Depolarization evoked by acetylcholine in mesenteric arteries of hypertensive rats attenuates endothelium-dependent hyperpolarizing factor

The conducted vasomotor response and the principles of electrical communication in resistance arteries

Biological tissues are fed by arterial networks whose task is to set blood flow delivery in accordance with energetic demand. Coordinating vasomotor activity among hundreds of neighboring segments is an essential process, one dependent upon electrical information spreading among smooth muscle and endothelial cells. The "conducted vasomotor response" is a functional expression of electrical spread, and it is this process that lies at the heart of this critical review. Written in a narrative format, this review first highlights historical manuscripts and then characterizes the conducted response across a range of preparations. Trends are highlighted and used to guide subsequent sections, focused on cellular foundations, biophysical underpinnings, and regulation in health and disease. Key information has been tabulated; figures reinforce grounding concepts and reveal a framework within which theoretical and experimental work can be rationalized. This summative review highlights that despite 30 years of concerted experimentation, key aspects of the conducted response remain ill defined. Of note is the need to rationalize the regulation and deterioration of conduction in pathobiological settings. New quantitative tools, along with transgenic technology, are discussed as a means of propelling this investigative field forward.

Chloride Ions, Vascular Function and Hypertension

Blood pressure is determined by cardiac output and systemic vascular resistance, and mediators that induce vasoconstriction will increase systemic vascular resistance and thus elevate blood pressure. While peripheral vascular resistance reflects a complex interaction of multiple factors, vascular ion channels and transporters play important roles in the regulation of vascular tone by modulating the membrane potential of vascular cells. In vascular smooth muscle cells, chloride ions (Cl⁻) are a type of anions accumulated by anion exchangers and the anion–proton cotransporter system, and efflux of Cl⁻ through Cl⁻ channels depolarizes the membrane and thereby triggers vasoconstriction. Among these Cl⁻ regulatory pathways, emerging evidence suggests that upregulation of the Ca²⁺-activated Cl⁻ channel TMEM16A in the vasculature contributes to the increased vascular contractility and elevated blood pressure in hypertension. A robust accumulation of intracellular Cl⁻ in vascular smooth muscle cells through the increased activity of Na⁺–K⁺–2Cl⁻ cotransporter 1 (NKCC1) during hypertension has also been reported. Thus, the enhanced activity of both TMEM16A and NKCC1 could act additively and sequentially to increase vascular contractility and hence blood pressure in hypertension. In this review, we discuss recent findings regarding the role of Cl⁻ in the regulation of vascular tone and arterial blood pressure and its association with hypertension, with a particular focus on TMEM16A and NKCC1.

Endothelium-Dependent Hyperpolarization (EDH) in Diabetes: Mechanistic Insights and Therapeutic Implications

Diabetes mellitus is one of the major risk factors for cardiovascular disease and is an important health issue worldwide. Long-term diabetes causes endothelial dysfunction, which in turn leads to diabetic vascular complications. Endothelium-derived nitric oxide is a major vasodilator in large-size vessels, and the hyperpolarization of vascular smooth muscle cells mediated by the endothelium plays a central role in agonist-mediated and flow-mediated vasodilation in resistance-size vessels. Although the mechanisms underlying diabetic vascular complications are multifactorial and complex, impairment of endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells would contribute at least partly to the initiation and progression of microvascular complications of diabetes. In this review, we present the current knowledge about the pathophysiology and underlying mechanisms of impaired EDH in diabetes in animals and humans. We also discuss potential therapeutic approaches aimed at the prevention and restoration of EDH in diabetes.

