Although cocaine-related deaths have increased significantly in the past decade, the mechanism(s) of mortality remain poorly defined. Case reports suggest a cardiovascular etiology (Young and Glauber, 1947; Isner, 1986) with some implicating conduction block (Young and Glauber, 1947; Nanji and Filipenko, 1984) as the underlying cause. Since cocaine is well known to possess potent local anesthetic activity in nerves, one possible mechanism for cocaine cardiotoxicity is the alteration of normal conduction within the heart by block of cardiac sodium channels. The effects of cocaine were therefore examined on cardiac sodium channels in single ventricular myocytes isolated from guinea pigs using the whole cell variant of the patch clamp technique. Cocaine (30-50 $\mu$M) was found to be a potent blocker of cardiac sodium channels at concentrations reportedly achieved following a fatal overdose in man. To examine the role of cocaine-induced cardiac sodium channel block on cardiac conduction in vivo, the effects of cocaine on the electrocardiogram (lead II) were also examined. Cocaine produced conduction disturbances in an anesthetized cat indicative of sodium channel block (widened QRS complex, idioventricular arrhythmias). Since approximately 30% of the cocaine-related deaths occur at a time when an appreciable amount of cocaine can be expected to have been converted to metabolites (2-5 hours after administration), the effects of cocaine's primary metabolites: ecgonine methyl ester, benzoylecgonine, and norcocaine were also examined to see if they possessed sodium channel blocking properties similar to cocaine. Norcocaine was found to be a potent blocker of cardiac sodium channels while ecgonine methyl ester and benzoylecgonine (at 100 $\mu$M) were found to lack cardiac sodium channel blocking ability. Cocaine has also been shown to cause seizures which may result in metabolic and/or respiratory acidosis. The effects of internal and external pH on the cocaine-sodium channel interaction were therefore examined. Open channel block produced by 30 $\mu$M cocaine was enhanced by either an acidic internal pH or a basic external pH. In contrast, the steady-state amount of inactivated channel block was found to be only weakly dependent upon pH. These studies provide a possible molecular mechanism for cocaine-related arrhythmias and sudden death / acase@tulane.edu

Glaucoma is a disease that affects over 1 million people in the United States, and is one of the three leading causes of blindness nationwide. Loss of vision from glaucoma is believed to be the result of damage to the axons of the retina ganglion cells as they pass through the lamina cribrosa, which spans the opening in the back of the eye wall called the scleral canal. It is known that elevations of intraocular pressure (IOP) are associated with this damage, but the exact mechanisms by which the level of IOP causes these changes are unknown and controversial The objective of this work is to investigate how the load-bearing connective tissues within and around the optic nerve head (ONH) respond to changes in intraocular pressure, and how these responses are altered when the tissues are damaged early in glaucoma. The connective tissues of the ONH provide support for the retinal ganglion cell axons as they pass through the wall of the eye, and are crucial in maintaining axonal health at the ONH. Since the ONH is the principal site of glaucomatous damage, understanding how these connective tissues respond to different loading conditions provide insight into the pathophysiology of the retinal axons in this disease Histologic measurements made in 4 mum serial sagittal sections show that acute increases in IOP can deform the load-bearing connective tissues of the ONH. This is true both when the IOP increase is from 0 to 10 mm Hg, and when the increase is from 10 to 30 or 45 mm Hg. Additionally, these measurements show that in eyes that have been given early experimental glaucoma, the magnitude of the deformations caused by a given IOP increase is larger than in normal eyes, and that these larger deformations have both a plastic and hyperelastic component. Using digitized three-dimensional reconstructions, these profound deformations can be visualized. Finally, using finite element modeling that incorporates these digital reconstructions, a better understanding was gained of how regional stresses and strains are related to these deformations, and how all of these factors are associated with the onset and progression of glaucoma / acase@tulane.edu

No description available.

No description available.

