Biomedical subjects

A J Lawrence

Publications and source records attributed to A J Lawrence.

At least 91 records · Page 5Linked to original sources

PubMed1996

Markers of adenosine removal in normotensive and hypertensive rat nervous tissue.

Adenosine mechanisms are altered in brain stem nuclei associated with cardiovascular control in spontaneously hypertensive rats (SHR). Therefore, in the present study we used a number of techniques to compare the binding of the adenosine transport inhibitor [3H]nitrobenzylthioinosine ([3H]NBMPR) as well as adenosine deaminase immunoreactivity (ADA-IR) in brain stems and nodose ganglia of SHR and age-matched normotensive Donryu rats (DRY). Saturation binding revealed a single class of [3H]NBMPR binding sites in the dorsal brain stem of both strains, with Kd and Bmax values of 65 +/- 9 pmol/L and 282 +/- 31 fmol/mg protein, respectively, in SHR and 129 +/- 2 pmol/L and 217 +/- 23 fmol/mg protein in DRY. The Kd for [3H]NBMPR was significantly lower in SHR than in DRY. In competition assays, NBMPR, dilazep, dipyridamole, and adenosine displaced [3H]NBMPR binding, with Kd values of 0.21 +/- 0.04, 57.16 +/- 16.20, 1340 +/- 100, and 87000 +/- 12500 nmol/L, respectively, in DRY and 0.17 +/- 0.04, 28.24 +/- 3.60, 621 +/- 100, and 32000 +/- 6820 in SHR. Kd values for all displacers were lower in SHR; however, only values for dipyridamole and adenosine reached statistical significance. Autoradiography of adenosine transport sites with [3H]NBMPR revealed that unilateral nodose ganglionectomy reduced [3H]NBMPR binding on the denervated side of the nucleus tractus solitarius by 20.6 +/- 1.1% in DRY and 18.7 +/- 2.3% in SHR. The density of [3H]NBMPR binding in nodose ganglia was significantly lower in SHR (0.99 +/- 0.06 Bq/mm2) than in DRY (1.25 +/- 0.08). Immunohistochemical studies demonstrated ADA-IR in the dorsal vagal complex, associated with both nerve cells and fibers. Measurement of ADA-IR in the dorsal vagal complex with an 125I-labeled secondary antibody revealed a significantly higher level of ADA-IR in SHR (122%) than in DRY. In the nodose ganglia, ADA-IR was associated with a population of vagal perikarya. The present study helps provide a molecular explanation for the previously reported impaired cardiovascular responses to intra-nucleus tractus solitarius microinjection of adenosine in hypertensive rats.

Adenosine↗

PubMed1995

5-Hydroxytryptamine3 receptor modulation of excitatory amino acid release in the rat nucleus tractus solitarius.

In vivo microdialysis was employed to measure release of endogenous L-glutamate (GLU) and L-aspartate (ASP) in the medial nucleus tractus solitarius of urethane anaesthetised rats. Basal extracellular levels of these amino acids were stable following a 90 min equilibration period (6.3 +/- 0.24 and 3.4 +/- 0.6 pmol/20 microliters sample of GLU and ASP, respectively). Basal levels of endogenous extracellular GLU and ASP were increased over 2-fold and 3-fold, respectively, following local administration of the selective 5-hydroxytryptamine (5-HT3) receptor agonist phenylbiguanide (300 microM). Intracerebral administration of the selective 5-HT3 receptor antagonist ondansetron (30 microM) blocked the effect of phenylbiguanide on GLU release whilst the effect on ASP was variable and complex. These data suggest that 5-HT3 receptor activation in the rat nucleus tractus solitarius can affect excitatory amino acid neurotransmission in this region of the medulla oblongata.

Animals↗

PubMed1995

Visualization of beta-adrenoceptor binding sites on human inferior vagal ganglia and their axonal transport along the rat vagus nerve.

