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PubMed2006

Dynamics of halide ion-water hydrogen bonds in aqueous solutions: dependence on ion size and temperature.

We have carried out a series of molecular dynamics simulations to investigate the dynamics of X(-)-water (X = F, Cl, Br, and I) and water-water hydrogen bonds in aqueous alkali halide solutions at room temperature and also of Cl(-)-water and water-water hydrogen bonds at seven different temperatures ranging from 238 to 318 K. The hydrogen bonds are defined by using a set of configurational criteria with respect to the anion(oxygen)-oxygen and anion(oxygen)-hydrogen distances and the anion(oxygen)-oxygen-hydrogen angle for an anion(water)-water pair. The results of the hydrogen bond dynamics are obtained for two different cutoff values for the angular criterion. In both cases, similar dynamical behavior of the hydrogen bonds is found with respect to their dependence on ion size and temperature. The fluoride ion-water hydrogen bonds are found to break at a much slower rate than water-water hydrogen bonds, while the lifetimes of chloride and bromide ion-water hydrogen bonds are found to be shorter than those of fluoride ion-water ones but still longer than water-water hydrogen bonds. The short-time dynamics of iodide ion-water hydrogen bonds is found to be slightly faster, while its long-time dynamics is found to be slightly slower than the corresponding water-water hydrogen bond dynamics. Correlations of the observed dynamics of anion(water)-water hydrogen bonds with those of rotational and translational diffusion and residence times of water molecules in ion(water) hydration shells are also discussed. With variation of temperature, the lifetimes of both Cl(-)-water and water-water hydrogen bonds are found to show Arrhenius behavior with a slightly higher activation energy for the Cl(-)-water hydrogen bonds.

Journal Article↗
PubMed2006

Ultrafast hydration dynamics in the lipidic cubic phase: Discrete water structures in nanochannels.

We report here our studies of hydration dynamics of confined water in aqueous nanochannels (approximately 50 A) of the lipidic cubic phase. By systematically anchoring the hydrocarbon tails of a series of tryptophan-alkyl ester probes into the lipid bilayer, we mapped out with femtosecond resolution the profile of water motions across the nanochannel. Three distinct time scales were observed, revealing discrete channel water structures. The interfacial water at the lipid surface is well-ordered, and the relaxation dynamics occurs in approximately 100-150 ps. These dynamically rigid water molecules are crucial for global structural stability of lipid bilayers and for stabilization of anchored biomolecules in membranes. The adjacent water layers near the lipid interface are hydrogen-bonded networks and the dynamical relaxation takes 10-15 ps. This quasi-bound water motion, similar to the typical protein surface hydration relaxation, facilitates conformation flexibility for biological recognition and function. The water near the channel center is bulklike, and the dynamics is ultrafast in less than 1 ps. These water molecules freely transport biomolecules near the channel center. The corresponding orientational relaxation at these three typical locations is well correlated with the hydration dynamics and local dynamic rigidity. These results reveal unique water structures and dynamical motions in nanoconfinements, which is critical to the understanding of nanoscopic biological activities and nanomaterial properties.

Lipids↗
PubMed1983

Haemodynamic responses to static and dynamic handgrip before and after autonomic blockade.

1. Six healthy men performed static and dynamic handgrip to local muscular fatigue in approximately 6 min under control conditions, i.e. without drugs and after combined parasympathetic and beta-adrenergic blockade with atropine and metoprolol. 2. From rest to exercise at fatigue, systolic, diastolic and mean arterial pressures increased by 32 +/- 4 and 39 +/- 3 mmHg, 24 +/- 3 and 26 +/- 4 mmHg, and 26 +/- 3 and 30 +/- 3 mmHg respectively for static and dynamic handgrip. There were no significant differences between the pressor responses for the two modes of contraction. Cardiac output increased significantly only during dynamic exercise. Total peripheral resistance increased by 2.3 +/- 1.0 units for static handgrip (P less than 0.05) and by 0.7 +/- 0.8 unit (P greater than 0.05) for dynamic handgrip. Autonomic blockade abolished the heart rate response to both static and dynamic handgrip. For both modes of contraction the systolic arterial pressure responses were 9-12 mmHg lower (P less than 0.05) after autonomic blockade, but the diastolic and mean pressure responses were not significantly affected. A significant increase in cardiac output persisted during dynamic exercise. The increase in peripheral resistance during static exercise tended to be greater after blockade. Plasma noradrenaline and adrenaline levels showed only minor elevations in response to static and dynamic handgrip and were not changed by autonomic blockade. 3. These data indicate that when performed to a common end-point with identical small muscle groups static and dynamic exercise produce an equally large pressor response, which is only slightly attenuated by autonomic blockade.

Adult↗
PubMed1995

Dynamical disease: Identification, temporal aspects and treatment strategies of human illness.

Dynamical diseases are characterized by sudden changes in the qualitative dynamics of physiological processes, leading to abnormal dynamics and disease. Thus, there is a natural matching between the mathematical field of nonlinear dynamics and medicine. This paper summarizes advances in the study of dynamical disease with emphasis on a NATO Advanced Research Worshop held in Mont Tremblant, Quebec, Canada in February 1994. We describe the international effort currently underway to identify dynamical diseases and to study these diseases from a perspective of nonlinear dynamics. Linear and nonlinear time series analysis combined with analysis of bifurcations in dynamics are being used to help understand mechanisms of pathological rhythms and offer the promise for better diagnostic and therapeutic techniques. (c) 1995 American Institute of Physics.

Journal Article↗
PubMed2005

Ab initio Ehrenfest dynamics.

We present an ab initio direct Ehrenfest dynamics scheme using a three time-step integrator. The three different time steps are implemented with nuclear velocity Verlet, nuclear-position-coupled midpoint Fock integrator, and time-dependent Hartree-Fock with a modified midpoint and unitary transformation algorithm. The computational cost of the ab initio direct Ehrenfest dynamics presented here is found to be only a factor of 2-4 larger than that of Born-Oppenheimer (BO) dynamics. As an example, we compute the vibration of the NaCl molecule and the intramolecular torsional motion of H2C=NH2+ by Ehrenfest dynamics compared with BO dynamics. For the vibration of NaCl with an initial kinetic energy of 1.16 eV, Ehrenfest dynamics converges to BO dynamics with the same vibrational frequency. The intramolecular rotation of H2C=NH2+ produces significant electronic excitation in the Ehrenfest trajectory. The amount of nonadiabaticity, suggested by the amplitude of the coherent progression of the excited and ground electronic states, is observed to be directly related to the strength of the electron-nuclear coupling. Such nonadiabaticity is seen to have a significant effect on the dynamics compared with the adiabatic approximation.

Journal Article↗
PubMed2006

Configurational constant pressure molecular dynamics.

We propose two new algorithms for generating isothermal-isobaric molecular dynamics. The algorithms are based on an extended phase space dynamics where two extra degrees of freedom, representing the thermostat and the barostat, are included. These new methods adopt a totally different approach towards molecular dynamics simulation in the isothermal-isobaric ensemble. They are fully configurational in the sense that only the particle positions are required in the control of the system temperature and pressure. Following on from the works of Delhommelle and Evans [Mol. Phys., 99, 1825 (2001)] and of Braga and Travis [J. Chem. Phys., 123, 134101 (2005)] concerning configurational canonical dynamics, these new algorithms can be seen as a natural extension to the isothermal-isobaric ensemble. We have validated both of our new configurational isothermal-isobaric schemes by conducting molecular dynamics simulations of a Lennard-Jones fluid and comparing the static and dynamic properties for a single state point. We find that both schemes generate similar results compared with schemes which use kinetic temperature and pressure control. We have also monitored the response of the system to a series of isothermal compressions and isobaric quenches. We find that the configurational schemes performed at least as well as the kinetic based scheme in bringing the system temperature and pressure into line with the set point values of these variables. These new methods will potentially play a significant role in simulations where the calculation of the kinetic temperature and pressure can be problematic. A well known example resides in the field of nonequilibrium simulations where the kinetic temperature and pressure require a knowledge of the streaming velocity of the fluid in order to calculate the true peculiar velocities (or momenta) that enter into their definitions. These are completely avoided by using our configurational thermostats and barostats, since these are independent of momenta. By extending the analysis of Kusnezov et al. [Ann. Phys., 204, 155 (1990)] in order to derive a set of generalized Nose-Hoover equations of motion which can generate isothermal-isobaric dynamics in a number of different ways, we are able to show that both of our new configurational barostats and Hoover's kinetic isothermal-isobaric scheme are special cases of this more general set of equations. This generalization can be very powerful in generating constant pressure dynamics for a variety of systems.

