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Biomedical subjects

G Duncan

Publications and source records attributed to G Duncan.

At least 145 records · Page 8Linked to original sources
PubMed1985

Characterization of a cation channel on the apical surface of the frog lens epithelium.

The properties of a single conductance pathway of the apical (fibre-facing) surface of the frog lens epithelium are reported. Using the patch-clamp technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981), the most common single-channel currents had an amplitude of 1.9 pA, the mean open time 2.1 ms and a conductance of 25 pS. One open-state time constant (to = 3.3 ms) and two closed-state time constants (tau c1 = 0.9 ms, tau c2 = 23.1 ms) were resolved. The channel current and the mean open time were both increased when Ca2+ was removed from the external solution and the open time distribution was no longer fitted by a single exponential. Multiple-channel events in cell-attached patches containing two or more identical channels were distributed in a binomial fashion and the probability that an individual channel was open, obtained by fitting the binomial distribution, was 0.039. The channel was found to have a Na+:K+ selectivity ratio of 3:1. When Ca2+ was removed from the pipette solution the probability that an individual channel was open increased to 0.137 and the Na+:K+ selectivity ratio increased to 4:1. Channel activity was observed in the presence of tetrodotoxin (10(-6) M) in the bathing medium and the pipette solution but was abolished by internal perfusion of the patch pipette with 0.5 x 10(-4) M amiloride. this apical conductance pathway is identified as an amiloride-sensitive cation channel. These channels are clustered in groups on the apical membrane, spontaneously active at the resting potential and with the possibility of altering their Na+:K+ selectivity. They represent a distinct type of channel, that differ from nerve and muscle Na+ channels in their manner of activation, but do share some common features with both Na+ and Ca2+ channels in excitable cells.

Amiloride↗
PubMed1984

Calcium and the physiology of cataract.

Calcium has long been known to play a role in cataract formation but techniques have only recently become available for investigating the physiological mechanisms. Previous studies showed that lens membrane permeability alters when the external calcium concentration falls below 1 mM, so it was interesting that values for human aqueous from cataract patients ranged from 0.45 to 2.0 mM. The mean value for the aqueous was one half that for the plasma. The calcium concentration in cataractous lenses ranged from 0.1 to 64 mM and lenses with a high calcium concentration also had a high sodium content. In lenses with near normal sodium content the highest calcium concentrations were associated with highly localized opacities, while nuclear cataracts had a low calcium content. The relationship between calcium and transparency was investigated in a rat lens system using ion-sensitive microelectrodes. The distribution of free calcium in the lens varied with age and was correlated with a change in the sensitivity of the lens to cold cataract and a change in lens birefringence. The highest free calcium levels were obtained from lenses incubated in 10 mM-calcium in the absence of glucose and these lenses showed most light scattering. Ion-sensitive microelectrode techniques applied to human lenses yielded calcium levels of 0.1 microM-2 mM. In lenses with dense, highly localized opacities the calcium distribution was not uniform and was highest in regions that scattered most light. The movement of calcium through individual membrane channels was investigated using patch clamp techniques. Three types of ionic channels have been identified in the lens. The smallest appears to be a calcium channel; the larger current fluctuations are associated with sodium and potassium movements. In organ culture studies of the bovine lens, a marked decrease in protein synthesis and net leakage of proteins was associated more strongly with an increase in calcium than with an increase in sodium. The stability of the lens protein gel thus seems to depend on maintaining a low internal level of calcium ions.

Aging↗
PubMed1984

Aqueous humour glucose concentration in cataract patients and its effect on the lens.

The glucose concentrations of the blood plasma and aqueous humour were measured in 56 cataract patients. The sodium concentration and colour of the lenses were determined after extraction. The mean plasma and aqueous glucose levels were 5.8 and 3.2 mM respectively in non-diabetic patients, while the values for diabetics were 14.2 and 7.8 mM. The sodium concentration of the lenses from non-diabetic patients appeared to consist of two distributions around 30 and 170 mM, corresponding to nuclear and cortical cataracts respectively. Only two lenses from the non-diabetic patients had sodium concentrations in the range 60-120 mM. In diabetic patients, however, 80% of the extracted lenses had sodium concentrations in this intermediate range. The data indicate that the osmotic stresses induced in the lenses of diabetic and non-diabetic patients are different. The diabetic lenses were also distributed in the middle range when nuclear colour was graded on a scale from I to V, while normal lenses were again normally distributed at either end of the scale.

Aged↗
PubMed1984

Effect of 8-methoxypsoralen on rat lens cations, membrane potential and protein levels.

Systemic application of 8-methoxypsoralen to rats, followed by u.v.-irradiation, induces a minor change in the lens membrane potential after one week, and by this time marked histological changes have already occurred. Alterations in lens sodium and potassium concentrations followed these early changes and coincided with the appearance of small light-scattering vacuoles when the lenses were examined in vivo with a slit lamp. Dramatic changes in the dry weight, soluble protein and calcium content of the lenses were apparent only after eight weeks from the start of treatment, and these changes coincided with the appearance of very large vacuoles in slit-lamp examinations of the eye.

