Biomedical subjects

D H Evans

Publications and source records attributed to D H Evans.

At least 19 recordsLinked to original sources

PubMed1997

Spectral pattern of neonatal cerebral blood flow velocity: comparison with spectra from blood pressure and heart rate.

The cerebral blood flow velocity (CBFV) frequency spectra were studied in 106 premature and term newborns (gestational age range. 24-42 wk) and compared with the heart rate (HR) and mean arterial blood pressure (BP) spectra over the 0.005-0.5 Hz range. CBFV, BP, and HR were shown to have similar but not identical spectral patterns. Adjustment of a l/f model to these spectra produced highly significant fittings, but the residuals were not independent. This condition was met only by the CBFV and BP spectra over a limited frequency range (0.005-0.06 Hz). These results provide a characterization of the CBFV spectra for a much larger population of newborns than hitherto available, indicating that under certain conditions CBFV and BP might show the properties of chaotic systems. In infants without major complications, gestational age (GA) did not have a significant influence on the CBFV spectrum, whereas the spectral power to 0.5 Hz of both BP and HR was found to increase with GA. The spectral power increased over the first 24 h of postnatal life for all three variables: only CBFV showed a significant spectral change in the low frequency (LF, 0.02-0.08 Hz) range. A matched group comparison, adjusted for GA and postnatal age, indicated a reduction in CBFV LF power for term infants with birth asphyxia when compared with normal infants, which was not reproduced in the HR spectra.

Aging↗

PubMed1996

SFV topoisomerase: sequence specificity in a genetically mapped interval.

Poxviral DNA topoisomerases are sequence-specific enzymes whose activities are thought to influence such diverse processes as transcription, DNA replication, and genetic recombination. To obtain further insights into the relatedness of these enzymes, and their influence on virus-mediated recombination, we have determined the target-specificity and other catalytic properties of the Shope fibroma virus (SFV) topoisomerase. SFV topoisomerase was expressed in Escherichia coli and purified as a glutathione S-transferase (GST) or (his)6-tagged fusion protein. The recombinant Leporipox-virus (SFV) enzyme displayed catalytic properties very similar to vaccinia topoisomerase. In particular SFV topoisomerase recognizes the same pentanucleotide motif [5'-(C/T)CCTT-3'] and promotes the same DNA relaxation, strand transfer, and strand cleavage reactions catalyzed by the Orthopoxviral (vaccinia) enzyme. The SFV enzyme can also efficiently cleave DNA 3' of the variant site 5'-CCCTG-3' in certain sequence contexts. These studies identified several sites where SFV topoisomerases interact with a recombinational substrate and permitted a comparison of recombination frequencies across intervals which did, or did not, span these sites. We failed to detect any effect of topoisomerase-recognition sites on recombination frequencies, except for a small (< 2-fold) stimulation seen when the substrates encoded a nearby poxviral promoter. This and other work shows that poxviral topoisomerases from several genera share common target specificities, but other enzymatic systems probably catalyze the high-frequency recombination seen in poxvirus-infected cells.

Amino Acid Sequence↗

PubMed1996

Tobacco plants expressing T4 endonuclease V show enhanced sensitivity to ultraviolet light and DNA alkylating agents.

DNA repair processes and UV-filtering pigments protect organisms from the cytotoxicity of UV light and endow plants with a high degree of natural UV resistance. In an attempt to further enhance this UV resistance we have constructed transgenic tobacco lines that express a DNA repair enzyme encoded by the bacteriophage T4 denV gene. The denV gene encodes endonuclease V, an enzyme which initiates base excision repair of cyclobutane pyrimidine dimers. Its presence is expected to provide transgenotes with a repair pathway complementary to, but likely distinct from, the repair pathways found in tobacco. The denV gene, flanked by a CaMV 35S promoter and poly(A) addition site, was introduced into tobacco and mature plants regenerated. The transgenotes expressed high levels of a UV-specific endonuclease and no such activity was found in control plants. Curiously, assays which detected several different biological endpoints showed that the denV+ transgenotes were also hypersensitive to UV-C light. This hypersensitivity segregated with the denV gene and was not caused by altered concentrations of UV-filtering pigments. Moreover, the denV+ transgenotes were also hypersensitive to high levels of baseless lesions that would be generated by a transgenically expressed beta-eliminating lyase such as endonuclease V.

