EETs relax airway smooth muscle via an EpDHF effect: BK_Cachannel activation and hyperpolarization

Abstract

Epoxyeicosatrienoic acids (EETs) are produced from arachidonic acid via the cytochrome P-450 epoxygenase pathway. EETs are able to modulate smooth muscle tone by increasing K(+) conductance, hence generating hyperpolarization of the tissues. However, the molecular mechanisms by which EETs induce smooth muscle relaxation are not fully understood. In the present study, the effects of EETs on airway smooth muscle (ASM) were investigated using three electrophysiological techniques. 8,9-EET and 14,15-EET induced concentration-dependent relaxations of the ASM precontracted with a muscarinc agonist (carbamylcholine chloride), and these relaxations were partly inhibited by 10 nM iberiotoxin (IbTX), a specific large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel blocker. Moreover, 3 microM 8,9- or 14,15-EET induced hyperpolarizations of -12 +/- 3.5 and -16 +/- 3 mV, with EC(50) values of 0.13 and 0.14 microM, respectively, which were either reversed or blocked on addition of 10 nM IbTX. These results indicate that BK(Ca) channels are involved in hyperpolarization and participate in the relaxation of ASM. In addition, complementary experiments demonstrated that 8,9- and 14,15-EET activate reconstituted BK(Ca) channels at low free Ca(2+) concentrations without affecting their unitary conductance. These increases in channel activity were IbTX sensitive and correlated well with the IbTX-sensitive hyperpolarization and relaxation of ASM. Together these results support the view that, in ASM, the EETs act through an epithelium-derived hyperpolarizing factorlike effect.

Mechanisms underlying epithelium-dependent relaxation in rat bronchioles: analogy to EDHF-type relaxation in rat pulmonary arteries

“…The epithelium possesses one or more factors that are important for the relaxation of the underlying bronchial smooth muscle layer (3,51). Thus, removing the epithelium reduces isoproterenol relaxation in canine bronchi (51).…”

“…In addition, a non-NO nonprostanoid epithelium-dependent hyperpolarizing factor (EpDHF) is also involved in epitheliumdependent relaxations. Epoxyeicosatrienoic acids (EETs) have been suggested to account for EpDHF-mediated relaxation in guinea pig airways (3). In comparison, opening of intermediate (IK Ca ) and endothelial subtype small (SK Ca 3) conductance calcium-activated potassium channels account for the endothelium-derived hyperpolarizing factor (EDHF)-type mechanisms leading to dilation in arteries (6,14,19).…”

Effects of ω-hydroxylase product on distal human pulmonary arteries

“…Since it has recently been reported that 20-HETE activates large-conductance Ca 2ϩ -activated K ϩ (BK Ca ) channels in human airway smooth muscle (ASM) (20), the relaxing effects of 20-HETE on HPA were assessed in the absence and presence of 10 nM IbTx, a specific BK Ca channels blocker (1). Figure 3A shows two sequential recordings in which IbTx pretreatment did not modify the contractile response to 1 M 5-HT but did result in a significant inhibition (30%) of the amplitude and rate of relaxation induced by 20-HETE compared with the control response.…”

Epoxyeicosatrienoic Acid Activates BK Channels in the Cortical Collecting Duct

“…15,17 BK channels are also expressed in the CCD 13,25 and have been suggested to be involved in mediating K secretion in the late distal tubule including connecting tubule (CT) in response to HK intake. 13,25 Thus, we examined whether CYP-epoxygenase-dependent AA metabolism regulates BK channels in the CCD.…”

“…3,10,11 Conversely, a large body of evidence has strongly indicated that Ca 2ϩ -activated BK channels are in-volved in K secretion in the CCD subjecting to high flow rates or in animals with increased K intake [12][13][14] ; however, the mechanism by which HK intake stimulates BK channel-dependent K secretion is not understood. Ca 2ϩ -activated BK channels are also expressed in smooth muscle cells and endothelial cells and have been shown to be activated by epoxyeicosatrienoic acid (EET), [15][16][17] a product generated by cytochrome P450 (CYP)-epoxygenase-dependent arachidonic acid (AA) metabolism. 18 CYP-epoxygenasedependent AA metabolites play an important role in the regulation of Na transport in the CCD.…”