Endothelium-Dependent Hyperpolarization (EDH) in Hypertension: The Role of Endothelial Ion Channels

Upon stimulation with agonists and shear stress, the vascular endothelium of different vessels selectively releases several vasodilator factors such as nitric oxide and prostacyclin. In addition, vascular endothelial cells of many vessels regulate the contractility of the vascular smooth muscle cells through the generation of endothelium-dependent hyperpolarization (EDH). There is a general consensus that the opening of small- and intermediate-conductance Ca2+-activated K⁺ channels (SKCa and IKCa) is the initial mechanistic step for the generation of EDH. In animal models and humans, EDH and EDH-mediated relaxations are impaired during hypertension, and anti-hypertensive treatments restore such impairments. However, the underlying mechanisms of reduced EDH and its improvement by lowering blood pressure are poorly understood. Emerging evidence suggests that alterations of endothelial ion channels such as SKCa channels, inward rectifier K⁺ channels, Ca2+-activated Cl- channels, and transient receptor potential vanilloid type 4 channels contribute to the impaired EDH during hypertension. In this review, we attempt to summarize the accumulating evidence regarding the pathophysiological role of endothelial ion channels, focusing on their relationship with EDH during hypertension.

Contribution of Ca²⁺-dependent Cl⁻ channels to norepinephrine-induced contraction of femoral artery is replaced by increasing EDCF contribution during ageing

The activation of Ca²⁺-dependent Cl⁻ channels during norepinephrine-induced contraction of vascular smooth muscle was suggested to depolarize cell membrane and to increase Ca²⁺ entry. Hypertension and ageing are associated with altered Ca²⁺ handling including possible activation of Ca²⁺-dependent Cl⁻ channels. Our study was aimed to determine Ca²⁺-dependent Cl⁻ channels contribution to norepinephrine-induced contraction during hypertension and ageing. Norepinephrine-induced concentration-response curves of femoral arteries from 6- and 12-month-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were recorded using wire myograph. Pretreatment with Ca²⁺-dependent Cl- channel inhibitor indanyloxyacetic acid 94 [R(+)-IAA-94](IAA) attenuated norepinephrine-induced contraction in all groups, but relatively more in WKY than SHR arteries. The attenuation of norepinephrine-induced contraction after Ca²⁺-dependent Cl⁻ channels blockade was partially reduced in 12-month-old WKY rats, but substantially diminished in 12-month-old SHR. IAA effect was enhanced after NO synthase inhibition but decreased by ageing. In 20-month-old WKY rats norepinephrine-induced contraction was not affected by IAA but was almost abolished after cyclooxygenase inhibition by indomethacin or niflumic acid. In conclusion, contribution of Ca²⁺-dependent Cl⁻ channels to norepinephrine-induced contraction diminished with age, hypertension development, and/or NO synthesis inhibition. Ca²⁺-dependent Cl⁻ channels are important for maintenance of normal vascular tone while their inactivation/closing might be a pathological mechanism.

Characteristics of ACh-induced hyperpolarization and relaxation in rabbit jugular vein

BACKGROUND AND PURPOSE

The roles played by endothelium-derived NO and prostacyclin and by endothelial cell hyperpolarization in ACh-induced relaxation have been well characterized in arteries. However, the mechanisms underlying ACh-induced relaxation in veins remain to be fully clarified.

EXPERIMENTAL APPROACH

ACh-induced smooth muscle cell (SMC) hyperpolarization and relaxation were measured in endothelium-intact and -denuded preparations of rabbit jugular vein.

KEY RESULTS

In endothelium-intact preparations, ACh (≤ 10⁻⁸ M) marginally increased the intracellular concentration of Ca²⁺ ([Ca²⁺](i)) in endothelial cells but did not alter the SMC membrane potential. However, ACh (10⁻¹⁰ -10⁻⁸ M) induced a concentration-dependent relaxation during the contraction induced by PGF(2α) and this relaxation was blocked by the NO synthase inhibitor N(ω) -nitro-l-arginine. ACh (10⁻⁸ -10⁻⁶ M) concentration-dependently increased endothelial [Ca²⁺](i) and induced SMC hyperpolarization and relaxation. These SMC responses were blocked in the combined presence of apamin [blocker of small-conductance Ca²⁺-activated K⁺ (SK(Ca) , K(Ca) 2.3) channel], TRAM 34 [blocker of intermediate-conductance Ca²⁺ -activated K⁺ (IK(Ca) , K(Ca) 3.1) channel] and margatoxin [blocker of subfamily of voltage-gated K⁺ (K(V) ) channel, K(V) 1].

