Regional heterogeneity in acetylcholine-induced relaxation in rat vascular bed: role of calcium-activated K+channels

Abstract
Ca+-activated K+-channels (KCa) regulate vasomotor tone via smooth muscle hyperpolarization and relaxation. The relative contribution of the endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation differs depending on vessel type and size. It is unknown whether these KCachannels are differentially distributed along the same vascular bed and hence have different roles in mediating the EDHF response. We therefore assessed the role of small- (SKCa), intermediate- (IKCa), and large-conductance (BKCa) channels in mediating acetylcholine-induced relaxations in both first- and fourth-order side branches of the rat superior mesenteric artery (MA1 and MA4, respectively). Two-millimeter segments of each MA were mounted in the wire myograph, incubated with Nω-nitro-l-arginine methyl ester (l-NAME, 100 μmol/l) and indomethacin (10 μmol/l), and precontracted with phenylephrine (10 μmol/l). Cumulative concentration-response curves to ACh (0.001–10 μmol/l) were performed in the absence or presence of selective KCachannel antagonists. Apamin almost completely abolished these relaxations in MA4 but only partially blocked relaxations in MA1. The selective IKCachannel blocker 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (TRAM-34) caused a significantly greater inhibition of the ACh-induced relaxation in MA4 compared with MA1. Iberiotoxin had no inhibitory effect in MA4 but blunted relaxation in MA1. Relative mRNA expression levels of SKCa(rSK1, rSK3, and rSK4 = rIK1) were significantly higher in MA4 compared with MA1. BKCa(rBKα1and rBKβ1) genes were similar in both MA1 and MA4. Our data demonstrate regional heterogeneity in SKCaand IKCafunction and gene expression and stress the importance of these channels in smaller resistance-sized arteries, where the role of EDHF is more pronounced.

This publication has 24 references indexed in Scilit: