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Biomedical subjects

G Duncan

Publications and source records attributed to G Duncan.

At least 163 records · Page 9Linked to original sources
PubMed1981

Impedance of the amphibian lens.

1. The electrical resistance of the perfused frog lens was measured using separate internal current passing and voltage measuring electrodes. 2. The resistance values obtained using voltage clamp and direct and alternating current techniques were in good agreement. 3. The voltage transients induced in response to current steps were multi-exponential in form. Increasing the external K concentration reduced both the amplitude of the voltage response and the rise time. 4. The impedance characteristics were investigated in more detail using alternating current analysis techniques. 5. In an equivalent-circuit modelling study it was assumed that there were two major pathways for current flow in the lens. The first through the surface membranes and the second through the inner fibre membranes via the narrow extracellular spaces. 6. The experimental impedance loci could not be adequately fitted by a simple two time constant model and a third time constant was introduced which may represent diffusion polarization effects in the extracellular spaces. 7. The three time constant model gave good and consistent fits to impedance data from a number of preparations. 8. The form of the impedance loci was also dependent on the external K concentration, but the only fitted parameter which changed consistently with external K was the surface membrane resistance (Rs).

Animals↗
PubMed1980

Evidence of vagal neuropathy in chronic alcoholics.

Autonomic function was tested in healthy control subjects and 20 chronic alcoholic patients with varying degrees of alcohol-related peripheral and central neurological damage. The alcoholics were divided into two groups according to the severity of their symptoms and signs. The alcoholic subjects had no postural hypotension. However, heart-rate responses to Valsalva's manoeuvre, deep breathing, change in posture, baroreceptor stimulation, and atropine were lower in those alcoholics with a greater degree of peripheral and central nervous damage than in the less severely affected alcoholics and the controls. These results suggest that chronic vagal damage may be a feature of alcoholic polyneuropathy.

Adult↗
PubMed1980

Characteristics of voltage-dependent conductance in the membranes of a non-excitable tissue: the amphibian lens.

1. The electrical conductance of the frog lens membranes was observed to increase when the lens was depolarized by current, and to decrease when the lens was hyperpolarized. 2. The total lens conductance (GT) could be described by a voltage-dependent component (GH) together with a voltage-insensitive component (GO). 3. Conductance measured at the resting potential increased from 9.7 to 14.4 x 10(-5) S when the lens was depolarized by 9.1 mV in 10(-4) M-ouabain. The increase of conductance could be explained by an increase of GH which resulted from the depolarization alone. 4. Potassium-rich solutions influenced the conductance by increasing GH as the result of depolarization and by decreasing (blocking) GO. Small increases in the external potassium concentration (from 2.5 to 5 mM), which did not depolarize the lens, resulted in a decrease in resting conductance due to the blockade of GO. 5. Conductance-voltage relationships established in 2.5 (control), 5, 12.5 and 25 mM external potassium could each be fitted to a single mathematical model by assuming that GO had been reduced for 5.0 (control) to 4.5, 3 and 1 x 10(-5) S respectively by those increased potassium concentrations. 6. Barium (2 mM) depolarized the lens by 14.3 mV and decreased the resting conductance by 39%. When the lens was depolarized by a step current pulse in the presence of barium, the conductance increased with time after the onset of the current. Hyperpolarization elicited less time dependence of the conductance. 7. Barium reduced the extent to which lens conductance was dependent on voltage.

Animals↗
PubMed1979

Towards an understanding of chronic pain mechanisms: the use of psychologic tests and a refined differential spinal block.

Forty patients with chronic pain below the waist level were evaluated in a multidisciplinary pain clinic using a refined differential spinal block (DSB) technique. The refinements consisted of verbal instructions to prevent biasing the patients, coupled with a thorough evaluation of verbal and physiologic responses to the block. When demographic and psychologic data were assessed according to pain mechanisms, a pattern of patient groups emerged along a chronic pain continuum. Stress, anxiety, depression, and hysteria, as well as the neurophysiologic and demographic factors, modified the responses to the block. Long-term follow-up of these patients, including repeat DSB procedures and confirmatory anatomic blocks of sympathetic and somatic nerves, validated these impressions. The findings indicate a link between pain mechanisms and psychosocial factors that may directly influence responses to DSB.

Adult↗
PubMed1979

The effect of somatostatin on metabolic and hormonal changes during and after exercise.

The effect of intravenous somatostatin on blood levels of metabolites and hormones has been examined in normal subjects who performed a 30-minute period of bicycle exercises at 70% maximal exercise capacity. The results have been compared with control studies in the same subjects. Measurements were made of blood levels of lactate, glucose, free fatty acids, glycerol, acetoacetate, 3-hydroxybutyrate, insulin, glucagon, growth hormone (hGH) and prolactin. Growth hormone and glucagon release were suppressed during exercise with somatostatin and there was a subsequent elevation during recovery. There was slight post-exercise depression of insulin, but no alteration of plasma prolactin secretion. Blood glucose was reduced during exercise with somatostatin and increased during recovery. The elevation of ketone bodies after exercise was greater in the investigation with somatostatin, but there were no significant changes in other metabolites. Somatostatin, although causing inhibition of hGH release, appeared to have no significant effect upon fatty acid mobilization during exercise.

