Dihydropyridines Inhibit Acetylcholine-Induced Hyperpolarization in Cochlear Artery via Blockade of Intermediate-Conductance Calcium-Activated Potassium Channels

Abstract

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 2ϩ -activated K ϩ 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 ϭ 116 nM) and nitrendipine and by the calmodulin antagonist trifluoperazine, but it was not suppressed by the bigconductance 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 2ϩ channel (I L ) blockers, 10 M verapamil exerted a 20% inhibition on ACh-induced hyperpolarization, whereas diltiazem and the metal ion Ca 2ϩ channel blockers Cd 2ϩ and Ni 2ϩ 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.Several vasoactive agents, such as acetylcholine (ACh), substance P, and bradykinin, cause robust hyperpolarization and vasodilation when the endothelium is intact. The hyperpolarization and vasodilation have been attributed to endothelium-derived hyperpolarization factor (EDHF) or endothelium-derived relaxation factor (Faraci and Heistad, 1998;Busse et al., 2002). The EDHF is an important mechanism regulating blood flow to organs and tissues and is implicated in vascular pathology, such as ischemia, hypertension, atherosclerosis, diabetic and aging vascular malfunction (Faraci and Heistad, 1998), and coronary vascular spasms (Konidala and Gutterman, 2004).The EDHF seems to be a variable combination of gap junction coupling, endothelial release of K ϩ , NO, epoxyeicosatrienoic acids, and prostanoid in various vascular beds and in different animal species (Busse et al., 2002). We reported recently that ACh induced hyperpolarization and dilation in the cochlear spiral modiolar artery (SMA) via the EDHF (Jiang et al., 2005) and found that ACh-induced EDHF in the SMA was a complex. The induced hyperpolarization originated in the endothelial cell (EC) by activating Ca 2ϩ -acti-

2-Aminoethoxydiphenyl borate blocks electrical coupling and inhibits voltage-gated K+ channels in guinea pig arteriole cells

“…2-APB has long been known to block the inositol 1,4,5-trisphosphate receptor (IP 3 R) and store-operated Ca 2ϩ entry (42). These two Ca 2ϩ signaling pathways are functional mechanisms in VSMCs and ECs (18,30,33,38,46). Two compounds, nitrendipine and XeC, which act on two different IP 3 R signaling steps, were tested.…”

“…Conventional intracellular recordings were conducted from cells of the SMA, BA, and MA as previously described (33). Briefly, a 2-to 5-mm-long segment of the SMA or other arteriole branches of 40 -80 m in OD was pinned with minimum stretch to the silicon rubber layer (Sylgard 184, Dow Corning) in the bottom of the bath chamber (volume: 0.5 ml) and continuously superfused with Krebs solution at 35°C.…”

ACh-induced depolarization in inner ear artery is generated by activation of a TRP-like non-selective cation conductance and inactivation of a potassium conductance

“…It is now known that the EDHF is often a variable combination of gap junction coupling, endothelial release of K + , nitric oxide (NO), epoxyeicosatrienoic acids (EETs) and prostanoid in various vascular beds (Griffith, 2004;Jiang et al, 2005). In guinea pig cochlear SMA, we demonstrated that ACh primarily stimulates the intermediate conductance K Ca (IK) in endothelial cells (ECs) causing efflux of K + ion and thus hyperpolarizing these cells Jiang et al, 2007;Jiang et al, 2005). AChinduced hyperpolarization in the VSMC is mainly (60%) an electrical spread of the ECoriginated hyperpolarization via gap junctions; whereas, the K + release from the EC activates an inward rectifier potassium channel (K ir ) and the Na-K-ATP pump, causing the remaining 40% hyperpolarization of the VSMC.…”

Role of the potassium channel KCa3.1 in diabetic nephropathy

“…Although the exact role of EndMT in renal fibrosis is unclear, these studies all indicate that endothelial cells might be an important contributor to renal fibrosis. K Ca 3.1 has long been known to play a significant role in endothelial cell hyperpolarization and vasodilation [106][107][108][109][110][111][112], since K Ca 3.1 was first identified and characterized in endothelial cells by Cai et al [113] in 1998. K Ca 3.1 has been shown to be expressed in the vascular endothelium of rodents, humans and pigs [114,115].…”