Shape transform phasing of edgy nanocrystals

Diffraction patterns from small protein crystals illuminated by highly coherent X-rays often contain measurable interference signals between Bragg peaks. This coherent `shape transform' signal introduces enough additional information to allow the molecular densities to be determined from the diffracted intensities directly, without prior information or resolution restrictions. However, the various correlations amongst molecular occupancies/vacancies at the crystal surface result in a subtle yet critical problem in shape transform phasing whereby the sublattices of symmetry-related molecules exhibit a form of partial coherence amongst lattice sites when an average is taken over many crystal patterns. Here an iterative phase retrieval algorithm is developed which is capable of treating this problem; it is demonstrated on simulated data.

Properties of Nuclei up to A=16 using Local Chiral Interactions

We report accurate quantum Monte Carlo calculations of nuclei up to A=16 based on local chiral two- and three-nucleon interactions up to next-to-next-to-leading order. We examine the theoretical uncertainties associated with the chiral expansion and the cutoff in the theory, as well as the associated operator choices in the three-nucleon interactions. While in light nuclei the cutoff variation and systematic uncertainties are rather small, in ^{16}O these can be significant for large coordinate-space cutoffs. Overall, we show that chiral interactions constructed to reproduce properties of very light systems and nucleon-nucleon scattering give an excellent description of binding energies, charge radii, and form factors for all these nuclei, including open-shell systems in A=6 and 12.

Occupational exposure to ultrafine particles in police officers: no evidence for adverse respiratory effects

Background

Inhalation exposure to fine and ultrafine particles (UFPs) has been associated with respiratory diseases. However, little is known on the quality, threshold levels and concentration of these particles causing adverse health effects.

Methods

The impact of occupational exposure to submicrometer and UFPs was assessed in 30 healthy police shooting instructors by clinical investigation, self-assessment questionnaire, sputum and spirometry and compared to a control group. General laboratory chemistry parameters, circulating cytokines (interleukin [IL]-2, IL-4, IL-5, IL-6, IL-8, interferon-gamma [IFN-γ]), and granulocyte macrophage colony-stimulating factor (GM-CSF) in serum were measured. UFP exposure was recorded by Scanning Mobility Particle Sizer.

Results

Concentrations of submicrometer sized airborne particles (< 700 nm) measured between 3.34 × 10⁵/cm³ and 7.58 × 10⁵/cm³ at shooting sites, with highest concentrations found in the UFP range (< 100 nm). The size of the monodispersed particles ranged from 54.74 ± 16.25 nm to 98.19 ± 22.83 nm. Short term exposure (4 h) to high levels of UFPs caused an increase of IFN-γ in exposed subjects (p = 0.022). 24 h after exposure a significant decrease of IgG, albumin fibrinogen and factor VII was found. Neither directly after 4 h of high levels UFPs exposure nor 24 h after exposure subjective complaints or objective measurements indicating adverse respiratory effects in exposed subjects were found.

Conclusions

No consistent indications for adverse respiratory or inflammatory effects directly following exposure and 24 h after exposure to high levels of UFPs in our study group were detected. However we showed the assessment of short-term exposure effects at a genuine occupational setting, which might is relevant when a risk assessment of high level occupational exposures to UFPs is considered.

Accurate First Principles Calculation of Many-Body Interactions

The electronic structure Schrödinger equation is solved for the van der Waals complexes spin-polarized H2 and H3, and the closed-shell systems He2 and He3 by Monte Carlo methods. Two types of calculations are performed, variational Monte Carlo, which gives an upper bound to the eigenvalue of the Schrödinger equation, and Green's function Monte Carlo, which can solve the Schrödinger equation exactly within statistical sampling errors. The simulations are carried out on an ETA-10 supercom puter, and already existing computer codes were exten sively modified to ensure highly efficient coding. A major component of the computations was the develop ment of highly optimized many-electron wave functions. The results from the variational Monte Carlo simulations are reported for both the two- and three-body interac tion energies.

Chiral Three-Nucleon Interactions in Light Nuclei, Neutron-α Scattering, and Neutron Matter

We present quantum Monte Carlo calculations of light nuclei, neutron-α scattering, and neutron matter using local two- and three-nucleon (3N) interactions derived from chiral effective field theory up to next-to-next-to-leading order (N(2)LO). The two undetermined 3N low-energy couplings are fit to the (4)He binding energy and, for the first time, to the spin-orbit splitting in the neutron-α P-wave phase shifts. Furthermore, we investigate different choices of local 3N-operator structures and find that chiral interactions at N(2)LO are able to simultaneously reproduce the properties of A=3,4,5 systems and of neutron matter, in contrast to commonly used phenomenological 3N interactions.

