Triiodothyronine Acutely Increases Blood Flow in the Ventricles and Kidneys of Anesthesized Rabbits

Thyroid hormone (triiodothyronine [T₃]) has various nongenomic effects, including alterations in glucose and fatty acid metabolism, augmentation of intracellular Ca²⁺, enhancement of myocardial contractility, and vascular dilatation. However, its effect on regional blood flow remains to be established. We have measured the effect of T₃ on blood flow in major organs of anesthetized rabbits in vivo using the microsphere method. Under artificial respiration, nonradioactive microspheres (5 × 10⁵) labeled with barium were injected to measure blood flow at control level. Then, T₃ (50 µg/kg per milliliter) was administered and microspheres labeled with iodine (5 × 10⁵) were injected. The atria, ventricles, kidneys, and right upper limb were excised and their contents of microspheres were evaluated. Blood flow in the ventricles was significantly increased by T₃ (2.9 ± 0.3 versus 3.4 ± 0.3 mL/min per gram, vehicle versus T₃). Similarly, blood flow in the kidneys was significantly higher after T₃ injection (4.3 ± 0.5 versus 5.1 ± 0.5 mL/min per, vehicle versus T₃). The blood flow in the atria and skeletal muscles remained unchanged. These results indicate that the vasodilatory response to T₃ is not uniform and occurs preferentially in major organs such as cardiac ventricles and kidneys; this may be relevant to the T₃-induced improvement of cardiac function.