Ca -Activated K Channels Underlying the Impaired Acetylcholine-Induced Vasodilation in 2K-1C Hypertensive Rats

We tested the hypothesis that an abnormal function of K channels in vascular smooth muscle cells plays a key role in the impaired acetylcholine (ACh) vasodilation in aortas from two kidney-one clip (2K-1C) hypertensive rats and further investigated the K channel subtype involved in this altered response. ACh-induced endothelium-dependent relaxation was assessed in aortic rings from 2K-1C and normotensive two kidney (2K) rats. Glibenclamide, an ATP-sensitive K channel blocker, did not inhibit ACh-induced relaxation in aortic rings from 2K or 2K-1C rats. The voltage-dependent K channels inhibitor 4-aminopyridine attenuated ACh-induced relaxation in both groups. Charybdotoxin and iberiotoxin, blockers of Ca -sensitive (KCa) and large-conductance KCa (BKCa) channels, respectively, reduced ACh-induced relaxation in aortic rings from 2K rats without affecting this response in those from 2K-1C rats, abolishing the differences between groups. ACh-induced relaxation in vessels from both 2K and 2K-1C rats was unaffected by apamin, a small-conductance KCa blocker. NS1619 [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one], an activator of KCa, induced a smaller vasodilation in endothelium-denuded aortic rings from 2K-1C rats compared with those from 2K rats. Iberiotoxin reduced sodium nitroprusside-induced relaxation in endothelium-denuded aortic rings from 2K without affecting this response in those from 2K-1C rats. The inhibition of Na ,K ATPase with ouabain had no effects on ACh-induced relaxation in aortic rings from 2K-1C or 2K rats. These data indicate that a deficient functional activity of BKCa channels plays a key role in the impaired ACh vasodilation in aortas from 2K-1C rats. Several agonists, including acetylcholine (ACh), hyperpolarize the vascular smooth muscle cell membrane in an endothelium-dependent manner by the release of mediators such as nitric oxide (NO) (Tare et al., 1990), endotheliumderived hyperpolarizing factor (Chen et al., 1988, 1991), and prostacyclin (Parkington et al., 1995). ACh-induced hyperpolarization is not generated in high extracellular K concentration ([K ]o) solution, indicating that vascular smooth muscle hyperpolarization by endothelial factors is mediated by K channels activation, which in turn closes voltagedependent Ca channels, reduces cytosolic Ca concentration, and induces vasodilation (Chen et al., 1991; Feletou and Vanhoutte, 1999; Coleman et al., 2001). NO and endothelium-derived hyperpolarizing factor-mediated responses can also involve the Na ,K -ATPase, which is found in the plasma membrane of vascular smooth muscle cells (Busse et al., 2002) and is thought to be critically involved in the maintenance of cellular ionic homeostasis needed to regulate membrane potential and vascular smooth muscle tone (Marı́n and Redondo, 1999). Altered vascular tone, a characteristic feature of most forms of experimental and human hypertension, has been associated with impaired endothelium-dependent vasodilation and reduced NO signaling (Puddu et al., 2000; Schiffrin, 2001; Taddei and Salvetti, 2002). We previously reported that endothelium-dependent vasodilation and smooth muscle cell hyperpolarization are impaired in aortic segments from two kidney-one clip (2K-1C) hypertensive rats (Callera et al., This study was supported by grants from Fundação de Amparo à Pesquisa do Estado de São Paulo, Conselho Nacional de Desenvolvimento Cientifico e Tecnológico, and Programa de Apoio a Núcleos de Excelência. Article, publication date, and citation information can be found at http://jpet.aspetjournals.org. DOI: 10.1124/jpet.103.062810. ABBREVIATIONS: ACh, acetylcholine; NO, nitric oxide; [K ]o, extracellular K concentration; 2K-1C, two kidney-one clip; 2K, two kidney; KV, voltage-dependent K channel; KATP, ATP-sensitive K channels; BKCa large-conductance Ca 2 -sensitive K channel; SKCa, small-conductance Ca -sensitive K channel; OUA, ouabain; KCa, Ca 2 -sensitive channel; SBP, systolic blood pressure; PSS, physiological salt solution; 4-AP, 4-aminopyridine; TEA, tetraethylammonium; ChTX, charybdotoxin; IbTX, iberiotoxin; SNP, sodium nitroprusside; dAUC, difference(s) of the area under the concentration-response curve(s). 0022-3565/04/3093-1036–1042$20.00 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 309, No. 3 Copyright © 2004 by The American Society for Pharmacology and Experimental Therapeutics 62810/1149028 JPET 309:1036–1042, 2004 Printed in U.S.A.