Journal Article↗
PubMed2005

BetaIII-tubulin induces paclitaxel resistance in association with reduced effects on microtubule dynamic instability.

The development of resistance to paclitaxel in tumors is one of the most significant obstacles to successful therapy. Overexpression of the betaIII-tubulin isotype has been associated with paclitaxel resistance in a number of cancer cell lines and in tumors, but the mechanism of resistance has remained unclear. Paclitaxel inhibits cancer cell proliferation by binding to the beta-subunit of tubulin in microtubules and suppressing microtubule dynamic instability, leading to mitotic arrest and cell death. We hypothesized that betaIII-tubulin overexpression induces resistance to paclitaxel either by constitutively enhancing microtubule dynamic instability in resistant cells or by rendering the microtubules less sensitive to the suppression of dynamics by paclitaxel. Using Chinese hamster ovary cells that inducibly overexpress either betaI- or betaIII-tubulin, we analyzed microtubule dynamic instability during interphase by microinjection of rhodamine-labeled tubulin and time-lapse fluorescence microscopy. In the absence of paclitaxel, there were no differences in any aspect of dynamic instability between the two beta-tubulin-overexpressing cell types. However, in the presence of 150 nm paclitaxel, dynamic instability was suppressed to a significantly lesser extent (suppressed only 12%) in cells overexpressing betaIII-tubulin than in cells overexpressing similar levels of betaI-tubulin (suppressed 47%). The results suggest that overexpression of betaIII-tubulin induces paclitaxel resistance by reducing the ability of paclitaxel to suppress microtubule dynamics. The results also suggest that endogenous regulators of microtubule dynamics may differentially interact with individual tubulin isotypes, supporting the idea that differential expression of tubulin isotypes has functional consequences in cells.

Animals↗
PubMed1991

Inertial effects from single body segments in dynamic analysis of lifting.

Biomechanical calculations of loads on the locomotor system in lifting tasks are often based on static models. Several investigators have shown, though, that in a dynamic act like lifting, inertial effects are not generally negligible. A complete dynamic analysis includes the calculation of linear and angular accelerations of body segments and is rather complicated to carry out. So it was of interest to find out whether the contribution of single body segments to the dynamic effects of the whole body might be negligible. Ten male subjects lifted a 12.8 kg box using two different techniques and two speeds. Ground reactions and body segment movements were recorded and reaction forces and net moments at different joints were calculated with static, dynamic and 'semidynamic' methods. The latter method incorporated the ground reaction forces as measured from a force plate but was otherwise equal to the static method. The results from semidynamic analyses agreed well with those from complete dynamic analyses for the ankle, knee, hip, and L5/S1 joints. For example, the semidynamically calculated peak moments at L5/S1 differed on average less than 3% from the dynamically calculated values. The contributions of the lower extremities and the pelvis to the dynamic effects of the whole body seemed to be quite small. The less complicated semidynamic method offers a good alternative to a fully dynamic analysis for estimations of peak moments in the lower back, at least for the studied lifting techniques.

Adult↗
PubMed1996

One-Handed Juggling: A Dynamical Approach to a Rhythmic Movement Task.

