Thyroid control of sarcolemmal Na+/Ca2+exchanger and SR Ca2+-ATPase in developing rat heart

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Abstract
Thyroid hormone (TH) levels increase in the postnatal life and are essential for maturation of myocardial Ca2+handling. During this time, the sarcolemmal (SL) Na+/Ca2+exchanger (NCX) function decreases and the sarco endoplasmic reticulum (SR) Ca2+-ATPase (SERCA2) function increases. We examined the effects of postnatal hypo- or hyperthyroidism on NCX and SERCA2 in rat hearts. Animals were rendered hypothyroid by 0.05% 6- n-propyl-2-thiouracil in drinking water given to nursing mothers from days 2 to 21 postpartum. Hyperthyroidism was induced by daily injections of 10 μg/100 g body weight of 3,3′,5-triiodo-l-thyronine during this period. Ventricular steady-state mRNA and protein levels of NCX and SERCA2 were analyzed by Northern and Western blotting. These were compared with SL Na+gradient-induced and SR oxalate-supported Ca2+transports in isolated membranes. In hypothyroidism, NCX mRNA and protein were elevated by 66 and 80%, respectively, and SERCA2 mRNA and protein were reduced to 55 and 70%, respectively ( P < 0.05 vs. euthyroid). Corresponding differences were observed in the respective Ca2+transports. Conversely, reduced NCX (by 50%) and elevated SERCA2 (by 150%) activities were found in hyperthyroidism ( P < 0.05). The levels of NCX and SERCA2 mRNA and protein were, however, unchanged in hyperthyroidism, indicating that functional changes are not due to altered NCX and SERCA2 expression. In this case, a decline in noninhibitory phosphorylated phospholamban is a likely explanation for the elevated SR Ca2+transport. In conclusion, physiological TH levels appear to be essential for normal reciprocal changes in the expression and function of myocardial NCX and SERCA2 during postnatal development.

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