Expression, purification and crystallization of CTB-MPR, a candidate mucosal vaccine component against HIV-1

CTB-MPR is a fusion protein between the B subunit of cholera toxin (CTB) and the membrane-proximal region of gp41 (MPR), the transmembrane envelope protein of Human immunodeficiency virus 1 (HIV-1), and has previously been shown to induce the production of anti-HIV-1 antibodies with antiviral functions. To further improve the design of this candidate vaccine, X-ray crystallography experiments were performed to obtain structural information about this fusion protein. Several variants of CTB-MPR were designed, constructed and recombinantly expressed in Escherichia coli. The first variant contained a flexible GPGP linker between CTB and MPR, and yielded crystals that diffracted to a resolution of 2.3 Å, but only the CTB region was detected in the electron-density map. A second variant, in which the CTB was directly attached to MPR, was shown to destabilize pentamer formation. A third construct containing a polyalanine linker between CTB and MPR proved to stabilize the pentameric form of the protein during purification. The purification procedure was shown to produce a homogeneously pure and monodisperse sample for crystallization. Initial crystallization experiments led to pseudo-crystals which were ordered in only two dimensions and were disordered in the third dimension. Nanocrystals obtained using the same precipitant showed promising X-ray diffraction to 5 Å resolution in femtosecond nanocrystallography experiments at the Linac Coherent Light Source at the SLAC National Accelerator Laboratory. The results demonstrate the utility of femtosecond X-ray crystallography to enable structural analysis based on nano/microcrystals of a protein for which no macroscopic crystals ordered in three dimensions have been observed before.

Time-resolved protein nanocrystallography using an X-ray free-electron laser

We demonstrate the use of an X-ray free electron laser synchronized with an optical pump laser to obtain X-ray diffraction snapshots from the photoactivated states of large membrane protein complexes in the form of nanocrystals flowing in a liquid jet. Light-induced changes of Photosystem I-Ferredoxin co-crystals were observed at time delays of 5 to 10 µs after excitation. The result correlates with the microsecond kinetics of electron transfer from Photosystem I to ferredoxin. The undocking process that follows the electron transfer leads to large rearrangements in the crystals that will terminally lead to the disintegration of the crystals. We describe the experimental setup and obtain the first time-resolved femtosecond serial X-ray crystallography results from an irreversible photo-chemical reaction at the Linac Coherent Light Source. This technique opens the door to time-resolved structural studies of reaction dynamics in biological systems.

Signal, noise, and resolution in correlated fluctuations from snapshot small-angle x-ray scattering

It has been suggested that the three-dimensional structure of one particle may be reconstructed using the scattering from many identical, randomly oriented copies ab initio, without modeling or a priori information. This may be possible if these particles are frozen in either space or time, so that the conventional two-dimensional small-angle x-ray scattering (SAXS) distribution contains fluctuations and is no longer isotropic. We consider the magnitude of the correlated fluctuation SAXS (CFSAXS) signal for typical x-ray free-electron laser (XFEL) beam conditions and compare this against the errors derived with the inclusion of Poisson photon counting statistics. The resulting signal-to-noise ratio (SNR) is found to rapidly approach a limit independent of the number of particles contributing to each diffraction pattern, so that the addition of more particles to a "single-particle-per-shot" experiment may be of little value, apart from reducing solvent background. When the scattering power is significantly less than one photon per particle per Shannon pixel, the SNR grows in proportion to incident flux. We provide simulations for protein molecules in support of these analytical results, and discuss the effects of solvent background scatter. We consider the SNR dependence on resolution and particle size, and discuss the application of the method to glasses and liquids, and the implications of more powerful XFELs, smaller focused beams, and higher pulse repetition rates for this approach. We find that an accurate CFSAXS measurement may be acquired to subnanometer resolution for protein molecules if a 9-keV beam containing 10(13) photons is focused to a ~100-nm spot diameter, provided that the effects of solvent background can be reduced sufficiently.