The skill of rhythmically juggling a ball on a racket was investigated from the viewpoint of nonlinear dynamics. The difference equations that model the dynamical system were analyzed by means of local and nonlocal stability analyses. These analyses showed that the task dynamics offer an economical juggling pattern that is stable even for open-loop actuator motion. For this pattern, two types of predictions were extracted: (a) Stable periodic bouncing is sufficiently characterized by a negative acceleration of the racket at the moment of impact with the ball, and (b) a nonlinear scaling relation maps different juggling trajectories onto one topologically equivalent dynamical system. The relevance of these results for the human control of action was evaluated in an experiment in which subjects (N = 6) performed a comparable task of juggling a ball on a paddle. Task manipulations involved different juggling heights and gravity conditions of the ball. The following predictions were confirmed: (a) For stable rhythmic performance, the paddle's acceleration at impact is negative and fluctuations of the impact acceleration follow predictions from global stability analysis; and (b) for each subject, the realizations of juggling for the different experimental conditions are related by the scaling relation. These results permit one to conclude that humans reliably exploit the stable solutions inherent to the dynamics of the given task and do not overrule these dynamics by other control mechanisms. The dynamical scaling serves as an efficient principle for generating different movement realizations from only a few parameter changes and is discussed as a dynamical formalization of the principle of motor equivalence.

Journal Article↗
PubMed1999

Analytical propagation of errors in dynamic SPECT: estimators, degrading factors, bias and noise.

Dynamic SPECT is a relatively new technique that may potentially benefit many imaging applications. Though similar to dynamic PET, the accuracy and precision of dynamic SPECT parameter estimates are degraded by factors that differ from those encountered in PET. In this work we formulate a methodology for analytically studying the propagation of errors from dynamic projection data to kinetic parameter estimates. This methodology is used to study the relationships between reconstruction estimators, image degrading factors, bias and statistical noise for the application of dynamic cardiac imaging with 99mTc-teboroxime. Dynamic data were simulated for a torso phantom, and the effects of attenuation, detector response and scatter were successively included to produce several data sets. The data were reconstructed to obtain both weighted and unweighted least squares solutions, and the kinetic rate parameters for a two-compartment model were estimated. The expected values and standard deviations describing the statistical distribution of parameters that would be estimated from noisy data were calculated analytically. The results of this analysis present several interesting implications for dynamic SPECT. Statistically weighted estimators performed only marginally better than unweighted ones, implying that more computationally efficient unweighted estimators may be appropriate. This also suggests that it may be beneficial to focus future research efforts upon regularization methods with beneficial bias-variance trade-offs. Other aspects of the study describe the fundamental limits of the bias variance trade-off regarding physical degrading factors and their compensation. The results characterize the effects of attenuation, detector response and scatter, and they are intended to guide future research into dynamic SPECT reconstruction and compensation methods.

Algorithms↗
PubMed1995

Domains of tau protein, differential phosphorylation, and dynamic instability of microtubules.

The dynamic instability of microtubules is thought to be regulated by MAPs and phosphorylation. Here we describe the effect of the neuronal microtubule-associated protein tau by observing the dynamics of single microtubules by video microscopy. We used recombinant tau isoforms and tau mutants, and we phosphorylated tau by the neuronal kinases MARK (affecting the KXGS motifs within tau's repeat domain) and cdk5 (phosphorylating Ser-Pro motifs in the regions flanking the repeats). The variants of tau can be broadly classified into three categories, depending on their potency to affect microtubule dynamics. "Strong" tau variants have four repeats and both flanking regions. "Medium" variants have one to three repeats and both flanking regions. "Weak" variants lack one or both of the flanking regions, or have no repeats; with such constructs, microtubule dynamics is not significantly different from that of pure tubulin. N- or C-terminal tails of tau have no influence on dynamic instability. The two ends of microtubules (plus and minus) showed different activities but analogous behavior. These results are consistent with the "jaws" model of tau where the flanking regions are considered as targeting domains whereas the addition of repeats makes them catalytically active in terms of microtubule stabilization. The dominant changes in the parameters of dynamic instability induced by tau are those in the dissociation rate and in the catastrophe rate (up to 30-fold). Other rates change only moderately or not at all (association rate increased up to twofold, rates of rescue or rapid shrinkage decreased up to approximately twofold). The order of repeats has little influence on microtubule dynamics (i.e., repeats can be re-arranged or interchanged), arguing in favor of the "distributed weak binding" model proposed by Butner and Kirschner (1991); however, we confirmed the presence of a "hotspot" of binding potential involving Lys274 and Lys281 observed by Goode and Feinstein, 1994. Phosphorylation of Ser-Pro motifs by cdk5 (mainly Ser 202, 235, and 404) in the flanking regions had a moderate effect on microtubule dynamics while phosphorylation at the "Alzheimer"-site Ser262 MARK eliminated tau's interactions with microtubules. In both cases the predominant effects of phosphorylation are on the rates of tubulin dissociation and catastrophe whereas the effects on the rates of association or rescue are comparatively small.

