Dihydropyridines inhibit acetylcholine-induced hyperpolarization in cochlear artery via blockade of intermediate-conductance calcium-activated potassium channels.
Acetylcholine (ACh) induces hyperpolarization and dilation in a variety of blood vessels, including the cochlear spiral modiolar artery (SMA) via the endothelium-derived hyperpolarization factor (EDHF). We demonstrated previously that the ACh-induced hyperpolarization in the SMA originated in the endothelial cells (ECs) by activating a Ca(2+)-activated K(+) channel (K(Ca)); the hyperpolarization in smooth muscle cells was mainly an electrotonic spread via gap junction coupling. In the present study, using intracellular recording, immunohistology, and vascular diameter tracking techniques on in vitro SMA preparations, we found that 1) ACh-induced hyperpolarization was suppressed by intermediate-conductance K(Ca) (IK) blockers clotrimazole (IC(50) = 116 nM) and nitrendipine and by the calmodulin antagonist trifluoperazine, but it was not suppressed by the big-conductance K(Ca) blocker iberiotoxin. The immunoreactivity to anti-SK4/IK1 antibody was localized mainly in ECs. 2) The three dihydropyridines--nifedipine, nitrendipine, and nimodipine--all concentration-dependently inhibited the ACh-induced hyperpolarization, with an IC(50) value of 455, 34, and 3.2 nM, respectively. 3) Among other L-type Ca(2+) channel (I(L)) blockers, 10 microM verapamil exerted a 20% inhibition on ACh-induced hyperpolarization, whereas diltiazem and the metal ion Ca(2+) channel blockers Cd(2+) and Ni(2+) had no effect. 4) Nitrendipine and charybdotoxin abolished ACh-induced dilation in the SMA. We conclude that ACh-induced hyperpolarization in the SMA is generated mainly by activation of the IK in the ECs, and dihydropyridines suppress the EDHF-mediated hyperpolarization by blocking the IK channel, not the I(L) channel. The clinical relevance of this dihydropyridine action is discussed.
Jiang
Zhi-Gen
ZG
Oregon Hearing Research Center, Oregon Health and Science University, Portland, OR 97239, USA. jiangz@ohsu.edu
Shi
Xiao-Rui
XR
Guan
Bing-Cai
BC
Zhao
Hui
H
Yang
Yu-Qin
YQ
D000109:Acetylcholine / Q000494:pharmacology*; D000818:Animals; D001158:Arteries / Q000187:drug effects / Q000502:physiology; D001685:Biological Factors / Q000502:physiology; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D003051:Cochlea / Q000098:blood supply*; D004305:Dose-Response Relationship, Drug; D006168:Guinea Pigs; D007150:Immunohistochemistry; D051660:Intermediate-Conductance Calcium-Activated Potassium Channels / Q000032:analysis / Q000037:antagonists & inhibitors*; D008564:Membrane Potentials / Q000187:drug effects; D009568:Nitrendipine / Q000494:pharmacology*; D026902:Potassium Channel Blockers / Q000494:pharmacology*; D014661:Vasoconstriction / Q000187:drug effects
D001685:Biological Factors; D002121:Calcium Channel Blockers; D051660:Intermediate-Conductance Calcium-Activated Potassium Channels; D026902:Potassium Channel Blockers; C067410:endothelium-dependent hyperpolarization factor; D009568:Nitrendipine; D000109:Acetylcholine