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R A Webb

Publications and source records attributed to R A Webb.

At least 19 recordsLinked to original sources
PubMed2001

Diamagnetic persistent current in diffusive normal-metal rings.

We have measured a diamagnetic persistent current with flux periodicities of both h/e and h/2e in an array of thirty diffusive mesoscopic gold rings. At the lowest temperatures, the magnitudes of the currents per ring corresponding to the h/e- and h/2e-periodic responses are both comparable to the Thouless energy E(c) identical with Planck's over 2pi/tau(D), where tau(D) is the diffusion time. Taken in conjunction with earlier experiments, our results strongly challenge the conventional theories of persistent current. We consider a new approach associated with the saturation of the phase coherence time tau(phi).

Journal Article↗
PubMed2000

Multiple pulmonary nodules in AIDS: usefulness of CT in distinguishing among potential causes.

PURPOSE: To determine whether the computed tomographic (CT) appearances of multiple pulmonary nodules in patients with acquired immunodeficiency syndrome (AIDS) can help differentiate the potential infectious and neoplastic causes. MATERIALS AND METHODS: The thoracic CT scans obtained in 60 patients with AIDS and multiple pulmonary nodules were reviewed retrospectively by two thoracic radiologists who were blinded to clinical and pathologic data. The scans were evaluated for nodule size, distribution, and morphologic characteristics. CT findings were correlated with final diagnoses. RESULTS: Thirty-six (84%) of 43 patients with opportunistic infection had a predominance of nodules smaller than 1 cm in diameter, whereas 14 (82%) of 17 patients with a neoplasm had a predominance of nodules larger than 1 cm (P <.001). Of the 43 patients with opportunistic infection, 28 (65%) had a centrilobular distribution of nodules; only one (6%) of 17 patients with a neoplasm had this distribution (P <.001). Seven (88%) of eight patients with a peribronchovascular distribution had Kaposi sarcoma (P <.001). CONCLUSION: In patients with AIDS who have multiple pulmonary nodules at CT, nodule size and distribution are useful in the differentiation of potential causes. Nodules smaller than 1 cm, especially those with a centrilobular distribution, are typically infectious. Nodules larger than 1 cm are often neoplastic. A peribronchovascular distribution is suggestive of Kaposi sarcoma.

AIDS-Related Opportunistic Infections↗
PubMed1998

A novel Na+/HCO3--codependent choline transporter in the syncytial epithelium of the cestode Hymenolepis diminuta.

Absorption of exogenous choline by the cestode Hymenolepis diminuta was found to be both Na+- and HCO3--dependent and, at pH 6 to 7, accounted for up to 65% of the total choline uptake. Na+/HCO3- dependent choline uptake was activated at approximately 6 mM HCO3- (EC50 approximately 9 mM), and, above 100 mM Na+, the rate of uptake was directly proportional to the Na+ concentration. Atempts to uncouple Na+-dependent uptake from HCO3--dependent uptake were not successful: K+-depolarization was without effect on HCO3--dependent choline uptake, and use of valinoomycin to hyperpolarize the brush-border membrane resulted in inhibition of uptake. Na-/HCO3--dependent choline uptake was not associated with solvent drag. The Na+/HCO3--dependent choline uptake displayed a Q10 of 6.4 (27 degrees to 37 degrees) and a relatively high activation energy of 126 kJ x mol(-1). At pH 6.0 and 7.0, Na-/HCO3--dependent choline uptake rates were similar, but Na+/HCO3--dependent choline uptake was reduced at pH 5.0. The Na+/HCO3--dependent choline uptake, at pH 7.0, displayed a Kt of approximately 500 microM and a Vmax of 4.01 pmol x mg wet weight(-1) x min(-1). The Na+/HCO3--dependent choline uptake was hemicholinium-3 sensitive, but not significantly inhibited by 200 microM bumetanide, 100 microM amiloride, benzamil, or EIPA or by 1 mM 4,4'-diisothiocyano-2,2'-stilbene disulfonate (DIDS) or 4-acetamido-4'-isothiocvanostilbene-2,2'-disulfonic acid (SITS). Although it remains to be shown that HCO3- uptake is coupled directly to both choline and Na+ uptake, the data suggest that choline up take occurs via choline/Na+/HCO3--co-trans porter.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1998

The spatial and temporal concentrations of choline in the lumen contents of the small intestine of uninfected and Hymenolepis diminuta infected rats.

