Browsing by Subject "Rats"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Open Access
    Long-Term Left Ventricular Remodelling in Rat Model of Nonreperfused Myocardial Infarction: Sequential MR Imaging Using a 3T Clinical Scanner
    (2012) Saleh, Muhammad G; Sharp, Sarah-Kate; Alhamud, Alkathafi; Spottiswoode, Bruce S; van der Kouwe, André J W; Davies, Neil H; Franz, Thomas; Meintjes, Ernesta M
    Purpose. To evaluate whether 3T clinical MRI with a small-animal coil and gradient-echo (GE) sequence could be used to characterize long-term left ventricular remodelling (LVR) following nonreperfused myocardial infarction (MI) using semi-automatic segmentation software (SASS) in a rat model. Materials and Methods. 5 healthy rats were used to validate left ventricular mass (LVM) measured by MRI with postmortem values. 5 sham and 7 infarcted rats were scanned at 2 and 4 weeks after surgery to allow for functional and structural analysis of the heart. Measurements included ejection fraction (EF), end-diastolic volume (EDV), end-systolic volume (ESV), and LVM. Changes in different regions of the heart were quantified using wall thickness analyses. Results. LVM validation in healthy rats demonstrated high correlation between MR and postmortem values. Functional assessment at 4 weeks after MI revealed considerable reduction in EF, increases in ESV, EDV, and LVM, and contractile dysfunction in infarcted and noninfarcted regions. Conclusion. Clinical 3T MRI with a small animal coil and GE sequence generated images in a rat heart with adequate signal-to-noise ratio (SNR) for successful semiautomatic segmentation to accurately and rapidly evaluate long-term LVR after MI.
  • Thumbnail Image
    Item
    Open Access
    The pharmacological modification of reperfusion injury with particular reference to calcium fluxes in the isolated rat heart
    (1994) Du Toit, Eugene Francois; Opie, Lionel H
    Myocardial reperfusion injury is thought to be caused by reperfusion induced i) cytosolic Ca²⁺ overload and/or, ii) the formation of oxygen derived freeradicals. At the start of this study, data implicating cytosolic Ca²⁺ overload in the genesis of reversible reperfusion injury were inconclusive. Although several workers have approached this problem by measurements of cytosolic calcium ions, it was my aim to examine the potential sources of such calcium overload. The experiments reported in this thesis were therefore designed to examine the role of altered intracellular and transsarcolemmal Ca²⁺ fluxes in the genesis of reperfusion stunning and arrhythmias. The study was also aimed at elucidating the possible sources and entry pathways contributing to this proposed cytosolic Ca²⁺ overload. In order to investigate the possible role of altered reperfusion Ca²⁺ fluxes in reperfusion injury, we exposed the isolated working, and Langendorff perfused rat heart model to ischaemia and reperfusion to induce reperfusion stunning and arrhythmias. Hearts were pre-treated (before ischaemia) or reperfused with pharmacological compounds, or by interventions known to enhance or inhibit intracellular or transsarcolemmal Ca²⁺ fluxes. The severity of reperfusion stunning (mechanical dysfunction) was measured by reperfusion aortic output, coronary flow and left ventricular pressure. The incidence of reperfusion ventricular arrhythmias was measured by the incidence of ventricular tachycardia and/ or fibrillation. In selected studies, the metabolic status of hearts was evaluated using biochemical assays performed on myocardial tissue samples. Data obtained in these studies indicate that increased Ca²⁺ fluxes through sarcolemmal L-type Ca²⁺ channels during early reperfusion exacerbate stunning, while inhibition of these fluxes with the Ca²⁺ antagonist drug nisoldipine or by Mg²⁺ or Mn²⁺ improve reperfusion function. These data also suggest that although interventions increasing Ca²⁺ fluxes early in reperfusion exacerbate reperfusion stunning, these same interventions improve reperfusion function when performed later. The data also indicate that Ca²⁺ may enter the myocyte indirectly via activation of the Na⁺/H⁺ and Na⁺/Ca²⁺ exchanger during reperfusion. Inhibition of Na⁺/H⁺ exchange activity by HOE 694 during reperfusion attenuated reperfusion stunning and arrhythmias. Both activation of the Na⁺/H⁺ (and Na⁺/Ca²⁺) exchanger and Ca²⁺ influx via the Ca²⁺ channel could contribute to reperfusion induced Ca²⁺ overload and subsequent injury. The study also showed that altered intracellular Ca²⁺ oscillations play a role in reperfusion stunning and arrhythmias as shown by the use of the SR Ca²⁺ release channel blocker, ryanodine. Inhibition of the sarcoplasmic reticulum Ca²⁺ A TP-ase pump by two novel inhibitors, thapsigargin and cyclopiazonic acid, during ischaemia and early reperfusion improved reperfusion function and reduced the incidence of ventricular arrhythmias. function when unphysiologically high concentrations of the peptide were infused into the heart during reperfusion. Taken together, these data suggest that: 1) Ca²⁺ fluxes during early reperfusion (intracellular and transsarcolemmal) play a role in reperfusion injury, 2) that both the Ca²⁺ channel and Na⁺/H⁺ exchange activity contribute to reperfusion injury by possibly contributing to cytosolic Ca²⁺ overload and that, 3) altered intracellular Ca²⁺ oscillations through the SR play a role in both stunning and arrhythmias. Thus the proposal is that modulation of Ca²⁺ fluxes through either the sarcolemma or the sarcoplasmic reticulum, lessen reperfusion injury (stunning and arrhythmias). Although these data do not provide direct evidence of reperfusion Ca²⁺ overload, they support the concept that calcium ions play a role in the genesis of reversible reperfusion injury.
