Non-specific antagonism against thromboxane A2 of 7-ethoxycarbonyl-6,8-dimethyl-4-hydroxymethyl-1(2H)-phthalazinone (EG-626) in contraction of isolated smooth muscles

7-Ethoxycarbonyl-6,8-dimethyl-4-hydroxymethyl-1(2H)-phthalazinone (EG-626) was reported as an antagonist of thromboxane (Tx) A2 in the contraction of rabbit aorta. It was, however, observed that EG-626 did inhibit the contraction of superfused rabbit aorta, but also did inhibit that of rabbit coeliac artery, rat stomach strip and rat colon induced by TxA2, PG endoperoxides, angiotensin II and PGF2 alpha in non-specific manner. EG-626 had no effect on the biosynthesis of PG endoperoxides as well as TxA2. These results indicate that EG-626 is not a TxA2 antagonist, but has a general inhibitory effect on the smooth muscles. This inhibitory effect of EG-626 may be explained by the inhibition of phosphodiesterase.