Differential effects of triiodothyronine on rat left and right ventricular function and the influence of metoprolol

The influence of triiodothyronine (T3) on right ventricular functional parameters was tested and compared to the alterations induced in the left ventricle. Female Sprague-Dawley rats received daily injections of T3 (0.2 mg/kg s.c.) for 3 days and a constant i.v. infusion of 0.9% NaCl or the beta 1-receptor blocker metoprolol (1 mg/kg/h). The hyperthyroid state after 3 days administration of T3 was characterized by an increase in heart rate and cardiac output and a decrease in the systemic peripheral resistance. However, pulmonary vascular resistance was unchanged. Beta-Receptor blockade reduced heart rate to the control level without affecting the elevation in cardiac output. The T3 group showed a marked increase in right ventricular systolic pressure (RVSP) and mean pulmonary artery pressure, whereas left ventricular systolic pressure (LVSP) and mean aortic pressure were not significantly changed. Metoprolol had no effect on LVSP, and attenuated the increase in RVSP. Furthermore, T3 induced a considerable increase in dP/dtmax in both ventricles, which was reduced nearly to control level by concomitant metoprolol infusion. The T3-induced percentage weight gain of the right ventricular free wall (RV) was more pronounced than that of the left ventricle (LV) as indicated by the significant increase in the RV/LV weight and the RNA/DNA ratios. Metoprolol did not affect significantly the T3-induced left and right ventricular hypertrophy, but attenuated slightly the elevated RV/LV weight ratio. Our results indicate a different action of T3 on systemic and pulmonary circulation.