Acetylcholine-induced membrane potential changes in endothelial cells of rabbit aortic valve

Changes in vasodilation following myocardial ischemia/reperfusion in rats

BACKGROUND

Blockage of a coronary artery, usually caused by arteriosclerosis, can lead to life threatening acute myocardial infarction. Opening with PCI (percutaneous coronary intervention), may be lifesaving, but reperfusion might exacerbate the cellular damage, and changes in the endothelium are believed to be involved in this worsened outcome.

AIM

The aim of the present study was to compare endothelial dependent and independent vasodilatory effect after experimental myocardial ischemia/reperfusion (I/R).

METHODS

A well-established rat model of myocardial ischemia with 24 h of reperfusion was applied, followed by a study in a wire myograph.

RESULTS

Endothelial NO dependent relaxation in response to carbachol, was sensitive to arterial depolarization, and was unaffected by I/R. In contrast, endothelial NO dependent ADPβS signalling, which was not sensitive to arterial depolarization, was significantly reduced after I/R. Following I/R, an H₂O₂ dependent EDH induced dilation appears in response to both of the above agonists. In addition, calcitonin gene-related peptide (CGRP) induced vasodilation was reduced.

CONCLUSION

These data show that NO dependent ADPβS induced dilation is reduced after I/R. However, there is some compensation by released H₂O₂ causing an EDH. Combined with a loss of maximal dilation in response to CGRP, the reduced vasodilation could be an important factor in understanding the exacerbated damage after I/R.

Direct activation of Ca2+ and voltage-gated potassium channels of large conductance by anandamide in endothelial cells does not support the presence of endothelial atypical cannabinoid receptor

Endocannabinoid anandamide induces endothelium-dependent relaxation commonly attributed to stimulation of the G-protein coupled endothelial anandamide receptor. The study addressed the receptor-independent effect of anandamide on large conductance Ca²⁺-dependent K⁺ channels expressed in endothelial cell line EA.hy926. Under resting conditions, 10µM anandamide did not significantly influence the resting membrane potential. In a Ca²⁺-free solution the cells were depolarized by ~10mV. Further administration of 10µM anandamide hyperpolarized the cells by ~8mV. In voltage-clamp mode, anandamide elicited the outwardly rectifying whole-cell current sensitive to paxilline but insensitive to GDPβS, a G-protein inhibitor. Administration of 70µM Mn²⁺, an agent used to promote integrin clustering, reversibly stimulated whole-cell current, but failed to further facilitate the anandamide-stimulated current. In an inside-out configuration, anandamide (0.1-30µM) facilitated single BK_Ca channel activity in a concentration-dependent manner within a physiological Ca²⁺ range and a wide range of voltages, mainly by reducing mean closed time. The effect is essentially eliminated following chelation of Ca²⁺ from the cytosolic face and pre-exposure to cholesterol-reducing agent methyl-β-cyclodextrin. O-1918 (3µM), a cannabidiol analog used as a selective antagonist of endothelial anandamide receptor, reduced BK_Ca channel activity in inside-out patches. These results do not support the existence of endothelial cannabinoid receptor and indicate that anandamide acts as a direct BK_Ca opener. The action does not require cell integrity or integrins and is caused by direct modification of BK_Ca channel activity.

Cell-cell communication in the vessel wall

Intercellular communication between cells within the blood vessel wall plays an important role in the control of artery diameter. The endothelial cells lining the lumen of arteries can evoke smooth muscle hyperpolarization both by the release of a factor (EDHF) and by direct cell-cell coupling through gap junctions. Hyperpolarizing current can spread rapidly to cause widespread vasodilatation, and thus increase blood flow to that segment. In addition to the spread of current, small molecules, such as Ca2+, can also pass between cells, but at a much reduced rate. Instead of co-ordinating changes in diameter, intercellular Ca2+ signalling acts to amplify and, in special cases, modulate vascular responses. Together, direct cell-cell communication enables the blood vessel wall to act as a functional syncytium, which is influenced by surrounding tissues and nerves, and blood constituents.

Negative News: Cl− and HCO3− in the Vascular Wall

Cl− and HCO3− are the most prevalent membrane-permeable anions in the intra- and extracellular spaces of the vascular wall. Outwardly directed electrochemical gradients for Cl− and HCO3− permit anion channel opening to depolarize vascular smooth muscle and endothelial cells. Transporters and channels for Cl− and HCO3− also modify vascular contractility and structure independently of membrane potential. Transport of HCO3− regulates intracellular pH and thereby modifies the activity of enzymes, ion channels, and receptors. There is also evidence that Cl− and HCO3− transport proteins affect gene expression and protein trafficking. Considering the extensive implications of Cl− and HCO3− in the vascular wall, it is critical to understand how these ions are transported under physiological conditions and how disturbances in their transport can contribute to disease development. Recently, sensing mechanisms for Cl− and HCO3− have been identified in the vascular wall where they modify ion transport and vasomotor function, for instance, during metabolic disturbances. This review discusses current evidence that transport (e.g., via NKCC1, NBCn1, Ca2+-activated Cl− channels, volume-regulated anion channels, and CFTR) and sensing (e.g., via WNK and RPTPγ) of Cl− and HCO3− influence cardiovascular health and disease.

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.

Ca2+-activated K+ channels of small and intermediate conductance control eNOS activation through NAD(P)H oxidase

Ca(2+)-activated K(+) channels (K(Ca)) and NO play a central role in the endothelium-dependent control of vasomotor tone. We evaluated the interaction of K(Ca) with NO production in isolated arterial mesenteric beds of the rat. In phenylephrine-contracted mesenteries, acetylcholine (ACh)-induced vasodilation was reduced by NO synthase (NOS) inhibition with N(ω)-nitro-L-arginine (L-NA), but in the presence of tetraethylammonium, L-NA did not further affect the response. In KCl-contracted mesenteries, the relaxation elicited by 100 nM ACh or 1 μM ionomycin was abolished by L-NA, tetraethylammonium, or simultaneous blockade of small-conductance K(Ca) (SK(Ca)) channels with apamin and intermediate-conductance K(Ca) (IK(Ca)) channels with triarylmethane-34 (TRAM-34). Apamin-TRAM-34 treatment also abolished 100 nM ACh-activated NO production, which was associated with an increase in superoxide formation. Endothelial cell Ca(2+) buffering with BAPTA elicited a similar increment in superoxide. Apamin-TRAM-34 treatment increased endothelial NOS phosphorylation at threonine 495 (P-eNOS(Thr495)). Blockade of NAD(P)H oxidase with apocynin or superoxide dismutation with PEG-SOD prevented the increment in superoxide and changes in P-eNOS(Thr495) observed during apamin and TRAM-34 application. Our results indicate that blockade of SK(Ca) and IK(Ca) activates NAD(P)H oxidase-dependent superoxide formation, which leads to inhibition of NO release through P-eNOS(Thr495). These findings disclose a novel mechanism involved in the control of NO production.