Structure-factor analysis of femtosecond microdiffraction patterns from protein nanocrystals

A complete set of structure factors has been extracted from hundreds of thousands of femtosecond single-shot X-ray microdiffraction patterns taken from randomly oriented nanocrystals. The method of Monte Carlo integration over crystallite size and orientation was applied to experimental data from Photosystem I nanocrystals. This arrives at structure factors from many partial reflections without prior knowledge of the particle-size distribution. The data were collected at the Linac Coherent Light Source (the first hard-X-ray laser user facility), to which was fitted a hydrated protein nanocrystal injector jet, according to the method of serial crystallography. The data are single 'still' diffraction snapshots, each from a different nanocrystal with sizes ranging between 100 nm and 2 µm, so the angular width of Bragg peaks was dominated by crystal-size effects. These results were compared with single-crystal data recorded from large crystals of Photosystem I at the Advanced Light Source and the quality of the data was found to be similar. The implications for improving the efficiency of data collection by allowing the use of very small crystals, for radiation-damage reduction and for time-resolved diffraction studies at room temperature are discussed.

Phasing of coherent femtosecond X-ray diffraction from size-varying nanocrystals

The scattering between Bragg reflections from nanocrystals is used to aid solution of the phase problem. We describe a method for reconstructing the charge density of a typical molecule within a single unit cell, if sufficiently finely-sampled "snap-shot" diffraction data (as provided a free-electron X-ray laser) are available from many nanocrystals of different sizes lying in random orientations. By using information on the particle-size distribution within the patterns, this digital method succeeds, using all the data, without knowledge of the distribution of particle size or requiring atomic-resolution data.

Femtosecond protein nanocrystallography-data analysis methods

X-ray diffraction patterns may be obtained from individual submicron protein nanocrystals using a femtosecond pulse from a free-electron X-ray laser. Many "single-shot" patterns are read out every second from a stream of nanocrystals lying in random orientations. The short pulse terminates before significant atomic (or electronic) motion commences, minimizing radiation damage. Simulated patterns for Photosystem I nanocrystals are used to develop a method for recovering structure factors from tens of thousands of snapshot patterns from nanocrystals varying in size, shape and orientation. We determine the number of shots needed for a required accuracy in structure factor measurement and resolution, and investigate the convergence of our Monte-Carlo integration method.

Heavy-light fermion mixtures at unitarity

We investigate fermion pairing in the unitary regime for a mass ratio corresponding to a ;{6}Li-;{40}K mixture using quantum Monte Carlo methods. The ground-state energy and the average light- and heavy-particle excitation spectrum for the unpolarized superfluid state are nearly independent of the mass ratio. In the majority light system, the polarized superfluid is close to the energy of a phase separated mixture of nearly fully polarized normal and unpolarized superfluid. For a majority of heavy particles, we find an energy minimum for a normal state with a ratio of approximately 3ratio1 heavy to light particles. A slight increase in attraction to k_{F}a approximately 2.5 yields a ground state energy of nearly zero for this ratio. A cold unpolarized system in a harmonic trap at unitarity should phase separate into three regions, with a shell of unpolarized superfluid in the middle.

Equation of state of superfluid neutron matter and the calculation of the 1S0 pairing gap

We present a quantum Monte Carlo study of the zero-temperature equation of state of neutron matter and the computation of the 1S0 pairing gap in the low-density regime with rho < 0.04 fm(-3). The system is described by a nonrelativistic nuclear Hamiltonian including both two- and three-nucleon interactions of the Argonne and Urbana type. This model interaction provides very accurate results in the calculation of the binding energy of light nuclei. A suppression of the gap with respect to the pure BCS theory is found, but sensibly weaker than in other works that attempt to include polarization effects in an approximate way.

Tomographic femtosecond x-ray diffractive imaging

A method is proposed for obtaining three simultaneous projections of a target from a single radiation pulse, which also allows the relative orientation of successive targets to be determined. The method has application to femtosecond x-ray diffraction, and does not require solution of the phase problem. We show that the principal axes of a compact charge-density distribution can be obtained from projections of its autocorrelation function, which is directly accessible in diffraction experiments. The results may have more general application to time resolved tomographic pump-probe experiments and time-series imaging.

Auxiliary field diffusion Monte Carlo calculation of nuclei with A < or = 40 with tensor interactions

We calculate the ground-state energy of (4)He, (8)He, (16)O, and (40)Ca using the auxiliary field diffusion Monte Carlo method in the fixed-phase approximation and the Argonne v(6)' interaction which includes a tensor force. Comparison of our light nuclei results to those of Green's function Monte Carlo calculations shows the accuracy of our method for both open and closed-shell nuclei. We also apply it to (16)O and (40)Ca to show that quantum Monte Carlo methods are now applicable to larger nuclei.