DESIGN: Because of uncertainties regarding the complete antihypertensive mechanism of action of beta-adrenoceptor antagonists, the present study determined whether vagal afferent neurons of humans and rats possess beta-adrenoceptors. Such a location would provide an appropriate target for beta-blockers to modulate neurotransmission of barosensitive neurons, thereby affecting blood pressure. Therefore, in vitro receptor autoradiography of high-affinity beta-adrenoceptor binding sites was performed on slices of human and rat inferior vagal (nodose) ganglia with [125I]-pindolol. METHODS: Slide-mounted sections of human and rat inferior vagal ganglia were incubated with [125I]-pindolol in the absence or presence of propranolol (10 mumol/l) to define non-specific binding, atenolol (10 mumol/l) to inhibit binding to beta 1-adrenoceptors, or ICI 118551 (3 nmol/l) to inhibit binding to beta 2-adrenoceptors. Unilateral vagal ligation was also performed in the rat to study whether beta-adrenoceptors are subject to axonal transport along the vagus nerve. RESULTS: [125I]-pindolol bound with > 90% specific binding to sections both of human and of rat inferior vagal ganglia. Specific binding occurred over both neuronal perikarya and nerve fibres. In both species the beta 2-adrenoceptor subtype appeared to predominate, as defined by the differential ability of ICI 118551 (beta 2) and atenolol (beta 1) to inhibit the binding of [125I]-pindolol. Furthermore, unilateral vagal ligation in the rat caused an accumulation of specific binding adjacent to the ligature sites. CONCLUSIONS: We conclude that human and rat vagal afferent (and efferent) neurons possess beta-adrenoceptors that potentially could explain the mechanism of action of beta-adrenoceptor antagonists in the therapy of hypertension.

Animals↗

PubMed1995

Neurotransmitter mechanisms of rat vagal afferent neurons.

1. The present study has employed anatomical and neurochemical techniques to assess whether the amino acid, L-glutamate, may be considered as a potential neurotransmitter at rat vagal afferent neurons, with particular reference to baroreceptor afferents. 2. Slide-mounted sections of rat nodose ganglia were incubated with a high-titre antibody to glutamate, and visualization of the resulting immunoreactivity indicated glutamate-positive staining in a population of vagal afferent perikarya. In contrast, interstitial cells were devoid of immunostaining. 3. Release of endogenous glutamate was measured by in vivo microdialysis in the nucleus tractus solitarius, the site of central vagal afferent terminals, and could be evoked with a depolarizing stimulus of KCl in a calcium-dependent fashion. In addition, baroreceptor loading with an intravenous infusion of phenylephrine (30 micrograms/kg per min) increased the spontaneous efflux of glutamate to 148 +/- 28% of basal levels, which was paralleled by an increase in mean arterial pressure (approximately 40 mmHg). Release of glutamate was also elevated two-fold by intracerebral administration of S-nitroso-N-acetylpenicillamine (30 mumol/L), an effect that could be prevented by coadministration of methylene blue (10 mumol/L). 4. These data suggest that neuronal glutamate may be formed in a population of vagal afferent cell bodies, presumably to act on soma membrane receptors. Furthermore, the excitatory amino acid is released in a neurotransmitter-like fashion at the terminal region of vagal afferent neurons, where glutamate release is increased as a consequence of baroreceptor loading and also following activation of soluble guanylate cyclase. Thus, glutamate may be considered a candidate neurotransmitter of vagal baroreceptor afferent neurons, which may be modulated by nitric oxide or an endogenous nitrosothiol.

Animals↗

PubMed1995

Functional dopamine D2 receptors on rat vagal afferent neurones.