Connexins and gap junctions in the EDHF phenomenon and conducted vasomotor responses

It is becoming increasingly evident that electrical signaling via gap junctions plays a central role in the physiological control of vascular tone via two related mechanisms (1) the endothelium-derived hyperpolarizing factor (EDHF) phenomenon, in which radial transmission of hyperpolarization from the endothelium to subjacent smooth muscle promotes relaxation, and (2) responses that propagate longitudinally, in which electrical signaling within the intimal and medial layers of the arteriolar wall orchestrates mechanical behavior over biologically large distances. In the EDHF phenomenon, the transmitted endothelial hyperpolarization is initiated by the activation of Ca(2+)-activated K(+) channels channels by InsP(3)-induced Ca(2+) release from the endoplasmic reticulum and/or store-operated Ca(2+) entry triggered by the depletion of such stores. Pharmacological inhibitors of direct cell-cell coupling may thus attenuate EDHF-type smooth muscle hyperpolarizations and relaxations, confirming the participation of electrotonic signaling via myoendothelial and homocellular smooth muscle gap junctions. In contrast to isolated vessels, surprisingly little experimental evidence argues in favor of myoendothelial coupling acting as the EDHF mechanism in arterioles in vivo. However, it now seems established that the endothelium plays the leading role in the spatial propagation of arteriolar responses and that these involve poorly understood regenerative mechanisms. The present review will focus on the complex interactions between the diverse cellular signaling mechanisms that contribute to these phenomena.

Gap junctions in the control of vascular function

Direct intercellular communication via gap junctions is critical in the control and coordination of vascular function. In the cardiovascular system, gap junctions are made up of one or more of four connexin proteins: Cx37, Cx40, Cx43, and Cx45. The expression of more than one gap-junction protein in the vasculature is not redundant. Rather, vascular connexins work in concert, first during the development of the cardiovascular system, and then in integrating smooth muscle and endothelial cell function, and in coordinating cell function along the length of the vessel wall. In addition, connexin-based channels have emerged as an important signaling pathway in the astrocyte-mediated neurovascular coupling. Direct electrical communication between endothelial cells and vascular smooth muscle cells via gap junctions is thought to play a relevant role in the control of vasomotor tone, providing the signaling pathway known as endothelium-derived hyperpolarizing factor (EDHF). Consistent with the importance of gap junctions in the regulation of vasomotor tone and arterial blood pressure, the expression of connexins is altered in diseases associated with vascular complications. In this review, we discuss the participation of connexin-based channels in the control of vascular function in physiologic and pathologic conditions, with a special emphasis on hypertension and diabetes.

Are voltage-dependent ion channels involved in the endothelial cell control of vasomotor tone?

In the microcirculation, longitudinal conduction of vasomotor responses provides an essential means of coordinating flow distribution among vessels in a complex network. Spread of current along the vessel axis can display a regenerative component, which leads to propagation of vasomotor signals over many millimeters; the ionic basis for the regenerative response is unknown. We examined the responses to 10 s of focal electrical stimulation (30 Hz, 2 ms, 30 V) of mouse cremaster arterioles to test the hypothesis that voltage-dependent Na(+) (Na(v)) and Ca(2+) channels might be activated in long-distance signaling in microvessels. Electrical stimulation evoked a vasoconstriction at the site of stimulation and a spreading, nondecremental conducted dilation. Endothelial damage (air bubble) blocked conduction of the vasodilation, indicating an involvement of the endothelium. The Na(v) channel blocker bupivacaine also blocked conduction, and TTX attenuated it. The Na(v) channel activator veratridine induced an endothelium-dependent dilation. The Na(v) channel isoforms Na(v)1.2, Na(v)1.6, and Na(v)1.9 were detected in the endothelial cells of cremaster arterioles by immunocytochemistry. These findings are consistent with the involvement of Na(v) channels in the conducted response. BAPTA buffering of endothelial cell Ca(2+) delayed and reduced the conducted dilation, which was almost eliminated by Ni(2+), amiloride, or deletion of alpha(1H) T-type Ca(2+) (Ca(v)3.2) channels. Blockade of endothelial nitric oxide synthase or Ca(2+)-activated K(+) channels also inhibited the conducted vasodilation. Our findings indicate that an electrically induced signal can propagate along the vessel axis via the endothelium and can induce sequential activation of Na(v) and Ca(v)3.2 channels. The resultant Ca(2+) influx activates endothelial nitric oxide synthase and Ca(2+)-activated K(+) channels, triggering vasodilation.

Small- and intermediate-conductance Ca2+-activated K+ channels directly control agonist-evoked nitric oxide synthesis in human vascular endothelial cells

The contribution of small-conductance (SK(Ca)) and intermediate-conductance Ca(2+)-activated K(+) (IK(Ca)) channels to the generation of nitric oxide (NO) by Ca(2+)-mobilizing stimuli was investigated in human umbilical vein endothelial cells (HUVECs) by combining single-cell microfluorimetry with perforated patch-clamp recordings to monitor agonist-evoked NO synthesis, cytosolic Ca(2+) transients, and membrane hyperpolarization in real time. ATP or histamine evoked reproducible elevations in NO synthesis and cytosolic Ca(2+), as judged by 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) and fluo-3 fluorescence, respectively, that were tightly associated with membrane hyperpolarizations. Whereas evoked NO synthesis was unaffected by either tetraethylammonium (10 mmol/l) or BaCl(2) (50 micromol/l) + ouabain (100 micromol/l), depleting intracellular Ca(2+) stores by thapsigargin or removing external Ca(2+) inhibited NO production, as did exposure to high (80 mmol/l) external KCl. Importantly, apamin and charybdotoxin (ChTx)/ triarylmethane (TRAM)-34, selective blockers SK(Ca) and IK(Ca) channels, respectively, abolished both stimulated NO synthesis and membrane hyperpolarization and decreased evoked Ca(2+) transients. Apamin and TRAM-34 also inhibited an agonist-induced outwardly rectifying current characteristic of SK(Ca) and IK(Ca) channels. Under voltage-clamp control, we further observed that the magnitude of agonist-induced NO production varied directly with the degree of membrane hyperpolarization. Mechanistically, our data indicate that SK(Ca) and IK(Ca) channel-mediated hyperpolarization represents a critical early event in agonist-evoked NO production by regulating the influx of Ca(2+) responsible for endothelial NO synthase activation. Moreover, it appears that the primary role of agonist-induced release of intracellular Ca(2+) stores is to trigger the opening of both K(Ca) channels along with Ca(2+) entry channels at the plasma membrane. Finally, the observed inhibition of stimulated NO synthesis by apamin and ChTx/TRAM-34 demonstrates that SK(Ca) and IK(Ca) channels are essential for NO-mediated vasorelaxation.

Remifentanil-induced mechanical responses and membrane potential changes in human umbilical arteries

BACKGROUND

The aim of this study was to evaluate the characteristic features of the mechanical responses and membrane potential changes induced by remifentanil in human umbilical arteries (HUAs). The ionic mechanisms underlying the electrophysiological responses were pharmacologically assessed using two K(+) channel blockers.

METHODS

Thirty-eight HUAs were obtained. Contraction-relaxation, membrane potential changes and electrical responses of the HUAs were recorded.

RESULTS

Remifentanil produced concentration-dependent relaxation in both endothelium-intact and endothelium-denuded HUA rings. Remifentanil produced a significantly greater relaxation response in intact than in denuded HUA rings. In endothelium-intact rings, pre-treatment with L-nitroarginine [N(w)-NITRO-(L)-ARGININE (L-NO-ARG)] or indomethacin decreased the degree of remifentanil-induced relaxation. Remifentanil (10(-9)-10(-6) mol/l) produced a transient concentration-dependent membrane hyperpolarization, which was not decreased by pre-treatment with L-NO-ARG or indomethacin. It also produced a small concentration-dependent hyperpolarization in the presence of charybdotoxin or tetraethylammonium.

