Thyroid hormones differentially affect sarcoplasmic reticulum function in rat atria and ventricles |
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Authors: | Kaasik Allen Minajeva Ave Paju Kalju Eimre Margus Seppet Enn K |
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Institution: | (1) Department of Pathophysiology, University of Tartu, EE2400 Tartu, Estonia |
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Abstract: | The present study was undertaken to compare the effects of hypothyroidism and hyperthyroidism on sarcoplasmic reticulum (SR) Ca2+-pump activity, together with assessment of the functional role of SR in providing activator Ca2+ under these altered thyroid states. In response to a shift from hypothyroid to hyperthyroid state, a 10 fold and 2 fold increase in SR Ca2+-pump activity in atria and ventricles, respectively, were observed. This was associated with the 8-9 fold increases in atrial contractility (+dT/dt) and relaxation (-dT/dt), but only with a 3-4 fold increase in their ventricular counterparts. Also, the recirculation fraction of activator Ca2+ (RFA) increased to a far greater extent in atria (4 fold) than in papillary muscles, and the relative increment in inhibition of developed tension by ryanodine became 3 times larger in atria than in papillary muscles. A positive force-frequency relationship (FFR) was observed in hypothyroid atria, whereas the hyperthyroid atria, hypothyroid and hyperthyroid papillary muscles showed a negative FFR. These results suggest the greater role of transsarcolemmal (SL) Ca2+ and smaller role of SR Ca2+ in activating contraction in hypothyroid atria compared to other preparations. Thyroid hormones decrease the contribution of SL and increase that of SR in providing activator Ca2+ to the greater extent in atria than in ventricles. This effect of thyroid hormones is based on larger stimulation of SR Ca2+-pump in atria compared to ventricles. |
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Keywords: | atria ventricles thyroid hormones sarcoplasmic reticulum phospholamban Ca2+-pump force-frequency relationships ryanodine effects recirculation fraction of activator Ca2+ |
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