Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains xv, 188 p.; also includes graphics (some col.) Includes bibliographical references (p. 166-188). Available online via OhioLINK's ETD Center

Diabetes is known to result in a large number of alterations which affect various systems and organs. One of the more prominent disorders associated with diabetes is that of cardiac disease. Clinically, diabetics suffer from morbidity and mortality of cardiac origin to a greater extent than the nondiabetic population. Various functional studies have also revealed that the efficiency of diabetic hearts to function as pumps is lower than that of normal hearts. Experimentally, myocardial function of either rats or dogs made diabetic with either streptozotocin (STZ) or alloxan has been studied and a depression clearly demonstrated in both the species. The abnormalities of cardiac function in experimental diabetes are accompanied by depression of various enzyme systems in the heart. These include the ability of the sarcoplasmic reticulum (SR) to take up calcium; the myosin and actomyosin ATPase activities; and the Na⁺, K⁺ ATPase activity. All these changes can be prevented and reversed by insulin treatment suggesting that the myocardial problems seen in STZ or alloxan diabetic animals are due to diabetes and not direct toxicities of the drugs. It is not known whether the beneficial effects of in vivo insulin treatment are due to its direct myocardial effects or whether they are secondary to its effects mediated via normalisation of metabolism in diabetic animals. Thus, in the first part of the present investigation, we examined the direct effects of insulin on hearts from either control or diabetic rats using the isolated working heart preparation. Rats made diabetic with STZ (55 mg/kg) were sacrificed either 3 days or 6 weeks after induction of the disease and their hearts isolated and perfused in the working heart mode. Glucose concentrations varying from 5mM to 20mM were used in the perfusion medium, either in the presence or absence of insulin (5mU/mL). Left ventricular function was expressed as left ventricular developed pressure (LVDP) and the rates of contraction and relaxation (positive and negative dP/dt respectively) at various left atrial filling pressures. Three days after injecting STZ into rats, the animals exhibited hypoinsulinemia, hyperglycemia and their body weights although not significantly different from those of control animals, tended to be lower than the body weights of controls. Animals treated in this manner did not exhibit depression of cardiac function when compared with the myocardial function of control rats. Hearts from control rats exposed to regular insulin in the presence of 5mM glucose exhibited values of contractility which were significantly greater as compared with those obtained from control rat hearts not exposed to the hormone. When insulin was perfused along with a higher concentration of glucose (10mM), function of control rat hearts was affected to a significant extent. As opposed to the effects on control rat hearts, insulin failed to increase contractility in hearts from 3 day diabetic rats when either 5 or 10mM glucose was used in the perfusion medium. The study was then repeated using animals which had been diabetic for six weeks. At the time of sacrifice, these animals were hypoinsulinemic, hyperglycemic and weighed significantly less than their age-matched controls. Analysis of cardiac function revealed a significant depression in diabetic rats as compared with controls. Increasing glucose concentrations from 5 to 20mM in the perfusion medium did not affect the function of either control or diabetic rat hearts. Perfusion with regular insulin increased contractility in control rat hearts; the increase in contractility was not affected by increasing the glucose concentration from 5 to 10mM. However, contractility of diabetic rat hearts was not affected by insulin perfusion when either 5 or 10mM glucose was used in the perfusion medium. In order to eliminate the possibility of involvement of glucagon (which may contaminate commercial insulin preparations) in the effects of insulin on control rat hearts, part of the study was repeated using glucagon - free insulin. While the glucagon - free insulin increased contractility in control rat hearts, diabetic rat hearts were not affected. These results are identical to those obtained with regular insulin, suggesting that the effects of insulin observed were due to insulin itself. Although insulin treatment prevents and reverses diabetes - induced myocardial alterations in the rats, due to its widespread metabolic effects, it is not a good tool for investigating the specific factors which cause the cardiac abnormalities. In addition, a major problem with insulin treatment clinically is the fact that hypoglycemia can be associated with it, inadequate control occurs in some diabetics and secondary complications, such as myocardial problems, occur despite insulin treatment. It is thus desirable to have treatments which selectively affect certain aspects of diabetes so that the suspected underlying causes can be corrected specifically and their significance in causing the myocardial problems assessed. It would also be useful to have drug treatments which could either substitute for insulin or could be used in addition to the peptide. We have thus studied the effectiveness of certain treatments in preventing diabetes - induced myocardial alterations. The first one used was methyl palmoxirate, a fatty acid analog which is reported to reduce blood glucose levels in diabetic rats and dogs. The glucose - lowering effect is mediated via inhibition of fatty acid metabolism due to inhibition of carnitine acyl transferase resulting in inhibition of acyl carnitine formation and eventually inhibition of fatty acid transport across the mitochondrial membrane. Rats were treated with the drug (25mg/kg/day p.o.) three days after they were injected with either STZ or buffer. The treatment was carried out for 6 weeks and cardiac performance was then assesed. Untreated and treated diabetic rats were hypoinsulinemic, hyperglycemic and hyperlipedemic at the time of sacrifice. Cardiac function, which was depressed in diabetic animals, was still depressed despite the methyl palmoxirate treatment. However, the ability of the myocardial sarcoplasmic reticulum (SR) to take up calcium, which was depressed in diabetic rats, was normal in treated diabetic rats. Also, the levels of long chain acyl carnitines (LCAC) in the myocardial SR were normalised by methyl palmoxirate treatment in diabetic rats. In an effort to normalise diabetes - induced myocardial alterations in rats, we then attempted a combination of either methyl palmoxirate or carnitine (as both can prevent the depression of SR calcium uptake) with thyroid hormone treatment (as it can normalise myosin ATPase depression in diabetic rat hearts). The treatment protocol was identical to that described above (30µg/kg/day s.c. T₃ was used). Although the general features of both control and diabetic animals were not affected by either of the combination treatments, cardiac dysfunction in diabetic rats was prevented by methyl palmoxirate and T₃ treatment. Carnitine and T₃ treatment, on the other hand, affected the function of diabetic rat hearts only at the lower left atrial filling pressures. These results suggest that the combination treatment of methyl palmoxirate and T₃ affect parameters besides SR calcium uptake and myosin ATPase. This is because the combination of carnitine and T₃, which also supposedly affects same parameters as the other combination, could not prevent the myocardial alterations. One of the possible reasons for the effectiveness of the combination of methyl palmoxirate and T₃ could be that animals treated with methyl palmoxirate derived at least part of their metabolic energy (especially at higher left atrial filling pressures) from glucose and thus reduced the oxygen demand at higher filling pressures as opposed to the untreated diabetic rat hearts which depended completely on fatty acids for their metabolic energy demands. / Pharmaceutical Sciences, Faculty of / Graduate