CONCLUSION

In both endothelium-intact and endothelium-denuded HUAs, remifentanil induces concentration-dependent vasorelaxation and simultaneously releases nitric oxide, prostaglandins and possibly an endothelium-derived hyperpolarizing factor. In addition, it produces hyperpolarization in a dose-dependent manner. Hyperpolarization induced by remifentanil involves the activation of Ca(2+)-dependent and Ca(2+)-independent potassium channels regulated by intracellular Ca(2+).

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

OBJECTIVE

During blockade of endothelium-dependent hyperpolarizing factor (EDHF), acetylcholine evoked larger and faster depolarization in mesenteric arteries of spontaneously hypertensive rats (SHR) than normotensive Wistar-Kyoto (WKY) rats. We studied the mechanism underlying this response and its role in the attenuation of EDHF.

METHODS

Electrophysiology, computational modelling and myography were used to study changes in membrane potential and effects on contractility.

RESULTS

The large acetylcholine-evoked depolarization in SHR was accompanied by contraction, but this was not seen in WKY rats. The depolarization depended on release of intracellular Ca2+ but was unaffected by nonselective cation channel inhibitors, gadolinium, lanthanum or amiloride. The depolarization was significantly reduced by the Ca2+-dependent Cl- channel inhibitors, niflumic acid or flufenamic acid, or alterations in Cl- gradients using bumetanide (Na/K/Cl transporter inhibitor) or external Cl- replacement with isethionate. These drugs altered the time course of EDHF-evoked hyperpolarizations in SHR, making them indistinguishable from those in WKY rats. EDHF-induced relaxation was less sensitive to acetylcholine in SHR than in WKY rats, but this difference was eliminated following artery pretreatment with bumetanide. Computational modelling in which the SHR fast depolarizing response was selectively modulated mimicked physiologically acquired results obtained in SHR and WKY rats during Cl- -channel blockade.

CONCLUSIONS

Acetylcholine evokes a fast depolarization in SHR but not in WKY rats, mediated by the opening of Ca2+-dependent Cl- channels. The depolarization is responsible for a constriction that reduces EDHF-mediated relaxation. Data suggest that Ca2+-dependent Cl- channels may provide a novel therapeutic target for improvement of endothelial dysfunction during hypertension.

Effects of increased intracellular Cl- concentration on membrane responses to acetylcholine in the isolated endothelium of guinea pig mesenteric arteries

ACh-induced membrane responses in vascular endothelial cells that have been reported vary between preparations from a sustained hyperpolarization to a transient hyperpolarization followed by a depolarization; the reason for this variation is unknown. Using the perforated whole-cell clamp technique, we investigated ACh-induced membrane currents in freshly isolated endothelial layers having a resting membrane potential of less negative than -10 mV. A group of cells was electrically isolated using a wide-bore micropipette, and their membrane potential was well controlled. ACh activated K(+) and Cl(-) currents simultaneously. The K(+) current was blocked by a combination of charybdotoxin and apamin and appears to result from the opening of IK(Ca) and SK(Ca) channels. The Cl(-) current was partially blocked by tamoxifen, niflumic acid, or DIDS and appears to be produced by Ca(2+)-activated Cl(-) channels. When the pipettes contained 20 mM Cl(-), the ACh-induced K(+) conductance started decreasing during a 1-min application of ACh while the Cl(-) conductance continued, making the ACh-induced hyperpolarization sustained. When the pipettes contained 150 mM Cl(-), both conductances started decreasing during a 1-min application of ACh, making the ACh-induced hyperpolarization small and transient. [Cl(-)](i) is very likely modified by experimental procedures such as the cell isolation and the intracellular dialysis with the pipette solution. Such a variability in [Cl(-)](i) may be one of the reasons for the variations in the ACh-induced membrane response.

Reduced hyperpolarization in endothelial cells of rabbit aortic valve following chronic nitroglycerine administration

This study was undertaken to determine whether long-term in vivo administration of nitroglycerine (NTG) downregulates the hyperpolarization induced by acetylcholine (ACh) in aortic valve endothelial cells (AVECs) of the rabbit and, if so, whether antioxidant agents can normalize this downregulated hyperpolarization. ACh (0.03-3 microM) induced a hyperpolarization through activations of both apamin- and charybdotoxin-sensitive Ca2+-activated K+ channels (K(Ca)) in rabbit AVECs. The intermediate-conductance K(Ca) channel (IK(Ca)) activator 1-ethyl-2-benzimidazolinone (1-EBIO, 0.3 mM) induced a hyperpolarization of the same magnitude as ACh (3 microM). The ACh-induced hyperpolarization was significantly weaker, although the ACh-induced [Ca2+]i increase was unchanged, in NTG-treated rabbits (versus NTG-untreated control rabbits). The hyperpolarization induced by 1-EBIO was also weaker in NTG-treated rabbits. The reduced ACh-induced hyperpolarization seen in NTG-treated rabbits was not modified by in vitro application of the superoxide scavengers Mn-TBAP, tiron or ascorbate, but it was normalized when ascorbate was coadministered with NTG in vivo. Superoxide production within the endothelial cell (estimated by ethidium fluorescence) was increased in NTG-treated rabbits and this increased production was normalized by in vivo coadministration of ascorbate with the NTG. It is suggested that long-term in vivo administration of NTG downregulates the ACh-induced hyperpolarization in rabbit AVECs, possibly through chronic actions mediated by superoxide.

Membrane hyperpolarization is not required for sustained muscarinic agonist-induced increases in intracellular Ca2+ in arteriolar endothelial cells

OBJECTIVE

Hyperpolarization modulates Ca2+ influx during agonist stimulation in many endothelial cells, but the effects of hyperpolarization on Ca2+ influx in freshly isolated arteriolar endothelial cells are unknown. Therefore, the purpose of the present study was to characterize agonist-induced Ca2+ transients in freshly isolated arteriolar endothelial cells and to test the hypothesis that membrane hyperpolarization augments agonist-induced Ca2+ influx into these cells.

METHODS

Arterioles were removed from hamster cremaster muscles and arteriolar endothelial cells were enzymatically isolated. Endothelial cells were loaded with Fura 2-AM and the Fura 2 ratio measured photometrically as an index of intracellular Ca2+. The cells were then stimulated with the muscarinic, cholinergic agonist, methacholine, and the resulting Ca2+ transients were measured.

RESULTS

Methacholine (1 microM) increased the endothelial cell Fura 2 ratio from a baseline of 0.81 +/- 0.02 to an initial peak of 1.17 +/- 0.05 (n = 17) followed by a sustained plateau of 1.12 +/- 0.07. The plateau phase of the Ca2+ transient was inhibited by removal of extracellular Ca2+ (n = 12, p < .05), or the nonselective cation channel blockers Gd3+ (30 microM; n = 7, p < .05) or La3+ (50 microM; n = 7, p < .05) without significant effect on the baseline or peak (p > .05). The initial peak of methacholine-induced Ca2+ transients was inhibited by the IP3-receptor antagonist xestospongin D (10 microM, n = 5, p < .05). The methacholine-induced Ca2+ transients were accompanied by endothelial cell hyperpolarization of approximately 14-18 mV, as assessed by experiments using the potentiometric dye, di-8-ANEPPS as well as by patch-clamp experiments. However, inhibition of hyperpolarization by blockade of Ca2+-activated K+ channels with charybdotoxin (100 nM) and apamin (100 nM) (n = 5), or exposure of endothelial cells to 80 or 145 mM KCl (both n = 7) had no effect on the plateau phase of methacholine-induced Ca2+ transients (p > .05).

