published_or_final_version / Anatomy / Master / Master of Medical Sciences

published_or_final_version / Biological Sciences / Doctoral / Doctor of Philosophy

Contracting skeletal muscle releases ATP into the interstitial space where it is subsequently broken down to adenosine by the action of ecto-5’-nucleotidase. Both ATP and adenosine are vasodilators that contribute to the exercise hyperaemia. However, the mechanism for the release of ATP from muscle during exercise remains unknown. Cystic fibrosis transmembrane conductance regulator (CFTR) is involved in ATP release from muscle at low intracellular pH: this study was performed to investigate whether CFTR was involved in the ATP release from skeletal muscle during contractions. Experiments were performed in rats anaesthetised with sodium pentobarbitone and breathing spontaneously. A microdialysis probe was placed in one gastrocnemius muscle: ATP was determined in interstitial microdialysate samples using a bioluminescence assay. The sciatic nerve was stimulated to induce two bouts of muscle contractions, separated by a recovery period of 40 mins; one of the inhibitors was administered prior to the second bout of contractions. Muscle contractions elevated the interstitial ATP by 1500 to 3000%. In the control experiments, no drug was given: both the contractile force and the increase in interstitial ATP were reproducible in repeated contraction bouts. Infusion of a specific inhibitor of CFTR, CFTRinh-172, did not alter the contractile force, but significantly lowered the interstitial ATP during muscle contractions, suggesting that CFTR was involved in the contraction-induced ATP release. Similarly, infusion of the Protein Kinase A inhibitor, KT5720, significantly reduced interstitial ATP during muscle contractions without altering contractile force, suggesting that CFTR in skeletal muscle is activated through the cAMP/PKA pathway. The increase in interstitial ATP during muscle contraction was also inhibited by the Na/H exchanger inhibitor, amiloride, or the Na/Ca exchanger inhibitor, SN6. It has been also shown that two gap junction hemichannel inhibitors, gadolinium and carbenoxolone, could attenuate the increase of ATP during muscle contraction. These data suggest that CFTR, activated through the cAMP/protein kinase A pathway, is involved in the ATP release during muscle contraction, and that activation of the Na/H exchanger and Na/Ca exchanger was also required, indicating that the signal transduction mechanism for CFTR activation during muscle contractions may be similar to that which is reported to occur at low pH. The preliminary data showed that the gap junction hemichannels might mediate the ATP release from skeletal muscle cells during muscle contraction. / published_or_final_version / Physiology / Master / Master of Philosophy

published_or_final_version / abstract / Pharmacology / Master / Master of Philosophy

Two studies of voluntary movement in Parkinson's disease were carried out. In the first study, both parkinsonians and age-matched controls performed unconstrained prehensile movements in which subjects produced reaching and grasping (hand opening/closing) movements under varying conditions of movement amplitude, speed and object size. The act of prehension requires the intersegmental coordination of limb transport and grasp trajectories. Although parkinsonians were slower overall than controls, patients and controls similarly adjusted the spatial and temporal characteristics of their limb movement and grasp in response to changes in task demands. All groups showed increases in the speed of limb transport, the speed of hand opening and began hand opening proportionately earlier at faster movement speeds. All groups similarly increased hand opening velocity and initiated grasp earlier for smaller amplitude movements. Likewise, grasp was initiated earlier when reaching for wider objects. However, in contrast to controls, the onset time of hand opening during limb transport was delayed in these patients and was found to be more closely coupled with the timing of limb transport than in the controls. Moreover, patients showed greater curvature in their motion paths at the wrist during limb transport suggesting that the timing of joint motion (shoulder and elbow) may be different in these patients as well. Underlying differences between the groups in the temporal sequencing of movement are discussed. / In a second study, parkinson and control subjects performed continuous tracking movements in pursuit of sinusoidal and constant-speed target trajectories varying in frequency and amplitude. This task provided explicit temporal and spatial accuracy constraints by requiring subjects to reproduce the precise trajectory (i.e., velocity profile) of target movement. The results show that patients, similar to controls, were capable of modifying peak movement velocity while varying their times to reversal (i.e., movement durations) in response to changes in the movement time requirements of target motion. Indeed, both patients and controls were shown to alter the timing of movement deceleration in order to maintain their movement durations within the temporal limits of target movement. In contrast to controls, patients show progressive reduction in endpoint accuracy (undershooting the target) and, hence, reduced movement amplitudes, over the course of the trial. However, when endpoint accuracy requirements were reduced, by providing mechanical constraints on movement amplitude, patients were able to increase movement amplitudes while satisfying the temporal requirements of the task. These results are interpreted in terms of tradeoffs in performance between competing spatial and temporal demands of pursuit tracking. The significance of movement accuracy constraints on motor function in parkinsonian performance is discussed.