Quantum Monte Carlo calculations of symmetric nuclear matter

We present an accurate numerical study of the equation of state of nuclear matter based on realistic nucleon-nucleon interactions by means of auxiliary field diffusion Monte Carlo (AFDMC) calculations. The AFDMC method samples the spin and isospin degrees of freedom allowing for quantum simulations of large nucleonic systems and represents an important step forward towards a quantitative understanding of problems in nuclear structure and astrophysics.

Pfaffian pairing wave functions in electronic-structure quantum Monte Carlo simulations

We investigate the accuracy of trial wave functions for quantum Monte Carlo based on Pfaffian functional form with singlet and triplet pairing. Using a set of first row atoms and molecules we find that these wave functions provide very consistent and systematic behavior in recovering the correlation energies on the level of 95%. In order to get beyond this limit we explore the possibilities of multi-Pfaffian pairing wave functions. We show that a small number of Pfaffians recovers another large fraction of the missing correlation energy comparable to the larger-scale configuration interaction wave functions. We also find that Pfaffians lead to substantial improvements in fermion nodes when compared to Hartree-Fock wave functions.

1S0 Superfluid phase transition in neutron matter with realistic nuclear potentials and modern many-body theories

The 1S0 pairing in neutron matter is studied using realistic two- and three-nucleon interactions. The auxiliary field diffusion Monte Carlo method and correlated basis function theory are employed to get quantitative and reliable estimates of the gap. The two methods are in good agreement up to the maximum gap density and both point to a slight reduction with respect to the standard BCS value. In fact, the maximum gap is about 2.5 MeV at kF approximately 0.8 fm(-1) in BCS and 2.2-2.4 MeV at kF approximately 0.6 fm(-1)in correlated matter. In general, the computed medium polarization effects are much smaller than those previously estimated within all theories. Truncations of Argonne to simpler forms give the same gaps in BCS, provided the truncated potentials have been refitted to the same data set. The three-nucleon interaction provides an additional increase of the gap of about 0.35 MeV.

Green's function Monte Carlo method with exact imaginary-time propagation

We present a general formulation of the Green's function Monte Carlo method in imaginary-time quantum Monte Carlo which employs exact propagators. This algorithm has no time-step errors and is obtained by minimal modifications of the time-independent Green's function Monte Carlo method. We describe how the method can be applied to the many-body Schrödinger equation, lattice Hamiltonians, and simple field theories. Our modification of the Green's function Monte Carlo algorithm is applied to the ground state of liquid 4He. We calculate the zero-temperature imaginary-time diffusion constant and relate that to the effective mass of a mass-four "impurity" atom in liquid 4He.

Superfluid Fermi gases with large scattering length

We report quantum Monte Carlo calculations of superfluid Fermi gases with short-range two-body attractive interactions with infinite scattering length. The energy of such gases is estimated to be (0.44+/-0.01) times that of the noninteracting gas, and their pairing gap is approximately twice the energy per particle.

Structure, rotational dynamics, and superfluidity of small OCS-doped He clusters

The structural and dynamical properties of carbonyl sulfide (OCS) molecules solvated in helium clusters are studied using reptation quantum Monte Carlo, for cluster sizes n=3-20 He atoms. Computer simulations allow us to establish a relation between the rotational spectrum of the solvated molecule and the structure of the He solvent, and of both with the onset of superfluidity. Our results agree with a recent spectroscopic study of this system and provide a more complex and detailed microscopic picture of this system than inferred from experiments.

Comparative endocrinology of the insulin-like growth factor-binding protein

Emerging early in chordate evolution, the IGF-regulatory axis diverged from an insulin-like predecessor into a vertebrate regulatory system specializing in cell growth activation and allied anabolic functions. Essential to the divergence of the IGF and insulin systems was an early presence of soluble IGF-binding proteins (IGFBPs), which bind IGF peptides at much higher affinity than that of the insulin receptor but at comparable affinities to that of the IGF receptor. IGFBPs have no homology with IGF receptors. Instead, IGFBPs are a derived group of proteins within a superfamily of cysteine-rich growth factors, whose members are found throughout the animal taxa. While blocking IGF actions through the insulin receptor is a fundamental role, IGFBPs evolved within the vertebrate line into centralized, 'integrators' of the endocrine growth-regulatory apparatus. IGFBPs have substantial influences on the distribution and bioavailability of IGF peptides in the cellular and physiological environments, but they have a variety of other properties. The six principal mammalian IGFBPs exhibit an array of specialized properties that appear to be derived from a complex evolutionary history (including cell membrane association, interaction with proteins that post-translationally modify them, direct IGF-independent effects on cells, and others) and they are regulated by a diversity of 'outside' factors (e.g. other hormones, metabolic status, stress). Thus, IGFBPs are multifunctional integrators having diverse physiological 'agendas'. Much less is known about IGFBPs and their properties in the other vertebrate taxa. Increasingly, however, it is being recognized that they play equally important endocrine roles, in both conserved and non-conserved ways, when compared with those currently defined in mammals. This review highlights selected 'comparative aspects' in current IGFBP research, in an attempt to view this essential group of endocrine regulators from a wider, biological perspective.