CONCLUSIONS

Freshly isolated arteriolar endothelial cells display agonist-induced Ca2+ transients. For the muscarinic agonist, methacholine, these Ca2+ transients result from release of Ca2+ from intracellular stores through IP3 receptors, followed by sustained influx of extracellular Ca2+. While these changes in intracellular Ca2+ are associated with endothelial cell hyperpolarization, the methacholine-induced, sustained increase in intracellular Ca2+ appears to be independent from this change in membrane potential. These data suggest that arteriolar endothelial cells may possess a novel Ca2+ influx pathway, or that the relationship between intracellular Ca2+ and Ca2+ influx is more complex than that observed in other endothelial cells.

Endothelium-dependent smooth muscle hyperpolarization: do gap junctions provide a unifying hypothesis?

An endothelium-derived hyperpolarizing factor (EDHF) that is distinct from nitric oxide (NO) and prostanoids has been widely hypothesized to hyperpolarize and relax vascular smooth muscle following stimulation of the endothelium by agonists. Candidates as diverse as K(+) ions, eicosanoids, hydrogen peroxide and C-type natriuretic peptide have been implicated as the putative mediator, but none has emerged as a 'universal EDHF'. An alternative explanation for the EDHF phenomenon is that direct intercellular communication via gap junctions allows passive spread of agonist-induced endothelial hyperpolarization through the vessel wall. In some arteries, eicosanoids and K(+) ions may themselves initiate a conducted endothelial hyperpolarization, thus suggesting that electrotonic signalling may represent a general mechanism through which the endothelium participates in the regulation of vascular tone.

Endothelial potassium channels, endothelium-dependent hyperpolarization and the regulation of vascular tone in health and disease

1. The elusive nature of endothelium-derived hyperpolarizing factor (EDHF) has hampered detailed study of the ionic mechanisms that underlie the EDHF hyperpolarization and relaxation. Most studies have relied on a pharmacological approach in which interpretations of results can be confounded by limited specificity of action of the drugs used. Nevertheless, small-, intermediate- and large-conductance Ca2+-activated K+ channels (SKCa, IKCa and BKCa, respectively) have been implicated, with inward rectifier K+ channels (KIR) and Na+/K+-ATPase also suggested by some studies. 2. Endothelium-dependent membrane currents recorded using single-electrode voltage-clamp from electrically short lengths of arterioles in which the smooth muscle and endothelial cells remained in their normal functional relationship have provided useful insights into the mechanisms mediating EDHF. Charybdotoxin (ChTx) or apamin reduced, whereas apamin plus ChTx abolished, the EDHF current. The ChTx- and apamin-sensitive currents both reversed near the expected K+ equilibrium potential, were weakly outwardly rectifying and displayed little, if any, time- or voltage-dependent gating, thus having the biophysical and pharmacological characteristics of IKCa and SKCa channels, respectively. 3. The IKCa and SKCa channels occur in abundance in endothelial cells and their activation results in EDHF-like hyperpolarization of these cells. There is little evidence for a significant number of these channels in healthy, contractile vascular smooth muscle cells. 4. In a number of blood vessels in which EDHF occurs, the endothelial and smooth muscle cells are coupled electrically via myoendothelial gap junctions. In contrast, in the adult rat femoral artery, in which the smooth muscle and endothelial layers are not coupled electrically, EDHF does not occur, even though acetylcholine evokes hyperpolarization in the endothelial cells. 5. In vivo studies indicate that EDHF contributes little to basal conductance of the vasculature, but it contributes appreciably to evoked increases in conductance. 6. Endothelium-derived hyperpolarizing factor responses are diminished in some diseases, including hypertension, pre-eclampsia and some models of diabetes. 7. The most economical explanation for EDHF in vitro and in vivo in small vessels is that it arises from the activation of IKCa and SKCa channels in endothelial cells. The resulting endothelial hyperpolarization spreads via myoendothelial gap junctions to result in the EDHF-attributed hyperpolarization and relaxation of the smooth muscle.

Connexins: gaps in our knowledge of vascular function

Gap junctions are common features in the vasculature, long thought to provide a pathway for cell-cell signaling. Emerging understanding of the gap-junctional proteins (connexins) and new tools for their investigation now offer the opportunity to explore the vital role that the gap junctions may play in cardiovascular homeostasis and pathophysiology.

Clonidine induces rat aorta relaxation by nitric oxide-dependent and -independent mechanisms

The alpha1- and alpha2-adrenoceptors coexist in vascular smooth muscle cells producing vascular contraction and relaxation. This study was designed to investigate which is the mechanism activated by clonidine in the rat aorta, and the endothelial factors possibly involved in the relaxation induced by clonidine. The alpha2-adrenoceptors agonist clonidine relaxed rat aortas pre-contracted with phenylephrine, with or without endothelium. In non-contracted denuded arteries, clonidine produced contractions instead of relaxation. In intact endothelium aortic rings, clonidine induced greater relaxation than in denuded aortic rings. In aortas with intact endothelium, the NO-synthase inhibitor L-NAME (10 micromol/L) and the NO-scavenger hemoglobin (10 micromol/L) reduced the relaxation to clonidine. On the other hand, indomethacin (10 micromol/L) failed to alter the relaxation induced by clonidine. These results suggest the participation of NO, but not prostacyclin in clonidine-induced relaxation. In aortic rings pre-contracted with KCl (60 mmol/L) the relaxation induced by clonidine was abolished; however, the K+ channel blockers glibenclamide (K(ATP)), tetraethylamonium (K(Ca)), and the combination of apamin and charybdotoxin (K(Ca)) did not change the relaxation induced by clonidine. The relaxation induced by clonidine on PGF2alpha-contracted arteries was not affected by prazosin. However, in the absence of prazosin, clonidine had an additional contractile effect in PGF2alpha-contracted arteries. In conclusion, our results show that in rat aorta clonidine can activate alpha2-adrenoceptors in the smooth muscle cells and alpha2-adrenoceptors in the endothelial cells that activates NO production, but not prostacyclin and/or EDHF. In the absence of phenylephrine and prazosin, clonidine can also activate alpha1-adrenoceptors and rat aorta contraction.

