The thyroid hormones have direct effects on vascular smooth muscle and are potent vasorelaxants. In the present study, the effects of d- and l-thyroxine (d-T4 and l-T4), 3,5,3'-triiodo-d-thyronine (d-T3), and 3,5,3'-triiodo-l-thyronine (l-T3) on the isolated mesenteric artery of the rabbit and superprecipitation of actomyosin from bovine aorta were examined. These thyroid hormones dose dependently relaxed vascular strips previously contracted with 50 mM KCl in the presence of phentolamine (1 microM), propranolol (1 microM), and atropine (0.3 microM), and the order of the inhibitory potency was l-T4 greater than d-T4 greater than l-T3 greater than d-T3 for the contraction. Pretreatment with l-T4 (10 and 30 microM) inhibited the contractile response concomitant with the inhibition of the 20,000-Da myosin light chain phosphorylation, without significant suppression of the increase in La3+-resistant 45Ca influx and uptake (5 and 30 min) induced by 50 mM KCl, suggesting that the inhibitory effect of l-T4 may not be…