HF dimer in small helium clusters: interchange-tunneling dynamics in a quantum environment

We present diffusion quantum Monte Carlo calculations of the interchange tunneling splitting of (4)He(n)(HF)(2) clusters, n = 1-10. The tunneling splitting decreases rapidly for n = 1-4 clusters, and much more slowly for n>4. The decrease calculated for (4)He(n)(HF)(2) represents 74% of the reduction in the tunneling splitting measured recently for HF dimer in nanodroplets of more than 2000 He atoms. The first four He atoms quench the interchange tunneling very efficiently by virtue of occupying the equatorial ring which encircles the C(2h) transition state of the tunneling pathway.

Association between vitamin E and enhanced athletic performance in sled dogs

PURPOSE

To determine the association between prerace plasma vitamin E concentration and performance in sled dogs competing in the 1998 Iditarod Race.

METHODS

Prerace blood samples were collected from 670 dogs. Samples were analyzed for plasma vitamin E concentration while controlling for selected hematological and biochemical variables and signalment. Starting in teams of 16, exercise consisted of running up to 1159 miles pulling a laden sled and musher via checkpoints. The records of dogs that were withdrawn from the race for health reasons, fatigue, or strategic or technical reasons, and those of dogs that finished the race were analyzed. Multiple logistic regression and Cox proportional hazards analysis were used to determine factors associated with endurance. Multiple linear regression analysis was used to determine factors associated with team speed.

RESULTS

A total of 323 dogs (48%) were withdrawn from racing at various distances from the start. Median time to finish for 39 teams was 11.5 d and the winning time was 9.2 d. Dogs with prerace plasma vitamin E concentrations > 40.7 microg.mL-1 were 1.9 times more likely to finish (P = 0.0006) and had 1.8 times less of a risk of being withdrawn for every mile ran (P = 0.03) than were dogs with plasma vitamin E concentrations between 16.3 and 40.7 microg.mL-1. Neither a team's mean prerace vitamin E concentration, nor the proportion of dogs within a team with high (> 40.7 microg.mL-1) vitamin E concentration was associated with team speed.

CONCLUSIONS

Dogs with higher plasma vitamin E concentrations have enhanced endurance compared with dogs with lower plasma vitamin E concentrations, but the plasma vitamin E status of a team is not associated with team speed.

Vitamin E and exertional rhabdomyolysis during endurance sled dog racing

Exertional rhabdomyolysis (ER) is common in sled dogs, animals with high energy expenditures that consume high fat (60% of ingested calories) diets. Associations between pre-race plasma [vitamin E] and total antioxidant status (TAS) and risk of developing ER were examined in dogs competing in the 1998 Iditarod race. Pre-race blood samples were collected from 750 dogs and a second sample was collected from 158 dogs withdrawn from the race at various times. Plasma creatine kinase activity was used to identify withdrawn dogs with ER. There was no association between pre-race plasma [vitamin E] and risk of development of ER. Dogs that developed ER started the race with higher TAS, but when withdrawn, had lower TAS than unaffected dogs and had similar pre-race [vitamin E] but higher [vitamin E] at time of withdrawal. Hence, the risk of ER in sled dogs is not affected by plasma [vitamin E] before the race.

Down-regulation in the insulin-like growth factor (IGF) axis during hibernation in the golden-mantled ground squirrel, Spermophilus lateralis: IGF-I and the IGF-binding proteins (IGFBPs)