Cyclopiazonic acid decreases spontaneous transient depolarizations in guinea pig mesenteric lymphatic vessels in endothelium-dependent and -independent manners

Guinea pig mesenteric lymphatic vessels exhibit vasomotion through a pacemaker mechanism that involves intracellular Ca2+release and resultant spontaneous transient depolarizations (STDs) of the smooth muscle membrane potential. This study presents a detailed characterization of the effects of cyclopiazonic acid (CPA) on this pacemaker activity. Microelectrode recordings from smooth muscle in vessel segments revealed that application of CPA (1–10 μM) caused a hyperpolarization accompanied by a decrease in the frequency and amplitude of STDs. The CPA-induced hyperpolarization was abolished after destruction of the endothelium and in the presence of NG-nitro-l-arginine (100 μM) or 1 H-[1,2,4]oxadiazolol-[4,3- a]quinoxaline-1-one (10 μM), which suggests a contribution of endothelium-derived nitric oxide (EDNO) in this response. In the absence of EDNO-induced effects, CPA decreased the frequency and amplitude of STDs recorded before and in the presence of the thromboxane A2mimetic U-46619, norepinephrine, or thimerosal. CPA abolished U-46619-induced vasomotion as determined by measurement of constriction-associated intracellular Ca2+concentration using the ratiometric Ca2+indicator fura-2. The endothelial actions of CPA were compared with those of ACh, which is known to cause EDNO release in this preparation. Although CPA and ACh both increased endothelial intracellular Ca2+concentration and depolarized the membrane potential, the kinetics of action for both parameters were markedly slower for CPA than ACh. These results suggest that CPA first hyperpolarizes the lymphatic smooth muscle and decreases STD frequency and amplitude through endothelial release of EDNO, and second, consistent with the action of CPA to inhibit sarcoplasmic reticulum Ca2+-ATPase and deplete Ca2+stores, it further reduces STD activity. Inhibition of the lymphatic smooth muscle pacemaker mechanism is thought to abolish agonist-induced vasomotion.

Potassium Channels and Membrane Potential in the Modulation of Intracellular Calcium in Vascular Endothelial Cells

The endothelium plays a vital role in the control of vascular functions, including modulation of tone; permeability and barrier properties; platelet adhesion and aggregation; and secretion of paracrine factors. Critical signaling events in many of these functions involve an increase in intracellular free Ca(2+) concentration ([Ca(2+)](i)). This rise in [Ca(2+)](i) occurs via an interplay between several mechanisms, including release from intracellular stores, entry from the extracellular space through store depletion and second messenger-mediated processes, and the establishment of a favorable electrochemical gradient. The focus of this review centers on the role of potassium channels and membrane potential in the creation of a favorable electrochemical gradient for Ca(2+) entry. In addition, evidence is examined for the existence of various classes of potassium channels and the possible influence of regional variation in expression and experimental conditions.

Characterization of a charybdotoxin-sensitive intermediate conductance Ca2+-activated K+ channel in porcine coronary endothelium: relevance to EDHF

1. This study characterizes the K(+) channel(s) underlying charybdotoxin-sensitive hyperpolarization of porcine coronary artery endothelium. 2. Two forms of current-voltage (I/V) relationship were evident in whole-cell patch-clamp recordings of freshly-isolated endothelial cells. In both cell types, iberiotoxin (100 nM) inhibited a current active only at potentials over +50 mV. In the presence of iberiotoxin, charybdotoxin (100 nM) produced a large inhibition in 38% of cells and altered the form of the I/V relationship. In the remaining cells, charybdotoxin also inhibited a current but did not alter the form. 3. Single-channel, outside-out patch recordings revealed a 17.1+/-0.4 pS conductance. Pipette solutions containing 100, 250 and 500 nM free Ca(2+) demonstrated that the open probability was increased by Ca(2+). This channel was blocked by charybdotoxin but not by iberiotoxin or apamin. 4. Hyperpolarizations of intact endothelium elicited by substance P (100 nM; 26.1+/-0.7 mV) were reduced by apamin (100 nM; 17.0+/-1.8 mV) whereas those to 1-ethyl-2-benzimidazolinone (1-EBIO, 600 microM, 21.0+/-0.3 mV) were unaffected (21.7+/-0.8 mV). Substance P, bradykinin (100 nM) and 1-EBIO evoked charybdotoxin-sensitive, iberiotoxin-insensitive whole-cell perforated-patch currents. 5 A porcine homologue of the intermediate-conductance Ca(2+)-activated K(+) channel (IK1) was identified in endothelial cells. 6. In conclusion, porcine coronary artery endothelial cells express an intermediate-conductance Ca(2+)-activated K(+) channel and the IK1 gene product. This channel is opened by activation of the EDHF pathway and likely mediates the charybdotoxin-sensitive component of the EDHF response.

Altered effect of cyclopiazonic acid on endothelium-dependent relaxation in femoral arteries from hypertensive rats

The function of endoplasmic reticulum in hypertensive vascular endothelium has not been intensively studied. The current study was designed to investigate a role of intracellular Ca2+ stores in endothelium-dependent relaxations to acetylcholine using femoral arteries obtained from Wistar-Kyoto (WKYs) and spontaneously hypertensive rats (SHRs). Rings were prepared from the femoral arteries and changes in isometric tension were recorded. Endothelium-dependent relaxations induced by acetylcholine in rings contracted with serotonin were identical in WKYs and SHRs. Cyclopiazonic acid (CPA) inhibited the relaxation in SHRs but not in WKYs. In WKYs, acetylcholine evoked smaller relaxations in rings contracted with KCl than in those contracted with serotonin, whereas in SHRs the relaxation was not affected by the contractile agonists used. The relaxation in rings contracted with KCl was abolished by Nw-nitro-l-arginine methyl ester (l-NAME) and was reduced by CPA to a similar extent in both strains. In rings contracted with serotonin, l-NAME abolished the relaxation in SHRs, but the inhibitor only partially reduced the relaxation in WKYs. CPA did not alter the relaxation in the presence of l-NAME. Endothelium-independent relaxations to sodium nitroprusside were not affected by CPA. These results suggest that acetylcholine relaxes rat femoral arteries by releasing both nitric oxide and endothelium-derived hyperpolarizing factor (EDHF). In SHRs, the relaxation is preserved, but the release of EDHF is absent. CPA inhibits the relaxation mediated by nitric oxide, but not EDHF and, thus, inhibits the relaxation in SHRs but not in WKYs. Functional alteration of endoplasmic reticulum in the hypertensive endothelium cannot be detected.

Myoendothelial electrical coupling in arteries and arterioles and its implications for endothelium-derived hyperpolarizing factor

1. Considerable progress has been made over the past decade in evaluating the presence of electrical coupling between the endothelial and smooth muscle layers of blood vessels, prompted, in part, by ultrastructural evidence for the presence of myoendothelial junctions. 2. In a variety of vessels ranging in size from conduit arteries down to small arterioles, action potentials have been recorded from endothelial cells that were associated with constriction of the vessels and/or occurred in synchrony with and were indistinguishable from action potentials recorded from the smooth muscle. From these results, it is now firmly established that myoendothelial electrical coupling occurs in at least some blood vessels. 3. Spread of hyperpolarizing current from the endothelium to the smooth muscle is the most likely explanation of the smooth muscle hyperpolarization attributed to endothelium-derived hyperpolarizing factor. Because this hyperpolarization can evoke considerable relaxation of the smooth muscle, myoendothelial electrical coupling has important implications for endothelial regulation of the contractile activity of blood vessels.

cAMP facilitates EDHF-type relaxations in conduit arteries by enhancing electrotonic conduction via gap junctions

