Endothelial Calcium-activated Potassium Channels Research Articles

Differential contribution of renal cytochrome P450 enzymes to kidney endothelial dysfunction and vascular oxidative stress in obesity

Arachidonic acid (AA)-derived cytochrome P450 (CYP) derivatives, epoxyeicosatrienoic acids (EETs) and 20-hidroxyeicosatetranoic acid (20-HETE), play a key role in kidney tubular and vascular functions and blood pressure. Altered metabolism of CYP epoxygenases and CYP hydroxylases has differentially been involved in the pathogenesis of metabolic disease-associated vascular complications, although the mechanisms responsible for the vascular injury are unclear. The present study aimed to assess whether obesity-induced changes in CYP enzymes may contribute to oxidative stress and endothelial dysfunction in kidney preglomerular arteries. Endothelial function and reactive oxygen species (ROS) production were assessed in interlobar arteries of obese Zucker rats (OZR) and their lean counterparts lean Zucker rats (LZR) and the effects of CYP2C and CYP4A inhibitors sulfaphenazole and HET0016, respectively, were examined on the endothelium-dependent relaxations and O2− and H2O2 levels of preglomerular arteries. Non-nitric oxide (NO) non-prostanoid endothelium-derived hyperpolarization (EDH)-type responses were preserved but resistant to the CYP epoxygenase blocker sulfaphenazole in OZR in contrast to those in LZR. Sulfaphenazole did not further inhibit reduced arterial H2O2 levels, and CYP2C11/CYP2C23 enzymes were downregulated in intrarenal arteries from OZR. Renal EDH-mediated relaxations were preserved in obese rats by the enhanced activity and expression of endothelial calcium-activated potassium channels (KCa). CYP4A blockade restored impaired NO-mediated dilatation and inhibited augmented O2− production in kidney arteries from OZR. The current data demonstrate that both decreased endothelial CYP2C11/ CYP2C23-derived vasodilator H2O2 and augmented CYP4A-derived 20-HETE contribute to endothelial dysfunction and vascular oxidative stress in obesity. CYP4A inhibitors ameliorate arterial oxidative stress and restore endothelial function which suggests its therapeutic potential for the vascular complications of obesity-associated kidney injury.

Acute activation of endothelial AMPK surprisingly inhibits endothelium-dependent hyperpolarization-like relaxations in rat mesenteric arteries.

Endothelium-dependent hyperpolarizations (EDHs) contribute to the regulation of peripheral resistance. They are initiated through opening of endothelial calcium-activated potassium channels (KCa ); the potassium ions released then diffuse to the underlying smooth muscle cells, causing hyperpolarization and thus relaxation. The present study aimed to examine whether or not AMPK modulates EDH-like relaxations in rat mesenteric arteries. Arterial rings were isolated for isometric tension recording. AMPK activity and protein level were measured by ELISA and western blotting respectively. The AMPK activator, AICAR, reduced ACh-induced EDH-like relaxations and increased AMPK activity in preparations with endothelium; these responses were prevented by compound C, an AMPK inhibitor. AICAR inhibited relaxations induced by SKA-31 (opener of endothelial KCa ) but did not affect potassium-induced, hyperpolarization-attributable relaxations or increase AMPK activity in preparations without endothelium. A769662, another AMPK activator, not only caused a similar inhibition of relaxations to ACh and SKA-31 in preparations with endothelium but also inhibited hyperpolarization-attributable relaxations and augmented AMPK activity in rings without endothelium. Protein levels of total AMPKα, AMPKα1, or AMPKβ1/2 were comparable between preparations with and without endothelium. Activation of endothelial AMPK, by either AICAR or A769662, acutely inhibits EDH-like relaxations of rat mesenteric arteries. Furthermore, A769662 inhibits signalling downstream of smooth muscle hyperpolarization. In view of the major blunting effect of AMPK activation on EDH-like relaxations, caution should be applied when administering therapeutic agents that activate AMPK in patients with endothelial dysfunction characterized by reduced production and/or bioavailability of NO.

The role of potassium channels in the endothelial dysfunction induced by periodontitis.

Objective:Periodontitis is associated with endothelial dysfunction, which is clinically characterized by a reduction in endothelium-dependent relaxation. However, we have previously shown that impairment in endothelium-dependent relaxation is transient. Therefore, we evaluated which mediators are involved in endothelium-dependent relaxation recovery.Material and methods:Rats were subjected to ligature-induced experimental periodontitis. Twenty-one days after the procedure, the animals were prepared for blood pressure recording, and the responses to acetylcholine or sodium nitroprusside were obtained before and 30 minutes after injection of a nitric oxide synthase inhibitor (L-NAME), cyclooxygenase inhibitor (Indomethacin, SC-550 and NS- 398), or calcium-dependent potassium channel blockers (apamin plus TRAM- 34). The maxilla and mandible were removed for bone loss analysis. Blood and gingivae were obtained for C-reactive protein (CRP) and myeloperoxidase (MPO) measurement, respectively.Results:Experimental periodontitis induces bone loss and an increase in the gingival MPO and plasmatic CRP. Periodontitis also reduced endothelium-dependent vasodilation, a hallmark of endothelial dysfunction, 14 days after the procedure. However, the response was restored at day 21. We found that endothelium-dependent vasodilation at day 21 in ligature animals was mediated, at least in part, by the activation of endothelial calcium-activated potassium channels.Conclusions:Periodontitis induces impairment in endothelial-dependent relaxation; this impairment recovers, even in the presence of periodontitis. The recovery is mediated by the activation of endothelial calcium-activated potassium channels in ligature animals. Although important for maintenance of vascular homeostasis, this effect could mask the lack of NO, which has other beneficial properties.