The golden-mantled ground squirrel, Spermophilus lateralis, undergoes a profound winter hibernation that represents, among other changes, a prolonged period of starvation. In addition to dramatic metabolic and other physiological adaptations during hibernation which serve to reduce fuel energy expenditure, we have hypothesized that there may also be significant changes in the endocrine axis that regulates energetically-expensive somatic growth. As compared with euthermic, non-hibernating controls, hibernating S. lateralis were found to have 75%-reduced serum concentrations of insulin-like growth factor-I (IGF-I; from approximately 625 to approximately 150 ng/ml in both females and males, P < 0.05). While IGFBP-3 was the predominant IGFBP in serum of the euthermic controls, its levels were reduced to a similar degree in serum from the hibernating animals. IGFBP-4 was present at relatively low levels in the euthermic controls, and was reduced to undetectable levels in hibernating animals. Surprisingly, there was no IGFBP detectable in the 30 kDa range in either euthermic or hibernating S. lateralis, suggesting that IGFBP-1 does not play a role in hibernation-related changes in the IGF axis. In accordance with these endocrine changes, when serum from hibernating S. lateralis was added to cartilage explant cultures (at a 5% v/v concentration), it exhibited no ability to alter (35)S-proteoglycan synthetic rate, whereas serum from the euthermic squirrels significantly stimulated synthetic activity by 2-fold. These results suggest that part of hibernation adaptation in S. lateralis includes down-regulation in the growth-regulatory IGF axis. J. Exp. Zool. 289:66-73, 2001.

Matching the modules: cortical maps and long-range intrinsic connections in visual cortex during development

Visual cortical neurons exhibit a high degree of response selectivity and are grouped into small columns according to their response preferences. The columns are located at regularly spaced intervals covering the whole cortical representation of the visual field with a modular system of feature-selective neurons. The selectivity of these cells and their modular arrangement is thought to emerge from interactions in the network of specific intracortical and thalamocortical connections. Understanding the ontogenesis of this complex structure and contributions of intrinsic and extrinsic, experience-dependent mechanisms during cortical development can provide new insights into the way the visual cortex processes information about the environment. Available data about the development of connections and response properties in the visual cortex suggest that maturation proceeds in two distinct steps. In the first phase, mechanisms inherent to the cortex establish a crude framework of interconnected neural modules which exhibit the basic but still immature traits of the adult state. Relevant mechanisms in this phase are assumed to consist of molecular cues and patterns of spontaneous neural activity in cortical and corticothalamic interconnections. In a second phase, the primordial layout becomes refined under the control of visual experience establishing a fine-tuned network of connections and mature response properties.

Effect of distance traveled and speed of racing on body weight and serum enzyme activity of sled dogs competing in a long-distance race

OBJECTIVE

To determine the relationship of serum biochemical values and change in body weight with finishing status (retired from or finished the race), finishing order of a team, and distance traveled for dogs participating in a long-distance sled dog race.

ANIMALS

262 of 848 dogs that participated in the 1995 Iditarod Trail sled dog race.

DESIGN

Prospective study.

PROCEDURE

Body weight was recorded for 261 dogs before the race and again when these dogs retired from or completed the race. Using a nonrandom convenience sample of participating dogs, blood samples were obtained from 151 dogs that retired from the race and 111 dogs that completed the race.

RESULTS

Serum biochemical indices of skeletal muscle damage were significantly higher in dogs retiring during the first 500 miles of the race than in dogs retiring in the last 638 miles or finishing the race. Serum sodium concentration was less than the reference range in a significantly greater proportion of dogs that retired from the race than of dogs that completed the race. There was little relationship between finishing order and serum biochemical values. Dogs completing the race lost a mean of 8.9% of body weight, and amount of weight lost was not related to finishing order.

CLINICAL IMPLICATIONS

Results indicated that exertional rhabdomyolysis develops more often in dogs that retire during the initial 500 miles of a long-distance race, compared with dogs that complete the race. There is no detectable relationship between the speed with which the race is run (finishing order) and body weight loss or serum biochemical values.

The layout of orientation and ocular dominance domains in area 17 of strabismic cats

In the primary visual cortex of strabismic cats, the elimination of correlated activity between the two eyes enhances the segregation of the geniculocortical afferents into alternating ocular dominance domains. In addition, both tangential intracortical fibres and neuronal synchronization are severely reduced between neurons activated by different eyes. Consequently, ocular dominance columns belonging to different eyes are functionally rather independent. We wondered whether this would also affect the organization of orientation preference maps. To this end, we visualized the functional architecture of area 17 of strabismic cats with both optical imaging based on intrinsic signals and double labelling of orientation and ocular dominance columns with [14C]2-deoxyglucose and [3H]proline. As expected, monocular iso-orientation domains had a patchy appearance and differed for the two eyes, leading to a clear segregation of the ocular dominance domains. Comparison of 'angle maps' revealed that orientation domains exhibit a pinwheel organization as in normally reared cats. Interestingly, the map of orientation preferences did not show any breaks at the borders between ocular dominance columns: iso-orientation domains were continuous across these borders. In addition, iso-orientation contours tended to cross the borders of adjacent ocular dominance columns at right angles. These data suggest that the basic relations between the layout of orientation maps and ocular dominance columns are not disturbed by artificial decorrelation of binocular input. Therefore in cat area 17, the orientation map does not seem to be modified by experience-dependent changes of thalamic input connections. This suggests the possibility that use-dependent rearrangement of geniculocortical afferents into ocular dominance columns is due to Hebbian modifications whereby postsynaptic responsivity is constrained by the scaffold of the orientation map.

