Hypotensive Effects of Eugenosedin-A with Serotonin, Alpha- and Beta-Adrenoceptor Antagonistic Activities in Spontaneously Hypertensive and Normotensive Rats

Abstract

Eugenosedin-A is a newly synthesized compound with special serotonergic, α- and β₁-adrenergic blocking actions. Intravenous injection of eugenosedin-A significantly caused dose-dependent decreases in the mean arterial blood pressure and heart rate in normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). The effects of eugenosedin-A-decreased blood pressure and heart rate in SHR were more potent than in WKY. In in vitro experiments, eugenosedin-A competitively antagonized the serotonin-, norepinephrine- and clonidine-induced vasocontraction in a concentration-dependent manner in isolated thoracic aorta of WKY and SHR. We also observed that eugenosedin-A competitively antagonized the isoproterenol-induced positive inotropic effects in a concentration-dependent manner in the isolated left atrium of WKY and SHR. These findings clearly suggested that eugenosedin-A possesses α₁/α₂, β₁ and 5-HT_2A receptor-blocking activities. The order of pA₂ values in isolated tissues of WKY was 5-HT_2A > α₁/α₂ > β₁. However, the order of pA₂ values in isolated tissues of SHR was α₁/α₂ >5-HT_2A > β₁. Similarly, we found that the in vitro functional activity of eugenosedin-A is quite different between WKY and SHR. On the other hand, in the isolated rabbit ear artery sensitized with 16 mmol/l K⁺, eugenosedin-A antagonized 5-nonyloxytryptamine- and serotonin-induced vasocontractions, indicating that it also blocked 5-HT_1B and 5-HT_2A receptors. In radioligand binding experiments, eugenosedin-A had significant binding affinities on α₁/α₂, β₁, 5-HT_1B and 5-HT_2A receptors. Finally, we suggest that the hypotensive effects of eugenosedin-A can be attributed to its multiple actions on the blockade of 5-HT_1B, 5-HT_2A, α and β₁ receptors in both WKY and SHR strains.