Biomedical subjects

J Brooke

Publications and source records attributed to J Brooke.

13 recordsLinked to original sources

PubMed2001

Tunable quantum tunnelling of magnetic domain walls.

Perhaps the most anticipated, yet experimentally elusive, macroscopic quantum phenomenon is spin tunnelling in a ferromagnet, which may be formulated in terms of domain wall tunnelling. One approach to identifying such a process is to focus on mesoscopic systems where the number of domain walls is finite and the motion of a single wall has measurable consequences. Research of this type includes magnetotransport measurements on thin ferromagnetic wires, and magnetization experiments on single particles, nanomagnet ensembles and rare-earth multilayers. A second method is to investigate macroscopic disordered ferromagnets, whose dynamics are dominated by domain wall motion, and search the associated relaxation-time distribution functions for the signature of quantum effects. But whereas the classical, thermal processes that operate in these experiments are easily regulated via temperature, the quantum processes have so far not been tunable, making difficult a definitive interpretation of the results in terms of tunnelling. Here we describe a disordered magnetic system for which it is possible to adjust the quantum tunnelling probabilities. For this material, we can model both the classical, thermally activated response at high temperatures and the athermal, tunnelling behaviour at low temperatures within a unified framework, where the domain wall is described as a particle with a fixed mass. We show that it is possible to tune the quantum tunnelling processes by adjusting the 'mass' of this particle with an external magnetic field.

Journal Article↗

PubMed2000

Nonlinear electric field effects at a continuous mott-hubbard transition

We characterize the non-Ohmic portion of the conductivity at temperatures T<1 K in the highly correlated transition metal chalcogenide Ni(S,Se)(2). Pressure tuning of the T = 0 metal-insulator transition reveals the influence of the quantum critical point and permits a direct determination of the dynamical critical exponent z = 2.7(+0.3)(-0.4). Within the framework of finite temperature scaling, we find that the spatial correlation length exponent nu and the conductivity exponent &mgr; differ.

Journal Article↗

PubMed2000

Recognition and sex categorization of adults' and children's faces: examining performance in the absence of sex-stereotyped cues.

The ability of children and adults to classify the sex of children's and adults' faces using only the biologically based internal facial structure was investigated. Face images of 7- to 10-year-old children and of adults in their 20s were edited digitally to eliminate hairstyle and clothing cues to sex. Seven-year-olds, nine-year-olds, and adults classified a subset of these faces by sex and were asked, subsequently, to recognize the faces from among the entire set of faces. This recognition task was designed to assess the relationship between categorization and recognition accuracy. Participants categorized the adult faces by sex at levels of accuracy varying from just above chance (7-year-olds) to nearly perfect (adults). All participant groups performed less accurately for children's faces than for adults' faces. The 7-year-olds were unable to classify the children's faces by sex at levels above chance. Finally, the faces of children and adults were equally recognizable--a finding that has theoretical implications for understanding the relationship between categorizing and identifying faces.

Adult↗

PubMed1999

Quantum annealing of a disordered magnet

Traditional simulated annealing uses thermal fluctuations for convergence in optimization problems. Quantum tunneling provides a different mechanism for moving between states, with the potential for reduced time scales. Thermal and quantum annealing are compared in a model disordered magnet, where the effects of quantum mechanics can be tuned by varying an applied magnetic field. The results indicate that quantum annealing hastens convergence to the optimum state.

Journal Article↗

PubMed1998

The evaluation of sexual function in men presenting with symptomatic benign prostatic hyperplasia.

OBJECTIVE: To evaluate quantitatively and qualitatively the degree of sexual dysfunction in an unselected population of men attending a prostate-assessment clinic using a sexual-function inventory, and to ascertain the degree of correlation between sexual dysfunction, urinary symptoms and age. PATIENTS AND METHODS: In all, 168 men with symptomatic BPH attending a prostate assessment clinic were investigated prospectively using the International Prostate Symptom Score (IPSS), BPH Impact Index (BPHII), a measurement of urinary flow rate and residual urine volume, and a sexual function questionnaire. The results were assessed using Spearman's rank order correlation to discern any correlations between the measured variables. RESULTS: The data from 140 patients were available for analysis; of these, low scores were obtained in 59% for sexual drive, in 56% for erections (with 46% of men satisfying the National Institute of Health criterion for impotence) and in 38% for ejaculation. There was a statistically significant rank order correlation between age and the sexual symptom scores for each of the three categories (sexual drive, erection and ejaculation), but no correlation between age and the problem assessment scores for these domains, suggesting that the older patients are just as bothered by their sexual dysfunction as the younger men. Furthermore, the BPHII scores correlated weakly but significantly with all aspects of sexual function, including overall sexual satisfaction, in contrast to the poor correlation seen with the total IPSS and sexual function scores. CONCLUSION: There is a significant number of patients with symptomatic BPH who have sexual dysfunction, with the proportion increasing with advancing age and with the older men still showing a high degree of bother from their symptoms. Sexual function scores were better correlated with BPHII scores than with the total IPSS, although some of the individual IPSS questions correlated well.