Amino Acid Sequence↗
PubMed1995

Vinblastine suppresses dynamics of individual microtubules in living interphase cells.

We have characterized the effects of vinblastine on the dynamic instability behavior of individual microtubules in living BS-C-1 cells microinjected with rhodamine-labeled tubulin and have found that at low concentrations (3-64 nM), vinblastine potently suppresses dynamic instability without causing net microtubule depolymerization. Vinblastine suppressed the rates of microtubule growth and shortening, and decreased the frequency of transitions from growth or pause to shortening, also called catastrophe. In vinblastine-treated cells, both the average duration of a pause (a state of attenuated dynamics where neither growth nor shortening could be detected) and the percentage of total time spent in pause were significantly increased. Vinblastine potently decreased dynamicity, a measure of the overall dynamic activity of microtubules, reducing this parameter by 75% at 32 nM. The present work, consistent with earlier in vitro studies, demonstrates that vinblastine kinetically caps the ends of microtubules in living cells and supports the hypothesis that the potent chemotherapeutic action of vinblastine as an antitumor drug is suppression of mitotic spindle microtubule dynamics. Further, the results indicate that molecules that bind to microtubule ends can regulate microtubule dynamic behavior in living cells and suggest that endogenous regulators of microtubule dynamics that work by similar mechanisms may exist in living cells.

Animals↗
PubMed2002

beta-Tubulin C354 mutations that severely decrease microtubule dynamics do not prevent nuclear migration in yeast.

Microtubule dynamics are influenced by interactions of microtubules with cellular factors and by changes in the primary sequence of the tubulin molecule. Mutations of yeast beta-tubulin C354, which is located near the binding site of some antimitotic compounds, reduce microtubule dynamicity greater than 90% in vivo and in vitro. The resulting intrinsically stable microtubules allowed us to determine which, if any, cellular processes are dependent on dynamic microtubules. The average number of cytoplasmic microtubules decreased from 3 in wild-type to 1 in mutant cells. The single microtubule effectively located the bud site before bud emergence. Although spindles were positioned near the bud neck at the onset of anaphase, the mutant cells were deficient in preanaphase spindle alignment along the mother-bud axis. Spindle microtubule dynamics and spindle elongation rates were also severely depressed in the mutants. The pattern and extent of cytoplasmic microtubule dynamics modulation through the cell cycle may reveal the minimum dynamic properties required to support growth. The ability to alter intrinsic microtubule dynamics and determine the in vivo phenotype of cells expressing the mutant tubulin provides a critical advance in assessing the dynamic requirements of an essential gene function.

Cell Cycle↗
PubMed1997

Comparison of ras-p21 bound to GDP and GTP: differences in protein and ligand dynamics.

