To determine whether thyroid hormones, triiodothyronine (T3) and thyroxine (T4), 
have any direct, nongenomic effects on vascular smooth muscle cells, we 
evaluated the effects of these hormones on rat coronary arteries. Bolus 
injection of T3 or T4 elicited a transient, dose-dependent decrease in coronary 
perfusion pressure (CPP), as well as an increase in arterial vasodilation. 
Vasodilation occurred immediately after injection, peaked at 15 seconds, and 
lasted 80 seconds. Reverse T3 had no effect on CPP or vasodilation. The rapidity 
of these effects suggests that they are not mediated by the T3-nuclear receptor, 
but are direct, nongenomic effects of thyroid hormones. Our results also suggest 
that thyroid hormones may play a role in preventing myocardial ischemia by 
inducing coronary artery vasodilation.