We have investigated the role of cAMP in NO- and prostanoid-independent relaxations that are widely attributed to an endothelium-derived hyperpolarizing factor (EDHF). Under control conditions EDHF-type relaxations evoked by acetylcholine (ACh) in rabbit iliac arteries were transient, but in the presence of the cAMP phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) or the cell permeant cAMP analog 8-bromo-cAMP, relaxations became sustained with their maxima potentiated approximately 2-fold. Relaxation was associated with transient approximately 1.5-fold elevations in smooth muscle cAMP levels with both mechanical and nucleotide responses being abolished by interrupting gap junctional communication with the connexin-mimetic peptide Gap 27 and by endothelial denudation. However, IBMX induced a sustained endothelium-independent approximately 2-fold rise in cAMP levels, which was not further amplified by ACh, suggesting that the contribution of cAMP to the EDHF phenomenon is permissive. After selective loading of the endothelium with calcein AM, direct transfer of dye from the endothelium to the media was enhanced by IBMX or 8-bromo-cAMP, but not by 8-bromo-cGMP, whereas Gap 27 promoted sequestration within the intima. ACh-induced hyperpolarizations of subintimal smooth muscle in arterial strips with intact endothelium were abolished by Gap 27 and the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine but were unaffected by IBMX. By contrast, in strips partially denuded of endothelium, IBMX enhanced the transmission of hyperpolarization from the endothelium to remote smooth muscle cells. These findings support the hypothesis that endothelial hyperpolarization underpins the EDHF phenomenon, with cAMP governing subsequent electrotonic signaling via both myoendothelial and homocellular smooth muscle gap junctions.

Cellular target of voltage and calcium-dependent K(+) channel blockers involved in EDHF-mediated responses in rat superior mesenteric artery

1. We have investigated the cellular target of K(+) channel blockers responsible for the inhibition of the EDHF-mediated relaxation in the rat mesenteric artery by studying their effects on tension, smooth muscle cell (SMC) membrane potential and endothelial cell Ca(2+) signal ([Ca(2+)](endo)). 2. In arteries contracted with prostaglandin F(2 alpha) (2.5 - 10 microM), relaxation evoked by ACh (0.01 - 3 microM) was abolished by a combination of charybdotoxin (ChTX, 0.1 microM) plus apamin (Apa, 0.1 microM) and was inhibited by 68+/-6% (n=6) by 4-aminopyridine (4-AP, 5 mM). 3. ACh(0.001 - 3 microM) increased [Ca(2+)](endo) and hyperpolarized SMCs with the same potency, the pD(2) values were equal to 7.2+/-0.08 (n=4) and 7.2+/-0.07 (n=9), respectively. SMCs hyperpolarization to ACh (1 microM) was abolished by high K(+) solution or by ChTX/Apa. It was decreased by 66+/-5% (n=6) by 4-AP. 4. The increase in [Ca(2+)](endo) evoked by ACh (1 microM) was insensitive to ChTX/Apa but was depressed by 58+/-16% (n=6) and 27+/-4% (n=7) by raising external K(+) concentration and by 4-AP, respectively. 5. The effect of 4-AP on [Ca(2+)](endo) was not affected by increasing external K(+) concentration. In Ca-free/EGTA solution, the transient increase in [Ca(2+)](endo) evoked by ACh (1 microM) was abolished by thapsigargin (1 microM) and was decreased by 75+/-7% (n=5) by 4-AP. 6. These results show that inhibition of EDHF-evoked responses by 4-AP may be attributed to a decrease in the Ca(2+) release activated by ACh in endothelial cells. The abolition of SMCs hyperpolarization to ACh by ChTX/Apa is not related to an interaction with the [Ca(2+)](endo).

Endothelial G protein beta-subunits trigger nitric oxide-but not endothelium-derived hyperpolarizing factor-dependent dilation in rabbit resistance arteries

A single subtype of heterotrimeric G protein-coupled receptor controls both nitric oxide (NO) (sensitive to L-arginine analogues) and endothelium-derived hyperpolarizing factor (EDHF) (sensitive to high-external K(+) and apamine) production by the vascular endothelium leading to dilation. We hypothesized that alpha- and betagamma-subunits of the G protein serve as distinct intermediates to produce NO and EDHF. In pressurized resistance arteries, selective pinocytotic endothelial incorporation of specific antibodies (Abs) directed against alpha(q/11)-subunits abolished acetylcholine (Ach)-mediated dilation but failed to influence oxymetazoline (Oxy, alpha(2)-adrenergic receptor agonist)-induced dilation. In contrast, alpha(i1-2)-subunit Abs prevented Oxy- but not Ach-induced dilation. Thus, as expected, endothelial muscarinic and alpha(2)-adrenoceptors couple to G(q) protein and G(i) proteins, respectively. beta-subunit Abs reduced both Ach- and Oxy-induced dilation. The beta-subunit Abs abolished the nitro-L-arginine (L-NNA)-sensitive component but did not impair the high-external K(+)-sensitive component of the dilation induced by Ach and Oxy. Thus, G protein beta-subunits primarily accounted for NO production. Neutralization of Hsp90 and inhibition of the phospholipase C by U73122 (1 micromol/L) or intracellular Ca(2+) buffering with BAPTA-AM (10 micromol/L) sharply reduced NO-dependent but not K(+)-sensitive dilation. In conclusion, mobilization of the G protein beta-subunit is pivotal to NO-dependent dilation triggered through muscarinic and alpha(2)-adrenergic receptors. In contrast, receptor-operated EDHF-dependent dilation was insensitive to beta-subunit Abs. Although not directly activating the NO pathway, alpha-subunit activation is an absolute prerequisite for receptor-operated endothelium-dependent dilation of resistance arteries.

Dominant role of an endothelium-derived hyperpolarizing factor (EDHF)-like vasodilator in the ciliary vascular bed of the bovine isolated perfused eye

1. The roles of the endothelium-derived nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor (EDHF) in mediating vasodilator responses to acetylcholine and bradykinin were assessed in the ciliary vascular bed of the bovine isolated perfused eye preparation. 2. Vasodilatation to acetylcholine or bradykinin was unaffected by the nitric oxide synthase inhibitor, L-NAME (100 microM), or the cyclo-oxygenase inhibitor, flurbiprofen (30 microM), but was virtually abolished following treatment with a high concentration of KCl (30 mM), or by damaging the endothelium with the detergent, CHAPS (0.3%, 2 min). 3. Acetylcholine-induced vasodilatation was unaffected by glibenclamide (10 microM), an inhibitor of ATP-sensitive K(+) channels (K(+)(ATP)), but was significantly attenuated by TEA (10 mM), a non-selective inhibitor of K(+) channels. 4. The small conductance calcium-sensitive K(+) channel (SK(+)(Ca)) inhibitor, apamin (100 nM), and the large conductance calcium-sensitive K(+) channel (BK(+)(Ca)) inhibitor, iberiotoxin (50 nM), had no significant effect on acetylcholine-induced vasodilatation. In contrast, the intermediate (IK(+)(Ca))/large conductance calcium-sensitive K(+) channel inhibitor, charybdotoxin (50 nM), powerfully blocked these vasodilator responses, and uncovered a vasoconstrictor response. 5. The combination of apamin (100 nM) with a sub-threshold concentration of charybdotoxin (10 nM) significantly attenuated acetylcholine-induced vasodilatation, but the combination of apamin (100 nM) with iberiotoxin (50 nM) had no effect. 6. In conclusion, blockade by a high concentration of KCl, by charybdotoxin, or by the combination of apamin with a sub-threshold concentration of charybdotoxin, strongly suggests that vasodilatation in the bovine isolated perfused eye is mediated by an EDHF.