This paper documents the first essential dynamics analysis of ras protein ligands and of the protein itself, showing important features of their dynamic properties. Essential dynamics analysis of 300 ps of full solvent molecular dynamics simulations revealed differences in structure and dynamics between GDP- and GTP-bound forms of H-ras-p21. Regions in the protein which exhibited a structural shift correspond to the switch regions described previously. Differences in dynamics between H-ras-p21 GDP and H-ras-p21 GTP may be related to interactions of ras with GAP and its receptor and effector. Molecular dynamics of free GDP (in the absence of protein) were performed in water for 2 ns and analysed using essential dynamics. The conformations of GDP and GTP when bound to the protein were compared with free GDP, revealing that the ligands bind to the protein in an energetically unfavourable conformation. GDP and GTP molecules from various other protein crystal structures were also analysed. These ligands adopt similar conformations to those seen in H-ras-p21.

Guanosine Diphosphate↗
PubMed2000

The effect of reduced dynamic range on speech understanding: implications for patients with cochlear implants.

OBJECTIVE: To determine the effect of reduced dynamic range on speech understanding when the speech signals are processed in a manner similar to a 6-channel cochlear implant speech processor. DESIGN: Signals were processed in a manner similar to a 6-channel cochlear implant processor and output as a sum of sine waves with frequencies equal to the center frequencies of the analysis filters. The amplitudes of the sine waves were compressed in a systematic fashion to simulate the effect of reduced dynamic range. The compressed signals were presented to 10 normal-hearing listeners for identification. RESULTS: There was a significant effect of compression for all test materials. The effect of the compression on speech understanding was different for the three test materials (vowels, consonants, and sentences). Vowel recognition was affected the most by the compression, and consonant recognition was affected the least by the compression. Feature analysis indicated that the reception of place information was affected the most. Sentence recognition was moderately affected by the compression. CONCLUSIONS: Dynamic range should affect the speech perception abilities of cochlear implant users. Our results suggest that a relatively wide dynamic range is needed for a high level of vowel recognition and a relatively small dynamic range is sufficient to maintain consonant recognition. We infer from this outcome that, if other factors were held equal, an implant patient with a small dynamic range could achieve moderately high scores on tests of consonant recognition but poor performance on vowel recognition, and that it is more likely for an implant patient with a large dynamic range to obtain high scores on vowel recognition than for an implant patient with a small dynamic range.

Cochlear Implants↗
PubMed2000

Effects of dynamic range and amplitude mapping on phoneme recognition in Nucleus-22 cochlear implant users.

OBJECTIVE: To determine the consequences for phoneme recognition of errors in setting threshold and loudness levels in cochlear implant listeners using a 4-channel continuous interleaved sampling (CIS) speech processor. DESIGN: Three Nucleus-22 cochlear implant listeners, who normally used the SPEAK speech processing strategy participated in this study. An experimental 4-channel CIS speech processor was implemented in each listener as follows. Speech signals were band-pass filtered into four broad frequency bands and the temporal envelope of the signal in each band was extracted by half-wave rectification and low-pass filtering. A power function was used to convert the extracted acoustic amplitudes to electric currents. The electric currents were dependent on the exponent of the mapping power function and the electrode dynamic range, which was determined by the minimum and maximum stimulation levels. In the baseline condition, the minimum and maximum stimulation levels were defined as the psychophysically measured threshold level (T-level) and maximum comfortable level (C-level). In the experimental conditions, the maximum stimulation levels were fixed at the C-level and the dynamic range (in dB) was changed by varying the minimum stimulation levels on all electrodes. This manipulation simulates the effect of an erroneous measurement of the T-level. Phoneme recognition was obtained as the dynamic range of electrodes was changed from 1 dB to 20 dB and as the exponent of the power-law amplitude mapping function was changed from 0.1 to 0.4. RESULTS: For each mapping condition, the electric dynamic range had a significant, but weak effect on vowel and consonant recognition. For a strong compression (p = 0.1), best vowel and consonant scores were obtained with a large dynamic range (12 dB). When the exponent of the mapping function was changed to 0.2 and 0.4, the dynamic range producing the highest scores decreased to 6 dB and 3 dB, respectively. CONCLUSIONS: Phoneme recognition with a 4-channel CIS strategy was only mildly affected by large changes in both electric threshold and loudness mapping. Errors in threshold by a factor of 2 (6 dB) and in the loudness mapping exponent by a factor of 2 were required to produce a significant decrease in performance. In these extreme conditions, the effect of the electric dynamic range on phoneme recognition could be due to two independent factors: abnormal loudness growth and a reduction in the number of discriminable intensity steps. The decrease in performance caused by a reduced electric dynamic range can be compensated by a more expansive power-law mapping function, as long as the number of discriminable intensity steps is moderately large (e.g., >8).