1. In the present study in vitro electrophysiology and receptor autoradiography were used to determine whether rat vagal afferent neurones possess dopamine D2 receptors. 2. Dopamine (10-300 microM) elicited a temperature- and concentration-dependent depolarization of the rat isolated nodose ganglion preparation. When applied to the tissue 15 min prior to agonist, raclopride (10 microM), clozapine (10 microM) or a mixture of raclopride and clozapine (10 microM each) all produced a threefold parallel shift to the right of the dopamine concentration-response curve. In contrast, SCH 23390 (100 nM), phentolamine and propranolol (1 microM each) failed to antagonize the dopamine-mediated depolarization. 3. [125I]-NCQ 298 (0.5 nM), a D2 selective radioligand, bound topographically to sections of rat brainstem. Densitometric quantification of autoradiograms revealed 93.8 +/- 0.5% specific binding of this salicylamide radioligand, as determined by raclopride (10 microM, n = 10 animals). Binding was highest in the nucleus tractus solitarius (NTS), particularly the medial and gelatinous subnuclei. In addition, specific binding was also observed in the interpolar spinal trigeminal nucleus and the inferior olive. 4. Unilateral nodose ganglionectomy caused a 36.6 +/- 3.0% reduction in specific binding in the denervated NTS compared to the contralateral NTS. Furthermore, the loss of binding was confined to the dorsal aspect of the medial subnucleus of the NTS. Sham surgery had no effect on the binding of [125I]-NCQ 298 in rat brainstem. 5. The present data provide evidence for the presence of functionally relevant dopamine D2 receptors on both the soma and central terminals of rat vagal afferent neurones. In addition, the majority of D2 receptors in the rat NTS appear to be located postsynaptically with respect to vagal terminals, and are presumably located either on ascending glossopharyngeal terminals, descending terminals from higher brain regions or on neuronal cell bodies within the NTS.

Afferent Pathways↗

PubMed1995

The provision of orthodontic services by general dental practitioners. 1. Methods and descriptive results.

Information regarding orthodontic service provision by general dental practitioners in Australia is limited. The aim of this survey was to determine the amount and variety of orthodontic services provided by general dental practitioners in the Melbourne Statistical Division, Victoria, Australia. A random sample of 307 dentists drawn from the Victorian Dentists Register was surveyed by mailed questionnaire: 218 (71%) replied. Data were collected using a fortnight log. During this time 59 per cent of the dentists saw at least one orthodontic patient; one dentist saw 66 orthodontic patients. Removable orthodontic appliances were used by 35 per cent of the dentists and fixed orthodontic appliances by 18 per cent. Twenty-six per cent provided comprehensive orthodontic treatment, 22 per cent aligned incisors, and 21 per cent corrected anterior crossbites. The general dental practitioners surveyed provided a wide range of preventive and interceptive orthodontic services to generally a small percentage of their patients.

Adult↗

PubMed1995

The provision of orthodontic services by general dental practitioners. 2. Factors influencing variation in service provision.

Previous work has shown that variations exist amongst general dental practitioners in the volume and type of orthodontic services provided, the type of orthodontic appliances used, and the objectives of the orthodontic treatment. The aims of this survey were to identify practitioner characteristics that account for variations in the level of orthodontic services provided and which distinguish providers and non-providers of orthodontic services. Multiple regression analysis revealed that four practitioner characteristics explained 41 per cent of the variance in the number of orthodontic patients treated. Dentists who treated more orthodontic patients: 1) treated more general practice patients; 2) frequently used multiple sources to keep up to date in orthodontics; 3) perceived their patient base to contain more children; and 4) were likely to have attended a Truitt course. Eleven variables best distinguished providers from non-providers of orthodontic treatment; 1, 2 and 3 above had the highest correlation with the discriminant function. The Null Hypothesis that selected characteristics of dentists providing orthodontic services were no different from those of dentists not providing orthodontic services was rejected. The provision of orthodontic services was associated with a higher level of continuing orthodontic education as well as treating more general practice patients, especially children.

Adult↗

PubMed1994

Visualization of dopamine D2 binding sites on human inferior vagal ganglia.

