[Renin-angiotensin system blocking agents reverse the myocardial hypertrophy in experimental hyperthyroid cardiomyopathy via altering intracellular calcium handling].

Objective: To explore the mechanisms of myocardial hypertrophy induced by Levothyroxine (L-Thy).

Methods: A rabbit model of hyperthyroidism was established by daily intraperitoneal injections of L-Thy (45 microg/kg per day) for 28 days. New Zealand rabbits were randomly divided into four groups (n = 10 each): control group, L-Thy group (L-Thy alone), imidapril group (L-Thy + 0.5 mg/kg imidapril), and valsartan group (L-Thy + 8 mg/kg valsartan). All rabbits were treated for 4 weeks. At the end of the treatments, all rabbits underwent echocardiography and IVS, LV and LVPW thickness were measured. Ventricular tissues were then collected.Cardiac hypertrophy index, cardiomyocyte diameter, structural and ultrastructural changes were obtained. Ventricular myocytes were isolated by enzymatic digestion method and intracellular Ca2+ concentration was determined with the fluorescent Ca2+ indicator Fluo3/AM and laser scanning confocal microscopy. Activity of Sarco/Endoplasmic reticulum Ca2+ ATPase (SERCA) was evaluated with P-NPP method.mRNA expression of L-type Ca2+ channel (LCC), ryanodine receptor (RyR), and SERCA was semi-quantified with RT-PCR. Protein of IP3R was localized by immunostaining and semi-quantified with pathological image analytic system.

Results: Compared with control group, rabbits treated with L-Thy displayed remarkable myocardial hypertrophy and morphological changes in both structure and ultrastructure levels. Increased intracellular Ca2+ concentration [(576.2 +/- 41.7) nmol/L vs. (314.6 +/- 35.6) nmol/L, P < 0.01] and decreased SERCA activity [(0.062 +/- 0.013) micromol x min(-1)xg(-1) vs. (0.133 +/- 0.022) micromol x min(-1)xg(-1), P < 0.01] were detected in L-Thy treated rabbits. RT-PCR analysis and (or) immunohistochemistry revealed decreased mRNA expression of LCC mRNA (0.48 +/- 0.11 vs. 0.75 +/- 0.16, P < 0.01) and increased RyR mRNA (1.19 +/- 0.21 vs. 0.73 +/- 0.15, P < 0.01), SERCA mRNA (1.01 +/- 0.08 vs. 0.76 +/- 0.09, P < 0.01) and IP3R protein (65.3 +/- 13.7 vs. 47.9 +/- 10.2, P < 0.01) expression in L-Thy treated rabbits. Both imidapril and valsartan could significantly attenuate cardiomyocyte hypertrophy and structural remodeling induced by L-Thy. Compared with L-Thy group, decreased intracellular Ca(2+) concentration [(376.4 +/- 32.5) nmol/L vs. (576.2 +/- 41.7) nmol/L, P < 0.01 and (392.6 +/- 41.2) nmol/L vs. (576.2 +/- 41.7) nmol/L, P < 0.01, respectively], and increased LCC mRNA (0.68 +/- 0.14 vs. 0.48 +/- 0.11, P < 0.01; 0.64 +/- 0.13 vs. 0.48 +/- 0.11, P < 0.01, respectively) and SERCA activity [(0.115 +/- 0.019) micromol x min(-1)xg(-1) vs. (0.062 +/- 0.013) micromol x min(-1)xg(-1), P < 0.01; (0.109 +/- 0.015) micromol x min(-1)xg(-1) vs. (0.062 +/- 0.013) micromol x min(-1)xg(-1), P < 0.01, respectively] were found in both imidapril and valsartan treated rabbits, but expression of RyR, SERCA and IP3R remained unchanged.

Conclusion: Intracellular Ca(2+) overload may play important roles in myocardial hypertrophy induced by L-Thy. Imidapril and valsartan may exert beneficial effects on hyperthyroid myocardial hypertrophy via altering intracellular calcium handling.