Contributory role of endothelium and voltage-gated potassium channels in apocynin-induced vasorelaxations.
OBJECTIVE
Although apocynin, the nicotinamide adenine dinucleotide phosphate oxidase inhibitor improves vascular function in hypertension, its specificity has been questioned. The present study examined whether apocynin-induced vasorelaxations involve endothelium and/or K channels in vascular cells.


METHODS
Aortas from Sprague-Dawley rats were suspended in organ baths for functional studies. Changes in intracellular calcium ([Ca]i) and nitric oxide ([NO]i) in rat endothelial cells were detected by fluorescence imaging. Whole-cell voltage-gated K (Kv) currents were recorded in vascular smooth muscle cells.


RESULTS
Apocynin-induced aortic relaxations were attenuated by the absence of endothelium, endothelial nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester, or nitric oxide-dependent soluble guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one. 4-Aminopyridine (4-AP, voltage-gated potassium channels blocker) attenuated apocynin-induced relaxations, its combined treatment with 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (ODQ) did not cause further inhibition. Apocynin increased [Ca]i and [NO]i in endothelial cells, which were abolished by 4-AP, indicating the involvement of voltage-gated potassium channels in endothelial cells. In addition, apocynin-stimulated increase in endothelial nitric oxide synthase phosphorylation at Ser depended on the presence of extracellular Ca. Notably, 4-AP also reduced apocynin-induced relaxations in the absence of endothelium and apocynin increased 4-AP-sensitive voltage-gated potassium channel currents in vascular smooth muscle cells.


CONCLUSIONS
This study provides novel data showing that apocynin-induced relaxations of rat aortas are mediated by 4-AP-sensitive stimulation of [Ca]i and [NO]i increases in endothelium and by activation of voltage-gated potassium channels in vascular smooth muscle cells.
Han
Wei-qing
WQ
State Key Laboratory of Medical Genomics, Department of Hypertension, Ruijin Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China.
Wong
Wing Tak
WT
Tian
Xiao Yu
XY
Huang
Yu
Y
Wu
Ling-yun
LY
Zhu
Ding-liang
DL
Gao
Ping-jin
PJ
D000098:Acetophenones / Q000494:pharmacology*; D000631:Aminopyridines / Q000494:pharmacology; D000818:Animals; D000975:Antioxidants / Q000494:pharmacology; D001011:Aorta / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D004730:Endothelium, Vascular / Q000166:cytology / Q000502:physiology*; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D008297:Male; D023421:Models, Animal; D009569:Nitric Oxide / Q000378:metabolism; D018408:Patch-Clamp Techniques; D026902:Potassium Channel Blockers / Q000494:pharmacology; D024642:Potassium Channels, Voltage-Gated / Q000037:antagonists & inhibitors / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D014664:Vasodilation / Q000187:drug effects* / Q000502:physiology*
D000098:Acetophenones; D000631:Aminopyridines; D000975:Antioxidants; D004791:Enzyme Inhibitors; D026902:Potassium Channel Blockers; D024642:Potassium Channels, Voltage-Gated; D009569:Nitric Oxide; C056165:acetovanillone; D002118:Calcium