The present study used in vitro autoradiography to assess whether high affinity D2 binding sites are present on sections of human inferior vagal ganglia, where arterial baroreceptor primary afferent cell bodies are located. Incubation of tissue sections with [125I]NCQ 298 (0.5 nM) revealed dense but non-uniform binding, with a well-defined topography, consistent with the localization of binding sites over cell bodies rather than nerve axons. Coincubation with the D2 antagonist raclopride (10 microM) completely abolished binding of [125I]NCQ 298. Densitometric quantification of autoradiograms estimated 82 +/- 4% specific binding for [125I]NCQ 298 (0.5 nM). These observations indicate the presence of D2 binding sites on sensory ganglia involved in cardiovascular control pathways in the human, and may help to explain some of the documented cardiovascular effects of dopamine.

Autoradiography↗

PubMed1994

Presynaptic adenosine A2a receptors on soma and central terminals of rat vagal afferent neurons.

The dorsal vagal complex of the medulla oblongata is a key centre involved in the regulation of numerous autonomic functions, including cardiovascular control. Adenosine has been implicated as a potential neuromodulator of the baroreceptor reflex, and therefore the current study has investigated the presence and characteristics of adenosine receptors on rat vagal afferent neurons. In the nodose-vagal grease gap preparation, the adenosine A2a agonist CGS-21680 evoked a depolarisation only in the presence of the selective adenosine A1 antagonist PACPX. Autoradiography using [3H]NECA (4 nM) with suppression of A1 binding enabled the first visualisation of high affinity adenosine A2 receptors in the nucleus tractus solitarius (NTS). Unilateral nodose ganglionectomy resulted in over 90% reduction in binding in the lesioned (ipsilateral) NTS compared to a sham control. Furthermore, local administration of CGS-21680 increased evoked glutamate release in the NTS, as measured by in vivo microdialysis. These data suggest the presence of presynaptic adenosine A2a receptors on both the soma and central terminals of rat vagal afferent neurons, and thereby support the hypothesis that adenosine may have a modulatory role in the baroreceptor reflex.

Adenosine↗

PubMed1994

Zinc and barium inhibit the phospholipase A2 from Naja naja atra by different mechanisms.

The mode of inhibition of the phospholipase A2 (PLA2) enzyme from the Chinese cobra (Naja naja atra) by Zn2+ is qualitatively different from inhibition by Ba2+. Inhibition by Ba2+ shows the kinetic characteristics of a conventional competitive inhibitor acting to displace Ca2+ from a single essential site, but Zn2+ has the paradoxical property of being more inhibitory at high than at low Ca2+ concentration. Kinetic analysis of the Ca(2+)-dependence of enzymic activity shows a bimodal response, indicating the presence of two Ca(2+)-binding sites with affinities of 2.7 microM and 125 microM respectively, and we propose that these can be identified with the two Ca(2+)-binding sites revealed by crystallographic analysis [White, Scott, Otwinowski, Gleb and Sigler (1990) Science 250, 1560-1563]. The results are consistent with the model that the enzyme is activated by two Ca2+ ions, one that is essential and can be displaced by Ba2+, and one that modulates the activity by a further 5-10-fold and which can be displaced by Zn2+. An alternative model is also presented in which the modulating Zn(2+)-binding site is a phenomenon of the lipid/water interface.

Barium↗

PubMed1994

Conductimetric assays for the hydrolase and transferase activities of phospholipase D enzymes.

Measurement of solution electrical conductance (conductimetry) is a simple direct assay method for the protogenic, hydrolytic reactions catalyzed by all phospholipase enzymes. The technique is especially suitable for assay of phospholipase D (PLD) enzymes where cleavage of zwitterionic substrates reinforces the pH dependent conductance change and allows the method to be used over a much wider pH range than the equivalent titrimetric assay. The ability to detect zwitterion cleavage enables the method to assay reactions in which phospholipase D transfers neutral, or anionic, alcohol species to the zwitterionic substrates phosphatidyl choline and phosphatidyl ethanolamine. The method can follow the sequential attack by different phospholipases and provides a simple technique for investigating the effect of substrate structure on susceptibility to various phospholipase enzymes. The results confirm that PLD from Streptomyces chromofuscus can attack lysophospholipids, but cannot transfer primary alcohols to the phosphatidyl residue, while the PLD from savoy cabbage is an efficient transferase, but cannot attack lysophospholipids. The data suggest that the bacterial PLD fails to act as a transferase because it hydrolyzes the transphosphatidylation products. Some phosphatidyl alcohols are more highly susceptible to PLA2 attack than the parent phosphatidyl choline derivatives.

