Renin-angiotensin system contribution to cardiac hypertrophy in experimental hyperthyroidism:: An echocardiographic study

The objective of this study was to evaluate, using echocardiography, the involvement of the renin-angiotensin system (RAS) in left ventricular (LV) hypertrophy development in experimental hyperthyroidism. Thyrotoxicosis was produced by a daily intraperitoneal injection of L-thyroxine (T-4), 0.1 mg/kg per day for 15 days in Wistar rats. Control (euthyroid) rats received intraperitoneal daily injection of the thyroxine solvent. Two series of experiments were per formed. In the first series, euthyroid (n = 10) and hyperthyroid (n = 14) rats were surgically prepared with a femoral artery catheter. After a 3-day recovery period, blood pressure and heart rate were measured and blood samples were collected in conscious and unrestrained rats. In the second series of experiment, measurement of LV geometry was realized with two-dimensional time-movement echocardiography on the 15th day of treatment in control conditions and after long-term treatment with the angiotensin II type 1 receptor antagonist valsartan (10 mg/kg per day for 15 days) in both euthyroid and hyperthyroid rats. The dose and duration of T-4 treatment was sufficient to induce a significant degree of hyperthyroidism with characteristic features including tachycardia, systolic hypertension, myocardial hypertrophy, hyperthermia, and weight loss. In addition, we measured an increase in free fractions of thyroid hormones, and a threefold increase in plasma lenin activity. Echocardiographic examinations in rats revealed a strong correlation between LV weight and echocardiographic LV mass. Hyperthyroid rats exhibited an increased LV mass with a marked increase in the LV end-diastolic posterior wall and septal thickness. Chronic treatment with valsartan prevented this concentric LV hypertrophy (p < 0.01), with full prevention of the LV posterior wall hypertrophy (p < 0.001) and decreased LV septal hypertrophy (p < 0.05). In conclusion, the cardiovascular alterations of hyperthyroidism were reproduced with thyroid hormone injections in rats. Activation of the RAS in hyperthyroid rats was accompanied by increased LV mass. Using valsartan, we demonstrated that the RAS impinged on the LV remodelling in our experimental hyperthyroidism model. A chronic treatment with an angiotensin II type I receptor antagonist prevented the development of the concentric LV hypertrophy associated with thyrotoxicosis.