No description available.

No description available.

M.Sc.(Med.), Faculty of Health Sciences, University of the Witwatersrand, 2010 / The progression from compensated cardiac hypertrophy to decompensation and cardiac failure is accompanied by cardiac dilatation. As cardiac failure has a poor prognosis, it is imperative to prevent the progression to cardiac dilatation and heart failure. In this regard, an understanding of the mechanisms of cardiac dilatation is vital to guide optimal therapy to prevent heart failure. Although a number of factors have been shown to contribute to the development of cardiac dilatation, to date the role of alterations in cardiac myocyte dimensions remains unclear. Hence, the aim of the current study was to determine whether changes in cardiac myocyte dimensions contribute to the process of cardiac dilatation. Methods: Two models of cardiac dilatation in pressure-overload induced cardiac hypertrophy were assessed. One model was a natural progression model, in which 18 spontaneously hypertensive rats (SHR), were assessed at 23 months of age (an age when left ventricular hypertrophy is noted to have progressed to left ventricular decompensation, dilatation and heart failure in approximately 50% of rats). The second model, a pharmacological model, was induced in 14 month old SHR (n=9) by chronic beta-adrenoreceptor activation [0.02mg/kg isoproterenol (ISO) twice daily for 4.5 months]. Chronic beta-adrenoreceptor activation in SHR, enhances the progression from compensated left ventricular hypertrophy to left ventricular dilatation. Nine normotensive Wistar Kyoto (WKY) rats were the controls for both models. Left ventricular dilatation was defined as an increase in left ventricular radius determined at controlled filling pressures using piezo-electric transducers. The classification of rats as being in heart failure was based upon the presence of pleuropericardial effusions and / or atrial thrombi. Cardiac myocytes were isolated and dimensions determined using both light microscopy and flow cytometry. Results: Left ventricular radius was increased in SHR-Failure compared to SHR-Non-Failure (p<0.01), and in SHR-ISO compared to SHR-Control (saline administration) (p<0.01), hence confirming the presence of cardiac dilatation in both models. Although, cardiac myocyte length vi was increased in all SHR groups compared to WKY (p<0.001), no differences were observed between SHR-Failure and SHR-Non-Failure, or between SHR-ISO and SHR-Control. No differences in cell length:width ratios or in cell widths were evident between the groups. The flow cytometry data confirmed the results obtained for cardiac myocyte lengths using microscopy. Moreover, a linear correlation (r=0.46, p=0.002) between flow cytometry and microscopy cardiac myocyte lengths was observed. Importantly, no relationships were evident between left ventricular radius and cardiac myocyte length (r=0.12, p=0.42 and r=0.14, p=0.35 for microscopic and flow cytometry lengths respectively). Conclusion: The results from the present study show that although pressure-overload hypertrophy is associated with lengthening of cardiac myocytes, no further changes occur with cardiac dilatation. Hence, alterations in cardiac myocyte dimensions do not contribute to the development of cardiac dilatation in pressure-overload models.

No description available.

While it is clear that myocardium responds to mechanical stimuli, it is unknown whether myocytes transduce stress or strain. It is also unknown whether myofibers maintain lateral connectivity or move freely over one another when myocardium is deformed. Due to the lack of information about the relationship between macroscopic and cellular deformations, we sought to develop an experimental method to examine myocyte deformations and to determine their degree of affinity. A set of protocols was established for specimen preparation, image acquisition, and analysis, and two experiments were performed according to these methods. Results indicate that myocyte deformations are non-affine; therefore, some cellular rearrangement must occur when myocardium is stretched.

While it is clear that myocardium responds to mechanical stimuli, it is unknown whether myocytes transduce stress or strain. It is also unknown whether myofibers maintain lateral connectivity or move freely over one another when myocardium is deformed. Due to the lack of information about the relationship between macroscopic and cellular deformations, we sought to develop an experimental method to examine myocyte deformations and to determine their degree of affinity. A set of protocols was established for specimen preparation, image acquisition, and analysis, and two experiments were performed according to these methods. Results indicate that myocyte deformations are non-affine; therefore, some cellular rearrangement must occur when myocardium is stretched.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references (leaves 96-105). Also available in electronic version. Access restricted to campus users.

Thesis (M. Phil.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 108-139).