Topoisomerase inhibiting chemotherapy is most appropriate in cancers containing high levels of topoisomerase. Study of these drugs is particularly difficult because two receptors, enzyme plus DNA, are involved. Genes of all three known human topoisomerases have neighboring oncogenes and those amplified receptors may be used to target topoisomerase inhibitors to transformed cells. To facilitate design of inhibitors, modeling tools of graphic imaging, sequence analysis, homology, molecular dynamics, energy evaluation, and QSAR were used. The principle findings are: (a) The widely accepted model of 5' attachment to DNA and 5' overhang is inconsistent with topoisomerase structure; 3' overhang is more likely. (b) The active site region near the catalytic tyrosine hydroxyl is well conserved between yeast and humans with some exceptions: 4 Å away Glu554->His566 is likely to modify the proton transport of the trans-esterification reaction conducted by this enzyme; two base pairs away, the yeast Asp 552 appears to probe the DNA major groove and becomes an oppositely charged Arg 565 in humans; 8-15 A away along the probable location of DNA backbone, the acid pair Glu-Asp 512-13 is conserved and repeated in humans as Glu-Asp-Glu-Asp (522-5); these residues might coordinate the required Mg2+ along with the conserved Asp 635 (644 in humans); the acidic triad 590-2 evolves to a hydroxy triad and might provide a proton transport pathway for the trans-esterification reaction. (c) Drug resistant mutant structure analysis suggests multiple sites of action on topoisomerase and suggests the region of DNA binding. (d) Prediction of intercalator binding to DNA is correctly ranked by molecular mechanics for amonafide and azonafide; however, there remains significant error from experimental cytotoxicity; force field error derives from the approximation of electron orbitals as van der Waals spheres, lack of polarization functions, poor handling of hydrogen bond variability, and force field parameterization from packed protein crystals. (e) Wide variability of the azonafide analogs on whole cell assays makes QSAR difficult and suggests other biochemical pathways are affected; the measurements need to be redone on a simple enzyme-DNA assay.

One concept central to theories of multijoint control concerns the selection of muscles for the appropriate joint motion. For multijoint movements, a given muscle torque at an individual joint can lead to flexion, extension, or very little motion, since mechanical effects coming from other segments interact with muscle torque. This study quantified the contribution of muscle torque to initial joint motion for horizontal arm movements throughout the workspace. Previous studies of arm mechanics have been limited to a few movements or have focused on one joint. In contrast, this study reports data for both the shoulder and elbow joints. Moreover, a large number of movements were used for which direction, excursion, and distance were manipulated. Using high speed video recordings and techniques of inverse dynamics, a ratio of muscle torque to total torque was computed for each movement as a measure of contribution of muscle torque to joint acceleration. One consistent finding was that the muscle torque contribution consistently differed between the shoulder and elbow for most of the workspace. At one joint, muscle torque directly contributed to acceleration with negligible interaction torque ('direct' muscle torque contribution), thus the joint appeared to act as the launcher of the arm. At the other joint, both muscle and interaction torques contributed to joint acceleration ('complex' contribution), thus the joint appeared to be responding to mechanical effects from motion of the launcher. This contrast between joints may provide a simplifying feature for multijoint arm control. Specifically, only one of the two joints has complex mechanics, while the other joint, surprisingly, has simplified mechanics similar to a single joint in isolation. Movements in this study also demonstrated a three fold covariance (muscle torque contribution, movement direction, and the relative excursions of the shoulder and elbow) regardless of distance. A covariance of movement features, historically viewed as a confound, may provide a further simplification for arm control by reducing the unknowns; namely, the muscle torque contribution is associated with a resultant direction and joint excursions, or a direction or set of excursions is achieved by the associated muscle torque contribution.

A model was developed to account for the static elastic behaviour of the lung tissue strip in terms of distributions of collagen and elastin fibers. Distributions of collagen fiber lengths and elastin fiber stiffnesses were determined by fitting the model to data from dog lung tissue strips. These distributions followed 1/f power-laws for more than 95% of the data. Computer simulations of two dimensional tissue strip models with 1/f distributions of collagen fiber lengths also predicted realistic stress-strain curves. The simulations illustrated the gradual development of geometric and stress heterogeneity throughout the tissue as the collagen fibers were recruited during stretch. This model suggests a mechanistic basis for the shape of the pressure-volume curve of whole lung. It also indicates how this curve may be affected by changes in tissue collagen and elastin similar to the changes occurring in the diseases of pulmonary emphysema and fibrosis. Nonparametric block-structured nonlinear models for describing both the static and dynamic stress-strain behaviour of the lung were applied to dog lung tissue strips and to whole rat lungs in vivo. Both the Wiener and Hammerstein models accounted for more than 99% of the tissue strip data, although the Hammerstein model was more consistently accurate across a range of perturbation amplitudes and operating stresses. Plastic dissipation of energy within the lung tissue strip was estimated at less than 20% of the total dissipation during slow sinusoidal cycling. The Hammerstein model was also the best of those investigated for describing the rat lung data in vivo, although there were dependencies of the model parameters on perturbation amplitude and operating point that indicate that a more complicated model is required for the whole lung. Finally, construction of a fiber recruitment model for the dynamic mechanical behaviour of lung tissue strips was attempted. However accurate reproduction of measured behaviour was no

