Mechanisms Underlying the Relaxation Induced by Bradykinin in Rat Aorta

Bradykinin (BK) plays a critical role in intervening relaxation in vascular smooth muscle through distinctive mechanisms. The current project is designed to study the role of PKA, calcium-dependent potassium (BKca, IKca, and SKca) channels, voltage-dependent potassium channel (Kv) and inward rectifier potassium channel (Kir) in BK interceded response and to determine functionally the mechanisms underlying BK mediated relaxation, utilizing standard tissue bath protocols. The results of the current work showed that BK inducing vasorelaxation is mediated by PKA which activate eNOS (endothelial nitric oxide synthase) followed by a subsequent release of nitric oxide (NO) from endothelial cells since the protein kinase inhibitor (H-89) significantly reduced E_maxand pIC50. Similarly, in rings pre-incubated with L-NAME (3×10⁻⁴M) significantly inhibited vasorelaxation induced by BK. Furthermore, both Kca and Kv played an interesting role in vasorelaxation mediated by BK (1×10⁻⁶ to 3×10⁻³ M) in phenylephrine (PE) precontracted aortic rings, pre-incubated with tetraethylammonium (TEA) which reduced E_maxwith a slight reduction of pIC50 as compared to the control. Also, aortic rings pre-incubated with IK(Ca) blocker (clotrimazole) markedly reduced E_maxbut with a tendency of pIC50 elevation. In the present study, voltage-dependent potassium channels blocker (4-AP) significantly reduced the relaxant effects of BK and reduced E_maxwith altering pIC50 values significantly. On the other hand, BaCl₂tends to enhance the relaxation induced by BK and significantly elevated E_max. Collectively, our findings indicate that BK-induced relaxation is dependent on the activation of PKA to release NO from endothelium which through a cascade of a signaling pathway, in turn, activate both IK(Ca) and Kv in rat artic vascular tissue.