Adult↗

PubMed1991

Biosynthesis of corticotrophin-releasing hormone (CRH) in mouse corticotrophic tumour cells expressing the human proCRH gene: intracellular storage and regulated secretion.

The mouse corticotrophic tumour cell line AtT-20 naturally synthesizes pro-opiomelanocortin (POMC) which is proteolytically processed to N-POMC(1-76), ACTH, beta-lipotrophin and beta-endorphin. The processed products are stored in secretory vesicles and released upon stimulation with specific secretagogues. AtT-20 cells which have been stably transfected with the human corticotrophin-releasing hormone (CRH) gene store and secrete immunoreactive CRH. The present results demonstrate that the CRH precursor is proteolytically processed in the transfected cells to yield the 41 amino acid neuropeptide CRH(1-41). On stimulation with the secretagogue noradrenaline, CRH(1-41) was released into the medium, while the precursor was not. Whilst treatment of wild-type AtT-20 cells with exogenous CRH(1-41) (1 nM) caused a fourfold stimulation of ACTH release above basal levels, the peptide had no effect on ACTH release from the stably transfected cells R1 and R4. These results suggest that the endogenous CRH produced by the transfected R1 and R4 cells may cause down-regulation of their CRH receptors, and thus exogenous CRH cannot cause further stimulation of ACTH release in these cells. We propose that the CRH precursor is correctly processed in the transfected AtT-20 cells (R1 and R4) and that the foreign prohormone is sorted into the secretory pathway.

Adrenocorticotropic Hormone↗

PubMed1990

Cytotoxic effects of dihydroorotase inhibitors upon human CCRF-CEM leukemia.

6-L-Thiodihydroorotate (TDHO) and 2-oxo-1,2,3,6-tetrahydropyrimidine-4,6-dicarboxylate (HDDP) are potent inhibitors of mammalian dihydroorotase in vitro (R. I. Christopherson, K. J. Schmalzl, E. Szabados, R. J. Goodridge, M. C. Harsanyi, M. E. Sant, E. M. Algar, J. E. Anderson, A. Armstrong, S. C. Sharma, W. A. Bubb, and S. D. Lyons, Biochemistry, 28: 463-470, 1989). Using human CCRF-CEM leukemia cells growing in culture, TDHO and HDDP as the free acids have 50% inhibitory concentration (IC50) values of 32 microM and greater than 1000 microM, respectively, whereas for TDHO methyl ester, the IC50 value is 25 microM, and for HDDP dimethyl ester, the IC50 value is 21 microM. These IC50 values were not affected by addition of dihydroorotate, uridine, or deoxycytidine to the culture medium. TDHO methyl ester (25 microM) had only slight inhibitory effects upon the dihydroorotase reaction of de novo pyrimidine biosynthesis in growing leukemia cells, cells arrested in G2 + M phases of the cell cycle. At 250 microM TDHO methyl ester, analysis of cell extracts by high-performance liquid chromatography showed that after 4 h carbamyl aspartate had accumulated from undetectable levels to 760 microM, whereas UTP decreased from 580 to 110 microM and CTP from 350 to 86 microM, indicating inhibition of dihydroorotase in growing leukemia cells. IMP accumulated from 63 to 350 microM, total guanylates increased while adenylates decreased, and the adenylate energy charge decreased from 0.91 to 0.69 after 4 h. The cellular concentration of 5-phosphoribosyl 1-pyrophosphate increased from 180 to 290 microM due to sparing from pyrimidine nucleotide biosynthesis resulting in complementary stimulation of the de novo purine pathway. HDDP dimethyl ester at concentrations of up to 250 microM had no discernable effect upon pyrimidine or purine nucleotide biosynthesis. At 25 microM HDDP-dimethyl ester, cells arrested in G2 + M phases initially, with accumulation of cells in G1/G0 at later times. These data suggest that the primary mechanisms of growth inhibition for TDHO and HDDP involve inhibition of cell cycle progression from late G2 or M phase to G1 phase and that blockade of the pyrimidine pathway by TDHO is a secondary effect found at higher concentrations.

Animals↗

PubMed1988

Effects of supine exercise on fetal heart rate in the second and third trimesters.

The effects of a brief period of supine exercise on fetal heart rate (FHR) was studied in 25 healthy, regularly exercising women, 12 in the late second trimester and 13 in the mid third trimester. Although statistically significant increases in maternal mean arterial pressure and pulse occurred, the exercise intensity was mild, with the average percent maximal pulse being 46 +/- 5% and 49 +/- 5% in the second and third trimester groups, respectively. Small increases in FHR were seen in both groups, which were insignificant statistically and physiologically. No patient experienced significant FHR abnormalities as a result of the supine exercise sequence. The findings suggest that pregnant women may continue submaximal supine exercise of short duration into the mid to late third trimester.

Blood Pressure↗

PubMed1972