Animals↗

PubMed1994

High-resolution two-part basic urea gels for analysis of venom phospholipase A2 isoforms.

The performance of acidic and basic urea polyacrylamide gels has been improved by adopting a two-part gel system with a concentration discontinuity to act as a stacking boundary and by increasing the urea concentration to 8 M. The contributions of primary amino-group and guanidino-group ionizations to mobility have been evaluated by acetylation and phenyl glyoxal treatment respectively. The chromogenic PLA2 detection method of Shier and Trotter (Analyt. Biochem. 87, 604, 1978) has been modified for use with basic urea PAGE. The results have confirmed the major findings of other workers, but have demonstrated the presence of many hitherto uncharacterised isoforms of PLA2 in a variety of whole snake venoms. The basic urea PAGE (BG) method is proposed as the basis of a simple and rapid method for the classification of PLA2 isoforms which should allow unambiguous identification of isoforms by referring bands for purified material to the isoform content of whole venoms.

Acetylation↗

PubMed1994

Addition-order dependent modulation of the sensitivity of rabbit erythrocyte membrane to bee venom phospholipase A2 by oleic acid, lysophosphatidyl choline and albumin.

The addition of exogenous oleic acid to erythrocyte membranes induces a characteristic membrane crenation and sensitises the cells to the lytic action of phospholipase A2 enzymes. Both effects are extremely sensitive to inhibition by endogenous lysophosphatidyl choline (LPC), but the strength of inhibition depends of the order in which the reagents are added to the cells. These responses are further enhanced when the reagents are extracted from the cell membranes by treatment with albumin. Thus the inhibitory action of LPC added before oleic acid increases when the reagents have been extracted but that of LPC added after oleic acid decreases after extraction. The results are discussed in terms of the stimulation of PLA2 activity by enhanced membrane curvature.

Albumins↗

PubMed1994

[3H]adenosine transport in rat dorsal brain stem using a crude synaptosomal preparation.

The neuromodulator adenosine, has been shown to have the highest density of central uptake sites in the nucleus tractus solitarius in the dorsal brain stem. The nucleus tractus solitarius is involved in the central regulation of reflex cardiovascular activity suggesting that adenosine may also be involved in central cardiovascular control. Thus, the present study has characterized the transport of [3H]adenosine into rat dorsal brain stem synaptosomes. The process was found to be saturable and highly dependent on temperature. Furthermore, [3H]adenosine transport in rat dorsal brain stem synaptosomes was far more sensitive to the removal of sodium ions than has been previously reported for rat cortical synaptosomes. In addition transport was rapid, being linear for at least 30 s at 37 degrees C, reaching equilibrium within 1 min and had an apparent Km value of 2.7 +/- 0.2 microM (n = 4) and a Vmax value of 135.5 +/- 17.8 pmol/mg protein/min (n = 4). These kinetic parameters are within an order of magnitude of adenosine uptake processes found in rat cerebral cortical synaptosomes. Transport of [3H]adenosine was significantly inhibited by an excess of unlabelled adenosine (1 mM) and the adenosine uptake inhibitor S(4-nitrobenzyl)-6-thioinosine (100 microM) while morphine (150 microM) and flurazepam (150 microM) were largely ineffective as inhibitors of the process, in contrast with previous findings in rat cortical synaptosomes. The present findings demonstrate the presence of a high affinity transport system for adenosine in the rat dorsal brain stem which appears to differ in some properties to uptake processes found in rat cortex.

