Possible role for K+ in endothelium-derived hyperpolarizing factor-linked dilatation in rat middle cerebral artery

Background and Purpose - Endothelium- derived hyperpolarizing factor ( EDHF) and K+ are vasodilators in the cerebral circulation. Recently, K+ has been suggested to contribute to EDHF-mediated responses in peripheral vessels. The EDHF response to the protease-activated receptor 2 ligand SLIGRL was characterized in cerebral arteries and used to assess whether K+ contributes as an EDHF. Methods - Rat middle cerebral arteries were mounted in either a wire or pressure myograph. Concentration-response curves to SLIGRL and K+ were constructed in the presence and absence of a variety of blocking agents. In some experiments, changes in tension and smooth muscle cell membrane potential were recorded simultaneously. Results - SLIGRL (0.02 to 20 mu mol/L) stimulated concentration and endothelium-dependent relaxation. In the presence of N-G-nitro-L-arginine methyl ester, relaxation to SLIGRL was associated with hyperpolarization and sensitivity to a specific inhibitor of IKCa, 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (1 mu mol/L), reflecting activation of EDHF. Combined inhibition of K-IR with Ba2+ (30 mu mol/L) and Na+/K+-ATPase with ouabain (1 mu mol/L) markedly attenuated the relaxation to EDHF. Raising extracellular [K+] to 15 mmol/ L also stimulated smooth muscle relaxation and hyperpolarization, which was also attenuated by combined application of Ba2+ and ouabain. Conclusions - SLIGRL evokes EDHF-mediated relaxation in the rat middle cerebral artery, underpinned by hyperpolarization of the smooth muscle. The profile of blockade of EDHF-mediated hyperpolarization and relaxation supports a pivotal role for IKCa channels. Furthermore, similar inhibition of responses to EDHF and exogenous K+ with Ba2+ and ouabain suggests that K+ may contribute as an EDHF in the middle cerebral artery.