Stereotactic radiosurgery with several static conformal beams shaped by a micro multileaf collimator (muMLC) is used for treating small irregularly shaped brain lesions. Specific requirements for this technique are a precise localization and positioning of the target (1mm) and a precise (1mm) and numerically accurate (+/-5%) dose delivery. In this work, a pencil beam algorithm based treatment planning software BrainScan 5.2 (Brainlab, Germany) is validated against measurements (diode, radiographic films) and Monte Carlo simulations (BEAMnrc and XVMC codes). The latter is required because of difficulties in obtaining precise and accurate dose measurements for small fields. A dedicated muMLC component module for the BEAMnrc code was developed as part of this project. Results show that Monte Carlo calculations agree with measured dose distributions to within 2% and/or 1 mm except for field sizes smaller than 1.2 cm where agreement is within 5% due to uncertainties in measured output factors. Comparison with the pencil beam algorithm calculations were performed for square and irregularly shaped fields at different incidence angles on rectangular and humanoid homogeneous phantoms. Results show that the pencil beam algorithm is suitable for radiosurgery although some differences were found in the comparison of interleaf leakage and beam profile penumbras.

The goal of this work is to study human whole-body gas exchange and cerebral autoregulation using a mathematical model. Previously, a human cardiopulmonary (CP) model [45, 47] was developed, which included heart, closed-loop blood circulation, gas exchange at lungs and baroreflex control of arterial pressure. In the current study, two major extensions to the model are made. First, a description of gas exchange in the peripheral tissues is added and is coupled with the lung gas exchanger via the circulatory loop with variable transport delays. A peripheral chemosensitive loop is also added to mimic the influence of blood gas composition on the heart and vasculature. The CP model is then used to predict the integrated cardiovascular and blood-tissue gas transport responses to pronounced changes in lung gas composition, and thus simulates changes encountered in apnea with and without passive oxygenation. The second extension of the CP model includes a more detailed description of cerebral circulation, cerebrospinal fluid (CSF) dynamics, brain gas exchange and cerebral blood flow (CBF) autoregulation. Two CBF regulatory mechanisms are described: autoregulation and CO2 reactivity. Central chemoreceptor control of ventilation is also added. This new model is subsequently used to study cerebral hemodynamic and brain gas exchange responses to test protocols commonly used in the assessment of CBF autoregulation (e.g., carotid artery compression and the thigh cuff test). The model closely mimics the experimental findings and provides biophysically based insights into the dynamics and interactions of the associated physiological systems. In summary, this work represents a bold effort in large-scale modeling of physiological systems. The presented model accurately describes the physiological systems and can explain how the cardiovascular, pulmonary and autonomic nervous systems interact in response to a variety of cardiopulmonary challenges, such as apnea, carotid artery compression and the thigh cuff test. With further refinement, the model may help investigators to better understand the complex biophysics of cardiopulmonary diseases such as sleep-related disorders of breathing (obstructive and central sleep apnea) and complications associated with head-injuries.

Arterial thrombosis is one of the important pathophysiological mechanisms that lead to cardiovascular diseases. In this thesis, we have made an attempt to better characterize the kinetic and molecular mechanisms that underlie the critical first step in arterial thrombosis, namely, the interaction between platelet glycoprotein (GP) Ib and von Willebrand factor (VWF). In the first part of the work, we evaluated the kinetics of interaction between platelet GP Ib-IX-V complex and VWF under arterial flow conditions. The GP Ibalpha subunit of GP Ib complex binds to VWF through the Al domain of VWF. Impaired GP Ib-VWF interaction due to GP Ibalpha mutations can result in bleeding abnormalities including platelet-type von Willebrand disease (ptVWD). We measured the cellular on- and off-rate constants of CHO cells expressing wild-type or gain- or loss-of-function mutant GP Ibalpha interacting with VWF-Al-coated surfaces at different shear stresses. We found that the gain-of-function mutant, K237V, rolled very slowly and continuously on VWF-Al surface while the loss-of-function mutant, Q232V, showed fast, saltatory movement compared to the wild-type (WT). The off-rate constants, calculated based on the analysis of lifetimes of transient tethers formed on surfaces coated with limiting densities of VWF-Al, revealed that the Q232V and K237V dissociated 1.25-fold faster and 2.2-fold slower than the WT. The cellular on-rate constant of WT, measured in terms of tethering frequency was 3-fold more and 3-fold less than Q232V and K237V, respectively. Thus, the gain- and loss-of-function mutations in GP Ibalpha affect both the association and dissociation kinetics of the GP Ibalpha-VWF-Al bond. In the second part of the work, we compared the interaction of unusually large multimers of VWF (ULVWF) and that of the normal plasma multimers of VWF (P-VWF) platelets. ULVWF multimers are implicated in the pathology of a thrombotic disorder, thrombotic thrombocytopenic purpura (TTP) due to their increased affinity for platelets. We found that the ULVWF multimers are more effective than the normal P-VWF multimers in mediating (a) platelet aggregation in solution at high shear stress; (b) ristocetin-modulated platelet agglutination and (c) platelet adhesion to immobilized VWF under arterial shear conditions.