Alkylating Agents↗

PubMed1996

ETB-type receptors mediate endothelin-stimulated contraction in the aortic vascular smooth muscle of the spiny dogfish shark, Squalus acanthias.

Using various agonists, and the specific antagonist BQ-123, we have examined the sensitivity to endothelin of the vascular smooth muscle of the ventral aorta of the spiny dogfish shark, Squalus acanthias. Human endothelin-1 produced significant contraction of isolated vascular smooth muscle rings, with an EC50 of 10 nmol.l-1. The presence of an intact endothelium did not alter this response but the magnitude of the contraction was greater in rings with an intact endothelium. The response to 0.2 mumol.l-1 endothelin-1 was equivalent to that of 0.1 mmol.l-1 acetylcholine, and significantly greater than that to 80 mmol.l-1 KCl, suggesting high sensitivity even to the heterologous, mammalian peptide. The Hill plot of the contractile response was a straight line with a slope of 1.12, indicating that a single receptor was mediating the response. Endothelin-1, endothelin-3, and sarafotoxin S6c produced similar concentration-response curves, and the response to endothelin-1 was insensitive to the ETA-specific inhibitor BQ-123. Our data are consistent with the hypothesis that the receptor involved in the contractile response to endothelin of shark aortic vascular smooth muscle is of the ETB-rather than the ETA-type.

Animals↗

PubMed1996

The effect of transcription on genetic recombination in poxvirus-infected cells.

We have examined the effects of transcription on recombination frequencies in poxvirus-infected cells. A synthetic poxviral promoter was shown to function as a hybrid early/late transcription element when fused to a luciferase reporter gene, and then cloned into genetically-marked recombination substrates. These lambda DNA substrates were transfected into cells infected with Shope fibroma virus (SFV) and the recombinants detected by recovering the transfected DNA, packaging it in vitro into infectious particles, and then assaying the yield of recombinants on Escherichia coli. Controls showed that the poxviral promoter conferred no replicative advantage, or disadvantage, on molecules encoding the promoter. Furthermore, the promoter had no detectable effect on the recombination frequency when recombination was measured in the interval immediately adjacent to the promoter-insertion site. However, the promoter did appear to stimulate recombination at a distance, in a manner that appeared to be dependent on the level of transcription, and the effect was observed regardless of whether or not the promoter was present on one or both of the recombinational substrates. The peak of recombinational enhancement was centered about 500 bp away from the promoter element, where the frequency of recombination was 30-50% higher than that seen when the recombinational substrates lacked the promoter. Possible explanations for these observations are discussed.

Base Sequence↗

PubMed1996

Compensation for phase and amplitude imbalance in quadrature Doppler signals.

Quadrature phase detection is widely used in ultrasound Doppler systems to obtain directional information about flow velocities in blood vessels. The presence of phase and/or gain imbalance between the quadrature outputs produces crosstalk between the forward and reverse channels which may introduce errors into mean and maximum velocity estimates and hamper the interpretation of sonograms. This technical note describes the assessment of the crosstalk in a commercial pulsed-wave Doppler system, and suggests methods of compensation for the causative imbalance which can be implemented using digital signal processing techniques. Examples of applying these methods to the outputs of the commercial system are given.

Blood Flow Velocity↗

PubMed1996

Power Doppler: an advance over colour Doppler for transcranial imaging?

