Tetrahydropapaveroline (THP), a condensation product of ethanol-derived acetaldehyde, has been shown to elicit a vasorelaxant response in rat thoracic aorta. This study examined the influence of hypertension on the THP-induced vasorelaxant responsiveness. Ring segments of thoracic aorta were isolated from Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) and isometric tension development was measured with a force transducer. In aorta, with or without intact endothelium, the contractile responses to potassium chloride (0-120 mM) were comparable between the WKY and the SHR groups. Hypertension did not affect the vasoconstrictive response to norepinephrine (0-10 microM) in vessels with intact endothelium, whereas it attenuated the norepinephrine-induced response in vessels without endothelium. THP (0.1-100 microM) elicited endothelium-intact as well as -denuded vasorelaxation in aorta from both WKY and SHR groups. Interestingly, the THP-induced endothelium-dependent vasorelaxation was significantly enhanced, whereas the THP-induced endothelium-independent vasorelaxation was not affected by hypertension. These data indicate that the THP-induced endothelium-dependent vasorelaxant response is altered by the hypertensive state.