Role of SK_Ca and IK_Ca in endothelium‐dependent hyperpolarizations of the guinea‐pig isolated carotid artery

Abstract

1 This study was designed to determine whether the endothelium-dependent hyperpolarizations evoked by acetylcholine in guinea-pig carotid artery involve a cytochrome P450 metabolite and whether they are linked to the activation of two distinct populations of endothelial K Ca channels, SK Ca and IK Ca. 2 The membrane potential was recorded in the vascular smooth muscle cells of the guinea-pig isolated carotid artery. All the experiments were performed in the presence of N o -L-nitro arginine (100 mM) and indomethacin (5 mM). 3 Under control conditions (Ca 2 þ : 2.5 mM), acetylcholine (10 nM to 10 mM) induced a concentrationand endothelium-dependent hyperpolarization of the vascular smooth muscle cells. Two structurally different specific blockers of SK Ca , apamin (0.5 mM) or UCL 1684 (10 mM), produced a partial but significant inhibition of the hyperpolarization evoked by acetylcholine whereas charybdotoxin (0.1 mM) and TRAM-34 (10 mM), a nonpeptidic and specific blocker of IK Ca, were ineffective. In contrast, the combinations of apamin plus charybdotoxin, apamin plus TRAM-34 (10 mM) or UCL 1684 (10 mM) plus TRAM-34 (10 mM) virtually abolished the acetylcholine-induced hyperpolarization. 4 In the presence of a combination of apamin and a subeffective dose of TRAM-34 (5 mM), the residual hyperpolarization produced by acetylcholine was not inhibited further by the addition of either an epoxyeicosatrienoic acid antagonist, 14,15-EEZE (10 mM) or the specific blocker of BK Ca , iberiotoxin (0.1 mM). 5 In presence of 0.5 mM Ca 2 þ , the hyperpolarization in response to acetylcholine (1 mM) was significantly lower than in 2.5 mM Ca 2 þ . The EDHF-mediated responses became predominantly sensitive to charybdotoxin or TRAM-34 but resistant to apamin. 6 This investigation shows that the production of a cytochrome P450 metabolite, and the subsequent activation of BK Ca , is unlikely to contribute to the EDHF-mediated responses in the guinea-pig carotid artery. Furthermore, the EDHF-mediated response involves the activation of both endothelial IK Ca and SK Ca channels, the activation of either one being able to produce a true hyperpolarization.

Mechanisms Underlying Enhanced Noradrenaline-Induced Femoral Arterial Contractions of Spontaneously Hypertensive Rats: Involvement of Endothelium-Derived Factors and Cyclooxygenase-Derived Prostanoids

“…20,30,39) At present, the best strategy for blocking EDHF signaling is the combined inhibition of small-and intermediate-conductance K Ca channels. 26,39,42) In the present study, we found that NAd-induced contraction in the presence of NOS inhibition was further amplified by additional co-treatment with inhibitors of small-and intermediate-conductance K Ca channels (apamin and TRAM-34, respectively) and this effect was greater in WKY rats than in SHR. Additionally, ACh-induced EDHF-type relaxation was impaired in the SHR femoral artery.…”

“…In addition to NO, EDHF plays an important role in the regulation of vascular tone. 20,39,42) Alterations in K + flux in both smooth muscle cells and endothelial cells are important in EDHF signaling; however, the precise mechanisms remain unclear because the signaling differs by vessel type, species, and disease state. 20,30,39) At present, the best strategy for blocking EDHF signaling is the combined inhibition of small-and intermediate-conductance K Ca channels.…”

Potentiation of EDHF-mediated relaxation by chloride channel blockers

“…Although the identification of EDHF remains under debate, K + , which effluxes from the endothelial cells through the Ca 2+ -activated K + channels (especially IK Ca and SK Ca channels) has been suggested as a proposed candidate of EDHF. Consistent to other studies [4] , we demonstrated that the EDHF-mediated relaxation could be abolished when the Ca 2+ -activated K + channels were blocked by apamin and charybdotoxin [17] . Indeed, the vasorelaxant effect of K + has been found for over 70 years [18] .…”

Reduced EDHF responses and connexin activity in mesenteric arteries from the insulin-resistant obese Zucker rat

“…Furthermore, 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34), a specific inhibitor of IK Ca channels, has been used in combination with apamin to confirm that IK Ca and SK Ca channels are involved in EDHFmediated vasodilation. The combination of TRAM-34 and apamin has been used to block EDHF responses in rat caudal [5], saphenous [6], middle cerebral [7] and mesenteric arteries [8] and guinea pig carotid arteries [9].…”