The spatial and temporal concentrations of free choline in the lumen of the small intestine of the uninfected and Hymenolepis diminuta-infected rat were investigated. In the unfed infected or uninfected rat, the choline concentrations ranged from approximately 500 microM in the duodenum to approximately 20 microM in the posterior ileum, with some segments in infected rats containing significantly higher choline levels than in uninfected rats. Following feeding, choline levels were significantly elevated to approximately 3 mM by 6 h, although these concentrations fell rapidly in transit down the intestine. By 12 h the choline levels were similar to those in the unfed rat. An initial small shift in the worm biomass toward the duodenum after feeding was followed by a redistribution of biomass along the length of the small intestine. The worm biomass, however, had little or no effect on the choline levels. The high concentrations of free choline observed in the anterior regions of the intestine are postulated to be predominantly determined by nutritional intake while the concentrations in the posterior region may in part be determined by blood choline levels. The high levels of choline indicate that choline is not limiting to support the growth of the worms.

Animals↗
PubMed1997

Novel benzamil-sensitive sodium-independent choline transport in the cestode Hymenolepis diminuta: evidence for sodium channels uptake at low pH.

The uptake of choline by the tegument of Hymenolepis diminuta was investigated. The Q10 at pH 7.0 was 1.7, with an Ea of 90 kJ.mol-1. Choline transport was pH sensitive: At pH 5.0, a Na(+)-independent mechanism predominated, which was inhibited by 100 nM benzamil, 130 mM Na+, and 300 microM verapamil. At pH 7.0, the Na(+)-independent mechanism was inhibited by 130 mM Na+, amiloride, and EIPA with IC50's of 130 microM and 30 microM, respectively, and by benzamil with IC50's of 100 pM (high-potency Benzamil Sensitive Component; HBSC) and 70 microM (low-potency Benzamil Sensitive Component; LBSC). Calcium-free saline enhanced choline uptake non-specifically. Lanthanum3+, Gd3+, gramicidin, nigericin, and high-K+ did not affect choline uptake at pH 5.0 or pH 7.0, and 10 microM verapamil was without effect at pH 5.0, suggesting no significant role for the electrical potential difference across the brush-border membrane, a Na+/H+ antiporter, a Na+/Ca2+ antiporter, or Ca2+ channels in choline uptake. Under physiological conditions, the HBSC accounts for approximately 25% of the total choline taken up at pH 5.0, while the LBSC accounts for approximately 55% of the choline taken up at pH 7.0. The data suggest novel choline transporting mechanisms; an HBSC which displays properties in common with apical Na+ channels, and a unique LBSC of choline transport.

Amiloride↗
PubMed1996

The stimulatory effect of L-glutamate and related agents on inositol 1,4,5-trisphosphate production in the cestode Hymenolepis diminuta.