The aim of this study is to compare the ability of mean frequency-based colour Doppler (CD) and power Doppler (PD) to identify the basal cerebral arteries in normal subjects. Fifty volunteers were recruited into the study. The temporal acoustic window was used to image the basal cerebral arteries bilaterally. The number of positively identified vessels was recorded, first using CD and then with PD. Our results show that both modalities were equally effective at demonstrating the anterior cerebral artery (ACA), the middle cerebral artery mainstream (MCA M1) and the precommunicating segment of the posterior cerebral artery (PCA P1; ACA: 82% and 83%; MCA M1: 84% and 89%; PCA P1: 81% and 83%; CD and PD, respectively). However, the M2 portion of the MCA (CD: 25%, PD: 54%; difference 29% [95% CI 20-38%]), the P2 portion of the PCA (CD: 67%, PD: 75%; difference 8% [95% CI 3-13%]) and the posterior communicating artery (CD: 15%, PD 32%; difference 17% [95% CI 10-24%]) were all identified in a significantly greater proportion of subjects using PD. Although its clinical value remains undetermined, PD offers significant advantages over colour-coded sonography in imaging small-calibre and low-flow arteries and those which run at unfavourable angles to the ultrasound beam.

Adolescent↗

PubMed1996

Differentiation between emboli and artefacts using dual-gated transcranial Doppler ultrasound.

It is well documented that transcranial Doppler ultrasound has the ability to detect cerebral emboli. During intraoperative patient monitoring studies, many signals due to artefact (probe motion, patient movement or surgical manipulation) are also detected and can be difficult to distinguish from genuine embolic events. We have constructed a Doppler system that can simultaneously range-gate at two separate depths, in order to test the hypothesis that it should be possible to distinguish between emboli and artefact by comparing the signal from the two separate regions within the vessel. The classification algorithm is based on the principle that emboli propagate with blood motion (whereas artefacts do not) and thus will be detected sequentially at different depths along the insonated cerebral artery. One hundred thirty-eight (presumed) embolic and 170 artefact signals were analysed. The median (interquartile range) gate separation was 10.01 mm (7.41-10.78 mm). The time delay between detection of embolic signals in the two channels was 11.04 ms (6.24-16.41 ms, but was only 0.08 ms (-0.48(-)+0.64 ms) for artefact (p < 0.0001). Dual-gated Doppler ultrasound is a conclusive and independent method that differentiates emboli from artefact. Incorporation of this system for long-term monitoring may eliminate the need for an experienced observer to be present.

Artifacts↗

PubMed1996

The effect of noise and high-pass filtering on the estimation of mean blood velocity using wide and narrow ultrasound beams.

Recently, a new method of analysis has been proposed for the calculation of a Doppler frequency proportional to mean blood velocity for the case where the Doppler beam is assumed to be of negligible thickness compared to the vessel diameter, and the velocity profile is axisymmetric and monotonic increasing from the vessel wall to the vessel center. Such analysis of the Doppler signal is an alternative to that commonly performed under the assumption that the beam insonates the vessel uniformly. Errors in each method are found and compared for the case where the Doppler signal is contaminated by noise, and for the case where the signal is subjected to an ideal high-pass filter. The frequency resulting from the new method of analysis is affected by low-frequency perturbations approximately twice as much as that resulting from the standard method. However, the new method is much more immune to high frequency perturbations. If each method is used with the beam shape for which it is appropriate then, for a given velocity profile, each method is equally affected by the use of the same ideal high-pass filter.

Artifacts↗

PubMed1996

The effect of geometrical spectral broadening on the estimation of mean blood velocity using wide and narrow ultrasound beams.

If an ultrasound beam uniformly insonates the cross section of a blood vessel then the Doppler signal can be analyzed to give a frequency proportional to the spatial mean blood velocity. This is also possible if the beam can be thought of as negligibly thin compared to the blood vessel radius, centrally placed, and the blood velocity profile is axisymmetric and monotonic, although the analysis takes a different form. The immunity of these mean velocity estimators to broadening of the ideal frequency spectrum is studied. If the broadening of a frequency component is such that its mean frequency, weighted by intensity, is unchanged then the analysis with a uniformly insonating beam still leads to the correct mean velocity. In contrast, for any such broadening, the analysis if the beam is negligibly thin produces an underestimate of the mean velocity. Error expressions are derived for idealized cases and some practical results given.

Blood Flow Velocity↗

PubMed1995

DNA strand transfer catalyzed by the 5'-3' exonuclease domain of Escherichia coli DNA polymerase I.