Free radicals are thought to be involved in the destructive process of beta cells in Type 1 diabetes mellitus. Studies were performed to test the hypotheses (1) that malondialdehyde (MDA), a by-product of lipid peroxidation, affects $ beta$-cell function and integrity in vitro and (2) that such effects might be prevented in the BB rat (a model of spontaneous autoimmune diabetes) in vivo by administration of $ alpha$-phenyl-N-tert-butylnitrone (PBN), a free radical spin trap. First, islets of Wistar-Furth rats were studied at 12, 24 and 40 hr of culture in either 5.5, 11 or 16.5 mM glucose, and MDA at a range of concentrations ($6 times10 sp{-12}$-10$ sp{-3}$M). High concentrations of MDA inhibited glucose-stimulated insulin release without corresponding decreases in islet insulin content, suggesting that in situations with high predicted islet free radical content (e.g., autoimmune insulitis) beta cell function may be affected even before the cells are destroyed. Second, 28 diabetes-prone (BBdp) and 13 non diabetes-prone (BBn) rats were given PBN (20 mg/kg) s.c. 2x/day and 27 BBdp and 12 BBn rats received an equal volume of saline. PBN was able to decrease MDA in the absence of the autoimmune process and is remarkably non-toxic. However, it did not prevent diabetes for reasons which may include its concentration at the site of the inflammatory process or specificity to types of radicals trapped. Because it did decrease MDA, either a higher dose or a combination of PBN with other agents may hold promise for disease prevention.

No description available.

No description available.

Intracerebral hemorrhage (ICH) is a severe form of stroke, with a high mortality rate and often resulting in irreversible neurological deterioration. Although animal studies have provided insight into the etiology of the disease, many of the causative genes and mechanisms implicated in cerebral-vascular malformations are unknown. Treatment options remain ineffective. With the present models, the pathophysiological consequences of ICH can only be assessed in situ and after histological analysis. Furthermore, common deficiencies of the current models include the heterogeneity, low expression and low reproducibility of the desired phenotype. Hence, there is a requirement for novel approaches to model ICH pathogenesis. Zebrafish (Danio rerio) has gained recognition as a vertebrate model for stroke research. Through a combination of pharmacological blockers, metabolite rescue, genetic approaches, and confocal imaging analysis, I demonstrate a requirement for the 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) pathway in regulating developmental cerebral-vascular stabilization. A transient loss in HMGCR function induces ICH, characterised by progressive dilation of blood vessels, vascular permeability and vessel rupture. These effects are likely due to reduced prenylation of Rho GTPases, evidenced by morpholino-mediated blocking of the prenylation pathway and in vivo assessment of endothelial-specific localization of cdc42, a Rho GTPase family protein. These results are in conformity with recent clinical and experimental evidence. I have further shown that this model consistently replicates common pathoghysiological processes associated with ICH. The hemorrhages are associated with the disruption of the blood-brain barrier, vessel disintegration, hematoma expansion and edema into the adjacent brain regions. Also, enhanced apoptosis, activation of inflammatory mediators in the periphery of the hematoma, enriched heme oxygenase 1 (HO-1) expression and localised thrombosis were observed in these embryos. I show that the patterning and distribution of catecholaminergic neurons, response to sensory stimulus and swimming speed were impaired as a consequence of ICH. These results suggest that HMGCR contributes to cerebral-vascular stabilisation through Rho GTPase mediated-signalling and that zebrafish can serve as a powerful paradigm for the systemic analysis of the etiological and pathophysiological underpinnings of ICH and can help establish the basis for future studies into screening for putative therapeutics and elucidating mechanisms aiding functional recovery.