Animals↗
PubMed1984

The ultrastructure of the lens of the cephalopod Sepiola: a scanning electron microscopic study.

The ultrastructure of the eye lens of Sepiola atlantica was investigated using scanning electron microscopy. The main lens elements in both the anterior and posterior half of the Sepiola lens are plate-like configurations with fiber-like extensions at their margins. Anteriorly the plates are plano-convex, posteriorly subspherical. The central, primordial, posterior plates are spherical with no marginal extensions. The plates are mutually anchored by protrusions and invaginations and by push-button attachments. The posterior and anterior halves are separated by a septum which consists of concentric zones of radially orientated elongated cells. The marginal extensions of the plates and the septal elements are closely associated. The unique structure of the septum makes it a good candidate for the high resistance barrier between the posterior and anterior halves of the Sepiola lens (Jacob and Duncan, 1981).

Animals↗
PubMed1984

Influence of external calcium and glucose on internal total and ionized calcium in the rat lens.

Free calcium in the rat lens, measured by ion-sensitive electrodes, is 1.8 microM while the total, measured by atomic absorption, is of the order of 600 microM. The measured free calcium concentration (pCa) varies with the depth below the surface. It is lowest in the region 100-400 micron below the capsule and again in the nucleus, while the intervening perinuclear cortex has a relatively high free calcium. In young rats (less than 16 weeks) the free calcium in the posterior and anterior cortical regions is the same, while in the older lenses the free calcium is lower in the anterior and the regional variation is greater. Rat lenses incubated in a medium of similar ionic composition to aqueous humour for 15-24 h maintained a low level of free calcium. The maintenance of low internal calcium (both free and total) was dependent on external glucose and on removing glucose the intracellular free calcium increased from 5 to 15 microM while the total calcium increased from 600 to over 1000 microM. Following incubation in high calcium (10 mM), the free and total calcium increased to 40 and 3000 microM respectively. Omitting glucose from the high-calcium solution led to a further increase in both free and total calcium to 400 and 10000 microM respectively. The incubated control lenses maintained their normal sodium and potassium levels and resting potential, while removing glucose gave rise to an increase in sodium, a decrease in potassium and a depolarization of the membrane potential. Increasing external calcium also depolarized the membrane potential, but there was no change in internal sodium and potassium.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗
PubMed1983

The role of divalent cations in controlling amphibian lens membrane permeability; the mechanisms of toxic cataracts.

The effects of a range of divalent ions on lens sodium and potassium permeability characteristics were studied in calcium competition and replacement experiments. Resting voltage and conductance were measured and also voltage-independent conductance. Strontium and manganese were the only divalent ions able to maintain, in the absence of calcium, both sodium and potassium permeability at or near the control level. Neither cobalt nor magnesium had any effect on lens voltage of conductance in the presence of calcium, but neither of these ions could maintain lens permeability properties in the absence of calcium. Cadmium and barium had little effect on sodium permeability, but the former increased potassium permeability while the latter reduced it. Barium was the only divalent studied that appeared to inactivate voltage-sensitive potassium channels in the presence of calcium. Nickel, zinc and copper increased both sodium and potassium permeability in the presence of calcium and so they are likely to be particularly damaging to the lens. Copper was extremely toxic since it was able to overturn the regulatory influence of calcium when it was present in concentrations as low as 10(-6)M.

Animals↗
PubMed1983

Specificity of glucose transport inhibitors in the frog lens.

The unidirectional fluxes (influx and efflux) of glucose across frog lens membranes were investigated using the radio-labelled sugar analogue 3-O-methyl-D-[U-14C]glucose. The effect of various inhibitors of sugar transport on the movement of 3-O-methylglucose was studied and the specificity of the inhibition was estimated by carrying out concomitant measurements of lens sodium content. The movement of 3-O-methylglucose into the lens was rapid, and 50% equilibration occurred within 5 hr. The influx was reduced in the presence of phloretin, phloridzin, ouabain, iodoacetate and cytochalasin B, but only in the case of the latter was there no concomitant change in lens sodium content. Only cytochalasin B can therefore be regarded as a specific inhibitor of glucose transport. The efflux of 3-O-methylglucose was followed after 16 hr incubation with [14C]3-O-methylglucose. The efflux kinetics had a double exponential form and the half-time of the slower phase was 165 mins. The efflux of the slow phase was found to be sensitive to the presence of inhibitors. Phloretin and cytochalasin B produced the most marked reduction in efflux rate, but again only in the latter case was the effect reversible. Bidirectional transport of glucose therefore occurs in the lens and movement in both directions is reduced by cytochalasin B, which appears to be the only inhibitor of transport so far studied that does not disturb lens ion levels.

3-O-Methylglucose↗
PubMed1983

Amino acid transport and crystallin synthesis in the bovine lens.