Adult↗
PubMed1979

The properties of bovine lens membranes measured by a conventional double-chamber method.

1. When the bovine lens was mounted in a divided chamber, an asymmetry potential of 4.3 mV (anterior face positive) was measured across the lens and values of 0.48 and 0.60 were obtained for P(Na)/P(K) of the anterior and posterior faces respectively.2. The half-times for the depolarization of the anterior and posterior face potentials on increasing the external K concentration were 20 and 14 min respectively compared to less than 2 min for the corresponding change determined previously from the totally immersed lens.3. The electrical resistance of the anterior surface was significantly smaller than that of the posterior and both resistances were much smaller than the value previously obtained from the bovine lens immersed in solution.4. The K permeability of the anterior surface, measured by (42)K efflux experiments, was greater than the value for the posterior surface and both were again very much higher than the value obtained for the totally immersed lens.5. The discrepancies between the present double-chamber preparation and the data obtained previously can be explained if it is assumed that the capsule provides a short-circuit pathway between the chamber port and the lens membranes. As the capsule is thicker at the anterior face, the short circuit there would be greater and this may explain many previously observed ;asymmetry' properties for the lens.

Animals↗
PubMed1979

Respiratory effects of fentanyl, diazepam, and methohexital sedation.

The respiratory depressant effects of fentanyl, diazepam, and methohexital were studied in 18 patients who were breathing room air. Two patients had 70% oxygen supplementation administered by a nasal inhaler. Varying degrees of hypoxia occurred when the narcotic was given but hypoxia was not seen in patients who received diazepam alone. The hypoxic changes may be attributed to various physiological responses and can probably be prevented with administration of adequate oxygen.

Anesthesia, Dental↗
PubMed1979

An analysis of the wave forms of photoreceptor potentials in the retina of the chephalopod Sepiola atlantica.

1. Simultaneous intracellular (Vi) and extracellular (Ve) recordings have been made from photoreceptor cells in the retina of the cephalopod Sepiola atlantica. 2. The depolarization of the distal (rhodopsin containing) membrane (Vm) was derived from the Vi and Ve records. 3. All three potentials had very similar time courses in response to short flashes of low intensity but the Vi and Vm responses were much greater in amplitude. 4. At high intensities the amplitudes of Vi and Ve were similar but the wave forms were quite different and it was postulated that a voltage-sensitive potassium conductance change in the proximal membranes of the cell was mainly reponsible for the observed differences. 5. The wave forms of the three responses following long (500 msec) flashes were quite different at all intensities tested. At high intensities there was a slow sag phase in the Vi response, that was mirrored by a slow rise phase in the Ve responses. The reconstructed Vm response was flat during this period of change in the other two responses. 6. The recoveries to the base line of all responses following both long and short flashes were similar in that at high intensities they consisted of at least two phases. The fast phase had a time constant less than 1 sec and the slow component had a time constant of greater than 10 sec. 7. The maximum rates of change of voltage (dV/dt) in both short and long flash responses were linear over a much wider range of intensity values than were the response amplitudes and the rates continued to increase after the peak response amplitude had saturated. 8. The characteristics of the cephalopod responses were compared with those from vertebrate rods and cones and several marked similarities were found.

Animals↗
PubMed1978

Two components of extracellularly-recorded photoreceptor potentials in the cephalopod retina: differential effects of Na+, K+ and Ca2+.

The ERG of the isolated, superfused half-eye of the cephalopod Sepiola atlantica, evoked by a brief (10 micro second) light flash, has been studied by recording intraretinal potentials with glass microelectrodes. The intensity-response characteristics of the potentials recorded at an electrode fixed at the surface (Vs) can be fitted by a simple equation derived from an equivalent circuit model based on a sodium conductance increase mechanism. Raising the external potassium level reduces the maximal response (deltaVm), but does not alter the half-saturation intensity value (I0). Reducing external sodium does not affect deltaVm, but increase I0. Reducing external calcium also does not affect delta Vm, but decrease I0. These effects are adequately described by the model if it is also assumed (a) that changing the external sodium does not significantly alter the transmembrane sodium gradient, and (b) that sodium and calcium ions compete for the sensitivity control mechanism. Differential-depth recording between the fixed electrode at the surface and another electrode that could be moved into the retina revealed that the two component appearance of the transretinal ERG arose from the superposition of two vitreal-negative waveforms. An initial "fast" component was mainly recorded in the photoreceptive distal segments while a "slow" component was prominent in the more proximal regions of the retina. Perfusion with high K+ salines resulted in a decrease in the amplitudes of both fast and slow components of the response whereas reducing external Na+ reduced the amplitude of the fast component at all light intensities but reduced the amplitude of the slow component only at low intensities. The amplitudes of both the fast and slow components increased on reducing external calcium, but the rate of rise and fall of the fast component was independent of external calcium. The rate of rise of the slow component was also independent of the external Ca2+ level but a minimum in the recovery time (tF) was shifted to a lower intensity value at lower calcium concentrations. The shift of the minimum was to a higher intensity value with lowered sodium perfusing solutions. On the basis of the differential sensitivity of the two components to ion changes, as well as stimulus intensity and intraretinal distribution of the components, it is suggested that they reflect two distinct processes in the light-evoked potential of the photoreceptor cells.

Animals↗