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BACKGROUND Apelin is a newly discovered peptide hormone and originally discovered endogenous apelin receptor ligand. OBJECTIVE In this study, we aimed to investigate the possible roles of potassium channel subtypes in the vasorelaxant effect mechanisms of apelin. METHODS The vascular rings obtained from the thoracic aortas of the male Wistar Albino rats were placed into the isolated tissue bath system. The resting tension was set to 2 g. The aortic rings were precontracted with 10-5 M phenylephrine (PHE) or 45 mM KCl after the equilibration period. Pyroglutamyl-apelin-13 ([Pyr1]apelin-13), which is the dominant apelin isoform in the human cardiovascular tissues and human plasma, was applied cumulatively (10-10-10-6 M) to the aortic rings in the plateau phase. The experimental protocol was repeated in the presence of specific K+ channel subtype blockers to determine the role of K+ channels in the vasorelaxant effect mechanisms of apelin. RESULTS [Pyr1]apelin-13 induced a concentration-dependent vasorelaxation (p < 0.001). The maximum relaxation level was approximately 52% according to PHE-induced contraction. Tetraethylammonium, iberiotoxin, 4-Aminopyridine, glyburide, anandamide, and BaCl2 statistically significantly decreased the vasorelaxant effect level of [Pyr1]apelin-13 (p < 0.001). However, apamin didn't statistically significantly change the vasorelaxant effect level of [Pyr1]apelin-13. CONCLUSION In conclusion, our findings suggest that BKCa, IKCa, Kv, KATP, Kir, and K2P channels are involved in the vasorelaxant effect mechanisms of apelin in the rat thoracic aorta.