EDHF is not K+ but may be due to spread of current from the endothelium in guinea pig arterioles

Endothelium-derived hyperpolarizing factor (EDHF)-attributed hyperpolarizations and relaxations were recorded simultaneously from submucosal arterioles of guinea pigs with the use of intracellular microelectrodes and a video-based system, respectively. Membrane currents were recorded from electrically short segments of arterioles under single-electrode voltage clamp. Substance P evoked an outward current with a current-voltage relationship that was well described by the Goldman-Hodgkin-Katz equation for a K+ current, consistent with the involvement of intermediate- and small-conductance Ca2+-activated K+ channels. 1-Ethyl-2-benzimidazolinone relaxed the arterioles and evoked hyperpolarizations that were blocked by charybdotoxin, but not by iberiotoxin. Application of K+induced depolarization under conditions in which EDHF evoked hyperpolarization. The Ba2+-sensitive component of the K+-induced current was inwardly rectifying, in contrast to the outwardly rectifying current evoked by substance P. EDHF-attributed hyperpolarizations in dye-identified smooth muscle cells were indistinguishable from those recorded from dye-identified endothelial cells in the same arterioles. These results provide evidence that EDHF is not K+ but may involve electrotonic spread of hyperpolarization from the endothelial cells to the smooth muscle cells.

The uptake and metabolism of fructose-1,6-diphosphate in rat cardiomyocytes

Fructose-1,6-diphosphate (FDP) is a glycolytic intermediate which has been theorized to increase the metabolic activity of ischemic tissues. Here we examine the effects of externally applied FDP on cardiomyocyte uptake and metabolism. Adult rat cardiomyocytes were isolated and exposed to varying concentrations (0, 5, 25 and 50 mM) of FDP for either 1, 16 or 24 h of hypoxia (95% N2/5% CO2), each time period followed by a 1 h reoxygenation (95% air/5% CO2). The uptake of FDP by rat cardiomyocytes was more concentration-dependent than time-dependent. Furthermore, the uptake of FDP by the cardiomyocytes was similar in the hypoxia and normoxia treated cells. Alamar Blue, a redox indicator that is sensitive to metabolic activity, was used to monitor the effects of the FDP on cardiomyocyte metabolism. In the 1 h hypoxia or normoxia group, the 5, 10 and 25 mM FDP showed a significant increase in metabolism compared to the control cells. When the length of hypoxia was extended to 16 h, all doses of FDP were greater than control. And at the 24 h hypoxia or normoxia time period, only the 10, 25 and 50 mM FDP groups were greater than control. The results indicate a non-linear trend between the external concentration of FDP and the changes noted in metabolism. The findings from this study indicate that a narrow concentration range between 5-10 mM augments cardiomyocyte metabolism, but higher or lower doses may have little additional affect.

K+ currents underlying the action of endothelium-derived hyperpolarizing factor in guinea-pig, rat and human blood vessels

Membrane currents attributed to endothelium-derived hyperpolarizing factor (EDHF) were recorded in short segments of submucosal arterioles of guinea-pigs using single microelectrode voltage clamp. The functional responses of arterioles and human subcutaneous, rat hepatic and guinea-pig coronary arteries were also assessed as changes in membrane potential recorded simultaneously with contractile activity. The current-voltage (I-V) relationship for the conductance due to EDHF displayed outward rectification with little voltage dependence. Components of the current were blocked by charybdotoxin (30-60 nM) and apamin (0.25-0.50 microM), which also blocked hyperpolarization and prevented EDHF-induced relaxation. The EDHF-induced current was insensitive to Ba2+ (20-100 microM) and/or ouabain (1 microM to 1 mM). In human subcutaneous arteries and guinea-pig coronary arteries and submucosal arterioles, the EDHF-induced responses were insensitive to Ba2+ and/or ouabain. Increasing [K+]o to 11-21 mM evoked depolarization under conditions in which EDHF evoked hyperpolarization. Responses to ACh, sympathetic nerve stimulation and action potentials were indistinguishable between dye-labelled smooth muscle and endothelial cells in arterioles. Action potentials in identified endothelial cells were always associated with constriction of the arterioles. 18beta-Glycyrrhetinic acid (30 microM) and carbenoxolone (100 microM) depolarized endothelial cells by 31 +/- 6 mV (n = 7 animals) and 33 +/- 4 mV (n = 5), respectively, inhibited action potentials in smooth muscle and endothelial cells and reduced the ACh-induced hyperpolarization of endothelial cells by 56 and 58 %, respectively. Thus, activation of outwardly rectifying K+ channels underlies the hyperpolarization and relaxation due to EDHF. These channels have properties similar to those of intermediate conductance (IKCa) and small conductance (SKCa) Ca2+-activated K+ channels. Strong electrical coupling between endothelial and smooth muscle cells implies that these two layers function as a single electrical syncytium. The non-specific effects of glycyrrhetinic acid precludes its use as an indicator of the involvement of gap junctions in EDHF-attributed responses. These conclusions are likely to apply to a variety of blood vessels including those of humans.

Intercellular Ca2+ signalling: the artery wall

The fact that many cells in the cardiovascular system are coupled via gap junctions enables direct intercellular Ca2+ signalling. Ca2+ ions and/or inositol trisphosphate (IP3) can pass through these aqueous pores. Cell-cell coupling can occur between cells of the same type, or via special junctions between adjacent cells of different types. Homocellular coupling acts to amplify and prolong Ca2+ signals, whereas heterocellular coupling allows complex interactions between the cells, including the modulation of their respective responses. This review will focus on the interactions between cells that form the blood vessel wall, illustrating how cell-cell communication defines important physiological functions.

Role of EDHF in the vasodilatory effect of loop diuretics in guinea-pig mesenteric resistance arteries

