Endothelium-dependent relaxation is resistant to inhibition of nitric oxide synthesis, but sensitive to blockade of calcium-activated potassium channels in essential hypertension

Abstract

In human essential hypertension (EH), endothelium-dependent relaxation can occur independent of nitric oxide (NO) and prostacyclin (PGI 2 ). Recent in vivo data suggest that rapid compensatory upregulation of endothelial cytochrome P450 epoxygenase 2C9 occurs to preserve vasorelaxation under conditions of decreased NO bioavailability. As one of the vascular actions of CYP2C9 is to modulate small and intermediate conductance endothelial calcium-activated potassium channels (SK Ca and IK Ca ), we examined whether endothelium-dependent relaxation is sensitive to inhibitors of these channels (apamin and charybdotoxin) in resistance-sized vessels from human with EH. Subcutaneous gluteal biopsies were performed on 12 humans with EH and 12 matched control subjects. Resistance arteries were dissected and relaxation responses to carbachol were assessed ex vivo using wire myography in the presence of: (i) N G -nitro-L-arginine (L-NOARG)/indomethacin; and (ii) apamin/charybdotoxin.Maximal carbachol relaxation was impaired in EH vs control subjects. No differences in responses were observed with the endothelium-independent agonist, Snitroso-N-acetyl-penicillamine. Relaxation to carbachol was attenuated following incubation with L-NOARG/indomethacin in vessels from control subjects (Po0.01 analysis of variance (ANOVA)), but not in vessels from patients with EH. The reverse pattern was seen following apamin/charybdotoxin with carbachol relaxation attenuated only in EH vessels (Po0.001 ANOVA). Endotheliumdependent relaxation is resistant to endothelial nitric oxide synthase inhibition but sensitive to blockade of calciumactivated potassium channels in human EH. Studies with more specific inhibitors are required to determine whether this response is mediated by endothelial potassium channel subtypes (SK Ca and IK Ca ).

The Intermediate-Conductance Ca2+-Activated K+ Channel (KCa3.1) in Vascular Disease

“…Therefore, as it relates to the rat circulatory system, the contribution of EDHF to vasorelaxation appears inversely related to vessel size. While numerous studies in different animal models have demonstrated a role of K Ca 3.1 in EDHF-mediated vasodilation [10,[27][28][29][30][31][32][33][34][35][36][37], few have investigated the role of K Ca 3.1 in NO-mediated vasodilation [28]. Sheng et al [26] recently demonstrated inhibition of NO production by the combination of apamin and charybdotoxin or TRAM-34, suggesting that K Ca 3.1 channels are necessary for NOmediated vasodilation.…”

“…Kohler et al [29] observed that K Ca 3.1 -/-mice not only had blunted acetylcholine-induced hyperpolarization of both endothelial and SMCs, but were also hypertensive, providing evidence for the role of K Ca 3.1 in EDHF-induced relaxation and consequent regulation of peripheral vascular resistance [29]. The delineation of the mechanism of K Ca 3.1 activation in endothelial-dependent dilation is challenging; as the response to EDHF is complex, putatively involving K + [34], epoxyeicosatrienoic acids (EETs) [39], cAMP, cytochrome P450 2C, hydrogen peroxide and gap junctions [37,40]. To add to the complexity, the relative contribution of each varies by species, vascular bed and pathology [39].…”

“…While numerous studies have examined the effect of K Ca 3.1 on vessel relaxation in animals, only recent investigations have demonstrated their role in human arteries [27,37], Gillham et al [27] demonstrated in human omental and myometrial arteries preconstricted with either arginine vasopressin or U46619 (thromboxane A2 agonist) that combinations of K Ca 2 and K Ca 3.1 channel subtypes were responsible for EDHF-induced relaxation [27]. Furthermore, Sainsbury et al [37] used apamin and charybdotoxin to demonstrate in resistance vessels from human patients with essential hypertension that endothelium-dependent relaxation relied on K Ca 2 and K Ca 3.1 channels, but was independent of nitric oxide synthase [37].…”

“…Since Garland and Plane [25] first demonstrated that endothelium-derived hyperpolarizing factor (EDHF)-mediated responses were blocked by apamin and charybdotoxin, numerous studies have demonstrated that K Ca 3.1 is important for endothelial cell hyperpolarization and vasodilation [10,[26][27][28][29][30][31][32][33][34][35][36][37][38]. Kohler et al [29] observed that K Ca 3.1 -/-mice not only had blunted acetylcholine-induced hyperpolarization of both endothelial and SMCs, but were also hypertensive, providing evidence for the role of K Ca 3.1 in EDHF-induced relaxation and consequent regulation of peripheral vascular resistance [29].…”

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].…”