Characterization of K ATP-channels in rat basilar and middle cerebral arteries: Studies of vasomotor responses and mRNA expression

Abstract

Changes in the activity of K + channels represent a major mechanism that regulates vascular tone. Cerebrovascular adenosine 5′-triphosphate-sensitive K +(K ATP) channels were characterized in studies of the molecular expression and vasomotor reactivity to different K ATP channel openers in rat basilar and middle cerebral arteries. Both arteries showed strong mRNA expression of the subunits of the pore-forming inward-rectifying K + channel type 6.1 (Kir6.1), Kir6.2 and the connected sulfonylurea receptor (SUR) subunits, SUR1 and SUR2B, while only weak bands for SUR2A were seen. The K ATP channel openers induced relaxation of prostaglalndin F 2α-precontracted isolated basilar and middle cerebral arteries with the order of potency N-Cyano- N-(1,1-dimethylpropyl)- N″-3pyridylguanidine (P-1075) > levcromakalim > N-(4-Phenylsulfonylphenyl)-3,3,3-trifluoro-2-hydroxy-2-methylpropanamide (ZM226600) > pinacidil > diazoxide. The responses induced by levcromakalim, ZM226600 and diazoxide were significantly more potent in basilar arteries than in middle cerebral arteries, while pinacidil and P-1075 were equipotent. Endothelium removal decreased ( P < 0.05) the sensitivity (pIC 50) of basilar arteries, but not of middle cerebral arteries, to pinacidil, levcromakalim, P-1075 and ZM226600. The maximum relaxant response to P-1075 was stronger ( P < 0.005) in basilar arteries with endothelium than without endothelium. Correlation of the relaxant potency of K ATP channel openers in rat basilar and middle cerebral arteries with historical measurements of affinity obtained in COS-7 cell lines expressing either SUR1, SUR2A or SUR2B showed that vasodilatation by K ATP channel openers correlated with binding to either the SUR2A or the SUR2B subunit. Glibenclamide was a blocker of relaxation induced by pinacidil, levcromakalim, P-1075 and ZM226600 in basilar arteries. Only a weak antagonistic effect of glibenclamide on pinacidil-, levcromakalim- and ZM226600-induced relaxations was found in middle cerebral arteries. The subunit profile and the observed pharmacological properties suggest that the K ATP channels expressed in rat basilar and middle cerebral artery are likely to be composed of SUR2B co-associated with Kir6.1 or Kir6.2. In basilar arteries, but not in middle cerebral arteries, endothelial K ATP channels may be involved.