Actions of prostaglandin I2 and thromboxane A2 on vascular smooth muscle tissues

Actions of prostaglandin I2 (PGI2) and thromboxane A2 (TXA2) on vascular smooth muscles were investigated in relation to the function of the endothelium. In intact vascular smooth muscle tissues of the thoracic aorta, PGI2-Na, used as a substitute substrate of PGI2, relaxed the precontracted tissue, in a dose dependent manner, in intact tissues and also after mechanical ablation of the endothelium. Acetylcholine (ACh) relaxed the tissue precontracted by noradrenaline, in the presence or absence of indomethacin; however, the relaxation required the presence of an intact endothelium. On the other hand, increased amounts of PGI2 with the application of acetylcholine, as estimated from the amount of 6-keto-PGF1 alpha, were markedly attenuated by indomethacin. In smooth muscles of this tissue without the endothelium, ACh synthesized lesser amounts of PGI2, while PGI2-Na increased the amount of cyclic AMP. Thus, PGI2 was synthesized in both the endothelium and smooth muscles. The former produces a larger amount of PGI2 than the latter, but the PGI2 synthesized in smooth muscles may act more potently on the smooth muscle than does that synthesized in the endothelium. In the canine coronary artery, mechanical responses induced by TXA2, as estimated from actions of 9, 11,-epithio-11, 12-methano-thromboxane A2 (STA2) were enhanced after ablation of the endothelium. The minimum concentration of STA2 required to produce the contraction was above 1 nM and the maximum amplitude was evoked with 30 nM. The amplitude of the STA2-induced contraction was reduced in Ca-free solution or with the application of nifedipine. However, prazosin, propranolol or atropine had no effect on the STA2-induced contraction.(ABSTRACT TRUNCATED AT 250 WORDS)