Bovine lenses were incubated in a defined, bicarbonate-free culture medium (EMEM) and the kinetics of amino acid uptake and protein synthesis investigated. The kinetics were interpreted in terms of a simple multi-compartment model. [14C]tyrosine was found to be totally exchangeable in the incubated lens and the rate constant for the exponential increase in activity was 0 . 0175 hr-1. The rate of influx was markedly reduced by incubating in the presence of ouabain (10(-5) M), which also caused a concomitant disturbance of the normal sodium and potassium distributions. The soluble proteins from the incubated lenses were fractionated on Sephadex G-200 and the rate of incorporation into the crystallins was shown to fall into two classes. The rate of synthesis of alpha and beta L crystallin was relatively rapid (rate constants approximately equal to 0 . 004 hr-1), while the synthesis rates of beta H and beta S/gamma were both much slower (0 . 001 hr-1). The efflux kinetics of [14C]tyrosine were determined and the rate of decrease of the free amino acid pool was identical to the rate of increase determined from an influx experiment. Hence the lenses are in a steady state with respect to free tyrosine throughout the incubation period (up to 160 hr). All classes of proteins continued to be synthesized during efflux experiments and there was no evidence for a breakdown of alpha or beta L crystallin during the time-course of these experiments. Ouabain slowed the rate of loss of tyrosine from the free amino acid pool, and this was interpreted in terms of an ouabain-induced decrease in synthesis rate rather than as a decrease in efflux rate from the lens. There was in fact a very marked decrease in the incorporation of [14C]tyrosine into the alpha and beta L crystallins on exposure to ouabain, and this decrease was apparent before any change in activity in the amino acid pool.

Amino Acids↗
PubMed1982

Changes in lens amino acid transport and protein metabolism during osmotic cataract produced by ouabain.

Osmotic (cortical) cataract was induced in bovine lenses during long-term organ culture by adding the cardiac glycoside, ouabain, to the incubation medium. This insult produces a much more rapid effect on protein synthesis than it does on amino acid influx, efflux or net protein loss. The kinetics of amino acid transport and protein synthesis were studied in detail using 14C-tyrosine in the incubation medium and a simple mathematical model was developed to help interpret the data. During efflux experiments, loss of radioactivity could be explained in terms of a single exponential process and ouabain was found to reduce the rate constant of loss after a delay of 50 hours. There was no delay, however, on the effect of ouabain on protein synthesis which was significantly reduced compared to control values. There was no effect of ouabain on net protein loss from the lens until after 120 hours, when significant amounts of 14-C labelled protein began to appear in the washing medium. This initiation of protein loss was accompanied by an increase in the amount of light scattered from the lens. Further studies are underway to determine which of the protein fractions are lost from the lens during osmotic insult.

Amino Acids↗
PubMed1981

Electrical coupling between fibre cells in amphibian and cephalopod lenses.

The lenses of vertebrate and cephalopod eyes differ ontogenetically and in other respects. The vertebrate lens, derived from a single cell type, consists mainly of long fibre cells continuously produced by division and elongation of columnar epithelial cells near the lens equator. Almost 50% of the fibre cell surface consists of junctional complexes and the internal resistance, from point to point within the lens, is low compared with the surface membrane resistance. Thus the vertebrate lens is expected to behave as a well coupled syncytial system. The cephalopod lens, however, is formed by the fusion of two distinct cell types; the anterior segment has the same ontogenetic origin as the cornea but the posterior segment shares a common origin with the retina, and the plane of contact of the two cell types can be seen in light-microscope sections. Most of the lens is composed of long fibre cells similar in appearance to those found in the vertebrate lens, and membrane junctional regions between adjacent fibres have also been tentatively identified. We now describe electrophysiological investigations of cellular communications in the cephalopod lens, which show marked differences in the intercellular electrical coupling within the vertebrate (amphibian) and cephalopod lens.

Action Potentials↗
PubMed1981

Role of sensory nerves in the cardiovascular and respiratory changes with isometric forearm exercise in man.

1. To investigate the stimulus to the cardiovascular and respiratory systems during isometric exercise, two patients with sensory neuropathies affecting forearm afferent nerves were studied and their circulatory and respiratory responses compared with those of normal subjects. The contribution of pain to the cardiorespiratory changes was also investigated in normal subjects by using hypnosis to relieve pain during and after isometric exercise. 2. The patients and normal subjects performed fatiguing isometric forearm exercises on a handgrip dynamometer on at least two occasions, once with normal forearm circulation and once with the forearm circulation occluded by an arterial cuff during and after exercise. Blood pressure, heart rate, minute ventilation, oxygen consumption and ventilatory equivalent were measured before, during and after exercise. 3. During exercises with and without occlusion increases in blood pressure and heart rate in patients and in subjects under hypnosis were similar to those in control subjects. Changes in oxygen consumption were also similar to those of controls, but the patients showed a smaller rise in ventilation and no increase in ventilatory equivalent. 4. After exercise in normal subjects occlusion of the forearm circulation produced continued elevation of blood pressure and respiration. Blood pressure also remained elevated after exercise with occlusion in subjects under hypnosis, but not in patients with sensory loss. Respiration did not remain elevated in either subjects under hypnosis or patients. 5. The results suggest that cardiovascular and respiratory changes during isometric exercise can occur without involvement of muscle sensory nerves and that pain contributes substantially to the increase in respiration. Normal sensory innervation is apparently necessary only for maintenance of raised blood pressure after exercise with occlusion.

Adolescent↗