  • Thumbnail Image
    Item
    Open Access
    Rat angiotensin-converting enzyme : tissue specific expression during pharmacological inhibition
    (1995) Brice, Edmund Andrew William; Kirsch, Ralph E; Parker, M Iqbal
    The renin-angiotensin system plays a central role in the maintenance of blood pressure. Angiotensin II, the main effector of this system, results from the action of angiotensin-converting enzyme (ACE) on angiotensin I. Angiotensin II, maintains vasomotor tone via its vasoconstrictor action, and also increases salt and water retention by stimulating the release of aldosterone. ACE inhibitors, such as captopril, enalapril and lisinopril, are highly effective in the treatment of hypertension and congestive cardiac failure. Previous studies have suggested that angiotensin converting enzyme (ACE) production may be enhanced during pharmacological inhibition of the enzyme. Little is known, however about the mechanism of this induction. After demonstrating increases in circulating ACE protein in cardiac failure patients receiving the ACE inhibitor captopril, a rat model was used to study this effect. A sensitive enzyme linked immunosorbent assay for rat ACE was developed and a partial cDNA for rat ACE cloned to enable examination of ACE mRNA and protein expression during enzyme inhibition with enalapril. Rat lung ACE mRNA increased by 156% (p<0.05) and ACE protein doubled within 3 hours of administering a single dose of enalapril. Testicular ACE mRNA also increased by 300% (p<0.05) within 2 hours and returned to pretreatment levels by 6 hours. The angiotensin II antagonist saralasin similarly caused a significant (p<0.0001) 800% enhancement of mRNA expression. Aldosterone pretreatment of rats prior to enalapril administration was found to abolish this mRNA induction. These findings indicate that increased ACE expression during inhibition results from reduced levels of angiotensin II with consequent reduced stimulation of the angiotensin 11 receptor and its effects, such as aldosterone release. This suggests that ACE levels are regulated by a negative feedback loop involving the distal components of the renin-angiotensin system, namely angiotensin II and aldosterone. In situ hybridisation and immunohistochemical techniques were employed to localise the site of this inductive response in rat tissue sections. It was found that lung macrophages were markedly induced to produce ACE, as was ACE in seminiferous tubules. ACE induction was also noted in the expected sites of renal tubular epithelium and glomerular tissue. Interestingly, ACE expression was also enhanced in cardiac valves. In these studies it has been conclusively demonstrated that new ACE expression is induced by enzyme inhibitor therapy. A variety of techniques have been developed that will allow futher study of ACE in rat tissues.