Vascular potassium channels in NVC.

It has long been proposed that the external potassium ion ([K(+)]0) works as a potent vasodilator in the dynamic regulation of local cerebral blood flow. Astrocytes may play a central role for producing K(+) outflow possibly through calcium-activated potassium channels on the end feet, responding to a rise in the intracellular Ca(2+) concentration, which might well reflect local neuronal activity. A mild elevation of [K(+)]0 in the end feet/vascular smooth muscle space could activate Na(+)/K(+)-ATPase concomitant with inwardly rectifying potassium (Kir) channels in vascular smooth muscle cells, leading to a hyperpolarization of vascular smooth muscle and relaxation of smooth muscle actin-positive vessels. Also proposed notion is endothelial calcium-activated potassium channels and/or inwardly rectifying potassium channel-mediated hyperpolarization of vascular smooth muscle. A larger elevation of [K(+)]0, which may occur pathophysiologically in such as spreading depression or stroke, can trigger a depolarization of vascular smooth muscle cells and vasoconstriction instead.

Role of in vivo vascular redox in resistance arteries.

Vascular thiol redox state has been shown to modulate vasodilator functions in large conductance Ca2+ -activated K+ channels and other related channels. However, the role of vascular redox in small resistance arteries is unknown. To determine how in vivo modulation of thiol redox state affects small resistance arteries relaxation, we generated a transgenic mouse strain that overexpresses thioredoxin, a small redox protein (Trx-Tg), and another strain that is thioredoxin-deficient (dnTrx-Tg). The redox state of the mesenteric arteries (MAs) in Trx-Tg mice is found to be predominantly in reduced state; in contrast, MAs from dnTrx-Tg mice remain in oxidized state. Thus, we created an in vivo redox system of mice and isolated the second-order branches of the main superior MAs from wild-type, Trx-Tg, or dnTrx-Tg mice to assess endothelium-dependent relaxing responses in a wire myograph. In MAs isolated from Trx-Tg mice, we observed an enhanced intermediate-conductance Ca2+ -activated potassium channel contribution resulting in a larger endothelium-dependent hyperpolarizing (EDH) relaxation in response to indirect (acetylcholine) and direct (NS309) opening of endothelial calcium-activated potassium channels. MAs derived from dnTrx-Tg mice showed both blunted nitric oxide-mediated and EDH-mediated relaxation compared with Trx-Tg mice. In a control study, diamide decreased EDH relaxations in MAs of wild-type mice, whereas dithiothreitol improved EDH relaxations and was able to restore the diamide-induced impairment in EDH response. Furthermore, the basal or angiotensin II-mediated systolic blood pressure remained significantly lower in Trx-Tg mice compared with wild-type or dnTrx-Tg mice, thus directly establishing redox-mediated EDH in blood pressure control.

Activation of Endothelial Calcium-Activated Potassium Channels Limits Nerve-Mediated Vasoconstriction in Resistance Arteries

Nerve-mediated vasoconstriction of resistance arteries is modulated by the endothelium. Increases in shear stress exerted by the blood on the endothelium triggers release of nitric oxide (NO) to limit further reductions in vessel diameter. In addition to releasing NO, increases in shear stress also activate endothelial Ca2+-activated potassium (KCa) channels. The resultant membrane hyperpolarization spreads electrotonically via gap junctions to the underlying smooth muscle cells to reduce Ca2+ influx and so attenuate vasoconstriction.

Endothelial calcium-activated potassium channels as therapeutic targets to enhance availability of nitric oxide

The vascular endothelium plays a critical role in vascular health by controlling arterial diameter, regulating local cell growth, and protecting blood vessels from the deleterious consequences of platelet aggregation and activation of inflammatory responses. Circulating chemical mediators and physical forces act directly on the endothelium to release diffusible relaxing factors, such as nitric oxide (NO), and to elicit hyperpolarization of the endothelial cell membrane potential, which can spread to the surrounding smooth muscle cells via gap junctions. Endothelial hyperpolarization, mediated by activation of calcium-activated potassium (K(Ca)) channels, has generally been regarded as a distinct pathway for smooth muscle relaxation. However, recent evidence supports a role for endothelial K(Ca) channels in production of endothelium-derived NO, and indicates that pharmacological activation of these channels can enhance NO-mediated responses. In this review we summarize the current data on the functional role of endothelial K(Ca) channels in regulating NO-mediated changes in arterial diameter and NO production, and explore the tempting possibility that these channels may represent a novel avenue for therapeutic intervention in conditions associated with reduced NO availability such as hypertension, hypercholesterolemia, smoking, and diabetes mellitus.

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

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, S-nitroso-N-acetyl-penicillamine. Relaxation to carbachol was attenuated following incubation with L-NOARG/indomethacin in vessels from control subjects (P<0.01 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 (P<0.001 ANOVA). Endothelium-dependent relaxation is resistant to endothelial nitric oxide synthase inhibition but sensitive to blockade of calcium-activated 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)).