1. Relaxing effect of loop diuretics, piretanide and furosemide in comparison with acetylcholine (ACh) was investigated in guinea-pig isolated mesenteric resistance arteries. 2. Concentration-response curves to ACh (0.001 - 10 microM) and diuretics (0.0001 - 1 microM) were constructed in noradrenaline (10 - 30 microM)-precontracted arteries incubated either in normal physiological salt solution (PSS) or in 30 mM KCl PSS (K-PSS). 3. In PSS, maximal relaxations (R(max)) and pD(2) to ACh were 87+/-2% and 7.1+/-0.1 (n=10). L-N(G)-nitro-arginine methyl ester (L-NAME, 100 microM) reduced R(max) by 20% (P<0.01, n=7) and pD(2) by 10% (P<0.01). In contrast, indomethacin (10 microM) increased R(max) by 19% (P<0.01, n=8) and pD(2) by 10% (P<0.05). Combination of L-NAME+indomethacin reversed the effect observed with either of these inhibitors used alone. In K-PSS, R(max) was attenuated by 40% (P<0.001, n=6) compared to PSS. L-NAME reduced R(max) by 65% (P<0.01, n=5) and increased pD(2) by 15 fold. L-NAME+indomethacin suppressed the resistant relaxation. 4. In PSS+L-NAME+indomethacin, inhibitors of small (SK(Ca); apamin, 0.1 microM) and large (BK(Ca); iberiotoxin and charybdotoxin, 0.1 microM) conductance Ca(2+)-sensitive K(-)-channels used alone had little effect on the ACh-response. Combination of apamin+iberiotoxin reduced R(max) by 40% (P<0.05, n=7) while apamin+charybdotoxin fully abolished the resistant relaxation. 5. In PSS, piretanide and furosemide induced relaxation with R(max): 89+/-3% vs 84+/-5% and pD(2): 8.5+/-0.1 vs 7.7+/-0.2 (P<0.01) for piretanide (n=11) and furosemide (n=10), respectively. Endothelial abrasion suppressed relaxation to diuretics. L-NAME and indomethacin used alone or in combination did not significantly modify the response to diuretics. 6. In K-PSS, piretanide-induced relaxation was abolished whereas that to furosemide was reduced by 70% (P<0.001, n=9) compared to PSS and was suppressed by L-NAME+indomethacin. In PSS+L-NAME+indomethacin, apamin slightly reduced relaxation to diuretics whereas charybdotoxin or iberiotoxin abolished the response. 7. These results indicate that ACh-evoked relaxation is mediated by both NO/PGl(2)-dependent and -independent mechanisms. The EDHF-dependent component relies on activation of Ca(2+)-activated K(+) channels, is sensitive to a combination of apamin+charybdotoxin and to a smaller degree to a combination of apamin+iberiotoxin. Loop diuretic-induced relaxation is endothelium-dependent, appears to be mediated by NO, PGl(2) and EDHF for furosemide and EDHF only for piretanide. For the two diuretics, opening of BK(Ca) channels may be involved in the relaxation.

Role of endothelial cell hyperpolarization in EDHF-mediated responses in the guinea-pig carotid artery

1. Experiments were performed to identify the potassium channels involved in the acetylcholine-induced endothelium-dependent hyperpolarization of the guinea-pig internal carotid artery. Smooth muscle and endothelial cell membrane potentials were recorded in isolated arteries with intracellular microelectrodes. Potassium currents were recorded in freshly-dissociated smooth muscle cells using patch clamp techniques. 2. In single myocytes, iberiotoxin (0.1 microM)-, charybdotoxin (0.1 microM)-, apamin (0.5 microM)- and 4-aminopyridine (5 mM)-sensitive potassium currents were identified indicating the presence of large- and small-conductance calcium-sensitive potassium channels (BK(Ca) and SK(Ca)) as well as voltage-dependent potassium channels (K(V)). Charybdotoxin and iberiotoxin inhibited the same population of BK(Ca) but a conductance specifically sensitive to the combination of charybdotoxin plus apamin could not be detected. 4-aminopyridine (0. 1 - 25 mM) induced a concentration-dependent inhibition of K(V) without affecting the iberiotoxin- or the apamin-sensitive currents. 3. In isolated arteries, both the endothelium-dependent hyperpolarization of smooth muscle and the hyperpolarization of endothelial cells induced by acetylcholine or by substance P were inhibited by 5 mM 4-aminopyridine. 4. These results indicate that in the vascular smooth muscle cells of the guinea-pig carotid artery, a conductance specifically sensitive to the combination of charybdotoxin plus apamin could not be detected, comforting the hypothesis that the combination of these two toxins should act on the endothelial cells. Furthermore, the inhibition by 4-aminopyridine of both smooth muscle and endothelial hyperpolarizations, suggests that in order to observe an endothelium-dependent hyperpolarization of the vascular smooth muscle cells, the activation of endothelial potassium channels is likely to be required.

Role of gap junctions and EETs in endothelium-dependent hyperpolarization of porcine coronary artery

1. The effects of endothelium-derived hyperpolarizing factor (EDHF: elicited using substance P or bradykinin) were compared with those of 11,12-EET in pig coronary artery. Smooth muscle cells were usually impaled with microelectrodes through the adventitial surface. 2. Substance P (100 nM) and 11,12-EET (11,12-epoxyeicosatrienoic acid; 3 microM) hyperpolarized endothelial cells in intact arteries. These actions were unaffected by 100 nM iberiotoxin but were abolished by charybdotoxin plus apamin (each 100 nM). 3. Substance P (100 nM) and bradykinin (30 nM) hyperpolarized intact artery smooth muscle; Substance P had no effect after endothelium removal. 11,12-EET hyperpolarized de-endothelialized vessels by 12.6+/-0.3 mV, an effect abolished by 100 nM iberiotoxin. 4. 11,12-EET hyperpolarized intact arteries by 18.6+/-0.8 mV, an action reduced by iberiotoxin, which was ineffective against substance P. Hyperpolarizations to 11, 12-EET and substance P were partially inhibited by 100 nM charybdotoxin and abolished by further addition of 100 nM apamin. 5. 30 microM barium plus 500 nM ouabain depolarized intact artery smooth muscle but responses to substance P and bradykinin were unchanged. 500 microM gap 27 markedly reduced hyperpolarizations to substance P and bradykinin which were abolished in the additional presence of barium plus ouabain. 6. Substance P-induced hyperpolarizations of smooth muscle cells immediately below the internal elastic lamina were unaffected by gap 27, even in the presence of barium plus ouabain. 7. In pig coronary artery, 11,12-EET is not EDHF. Smooth muscle hyperpolarizations attributed to 'EDHF' are initiated by endothelial cell hyperpolarization involving charybdotoxin- (but not iberiotoxin) and apamin-sensitive K(+) channels. This may spread electrotonically via myoendothelial gap junctions but the involvement of an unknown endothelial factor cannot be excluded.

K(+) channel blockers and cytochrome P450 inhibitors on acetylcholine-induced, endothelium-dependent relaxation in rabbit mesenteric artery

Acetylcholine caused an endothelium-dependent relaxation in isolated rabbit mesenteric small artery in the presence of nitro L-arginine and indomethacin. The acetylcholine-induced relaxation was attenuated by high K(+) solution, suggesting that the response is mediated by a membrane potential-sensitive mechanism, presumably an endothelium-derived hyperpolarizing factor. The acetylcholine-induced relaxation was also inhibited with tetraethylammonium, 4-aminopyridine and charybdotoxin, but not with Ba(2+), apamin, iberiotoxin nor glibenclamide. The relaxation was abolished by a combination of apamin and charybdotoxin, but iberiotoxin could not replace charybdotoxin in this combination. The responses to charybdotoxin and 4-aminopyridine were synergistic but neither apamin nor iberiotoxin increased the effect of 4-aminopyridine. Clotrimazole and proadifen inhibited the acetylcholine-induced relaxation, but these drugs also inhibited the cromakalim-induced relaxation, while protoporphyrin IX inhibited the acetylcholine- but not cromakalim-induced relaxation. 17-Octadecynoic acid and 1-aminobenzotriazole did not affect the response to acetylcholine. Four regioisomers of epoxyeicosatrienoic acids did not relax endothelium-denuded artery. A gap junction inhibitor 18alpha-glycyrrhetinic acid attenuated the relaxation to acetylcholine. It is suggested that in rabbit mesenteric artery, the acetylcholine-induced, nitric oxide- and prostacyclin-independent relaxation is mainly mediated by 4-aminopyridine- and charybdotoxin-sensitive K(+) channels and that the relaxation is not mediated through cytochrome P450 enzyme metabolites. The contribution of heterocellular gap junctional communication to the relaxation is discussed.