Electrocardiographic characteristics of endurance-trained Alaskan sled dogs

OBJECTIVE

To determine electrocardiographic characteristics of endurance-trained Alaskan sled dogs.

DESIGN

Case series.

ANIMALS

319 Alaskan sled dogs entered to compete in the 1994 Iditarod Trail Sled Dog Race.

PROCEDURE

ECG were recorded while dogs were standing and were analyzed digitally.

RESULTS

Amplitudes of P waves (median, 0.40 mV; fifth to 95th percentile range, 0.11 to 0.61 mV) and R waves in lead II (median, 3.02 mV; fifth to 95th percentile range, 1.49 to 4.40 mV) were high; durations of P waves in lead II (median, 61 milliseconds; fifth to 95th percentile range, 36 to 96 milliseconds), QRS complexes (median, 64 milliseconds; fifth to 95th percentile range, 52 to 80 milliseconds), and QT intervals (median, 236 milliseconds; fifth to 95th percentile range, 208 to 277 milliseconds) were prolonged. Median value for mean axis of ventricular depolarization was 57 degrees (fifth to 95th percentile range, 19 to 90 degrees). Atrial and ventricular premature depolarizations were observed in 3 (0.9%) and 4 (1.3%) of 319 dogs, respectively, and paroxysmal ventricular tachycardia was detected in 1 (0.3%).

CLINICAL IMPLICATIONS

Results suggest that electrocardiographic characteristics of endurance-trained Alaskan sled dogs differ from those reported for nonsled dogs, probably as a result of effects of endurance training on heart size. Some of these characteristics could be mistaken as evidence of pathologic cardiac hypertrophy.

Functional specificity of long-range intrinsic and interhemispheric connections in the visual cortex of strabismic cats

The development of both long-range intracortical and interhemispheric connections depends on visual experience. Previous experiments showed that in strabismic but not in normal cats, clustered horizontal axon projections preferentially connect cell groups activated by the same eye. This indicates that there is selective stabilization of fibers between neurons exhibiting correlated activity. Extending these experiments, we investigated in strabismic cats: (1) whether tangential connections remain confined to columns of similar orientation preference within the subsystems of left and right eye domains; and (2) whether callosal connections also extend predominantly between neurons activated by the same eye and preferring similar orientations. To this end, we analyzed in strabismic cats the topographic relationships between orientation preference domains and both intrinsic and callosal connections of area 17. Red and green latex microspheres were injected into monocular iso-orientation domains identified by optical imaging of intrinsic signals. Additionally, domains sharing the ocular dominance and orientation preference of the neurons at the injection sites were visualized by 2-deoxyglucose (2-DG) autoradiography. Quantitative analysis revealed that 56% of the retrogradely labeled cells within the injected area 17 and 60% of the transcallosally labeled neurons were located in the 2-DG-labeled iso-orientation domains. This indicates: (1) that strabismus does not interfere with the tendency of long-range horizontal fibers to link predominantly neurons of similar orientation preference; and (2) that the selection mechanisms for the stabilization of callosal connections are similar to those that are responsible for the specification of the tangential intrinsic connections.

The perceptual grouping criterion of colinearity is reflected by anisotropies of connections in the primary visual cortex

An important step in the processing of visual patterns is the segmentation of the retinal image. Neuronal responses evoked by the contours of individual objects need to be identified and associated for further joint processing. These grouping operations are based on a number of Gestalt criteria. Here we report that connections in the visual cortex of the cat exhibit a highly significant anisotropy, preferentially linking neurons activated by contours that have similar orientation and are aligned colinearly. These anatomical data suggest a close relation between the perceptual grouping criterion of colinearity and the topology of tangential intracortical connections. We propose that tangential intracortical connections support perceptual grouping by modulating the saliency of distributed cortical responses in a context-dependent way. The present data are compatible with the hypothesis that the criteria for this grouping operation are determined by the architecture of the tangential connections.