Abdominal Aortic Aneurysms (AAA) are amongst the top ten most common cause of death in those over 55 years of age. The disease is usually asymptomatic, often being diagnosed incidentally. Once diagnosed, elective repair of an AAA results in excellent long-term survival with a 3-5% operative mortality. However, up to one half of patients present with ruptured aneurysms, a complication that carries an 80% mortality in the community, and of those reaching hospital, a 50% mortality. Clearly early diagnosis and treatment results in improved survival. Screening for AAA, with ultrasound, would detect aneurysms early, prior to rupture. However, debate continues over the cost effectiveness of population based screening programmes. The identification of a sub-population at a higher risk of developing AAA would increase the yield of a screening prograrmne. A number of populations have been examined, none of which have received international acceptance. About 20% of patients with an AAA have a family history of an aneurysm. The disease is also considered to be a disease of Caucasians, both facts suggesting a strong genetic component to the disease. Perhaps a genetically identified sub-population at a high risk of developing an AAA would prove to be an ideal population for screening. This thesis examines the incidence of aneurysms and the family histories of patients with AAA in the Otago region of New Zealand. Almost twenty percent of the population has a family history of AAA. DNA was collected from each of these patients for genetic analysis. The population was divided into familial AAA and non-familial AAA for the purpose of genetic analysis and compared to a control population. AAA is believed to be a disease of Caucasians; a non-Caucasian population with a low incidence of AAA may prove to be a good control population for genetic studies. A literature review demonstrated a higher incidence of AAA in Caucasians than other ethnic groups and within Caucasians a higher incidence in patients of Northern European origin. The incidence was low in Asian communities, even in studies involving of migrant Asian populations. The New Zealand Maori are believed to have originated from South East Asia, therefore could be expected to have a low incidence of AAA and would make an ideal control population for genetic studies. A pilot study was undertaken to examine the incidence of AAA in the New Zealand Maori. The age standardised incidence of AAA proved to be at least equal in Maori to non-Maori, with a more aggressive form of the disease in Maori, manifesting with a younger age at presentation and a higher incidence of ruptured aneurysms at diagnosis. It is well known that at the time of surgery, an AAA is at the end stage in its life. At this time, inflammation and matrix metalloproteinases (MMP) enzymes are prevalent within the aneurysm wall and have destroyed the wall of the aorta. One of the most important genetic pathways regulating these enzymes is the plasminogen activator inhibiter 1-Tissue plasminogen activator-plasmin pathway. Genetic analysis of this pathway demonstrated an association of the 4G5G polymorphism in the promoter of the PAl-1 gene with familial AAA. In this insertion:deletion polymorphism, the 5G variant binds an activator and repressor, resulting in reduced PAI-1 expression and ultimately increased MMP activation. This allele was associated with familial aneurysms, 47% versus 62% non-familial AAA and 61% controls (p=0.024). A polymorphism within the tissue plasminogen activator gene was also examined and no association was found with AAA. Another way the MMPs expression could be increased is from mutations or polymorphisms in their own genetic structure. Stromelysin 3 is itself a MMP capable of destroying the aortic wall and it has a role in activating other MMPs. A 5A6A insertion:deletion polymorphism exists in the promoter of this gene. The 5A allele variant results in increased stromelysin expression and is associated with AAA 46% versus 33% in controls p=0. 0006. The actions of the MMPs are themselves inhibited by the tissue inhibitors of matrix metalloproteinases. The TIMP genes have been sequenced; two polymorphisms have been identified in the non-coding promoter area of the TIMP 1 gene. Further studies are necessary to examine the effect of these polymorphisms. Inflammation has been implicated in aneurysm progression. One of the roles of the inflammatory cells found in an aneurysm is to deliver the MMP�s to the AAA. The HLA system is integral in controlling this inflammation and was therefore examined. From this series of studies it is concluded that there is a genetic component to AAA. This thesis presents the first genetic polymorphism associated with familial AAA and explores the role of a genetic pathway in the formation of AAA.