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Studies on the effect of experimental insulin-dependent diabetes mellitus and hypothyroidism on rat cardiac and saroplasmic reticulum function Black, Shawn Clive


The objective of these studies was to investigate mechanisms whereby cardiac sarcoplasmic reticulum (SR) calcium transport activity may be influenced by changes in the lipid environment of the SR membrane in the experimental endocrine disease states hypothyroidism and insulin-dependent diabetes mellitus. These endocrine disease states were studied to determine, respectively, if SR function is influenced by endogenous acylcarnitine associated with the SR membrane and if SR phospholipid acyl composition plays a role in diabetes-induced cardiomyopathy. The effects of endogenous acylcarnitines on SR calcium transport in hypothyroidism were of interest since it has previously been implicated that acyl carnitines play a regulatory role in SR function. SR calcium transport was not affected at two weeks, but was significantly reduced at four, six and eight weeks following thyroidectomy. Endogenous acyl carnitines were detectable in the SR membrane fraction isolated from both euthyroid control and thyroidectomized animals. The level of acyl carnitine associated with the SR did not correlate with calcium transport activity. Since acylcarnitine did not appear to play a role in the reduced SR calcium transport, SR calcium pump protein was quantified. The reduced SR calcium transport of thyroidectomized animals, manifest at four weeks, was shown to correlate with a reduction in SR acylphosphoprotein level. Therefore the reduced SR calcium transport activity of hypothyroidism is not related to the level of SR acyl carnitine, but rather a hypothyroid-induced reduction in SR calcium pump sites. Since omega-3 fatty acids affect parameters relevant to diabetes-induced cardiomyopathy, it was of interest to determine the cardiac effects of omega-3 fatty acid treatment of streptozocin (STZ)-induced diabetic animals. Omega-3 fatty acid treatment significantly reduced the development of diabetic cardiomyopathy and improved isolated cardiac SR calcium transport activity of STZ-induced diabetic animals. To determine if the cardiac and SR changes were related to membrane changes induced by omega-3 fatty acids, the fatty acyl composition of phospholipid was determined. Phospholipid analysis of cardiac phosphatidylcholine and phosphatidylethanolamine, and total SR phospholipid indicated modest changes in the omega-3 fatty acid component. Omega-3 fatty acid treatment produced slight (statistically insignificant) changes in SR cholesterol levels. Therefore a change in membrane phospholipid acyl composition may not account for the observed cardiac and SR functional changes.

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