The effects of L-glutamate, acetylcholine, and serotonin (5HT) were examined on generation of inositol 1,4,5-triphosphate [Ins(1,4,5)P3], in membrane preparations of the cestode Hymenolepis diminuta. Only L-glutamate and acetylcholine stimulated a significant elevation in Ins(1,4,5)P3. The response to L-glutamate was stereospecific; D-glutamate or L-aspartate were not as potent. A role for G-protein(s) was supported by the observations that sodium fluoride stimulated Ins(1,4,5)P3 generation, and the L-glutamate response was potentiated by GTP and GTP-S and was suppressed by GDPS. However, studies with pertussis and cholera toxins indicated that the putative G-protein(s) was not pertussis or cholera toxin sensitive. The pharmacological profile of the L-glutamate response was examined partially. Trans-ACPD was a very effective agonist at 10(-5)M. While 10(-3)M L-glutamate, NMDA, and AMPA significantly elevated Ins(1,4,5)P3 levels, quisqualate and kainate did not. The elevation of Ins(1,4,5)P3 levels by L-glutamate and NMDA was antagonized by the specific glutamatergic antagonists AP-5, AP-7, CNQX, and CPP. While the response to ACPD was antagonized by AP5, CPP and CPG, CNQX was without effect. Collectively, the data support the hypothesis that in the cestode H. diminuta, L-glutamate activation of a metabotropic (ACPD) and/or ionotropic-like AMPA/NMDA receptor subtypes proceeds via a G protein(s) to enhance phospholipase C activity, ultimately resulting in the elevation of Ins(1,4,5)P3 levels in the tissues.

Acetylcholine↗
PubMed1995

Electrical field-stimulated release of L-[G-3H]-glutamate from tissue slices of the cestode Hymenolepis diminuta.

The release of preloaded [3H]-L-glutamate by tissue slices of Hymenolepis diminuta was examined by the technique of electrical field stimulation. For tissues slices in balanced saline, a high-field strength of 1.78 A resulted in a highly elevated increase in the rate of release of radiolabel, which was suppressed in calcium-free saline. This technique may prove useful for screening helminth tissues for potential neurotransmitters.

Animals↗
PubMed1995

An acute dose of monosodium L-glutamate causes decreased fecundity and enhanced loss of the cestode Hymenolepis diminuta from rats.

A method for the rapid quantitative analysis of eggs in the feces of rats infested with Hymenolepis diminuta is described. In one experiment, eggs in the feces of individual rats were monitored for 8 days. Rats were provided with 1 ml saline vehicle or with 0.1 or 1 g monosodium glutamate (MSG) by stomach tube and egg production was monitored. Egg production decreased in saline vehicle-treated rats, but the decrease was significantly greater in the group treated with 1 g MSG. There was no significant change in fecal mass output from rats with different treatments. The number of worms harbored by rats treated with 1 g MSG was significantly lower than those of saline control-treated rats. In another experiment, neither anesthesia alone nor anesthesia with 1 g sucrose significantly reduced egg output as compared with that observed in untreated rats. Therefore, MSG has a significant negative effect on both the fecundity of H. diminuta and the numbers of worms harbored by rats.

Animals↗
PubMed1994

Occurrence and characterization of a low-affinity choline uptake mechanism in the internal tissues of the cestode Hymenolepis diminuta.

[3H]Choline was taken up by tissue slices of the cestode Hymenolepis diminuta by sodium-dependent and sodium-independent mechanisms. The sodium-dependent uptake was saturable, against an apparent concentration gradient, and by analysis of the kinetics of uptake could be delineated into a high-affinity choline uptake (HAChU) mechanism (Kt of approximately 2.0 microM; Vmax of approximately 0.15 pmol/mg wet weight tissue/min), and a low-affinity choline uptake (LAChU) mechanism (Kt of approximately 20.0 microM and a Vmax of approximately 2.0 pmol/mg wet weight tissue/min). Unlike the HAChU system, the LAChU system was unaffected by potassium. Furthermore, the responses of the LAChU and the HAChU transporters to pharmacological agents were distinctive, indicating that these transporters are two separate entities. Moreover, the entry or exit of choline via the LACh transporter was dependent upon the direction of the sodium gradient. Furthermore, the majority of released radiolabel from preloaded [3H]choline was associated with choline and was via a proteinaceous transporter. The present study is the first to provide direct evidence for a partial chemosmotic coupling of a LAChU system to the sodium electrochemical gradient.