Acoustic Stimulation↗
PubMed1999

Pressure but not angiotensin II-induced increases in wall mass or tone influences static and dynamic aortic mechanics.

OBJECTIVE: To distinguish between static (due to slow changes in pressure) and dynamic (due to pressure pulsatility) components of aortic compliance over a large pressure range in vivo and to examine the effects of increased vascular mass and smooth muscle tone on these components. METHODS: Using ultrasound wall tracking, aortic lumen area-pressure curves were generated in anaesthetized rats over a broad range of pressures by altering blood volume. The compliance coefficient calculated at each mean pressure was considered the dynamic compliance at that pressure; the slope of the diastolic lumen area-pressure curve represents static compliance. Experiments were performed in control rats and rats treated with angiotensin II (ANG II) acutely (500 ng/kg per min intravenously) to modify vascular tone or chronically (250 ng/kg per min subcutaneously for 2 weeks) to modify vascular mass. RESULTS: The dynamic compliance-pressure curve approximated a parabola. Maximal dynamic compliance (0.272+/-0.026 mm2/kPa in control rats) was achieved at near-normotensive pressure (+/-105 mm Hg). The diastolic lumen area-pressure curve showed an exponential relationship within a physiological range (30-130 mm Hg). ANG II-induced increases in aortic wall mass or smooth muscle tone did not modify the relationship between static or dynamic compliance and pressure. CONCLUSIONS: These findings demonstrate that static and dynamic mechanics of the rat thoracic aorta depend differently on blood pressure. Static compliance increases slightly with pressure in a physiological range, while dynamic compliance is auto-regulated around normotensive pressures. Neither static nor dynamic compliance of the rat thoracic aorta are influenced by ANG II-induced increases in aortic wall mass or smooth muscle tone.

Angiotensin II↗
PubMed2003

Dynamic accommodative changes in rhesus monkey eyes assessed with A-scan ultrasound biometry.

PURPOSE: Prior studies in humans measured time constants of biometric accommodative changes as a function of amplitude, and prior studies in monkeys used slit lamp videography to analyze dynamic lenticular accommodative movements. Neither of these studies related biometric changes to refractive changes. We wished to develop and test methodology to begin to test the hypothesis that ocular biometric changes are well correlated with accommodative refractive changes in rhesus monkeys. METHODS: Methodology is described to dynamically measure biometric accommodative changes with A-scan ultrasonography. Lens thickness, anterior chamber depth, and anterior segment length (anterior chamber depth plus lens thickness) were measured dynamically during Edinger-Westphal-stimulated accommodation in two eyes of one rhesus monkey. In addition, dynamic accommodative refractive changes were measured with infrared photorefraction. Functions were fit to the accommodative and disaccommodative responses to obtain time constants. Derivatives of these functions allow peak velocities to be determined for each amplitude. Dynamic changes in lens thickness and anterior chamber depth measured with A-scan biometry were compared with dynamic measures of accommodation using infrared photorefraction. RESULTS: Lens thickness and anterior segment length increase and anterior chamber depth decreases during accommodation. The biometric changes are well correlated with the accommodative optical changes. Peak velocities of accommodative changes in lens thickness and anterior chamber depth increase with amplitude and peak velocities for disaccommodation were higher than those for accommodation. CONCLUSIONS: Dynamic A-scan provides a method for dynamic analysis of the accommodative biometric changes during Edinger-Westphal-stimulated accommodation in monkeys, although the measurement resolution of this approach is limited.

Accommodation, Ocular↗