Adenosine↗

PubMed1994

L-glutamate as a neurotransmitter at baroreceptor afferents: evidence from in vivo microdialysis.

In vivo microdialysis was employed to measure release of endogenous L-glutamate and L-aspartate in the region of the dorsomedial medulla oblongata including the medial nucleus tractus solitarius of the anaesthetized rat. Basal extracellular levels of these amino acids were stable and increased over two-fold when the perfusate was changed to a high KCl (80 mM) artificial cerebrospinal fluid. This high K(+)-evoked release was calcium-dependent, while basal levels were insensitive to removal of calcium ions from the perfusate. An intravenous infusion of phenylephrine, which elevated blood pressure, caused a marked increase of both spontaneous and evoked release of glutamate. In contrast aspartate efflux was not significantly altered. The present data provide evidence that the excitatory amino acids, glutamate and aspartate, serve a neurotransmitter function in the nucleus tractus solitarius of the rat. Furthermore, the increase in glutamate release following baroreceptor activation with phenylephrine suggests that glutamate may be a neurotransmitter at baroreceptor afferent nerve terminals within the nucleus tractus solitarius. On the other hand, aspartate appears to be possibly an inter-neuronal transmitter in this brain region.

Animals↗

PubMed1994

Effect of lysophosphatidic acid on motility, polarisation and metabolic burst of human neutrophils.

The effect of lysophosphatidic acid (LPA) on human neutrophil activation was examined by a combination of automated tracking assays, cell shape measurements and assays of the metabolic burst by means of 7-dimethylamino-naphthalene-1,2-dicarbonic acid hydrazide (DNDH)-dependent chemiluminescence. LPA powerfully stimulated polarisation and motility. Polarisation became detectable at 2 microM LPA and virtually 100% of cells were polarised at 20 microM LPA. Cell motility increased with the degree of polarisation, and was diminished at high LPA concentration, but this decrease was reversed by albumin. LPA also inhibited the metabolic burst response to both n-formyl-methionyl-leucyl-phenylalanine (fMLP) and phorobol 12-myristate 13-acetate (PMA). Inhibition of the PMA-induced metabolic burst by LPA was not affected by pertussis toxin, showing that the effect was not mediated by the pertussis toxin-sensitive heterotrimeric G protein, and that inhibition of the PMA-stimulated metabolic burst by LPA could result from a direct action of LPA on the small cytosolic GTP-binding proteins. These results indicate that lysophosphatidic acid production by thrombin-activated platelets could play a significant role in the regulation of the inflammatory response.

Cell Movement↗

PubMed1994

Health and safety at work in orthodontic practice.

Health and safety is now of major importance in general and specialist dental practice. Risk assessment and management is essential to ensure that practices are healthy and safe for staff, patients and visitors. Practitioners are required to do all that is reasonably practicable to ensure this. At the same time there is also the need to keep practising costs at an economic level. To comply requires a sound working knowledge of the law and regulations. It also involves turning general policies into specific surgery procedures that prevent accidents and promote good practice. Six new sets of regulations on health, safety and welfare were introduced in 1993 in the UK that brings us into step with matching regulations throughout The European Community. The general requirements, the new regulations and their background are discussed in the first of a two part article. Many of the specific problems related to orthodontic practice are discussed in the second article. Many problems can be addressed inexpensively by modifying working practice but this will need to involve the whole dental team.

Accidents↗

PubMed1994

Health and safety at work in orthodontic practice--Part II.

On April 1st, 1992 practice inspections came into force and the remit of the inspector takes into consideration not only whether the practice is a safe place in which patients may be treated, but also whether the environment is safe for staff to work in. Gone are the days therefore when one could ignore health and safety at work, for to do so could result in a new practice not being added to a FHSA list or at the worst an existing practice made to close down!

Cross Infection↗