Rosmarinic Acid Induces Vasorelaxation via Endothelium-Dependent, Potassium Channel-Related, and Calcium-Modulated Pathways: Evidence from Rat Aortic Rings

<b>Background:</b> Hypertension and its complications are a major global health problem, and natural compounds with vasorelaxant effects are being investigated as potential antihypertensive agents. <b>Objective:</b> This study aimed to determine whether rosmarinic acid (RA) induces vasorelaxation in the rat thoracic aorta and to elucidate the underlying mechanisms. <b>Methods:</b> Isolated thoracic aortic rings, with or without endothelium, were precontracted with phenylephrine and subsequently exposed to cumulative concentrations of RA. The roles of endothelium-derived factors, potassium channels, and calcium signaling were evaluated using selective pharmacological inhibitors and activators. In addition, the involvement of the AMPK pathway, adenylate cyclase/cAMP pathway, PKC signaling, β-adrenergic receptors, muscarinic receptors, and angiotensin II in RA-induced vasorelaxation was investigated. <b>Results:</b> RA induced a concentration-dependent vasorelaxation in endothelium-intact thoracic aortic rings (<i>p</i> < 0.001; pD₂ = 7.67 ± 0.04). The vasorelaxant effect of RA was attenuated in endothelium-denuded vessels (pD2: 5.26 ± 0.18). The relaxation response was significantly attenuated by inhibitors of the PI3K/Akt/eNOS/NO/cGMP pathway and by blockers of BK_Ca, IK_Ca, and Kv potassium channels (<i>p</i> < 0.001). Furthermore, RA markedly inhibited both extracellular Ca²⁺ influx and intracellular Ca²⁺ release from the sarcoplasmic reticulum (<i>p</i> < 0.001). RA incubation also significantly reduced the contractions induced by angiotensin II (Ang II) and by the PKC activator PMA (<i>p</i> < 0.001). Other tested pathways had no significant influence on the vasorelaxant effect of RA (<i>p</i> > 0.05). <b>Conclusions:</b> These findings demonstrate that rosmarinic acid induces both endothelium-dependent and endothelium-independent vasorelaxation in the rat thoracic aorta through activation of the PI3K/Akt/eNOS/NO/cGMP pathway, opening of BK_Ca, IK_Ca, and Kv potassium channels, and suppression of Ca²⁺ mobilization. Additionally, inhibition of PKC- and angiotensin II-mediated vascular contraction contributes to RA-induced vasorelaxation. RA may therefore have therapeutic potential in the management of hypertension.