CONCLUSIONS AND IMPLICATIONS

In rabbit jugular vein, NO plays a primary role in endothelium-dependent relaxation at very low concentrations of ACh (10⁻¹⁰ -10⁻⁸ M). At higher concentrations, ACh (10⁻⁸ -3 × 10⁻⁶ M) induces SMC hyperpolarization through activation of endothelial IK(Ca) , K(V) 1 and (possibly) SK(Ca) channels and produces relaxation. These results imply that ACh regulates rabbit jugular vein tonus through activation of two endothelium-dependent regulatory mechanisms.

The thromboxane/endoperoxide receptor (TP): the common villain

The stimulation of thromboxane/endoperoxide receptors (TP) elicits diverse physiological/pathophysiological reactions, including platelet aggregation and contraction of vascular smooth muscle. Furthermore, the activation of endothelial TP promotes the expression of adhesion molecules and favors adhesion and infiltration of monocytes/macrophages. In various cardiovascular diseases, endothelial dysfunction is predominantly the result of the release of endothelium-derived contracting factors that counteract the vasodilator effect of nitric oxide produced by the endothelial nitric oxide synthase. Endothelium-dependent contractions involve the activation of cyclooxygenases, the production of reactive oxygen species along with that of endothelium-derived contracting factors, which diffuse toward the vascular smooth muscle cells and activate their TP. TP antagonists curtail the endothelial dysfunction in diseases such as hypertension and diabetes, are potent antithrombotic agents, and reduce vascular inflammation. Therefore, TP antagonists, because of this triple activity, may have a unique potential for the treatment of cardiovascular disorders.

Enalapril treatment alters the contribution of epoxyeicosatrienoic acids but not gap junctions to endothelium-derived hyperpolarizing factor activity in mesenteric arteries of spontaneously hypertensive rats

Reduction in endothelium-derived hyperpolarizing factor (EDHF)-mediated dilatory function in large, elastic arteries during hypertension is reversed after blood pressure normalization. We investigated whether similar mechanisms occurred in smaller mesenteric resistance arteries from aged Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHRs), and SHRs treated with the angiotensin-converting enzyme inhibitor, enalapril, using immunohistochemistry, serial-section electron microscopy, electrophysiology and wire myography. Unlike the superior mesenteric artery, EDHF relaxations in muscular mesenteric arteries were not reduced in SHRs, although morphological differences were found in the endothelium and smooth muscle. In WKY rats, SHRs and enalapril-treated SHRs, relaxations were mediated by small-, large-, and intermediate-conductance calcium-activated potassium channels, which were distributed in the endothelium, smooth muscle, and both layers, respectively. However, only WKY hyperpolarizations and relaxations were sensitive to gap junction blockers, and these arteries expressed more endothelial and myoendothelial gap junctions than arteries from SHRs. Responses in WKY rats, but not SHRs, were also reduced by inhibitors of epoxyeicosatrienoic acids (EETs), 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE) and miconazole, although sensitivity to EET regioisomers was endothelium-independent in all rats. Enalapril treatment of SHRs reduced blood pressure and restored sensitivity to 14,15-EEZE, but not to gap junction blockers, and failed to reverse the morphological changes. In conclusion, the mechanisms underlying EDHF in muscular mesenteric arteries differ between WKY rats and SHRs, with gap junctions and EETs involved only in WKY rats. However, reduction of blood pressure in SHRs with enalapril restored a role for EETs, but not gap junctions, without reversing morphological changes, suggesting a differential control of chemical and structural alterations.

Effects of pomace olive oil-enriched diets on endothelial function of small mesenteric arteries from spontaneously hypertensive rats