  • Thumbnail Image
    Item
    Open Access
    Somatosensory processing by rat medial pontomedullary reticular formation neurones : responses to innocuous and noxious thermal and mechanical stimuli
    (1991) Farham, Craig Jeffrey; Douglas, Rodney J
    This work examines somatosensory processing in "giant" neurones of the medial pontomedullary reticular formation (PMRF) in the rat, with particular emphasis on the response to cutaneous thermal stimuli. Thermal test stimuli were employed as these were deemed to be more precisely quantifiable than other forms of cutaneous stimulation. Activity was recorded from 235 PMRF neurones in 94 female Long Evans rats (270 to 320 g) anaesthetised with urethane (1,25g/kg, i.p.). Rectal temperature was closely controlled at 38 ± 0,5°C. Standard stereotactic and extracellular recording techniques were employed. PMRF giant neurones were identified by their stereotactic location, large, stable spike amplitudes of long duration, responses to cutaneous mechanical stimuli and receptive field properties, and spontaneous discharge characteristics. Ramp, step and sine wave cutaneous thermal stimuli (35-48 °C) were applied to the glabrous skin on the hindpaw by means of a computer-controlled Peltier device. The location of the units was confirmed by subsequent histology. One hundred and eleven neurones were located in nucleus reticularis pontis caudalis (NPC), and 124 in nucleus reticularis gigantocellularis (NGC). Mechanical stimulation excited 188 of 235 (80%) PMRF neurones (ON-m cells), and inhibited 40 (17%, OFF-m cells). Seven cells (3%) had mosaic receptive fields of excitation and inhibition (complex responses, CX-m). Twenty-eight percent of neurones were responsive to both weak and intense stimuli (mixed neurones). The remainder (72%) responded only to intense mechanical stimulation of the skin (high threshold neurones). The (excitatory or inhibitory) response of the mixed neurones to intense stimuli was generally greater than to mild stimuli, Receptive fields ranged in size from restricted (hindlimbs only) to very extensive (covering the entire body surface). Neurones with small receptive fields were almost exclusively of the high threshold type, and tended to be located in NGC, while mixed neurones tended to have larger receptive fields, and were located predominantly in NPC. Some portion of the hind limbs were represented in the receptive fields of all but one of the neurones studied, while the tail and/ or trunk were represented in 77%, and the forelimbs and face in 28% of receptive fields. Most of the cells responding to cutaneous mechanical stimulation had bilateral (usually symmetric) receptive fields. Spontaneous (background) activity occurred in the absence of any deliberate sensory stimulation in 72% of PMRF neurones. The frequency of spontaneous discharge rates ranged from O to 47 spikes/ s. The coefficient of variation of the spontaneous discharge rate of a given neurone was generally less than 20% (range O to 85%). Of the 235 identified mechanosensitive PMRF neurones, 203 (86%) also responded to cutaneous thermal stimulation (43-48 °C) of the ipsilateral hind paw. Eighty percent of these responded with increased discharge rates (ON-t cells), and 20% were inhibited (OFF-t cells). The polarities of response of individual PMRF neurones to mechanical and thermal stimuli, and to repeated ipsilateral and contralateral thermal stimuli, did not differ significantly. Following transient thermal stimulation, spontaneous discharge rates largely returned to pre-stimulus levels. The thresholds of response to slow ramp (0,15°C/s) and stepped (2°C/s) thermal stimuli occurred both in the innocuous and noxious temperature ranges (below and above 42°C, respectively). The threshold temperatures showed large variability to repeated identical thermal stimuli. Despite the poor reproducibility of the threshold responses, the distribution of thresholds to thermal ramp stimuli was consistently bimodal, with peaks occurring at 39 and 43°C. The bimodality persisted even when the ipsilateral and contralateral data were pooled. The modes of these threshold distributions conform to the maximum discharge ranges for warm and noxious cutaneous receptors. Thus, it is likely that thermal input to individual PMRF neurones is derived from both types of receptors. The responses of PMRF neurones to repeated thermal stimuli were stable and reproducible with respect to magnitude and time course. The average (static) and maximum (dynamic) responses to thermal stimuli were generally small: for example, the mean of the average responses to ramp stimuli was 5,9 spikes/s ± 11,0 SD, (range -28 to 40 spikes/s), and the mean of the maximum responses was 9,3 spikes/s ± 16,1 SD, (range -46 to 65 spikes/s). The absolute change in firing rate of individual PMRF neurones, and of the population, increased monotonically as a function of the intensity of stepped cutaneous thermal stimuli in the range 40 to 48 °C. However, their resolution, based on their average and maximum responses, was poor. Incorporating the post-stimulus responses into the comparisons between different stimulus intensities marginally increased the resolution of these neurones. Thus, while the majority of PMRF neurones are able to distinguish innocuous from noxious stimuli, few are capable of encoding stimulus intensity within the noxious range (above 43 °C). The majority (70%) of PMRF neurones responded to sustained thermal stimuli with a slow increase or decrease to a new static discharge rate which was maintained with little or no adaptation. Latency to onset of response to stepped thermal stimuli varied from 1 to 50 seconds, and the time to maximal response between 5-60 seconds. Many PMRF neurones also showed marked after-discharge for periods of up to 5 minutes after removal of the stimulus. The thermal receptive fields of over 90% of PMRF neurones were large, incorporating at least both hindlimbs. The extensive receptive field sizes of individual PMRF neurones provides evidence against them having a role in stimulus location. The large number of PMRF neurones showing multimodal convergence, their small magnitude responses, their slow response times, and their large receptive fields strongly suggest that these neurones are not participating in classical sensory discrimination. Rather, they may function as stimulus detectors or alternatively play a role in associative processes.