activity-evoked [K+] changes in normal and 5-AZ-treated nerves prior to 5 days of age. However, in older nerves there were striking changes in the compound action potential and activity-dependent K+ accumulation in 5-AZ-treated nerves compared to controls. This simple model of disrupted central nervous system gliogenesis should prove useful in analyzing a variety of questions regarding neuroglial interactions including the role of glial cells in ionic homeostasis of brain extracellular space.

Animals↗
PubMed1985

Effects of altered gliogenesis on activity-dependent K+ accumulation in the developing rat optic nerve.

Gliogenesis in the rat optic nerve is disrupted by neonatal treatment with the mitotic inhibitor 5-azacytidine (5-AZ). The rate of myelination and number of glial cells are markedly reduced in treated animals. We analyzed the physiological consequences of these chemically induced morphological alterations in terms of activity-dependent K+ accumulation in brain extracellular space and characteristics of the compound action potential (CAP). Nerves from 5-AZ-treated animals older than 5 days of age showed significantly higher activity-dependent 'ceiling levels' of extracellular K+ concentration ( [K+]o) than controls. This result is consistent with the hypothesis that glial cells are involved in K+ homeostasis at a cellular level and play a role in helping to set the ceiling level of activity-dependent K+ accumulation. The CAPs of 5-AZ-treated nerves older than 5 days of age were larger and generally of simpler configuration than those observed in control animals, perhaps due, among other factors, to the retained uniformity of axonal conduction velocity caused by inhibition of myelination.

Action Potentials↗
PubMed1985

Activity-dependent shrinkage of extracellular space in rat optic nerve: a developmental study.

Activity-dependent shrinkage of extracellular space (ECS) is described in a simple model of the mammalian central nervous system, the rat optic nerve. In response to neural activation, the ECS of the mature nerve rapidly and reversibly decreases by as much as 20%. Activity-dependent ECS shrinkage is not present in neonatal nerves but develops over the first 2 weeks of life, roughly coincident with the appearance of glial cells in this structure. These and other observations suggest that activity-dependent ECS shrinkage results from fluid and electrolyte movements into glial cells.

Aging↗
PubMed1984

Chloride conductance and extracellular potassium concentration interact to modify the excitability of rat optic nerve fibres.

The excitability of developing rat optic nerves has been studied under conditions in which extracellular Cl- was replaced with other anions. In nerves younger than 3 days old, replacing Cl- with propionate or SO4(2-) usually led to spontaneous and repetitive cycling of extracellular K+ concentration ([K+]o). [K+]o reached peaks of 8-12 mM and then fell transiently below the base-line level of 5 mM before increasing again. This cycling behaviour continued, with a wave-length of 1-2 min, for as long as 2 h. Nerves older than 5 days either did not cycle or did so only transiently. Substitution of ten different anions for Cl- indicated that a minimum hydrated radius, between that of BrO3- and HCO3-, was necessary to induce cycling behaviour. Cycling behaviour was abolished by the Na+-channel blocker tetrodotoxin. Reduction of the bath [K+] to 2.5 mM slowed the frequency of spontaneous cycles; a bath [K+] of 1 mM abolished them. When the temperature was lowered, cycle frequency slowed. Substitution of large anions for Cl- enhanced axonal excitability. This was inferred from the prevalence of spontaneous action potentials during cycling behaviour, and from the generation of relatively large evoked increases of [K+]o. Cycling behaviour is hypothesized to result from a repetition of the following three processes: (i) spontaneous axonal firing elicits a gradual increase in [K+]o which increases axonal excitability and facilitates further K+ release, (ii) axonal firing and K+ release are eventually halted by a combination of depolarization block, intracellular Na+ accumulation and hyperpolarization from electrogenic pumping, (iii) recovery of [K+]o to its minimal value depends on active K+ reuptake mediated by a highly stimulated axonal Na+-K+-ATPase. We conclude that a large proportion of the resting membrane conductance of optic nerve fibres is Cl- specific. A high Cl- conductance may stabilize fine central axons against the depolarizing effects of [K+]o increases.

Action Potentials↗
PubMed1983

Neuropathy in the hypereosinophilic syndrome.

