Effects of Prostaglandin I2 and a Selective Thromboxane Synthetase Inhibitor on the Arteriolar Constriction Caused by Norepinephrine

Abstract

Effects of prostaglandin (PG) I₂ and thromboxane (Tx) A₂ on the systemic blood pressure, the internal diameter of arterioles with a diameter of 20 to 30 μm, and the change caused by the topical administration of norepinephrine (NE) were studied in the rat mesentery. The change in diameter was measured by an image-shearing eyepiece and television microscopy. The rats were divided into three groups. Group I (9 rats) served as the non-treated, control group. In group II (6 rats), PGI₂ was given at the rate of 0.6 μg/kg/min for ten minutes. In group III (6 rats), (E)-3-[4-(1-imidazolylmethyl)phenyl]-2-propenoic acid hydrochloride monohydrate (OKY 046), which is a highly selective TxA ₂ synthetase inhibitor, was administered at the rate of 0.5 mg/kg/min for ten minutes. The hypotension caused by PGI₂ administration was accompanied by arteriolar constriction, although it disappeared with the increase in the systemic blood pressure five minutes after administration of PGI₂ was stopped. The systemic blood pressure and the internal diameter of arterioles showed no change in the OKY 046 and in the control group. The vasoconstrictive response to NE was inhibited by PGI₂ and OKY 046 for thirty to forty-five minutes and the inhibitory effects were similar in both cases. These results show that PGI₂ and a selective Tx synthetase inhibitor, OKY 046, attenuate the vascular response to NE.