Pomace olive oil (POM), an olive oil subproduct traditionally used in Spain, is a good source of minor components from the unsaponifiable fraction such as triterpenoids, mainly in the form of oleanolic acid, which induces vascular protection and vasodilatation. Our aim was to evaluate the effects of long-term intake of diets enriched in POM with high concentration in oleanolic acid on endothelial dysfunction associated to hypertension in small mesenteric arteries (SMA) from spontaneously hypertensive rats (SHR). During 12 weeks, rats (six rats per group) were fed either a control 2 % maize oil diet (BD), or high-fat diets containing 15 % refined olive oil (OL), pomace olive oil (POM), or pomace olive oil supplemented in oleanolic acid (POMO; up to 800 parts per million). Endothelial and vascular functions were assessed by relaxing or contracting responses to acetylcholine (ACh) or phenylephrine, respectively. The involvement of endothelium-derived relaxing factors in these responses was evaluated. In contrast to BD, SHR fed high-fat diets showed a biphasic response to ACh related to changes in eicosanoid metabolism. POM enhanced the endothelial function in SMA from SHR by increasing the endothelium-derived hyperpolarising factor (EDHF)-type component, whereas administration of POMO resulted in a similar contribution of NO/EDHF in the endothelial response to ACh. The present study shows that despite the lack of changes in blood pressure, consumption of POM improves endothelial function in SMA from SHR by improving the agonist-mediated EDHF/NO response. Thus, triterpenoids confer a protective role to POM against endothelial dysfunction in hypertension.

Endothelium-dependent contractions in SHR: a tale of prostanoid TP and IP receptors

In the aorta of spontaneously hypertensive rats (SHR), the endothelial dysfunction is due to the release of endothelium-derived contracting factors (EDCFs) that counteract the vasodilator effect of nitric oxide, with no or minor alteration of its production. The endothelium-dependent contractions elicited by acetylcholine (ACh) involve an increase in endothelial [Ca(2+)](i), the production of reactive oxygen species, the activation of endothelial cyclooxygenase-1, the diffusion of EDCF and the subsequent stimulation of smooth muscle cell TP receptors. The EDCFs released by ACh have been identified as PGH(2) and paradoxically prostacyclin. Prostacyclin generally acts as an endothelium-derived vasodilator, which, by stimulating IP receptors, produces hyperpolarization and relaxation of the smooth muscle and inhibits platelet aggregation. In the aorta of SHR and Wistar-Kyoto rats, prostacyclin is the principal metabolite of arachidonic acid released by ACh. However, in SHR aorta, prostacyclin does not produce relaxations but activates the TP receptors on vascular smooth muscle cells and produces contraction. The IP receptor is not functional in the aortic smooth muscle cells of SHR as early as 12 weeks of age, but its activity is not reduced in platelets. Therefore, prostacyclin in the rule protects the vascular wall, but in the SHR aorta it can contribute to endothelial dysfunction. Whether or not prostacyclin plays a detrimental role as an EDCF in other animal models or in human remains to be demonstrated. Nevertheless, because EDCFs converge to activate TP receptors, selective antagonists of this receptor, by preventing endothelium-dependent contractions, curtail the endothelial dysfunction in diseases such as hypertension and diabetes.

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

Adequate cochlear blood supply by the spiral modiolar artery (SMA) is critical for normal hearing. ACh may play a role in neuroregulation of the SMA but several key issues including its membrane action mechanisms remain poorly understood. Besides its well-known endothelium-dependent hyperpolarizing action, ACh can induce a depolarization in vascular cells. Using intracellular and whole-cell recording techniques on cells in guinea pig in vitro SMA, we studied the ionic mechanism underlying the ACh-depolarization and found that: (1) ACh induced a DAMP-sensitive depolarization when intermediate conductance KCa channels were blocked by charybdotoxin or nitrendipine. The ACh-depolarization was associated with a decrease in input resistance (R(input)) in high membrane potential (V(m)) ( approximately -40 mV) cells but with no change or an increase in R input in low Vm ( approximately -75 mV) cells. ACh-depolarization was attenuated by background membrane depolarization from approximately -70 mV in the majority of cells; (2) ACh-induced inward current in smooth muscle cells embedded in a SMA segment often showed a U-shaped I/V curve, the reversal potential of its two arms being near EK and 0 mV, respectively; (3) ACh-depolarization was reduced by low Na+, zero K+ or 20mM K+ bath solutions; (4) ACh-depolarization was inhibited by La3+ in all cells tested, by 4-AP and flufenamic acid in low Vm cells, but was not sensitive to Cd2+, Ni2+, nifedipine, niflumic acid, DIDS, IAA94, linopirdine or amiloride. We conclude that ACh-induced vascular depolarization was generated mainly by activation of a TRP-like non-selective cation channel and by inactivation of an inward rectifier K+ channel.