The conduction system in sudden death in Alaskan sled dogs during the Iditarod race and/or during training

Using serial section examination, we studied the conduction system in five Alaskan sled dogs that died suddenly; four during the Iditarod race and one during training. We compared our findings with the conduction system of three sled dogs of similar age that died of natural causes unrelated to the cardiovascular system. The conduction system of sudden death dogs revealed marked fibrosis of the sinoatrial (SA) node and/or its approaches and narrowing of the SA nodal artery in 3 dogs, fibrosis and marked fatty infiltration in and around the AV node in all 5, total isolation and/or tenuous connection of the AV node with its approaches in 4, fat and fibrosis in the AV bundle and bundle branches to a varying degree in all, and focal fibrotic scars in the left ventricle with fat and/or some disarray in 3. The control group revealed mild fibro-fatty changes in the conduction system without fibrotic scar areas in the heart. These findings are similar to the pathological findings in and around the conduction system in cases of sudden death in humans, especially trained athletes. These changes may form an anatomical substrate for an arrhythmic event in susceptible dogs during an altered physiological state.

Factors associated with finishing status for dogs competing in a long-distance sled race

OBJECTIVE

To determine whether selected factors were associated with finishing status in a long-distance sled dog race.

DESIGN

Prospective, observational study.

ANIMALS

248 dogs participating in the 1994 Iditarod trail sled race that were members of 13 teams that finished the race. Dogs were not selected randomly, but were members of teams that were easily accessible for examination before the race. The proportion of teams that were examined that finished the race (14/17) was similar to the proportion of teams that were not examined that finished the race (36/41).

PROCEDURE

Age, sex, body conformation (weight, length, and thoracic width and circumference), cardiac variables (heart rate, natural logarithm of heart period variance, PR interval, QRS duration, QT index, R wave amplitude in leads II and V3, mean electrical axis, presence of cardiac arrhythmias suggestive of myocardial disease), and athletic ranking as assessed by the musher were compared between dogs that finished the race (n = 128) and dogs that did not finish (n = 120).

RESULTS

The only factor found to differ significantly (P < 0.05) between finishers and nonfinishers was athletic ranking as assessed by the musher. Athletic rank and QRS duration or QRS duration normalized for body weight were identified by means of logistic regression as variables associated (P < 0.15) with finishing status. There was a significant (P < 0.0001), but weak (R2 = 0.18), linear relationship between race time and mean QRS duration for each team.

CLINICAL IMPLICATIONS

Athletic ranking as assessed by the musher was the most important of the studied variables in determining finishing status, whereas age, sex, body conformation, and body weight were unimportant. Duration of the QRS was of minor importance in determining finishing status.

Conformation of amine-modified DNA: 2-aminofluorene- and 2-(acetylamino)fluorene-modified deoxydinucleoside monophosphates with all possible nearest neighbors. A comparison of search and optimization methods

Although a significant part of the replication fork exists as single-stranded DNA, little is known about the effect of carcinogens and mutagens on single-strand conformation. Large-scale conformational searches with potential energy minimization, using the torsion angle space molecular mechanics program DUPLEX, were employed to explore the conformation of all 16 deoxydinucleoside monophosphates bearing 2-aminofluorene (AF) or 2-(acetylamino)fluorene (AAF) modification on guanine. We have thus examined the effect of 3' versus 5' modification, the presence or absence of the acetyl group, and the effect of four different neighbors in each case. The principal effect of the acetyl group appeared to be the destabilization of anti (and, to a lesser degree, borderline anti) conformations for modified guanine. This mattered little in the 5'-substituted dimers, where one conformational type predominated in the low-energy structures for the adducts of both AAF and AF: It was right-handed, with syn-guanine, imperfect base-base stacking, and fluorene to 3'-sugar contacts. Greater divergence was seen in the 3'-substituted series. The AAF-substituted 3'-adducts primarily displayed good base-fluorene stacking, with syn-guanine in contact with the 5'-sugar. The AF-substituted 3'-adducts displayed a variety of structures which included base-base and carcinogen-base stacked forms. Two novel forms were encountered [global minima for d(ApG-AF) and d(GpG-AF)], whose unusual structures suggest mutagenic capability. In order to address the multiple minimum problem, we conducted our searches of conformation space using two alternative optimization methods that also employ differing search strategies. We used the Powell algorithm, BOTM, with starting conformations that are selected combinations of rotamers, and the method of simulated annealing (SA), with random or arbitrary starting conformations. While both approaches were effective in defining the most important structures, SA was more successful than BOTM in locating the structures of lowest energy.