Doctoral Degrees (Medical Physiology)

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Browsing Doctoral Degrees (Medical Physiology) by browse.metadata.advisor "Du Toit, E. F."

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    The effect of creatine supplementation on myocardial metabolism and function in sedentary and exercised rats
    (Stellenbosch : University of Stellenbosch, 2010-12) Webster, Ingrid; Du Toit, E. F.; Huisamen, Barbara; University of Stellenbosch. Faculty of Health Sciences. Dept. of Biomedical Sciences. Medical Physiology.
    ENGLISH ABSTRACT: Background: There has been a dramatic increase in the use of dietary creatine supplementation among sports men and women, and by clinicians as a therapeutic agent in muscular and neurological diseases. The effects of creatine have been studied extensively in skeletal muscle, but knowledge of its myocardial effects is limited. Objectives: To investigate the effects of dietary creatine supplementation with and without exercise on 1) basal cardiac function, 2) susceptibility to ischaemia/reperfusion injury and 3) myocardial protein expression and phosphorylation and 4) mitochondrial oxidative function. Methods: Male Wistar rats were randomly divided into control or creatine supplemented groups. Half of each group was exercise trained by swimming for a period of 8 weeks, 5 days per week. At the end of the 8 weeks the open field test was performed and blood corticosterone levels were measured by RIA to determine whether the swim training protocol had any effects on stress levels of the rats. Afterwards hearts were excised and either freeze-clamped for biochemical and molecular analysis or perfused on the isolated heart perfusion system to assess function and tolerance to ischaemia and reperfusion. Five series of experiments were performed: (i) Mechanical function was documented before and after 20 minutes global ischaemia using the work heart model, (ii) A H2O filled balloon connected to a pressure transducer was inserted into the left ventricle to measure LVDP and ischaemic contracture in the Langendorff model, (iii) The left coronary artery was ligated for 35 minutes and infarct size determined after 30 minutes of reperfusion by conventional TTC staining methods. (iv) Mitochondrial oxidative capacity was quantified. (v) High pressure liquid chromatography (HPLC) and Western Blot analysis were performed on blood and heart tissue for determination of high energy phosphates and protein expression and phosphorylation. Results: Neither the behavioural studies nor the corticosterone levels showed any evidence of stress in the groups investigated. Hearts from creatine supplemented sedentary (33.5 ± 4.5%), creatine supplemented exercised rats (18.22 ± 6.2%) as well as control exercised rats (26.1 ± 5.9%) had poorer aortic output recoveries than the sedentary control group (55.9 ± 4.35% p < 0.01) and there was also greater ischaemic contracture in the creatine supplemented exercised group compared to the sedentary control group (10.4 ± 4.23 mmHg vs 31.63 ± 4.74 mmHg). There were no differences in either infarct size or in mitochondrial oxygen consumption between the groups. HPLC analysis revealed elevated phosphocreatine content (44.51 ±14.65 vs 8.19 ±4.93 nmol/gram wet weight, p < 0.05) as well as elevated ATP levels (781.1 ±58.82 vs 482.1 ±75.86 nmol/gram wet weight, p<0.05) in blood from creatine supplemented vs control sedentary rats. These high energy phosphate elevations were not evident in heart tissue and creatine tranporter expression was not altered by creatine supplementation. GLUT4 and phosphorylated AMPK and PKB/Akt were all significantly higher in the creatine supplemented exercised hearts compared to the control sedentary hearts. Conclusion: This study suggests that creatine supplementation has no effects on basal cardiac function but reduces myocardial tolerance to ischaemia in hearts from exercise trained animals by increasing the ischaemic contracture and decreasing reperfusion aortic output. Exercise training alone also significantly decreased aortic output recovery. However, the exact mechanisms for these adverse myocardial effects are unknown and need further investigation.
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    The impact of obesity and chronic PPAR Alpha agonist treatment on cardiac function, metabolism and ischaemic tolerance
    (Stellenbosch : Stellenbosch University, 2012-03) Smith, Wayne; Du Toit, E. F.; Lochner, Amanda; Stellenbosch University. Faculty of Health Sciences. Dept. of Biomedical Sciences. Medical Physiology.
    ENGLISH ABSTRACT: Background: Myocardial oxidative fuel supply is increased in obese conditions. How this metabolic environment and altered cardiometabolic phenotype associated with prediabetic obesity impacts on cardiac function and tolerance to ischaemia/reperfusion injury remains uncertain. While obese individuals are likely to be treated with PPARα agonists, controversy exists as to how activation of the PPARα receptor influences cardiovascular function and post-ischaemic recovery. Aims: To determine in a model of hyperphagia-induced obesity 1) whether protracted obesity is associated with left ventricular (LV) mechanical dysfunction; 2) the responsiveness of these hearts to insulin stimulation; 3) whether insulin can afford cardioprotection against ischaemia/reperfusion damage; and 4) how obesity and chronic PPARα agonist (K-111) treatment influences myocardial function, substrate metabolism, mitochondrial function and post-ischaemic outcomes. Methods: Male Wistar rats were fed standard rat chow or a high caloric diet. 1) In vivo LV mechanical function was assessed echocardiographically in 32 week fed animals. Ex vivo LV function was measured in the presence of glucose, insulin and/or fatty acid (FA); 2) Ex vivo myocardial insulin sensitivity was assessed by measuring insulin stimulated glycolytic flux in 16 week fed rats. Insulin was also administered prior to and during regional ischaemia to determine its effect on post-ischaemic function and infarct size; 3) K-111 was added to the drinking water during the last 10 weeks of feeding (feeding period of 18 weeks); a) Ventricular mitochondrial function was determined polarographically in the presence of either glutamate or palmitoyl-L-carnitine as substrates; b) Myocardial carbohydrate and lipid metabolism, and in a separate series of perfusions, myocardial infarct size were determined in the presence of physiological or high insulin (30 or 50μIU/ml) and FA (0.7 or 1.5mM) concentrations. Results: 1) Obese animals maintained normal in vivo LV mechanical function. Glucose perfused hearts from obese animals had depressed aortic outputs compared to the control group (32.58±1.2 vs. 46.17±0.91 ml/min; p<0.001) which was abolished by the presence of FA; 2) Hearts from obese animals had reduced insulin stimulated glycolytic flux rates (1.54±0.42 vs. 2.16±0.57 μmol/g ww/min, p<0.01). Although insulin reduced infarct size in the obese group (20.94±1.60 vs. 41.67±2.09 %, p<0.001), its cardioprotective effect was attenuated in the presence of FA; 3) By simulating the in vivo metabolic environment of control and obese animals in ex vivo perfusions, elevated insulin and FA levels associated with obesity increased infarct sizes in the obese group compared to the control group (47.44±3.13 vs. 37.17±2.63 %, p<0.05); 4) While chronic K-111 treatment reversed systemic metabolic abnormalities associated with obesity, neither obesity nor the drug influenced myocardial and mitochondrial function or postischaemic outcomes. K-111 was able to reduce palmitate oxidation in the obese group. Conclusion: Elevated levels of circulating FFA may be important in maintaining normal LV mechanical function in the obese condition. While obesity had no impact on myocardial mitochondrial function and post-ischaemic outcomes during comparable perfusion conditions, the specific metabolic environment associated with obesity may augment post-ischaemic injury. K-111 is effective in reducing obesity related metabolic abnormalities, but has no effects on myocardial function, mitochondrial function or ischaemic tolerance.