A protein which promotes DNA strand transfer between linear double-stranded M13mp19 DNA and single-stranded viral M13mp19 DNA has been isolated from recA- E.coli. The protein is DNA polymerase I. Strand transfer activity residues in the small fragment encoding the 5'-3' exonuclease and can be detected using a recombinant protein comprising the first 324 amino acids encoded by polA. Either the recombinant 5'-3' exonuclease or intact DNA polymerase I can catalyze joint molecule formation, in reactions requiring only Mg2+ and homologous DNA substrates. Both kinds of reactions are unaffected by added ATP. Electron microscopy shows that the joint molecules formed in these reactions bear displaced single strands and therefore this reaction is not simply promoted by annealing of exonuclease-gapped molecules. The pairing reaction is also polar and displaces the 5'-end of the non-complementary strand, extending the heteroduplex joint in a 5'-3' direction relative to the displaced strand. Thus strand transfer occurs with the same polarity as nick translation. These results show that E.coli, like many eukaryotes, possesses a protein which can promote ATP-independent strand-transfer reactions and raises questions concerning the possible biological role of this function.

Amino Acid Sequence↗

PubMed1995

Bioactivation of aromatic amines by recombinant human cytochrome P4501A2 expressed in Ames tester strain bacteria: a substitute for activation by mammalian tissue preparations.

The most widely used bioassay in genetic toxicology is the Ames test, which combines a bacterial mutagenicity assay (reversion of Salmonella typhimurium histidine-auxotrophic tester strains) with an exogenous bioactivation system (hepatic postmitochondrial supernatant or "S9"). The enzymatic activities of S9 prepared from the tissues of experimental animals are difficult to control. We show that the requirement for S9 can be obviated by the engineered expression of enzymes of bioactivation within the bacterial cell. With this strategy, reactive metabolites are produced inside the bacterial cell, proximate to the genetic target. Species boundaries can be crossed, and chimeric or mutant enzymes can be studied. We have constructed an Ames tester strain, expressing both aromatic amine N-acetyltransferase and human cytochrome P4501A2, which detects aromatic amine mutagenicity in the absence of S9.

Acetyltransferases↗

PubMed1995

Measurement of blood flow velocity in the basal cerebral circulation: advantages of transcranial color-coded sonography over conventional transcranial Doppler.

Unlike conventional transcranial Doppler (TCD), transcranial color-coded sonography (TCCS) enables imaging of the basal cerebral arteries using color-flow ultrasonography and correction for the angle of insonation when determining blood flow velocities. We present hemodynamic data from 20 normal subjects, each studied with TCD and TCCS. Velocities derived using TCCS with angle correction were significantly greater than those derived using TCD in all vessels (mean velocities [cm/sec; mean with 95% confidence intervals)--anterior cerebral artery: TCD 48 (45-50), TCCS 62 (58-66), p < .0001; middle cerebral artery: TCD 61 (58-64), TCCS 70 (66-74), p < .0001; posterior cerebral artery: TCD 43 (41-46), TCCS 54 (50-57), p < .0001; basilar artery: TCD 40 (34-45), TCCS 45 (38-52), p < .01. Pulsatility index values were significantly greater in all arteries when determined by TCCS, and resistance index values were significantly greater except in the basilar artery. Correcting for the angle of insonation using TCCS may enable estimation of blood flow velocities closer to the "true" values than those derived using conventional TCD.

Adult↗

PubMed1995

Comparison of cerebral blood flow velocity estimation with cranial ultrasound imaging for early prediction of outcome in preterm infants.