Peripheral nerve dysfunction was a prominent clinical feature in each of three patients with the hypereosinophilic syndrome (HES). The neuropathy, occurring at the onset of marked eosinophilia or at a time of its worsening, had a painful onset, evolved over 1-4 weeks, and affected both sensory and motor function. Electrodiagnostic studies demonstrated both multifocal and generalized nerve involvement, with the former predominating (multiple mononeuropathy). Nerve and muscle biopsies from two patients revealed severe axonal degeneration with neurogenic atrophy of muscle. The eosinophilia decreased dramatically with corticosteroid treatment and the neuropathy gradually improved, but all three patients manifested residual neurologic disability 17-22 months after onset. The neuropathy may be on an ischemic basis or may be related to neurotoxicity of eosinophil products.

Adult↗
PubMed1982

Activity-dependent K+ accumulation in the developing rat optic nerve.

Potassium-sensitive microelectrodes were used to study activity-dependent changes of extracellular potassium ion concentration ([K+]o) in rat optic nerves of different postnatal ages (1 day to adulthood). The maximum level to which [K+]o rose with optimal frequencies of stimulation depended on age: mean maximum evoked [K+]o was 17.2 microM in 1- to 3-day-old optic nerves and 9.8 microM in adult nerves. The ceiling [K+]o seen in immature optic nerves, which is uniquely large for a mammalian central nervous system structure, may result from a relatively enhanced rate of evoked K+ release.

Aging↗
PubMed1981

Modulation of parallel fiber excitability by postsynaptically mediated changes in extracellular potassium.

Field potentials and extracellular potassium concentration ([K+]o) were simultaneously monitored in the molecular layer of the rat cerebellar cortex during stimulation of the parallel fibers. The synaptic field potential elicited by stimulation was reduced by several methods. Reduction of synaptic field potentials was accompanied by a marked increase in the excitability of the parallel fibers. This change in excitability was related to the degree of extracellular K+ accumulation associated with parallel fiber stimulation. These findings support the proposal that increases in [K+]o associated with activity in postsynaptic elements can modulate the excitability of presynaptic afferent fibers.

Afferent Pathways↗
PubMed1981

Dye-coupling between glial cells in the guinea pig neocortical slice.

Physiologically identified glial cells in guinea pig neocortical slices were injected with the low molecular weight, fluorescent dye Lucifer yellow CH. The stained aggregates which resulted consisted of one brightly stained, central cell surrounded by numerous lightly stained cells. The central cell had well defined feathery processes and resembled a protoplasmic astrocyte. The surrounding cells appeared also to be glial cells but lacked sufficient detail to be further categorized. This first demonstration of dye-coupling between neocortical glial cells strongly suggests that these cells are connected together via low resistance junctions capable of passing ionic current as well as dye.

Animals↗
PubMed1979

Dysarthria--clumsy hand syndrome produced by capsular infarct.

A 64-year-old hypertensive man presented with the dysarthria--clumsy hand syndrome, manifested by dysarthria, dysphagia, central facial weakness, deviation of the tongue on protrusion, incoordination of the affected hand, and mild imbalance on walking. A computed tomograpphic scan demonstrated a resolving acute infarction in the vicinity of the genu of the internal capsule.

Deglutition Disorders↗
PubMed1978

Amino acid pharmacology of mammalian central neurones grown in tissue culture.

1. Spinal and cerebellar-brainstem areas of fetal mouse were dissociated and grown in tissue culture until large enough to permit stable intracellular recording. 2. The tissue-cultured neurones, growing as a monolayer and accessible under direct vision using phase contrast optics, allowed precise placement of intracellular recording and extracellular ionophoretic pipettes. 3. Ionophoresis of GABA and glutamate revealed a non-uniform distribution of responses over the cell surface, with a lack of spatial coincidence in sensitivity between the two. GABA inhibited and glutamate excited all cells tested. 4. GABA responses evoked at the cell body and on nearby process membrane were almost uniformly hyperpolarizing, while those at some peripheral process membrane were either hyperpolarizing, depolarizing or a combination of both events. All responses were associated with an increase in membrane slope conductance. 5. Membrane polarization showed that all hyperpolarizing events extrapolated to about the same inversion potential, which averaged about 9 mV more negative than resting potential (n = 95 cells). The depolarizing phases of responses evoked at peripheral membranes extrapolated to about 0 mV (n = 5 cells). 6. The hyperpolarization and increase in membrane conductance of GABA responses at the cell body were dependent on Cl- ions and the inversion potential of the response was dependent on the Cl- ion concentration gradient. The inversion potentials of GABA, glycine and beta-alanine responses were identical. 7. When matched in magnitude for evoked conductance increase, glycine responses decayed more rapidly than GABA. Glycine and beta-alanine voltage responses both decayed faster than GABA responses of comparable size. 8. In about half the cells tested sustained or rapidly repeated application of GABA and glycine transformed hyperpolarizing responses into depolarizations which were associated with a maintained conductance increase. Results from conditioning-test experiments with pairs of GABA and glycine responses suggest that the reversal of response polarity is due to a rapid redistribution of Cl- ions. 9. The limiting slope of log-log dose-response curves for GABA-induced conductance averaged about 2, while those for glutamate-induced depolarizations averaged about 1. The results suggest that two molecules of GABA and one molecule of glutamate participate in the respective post-synaptic responses. 10. The observation indicate that mammalian C.N.S. tissue grown in culture is a suitable model to study C.N.S. membrane pharmacology with increasing precision.