Animals↗
PubMed1991

Serotonin stimulates protein phosphorylation in the cestode Hymenolepis diminuta.

1. Serotonin stimulated the incorporation of 32P from [gamma-32P] ATP into crude membrane preparations (P2) of Hymenolepis diminuta in a dose-dependent manner (EC50 of approximately 0.79 microM). 2. This response was seen with several serotonin agonists, and was inhibited by several serotonin antagonists, which were identical to the previously described activation and inhibition of serotonin-sensitive adenylate cyclase. 3. Cyclic AMP produced a dose-dependent stimulation of 32P incorporation into the P2 fraction, with an EC50 of approximately 2.51 microM. 4. The targets for the serotonin stimulated incorporation of 32P were found to be in trypsin-labile proteins with Mr's of 134,000, 110,000, 82,000, 80,000 and 31,000.

Animals↗
PubMed1991

The effect of L-glutamate and related agents on adenylate cyclase in the cestode Hymenolepis diminuta.

The effect of the putative amino acid transmitter, L-glutamate, on adenylate cyclase in crude membrane preparations of the rat tapeworm Hymenolepis diminuta was investigated to determine if glutamate effects the generation of the second messenger cAMP. Addition of glutamate at 10(-3) and 5.5 x 10(-9) M resulted in significant elevations in basal activity of adenylate cyclase, while concentrations in the 10(-5)-10(-7) M range caused significant depressions below basal activity. Assays with glutamate agonists and other acidic compounds showed glutamate to be the only amino acid, dicarboxylic acid, or acidic compound capable of this pattern of stimulation and inhibition. While the response of adenylate cyclase to glutamate agonists suggested that an N-methyl-D-aspartic acid (NMDA) type receptor may be present, glutamate agents acting as NMDA antagonists in vertebrate systems were agonists. Metabolic end products of glycolysis stimulated adenylate cyclase, suggesting that these, along with metabolic glutamate may regulate glycolytic enzymes. Only 10(-3) M L-glutamate significantly stimulated adenylate cyclase activity in tissue slices, and this response was restricted to those slices rich in nervous tissues. L-Glutamate eliminated the 5-hydroxytryptamine (5-HT) stimulated adenylate cyclase response suggesting that glutamate can modulate the 5-HT stimulated elevations in adenylate cyclase activity. The data support the hypothesis that L-glutamate is a neurotransmitter-modulator in the cestode.

1-Methyl-3-isobutylxanthine↗
PubMed1991

Characterization of a high-affinity choline uptake mechanism in the cestode Hymenolepis diminuta.

Radiolabelled choline was taken up by tissue slices of the cestode Hymenolepis diminuta by a sodium-dependent and a sodium-independent mechanism. The sodium-dependent uptake was saturable, against a concentration gradient, displayed structural specificity, and was inhibited, in part, by hemicholinium-3. Kinetic analysis of the sodium-dependent choline uptake showed an apparent Kt = 2.0 microM and a Vmax = 0.146 pmol.mg-1 wet weight tissue.min-1, which is consistent with a high-affinity choline uptake (HAChU) mechanism. The rate of uptake or release of choline depended on the magnitude and direction of the sodium gradient, was diminished by high- or low-potassium, but was not chloride or sulphate dependent. A homoexchange mechanism for HAChU was not demonstrated. Evidence was obtained to suggest that HAChU or release of endogenous ACh is regulated by autoreceptors. The choline taken up by the HAChU mechanism was but slowly converted to ACh and other products.

Acetylcholine↗
PubMed1991

Anterior segment ischemia in chronic herpes simplex keratouveitis.