Seventy-four low birth weight infants underwent serial measurements of cerebral blood flow velocity (CBFV) using Doppler ultrasound together with ultrasound imaging of the brain. Surviving infants were examined and assessed using the Bayley scales of mental and motor development at 18 months post-term. There were no significant differences in mean CBFV between normal and impaired infants. Infants with abnormal neurological signs (n = 6) or developmental delay (n = 3) did not show the usual steady rise in CBFV during the first few days of life. Four of nine had a rise, then a fall in CBFV compared with 4 of 31 with complete data in the normal group. This difference is significant (chi 2 = 4.3, p = 0.03). The magnitude of the percentage increase between day 1 and day 3 was also smaller in the abnormal group (median 0% vs. 39%, Mann-Whitney p = 0.03). A structural abnormality seen on the cranial ultrasound image was a better predictor of adverse outcome than an abnormal CBFV pattern, with a better sensitivity and specificity (66% and 97% for imaging compared to 44% and 87% for CBFV). The addition of serial ultrasound Doppler measurements of CBFV did not improve the prediction of outcome obtained using ultrasound imaging alone.

Blood Flow Velocity↗

PubMed1995

Volumetric blood flow measurement by simultaneous Doppler signal and B-mode image processing: a feasibility study.

A system is described for the estimation of volumetric flow in arteries in real time by the simultaneous processing of the audio output of a commercial ultrasound scanner carrying the Doppler signal, and the video output carrying B-mode images of the lateral view of the artery. These images are processed to provide estimates of instantaneous internal vessel diameter. Combination with the relevant estimates of spatial mean velocity provides estimates of instantaneous volumetric flow. The major limitations of the system are the low rate of production of the B-mode images (17 Hz), the limited computing power and the need to tilt the transducer to produce an acceptably low Doppler angle. This tilting diminishes the quality of the imaging echoes from the arterial walls, making diameter measurement difficult. In principle, this approach to flow measurement represents an improvement on the conventional duplex technique where a single estimate of diameter is used. In practice, its value is limited by current transducer capabilities.

Adult↗

PubMed1995

Bias in mean frequency estimation of Doppler signals due to wall clutter filters.

Wall clutter filters used in Doppler ultrasound systems give rise to a bias in the mean frequency estimation. This effect has been explored for two second order filters: the initialised infinite impulse response (IIR)-type filter, and the finite impulse response (FIR) echo-canceller filter. These are used in conjunction with two mean frequency estimation algorithms: the FFT-based intensity-weighted-mean algorithm, and the autocorrelation algorithm. The bias is shown to be caused by the finite settling times of the filters. The IIR filter, although having a much better frequency response than the FIR filter, has a greater bias associated with it. A compromise between bias and frequency response is suggested and a simple method of bias correction is described.

Algorithms↗

PubMed1995

Effects of nonuniform insonation by catheter-tipped Doppler transducers on velocity estimation.

Studies of the coronary arteries using intravascular Doppler catheters have frequently been performed to assess the local circulation in various clinical situations. The parameters which are correlated with clinical observations are usually calculated from the intensity weighted mean velocity (IWMV) or the maximum velocity extracted from the Doppler power spectrum. It is known that these estimates are affected by nonuniform insonation of the blood vessel by the ultrasonic beam. The sample volume of a commercially available Doppler catheter has been theoretically and experimentally studied, and the effects of the nonuniform sampling of the vessel by the ultrasound beam examined. The results show that the velocity estimates are dramatically affected by both the position of the catheter within the artery, and the blood velocity profile. It is concluded that the maximum velocity estimator is more reliable than the IWMV estimator, but that results obtained with this estimator must also be treated with extreme caution.

Blood Flow Velocity↗

PubMed1995

Statistical bias and variance in blood flow estimation by spectral analysis of Doppler signals.

The stochastic nature of the Doppler signal is discussed as a source of variability and bias in estimation of mean blood velocity and flow performed using the Discrete or Fast-Fourier Transform. The estimators studied are those appropriate when the ultrasound beam is assumed to be wide enough to insonate the cross-section of the vessel uniformly, and assumed to be narrow enough to insonate only a diameter. Approximate expressions are derived theoretically for the biases and variances of these statistics when the Doppler power spectrum is uniform. For more complex spectra numerical evaluation is carried out by computer. Results for the double-sided spectra obtained from mixed flow are inferred from the single-sided cases. Typically, an estimate of instantaneous mean velocity has very little bias added (< 1%) but might have a standard error of approximately 10% of its mean value, and an estimate of flow in one cardiac cycle a standard error of approximately 1% of its mean value.

Bias↗