Alanine↗
PubMed1978

Pentobarbitone pharmacology of mammalian central neurones grown in tissue culture.

1. The effects of the barbiturate anaesthetic pentobarbitone on the membrane properties and amino acid pharmacology of mammalian C.N.S. neurones grown in tissue culture were studied using intracellular recording coupled with bath application, extracellular ionophoresis, or focal diffusion. 2. The addition of an anaesthetic concentration of pentobarbitone to the bathing medium abolished all spontaneous synaptic activity, but did not render individual cells electrically inexcitable nor prevent evoked synaptic acitivity. 3. Focal ionophoresis of pentobarbitone or diffusion from blunt micropipettes reversibly increased membrane conductance, effectively dampening excitability without directly affecting individual action potential characteristics. 4. Pentobarbitone-induced membrane conductance was reversibly blocked by picrotoxin. The inversion potential of the pentobarbitone voltage response depended on Cl- ion gradients and was similar to that of GABA. 5. Pentobarbitone reversibly enhanced the conductance increase produced by GABA with a variable slowing of response kinetics, shifting GABA dose-response curves to the left. Responses to glycine and beta-alanine were not affected. 6. Higher ionophoretic currents of pentobarbitone, which measurably increased membrane conductance, attenuated and markedly slowed GABA responses. Similar effects on GABA responses were observed by superimposing GABA pulses on low level GABA currents. 7. Pentobarbitone, in the absence of an increase in membrane conductance, reversibly depressed depolarizing responses to glutamate without changing response kinetics. Slower responses to acetylcholine which were associated with an apparent decrease in membrane conductance were not affected by the drug. 8. Analysis of double-reciprocal plot data suggested a non-competitive type of antagonism between pentobarbitone and glutamate. Pentobarbitone depression of glutamate was not affected by picrotoxin. 9. Both GABA and glutamate responses appeared to be equally sensitive to pentobarbitone. Specific interaction of the drug with amino acid receptor-coupled events is indicated by the requirement for pentobarbitone pipette placement close to the amino acid response site. 10. The results suggest that pentobarbitone depresses neuronal excitability by (1) directly activating post-synaptic GABA-receptor coupled Cl- conductance, (2) potentiating post-synaptic GABA-induced conductance events, probably at the level of the GABA receptor, and (3) depressing post-synaptic glutamate-induced excitation, probably at the level of the conductance mechanism.

Animals↗
PubMed1977

Ionic determinants of excitability in cultured mouse dorsal root ganglion and spinal cord cells.

The ionic components of the action potentials of mouse spinal cord (SC) cells and dorsal root ganglion (DRG) cells were studied in dissociated cell cultures. It was found that the action potentials of SC cells required Na+ in the medium and were blocked by tetrodotoxin (TTX) (1 micron). Action potentials of DRG cells, on the other hand, were not blocked by TTX (up to 10 micron) and were observed in Na-free media in the presence of 8 mM Ca2+. In low Na (31 mM), low Ca2+ (0.1 mM) medium, action potentials were not observed but could be obtained if the Ca2+ concentration was increased. Action potentials of DRG cells investigated in low Na concentration in the presence of 1 mM or 8 mM Ca2+ became larger in amplitude and shorter in duration when the sodium concentration was increased. Na+ has this effect even in the presence of TTX. It is concluded that the action potentials of SC cells result mainly from a TTX-sensitive Na component. The action potentials of DRG cells on the other hand have both a TTX-insensitive Na component and a Ca2+ component.

Action Potentials↗