Six patients with chronic herpes simplex keratouveitis developed a rapidly progressive ocular inflammation unresponsive to corticosteroid and antiviral therapy. Severe secondary glaucoma unresponsive to maximum medical therapy ensued and was treated by cyclocryotherapy in five patients. Ocular ischemia with secondary corneal and scleral calcification subsequently developed in all six patients. All had also received prolonged topical therapy with topical antiviral agents, corticosteroids, beta adrenergic blockers and epinephrine compounds. Three eyes eventually required enucleation for the relief of pain; one stabilized, and two others became phthisical. One of the phthisical eyes developed a secondary fungal endophthalmitis. Histopathologic examination of the three enucleated globes revealed extensive corneal, scleral and conjunctival calcification, secondary angle closure, iris and ciliary body necrosis, focal choroiditis, retinal necrosis and atrophy. The syndrome recognized in these patients appears to be a rare but devastating complication of herpes simplex keratouveitis, possibly exacerbated by the application of cyclocryotherapy and other factors.

Aged↗
PubMed1990

Acetylcholine-like immunoreactivity in the cestode Hymenolepis diminuta.

The localization of acetylcholine in tissues of the cestode Hymenolepis diminuta was determined, following derivatization, using an antibody raised against choline-glutaraldehyde-protein. Specific immunoreactivity was observed in the rostellum and beneath the suckers of the scolex, in the longitudinal nerve cords, in cells adjacent to some deep longitudinal muscles, in the genital primordium, in the wall of the cirrus sac, and in the external and internal seminal vesicle. The distribution of acetylcholine-like immunoreactivity in relation to that of serotonin and glutamate, and the distribution of acetylcholinesterase in H. diminuta is discussed.

Acetylcholine↗
PubMed1989

Release of exogenously-supplied and endogenous serotonin from tissue slices of Hymenolepis diminuta.

The release of exogenously-supplied [3H]serotonin and of endogenous serotonin from H. diminuta tissue slices was studied using high [K+] depolarization. The release of [3H]serotonin was not calcium-dependent or magnesium-antagonized. While high-magnesium antagonism of endogenous serotonin release was inconclusive, this release was calcium-dependent. These findings suggest that, while exogenous serotonin is not taken into and released from nervous tissue, endogenous serotonin appears to be released from nervous tissue in a neurotransmitter-like manner.

Animals↗
PubMed1989

Distribution of histamine in the lumen contents of the small intestine of uninfected and Hymenolepis diminuta-infected rats.

Using a specific radioenzymatic assay, histamine was detected and measured in the lumen contents of six different segments of the small intestine of uninfected rats and rats infected with Hymenolepis diminuta as well as in worm tissues. The distribution of histamine in the lumen of the small intestine of uninfected rats was found to range from 1.4 +/- 0.1 microM in the first (anterior) segment to 0.59 +/- 0.13 mM in the sixth (posterior) segment. There were no significant differences between these concentrations and those found in the lumen contents of intestine from rats infected with H. diminuta. On the other hand, although most H. diminuta was confined to the second and third segments, the concentration of histamine associated with the worm tissues (5.4 +/- 0.4 microM) was significantly lower than that in Hymenolepis-containing intestinal segments. The data suggest that established infections of H. diminuta do not cause a significant increase in histamine levels in host's intestinal lumen, nor do they affect the spatial gradient in the lumen.

Animals↗
PubMed1989

Demonstration of intense glutamate-like immunoreactivity in the longitudinal nerve cords of the cestode Hymenolepis diminuta.

Glutamate-like immunoreactivity in the cestode Hymenolepis diminuta was studied at the light microscopic level using an antibody specifically directed against a reduced conjugate of glutamate linked to protein by glutaraldehyde. Relatively low levels of glutamate-like immunoreactivity were present in the subtegumental region, which was presumed to reflect an active metabolic pool. Relatively high levels of this reactivity were observed in the longitudinal nerve cords and associated cells, and somewhat lower levels were occasionally noted in ring commissures. The present data in conjunction with previous studies on glutamate in H. diminuta, support the hypothesis that glutamate is an excitatory neurotransmitter in the platyhelminths.

Animals↗