Stephanie E. Wölfle

Impaired Endothelium-Derived Hyperpolarizing Factor-Mediated Dilations and Increased Blood Pressure in Mice Deficient of the Intermediate-Conductance Ca2+-Activated K+ Channel

The endothelium plays a key role in the control of vascular tone and alteration in endothelial cell function contributes to several cardiovascular disease states. Endothelium-dependent dilation is mediated by NO, prostacyclin, and an endothelium-derived hyperpolarizing factor (EDHF). EDHF signaling is thought to be initiated by activation of endothelial Ca2+-activated K+ channels (KCa), leading to hyperpolarization of the endothelium and subsequently to hyperpolarization and relaxation of vascular smooth muscle. In the present study, we tested the functional role of the endothelial intermediate-conductance KCa (IKCa/KCa3.1) in endothelial hyperpolarization, in EDHF-mediated dilation, and in the control of arterial pressure by targeted deletion of KCa3.1. KCa3.1-deficient mice (KCa3.1−/−) were generated by conventional gene-targeting strategies. Endothelial KCa currents and EDHF-mediated dilations were characterized by patch-clamp analysis, myography and intravital microscopy. Disruption of the KCa3.1 gene abolished endothelial KCa3.1 currents and significantly diminished overall current through KCa channels. As a consequence, endothelial and smooth muscle hyperpolarization in response to acetylcholine was reduced in KCa3.1−/− mice. Acetylcholine-induced dilations were impaired in the carotid artery and in resistance vessels because of a substantial reduction of EDHF-mediated dilation in KCa3.1−/− mice. Moreover, the loss of KCa3.1 led to a significant increase in arterial blood pressure and to mild left ventricular hypertrophy. These results indicate that the endothelial KCa3.1 is a fundamental determinant of endothelial hyperpolarization and EDHF signaling and, thereby, a crucial determinant in the control of vascular tone and overall circulatory regulation.

Genetic deficit of SK3 and IK1 channels disrupts the endothelium-derived hyperpolarizing factor vasodilator pathway and causes hypertension.

Background— It has been proposed that activation of endothelial SK3 (KCa2.3) and IK1 (KCa3.1) K+ channels plays a role in the arteriolar dilation attributed to an endothelium-derived hyperpolarizing factor (EDHF). However, our understanding of the precise function of SK3 and IK1 in the EDHF dilator response and in blood pressure control remains incomplete. To clarify the roles of SK3 and IK1 channels in the EDHF dilator response and their contribution to blood pressure control in vivo, we generated mice deficient for both channels. Methods and Results— Expression and function of endothelial SK3 and IK1 in IK1−/−/SK3T/T mice was characterized by patch-clamp, membrane potential measurements, pressure myography, and intravital microscopy. Blood pressure was measured in conscious mice by telemetry. Combined IK1/SK3 deficiency in IK1−/−/SK3T/T (+doxycycline) mice abolished endothelial KCa currents and impaired acetylcholine-induced smooth muscle hyperpolarization and EDHF-mediated dilation in conduit arteries and in resistance arterioles in vivo. IK1 deficiency had a severe impact on acetylcholine-induced EDHF-mediated vasodilation, whereas SK3 deficiency impaired NO-mediated dilation to acetylcholine and to shear stress stimulation. As a consequence, SK3/IK1-deficient mice exhibited an elevated arterial blood pressure, which was most prominent during physical activity. Overexpression of SK3 in IK1−/−/SK3T/T mice partially restored EDHF- and nitric oxide–mediated vasodilation and lowered elevated blood pressure. The IK1-opener SKA-31 enhanced EDHF-mediated vasodilation and lowered blood pressure in SK3-deficient IK1+/+/SK3T/T (+doxycycline) mice to normotensive levels. Conclusions— Our study demonstrates that endothelial SK3 and IK1 channels have distinct stimulus-dependent functions, are major players in the EDHF pathway, and significantly contribute to arterial blood pressure regulation. Endothelial KCa channels may represent novel therapeutic targets for the treatment of hypertension.

Myoendothelial Coupling Is Not Prominent in Arterioles Within the Mouse Cremaster Microcirculation In Vivo

A smooth muscle hyperpolarization is essential for endothelium-dependent hyperpolarizing factor–mediated dilations. It is debated whether the hyperpolarization is induced by a factor (endothelium-derived hyperpolarizing factor) and/or is attributable to direct current transfer from the endothelium via myoendothelial gap junctions. Here, we measured membrane potential in endothelial cells (EC) and smooth muscle cells (SMC) in vivo at rest and during acetylcholine (ACh) application in the cremaster microcirculation of mice using sharp microelectrodes before and after application of specific blockers of Ca2+dependent K+ channels (KCa). Moreover, diameter changes in response to ACh were studied. Membrane potential at rest was lower in EC than SMC (−46.6±1.0 versus −36.5±1.0mV, P<0.05). Bolus application of ACh induced robust hyperpolarizations in EC and SMC, but the amplitude (11.1±0.9 versus 5.1±0.9mV, P<0.05) and duration of the response (10.7±0.8 versus 7.5±1.0s, P<0.05) were larger in EC. Blockers of large conductance KCa (charybdotoxin or iberiotoxin) abrogated ACh-induced hyperpolarizations in SMC but did not alter endothelial hyperpolarizations. In contrast, apamin, a blocker of small conductance KCa abolished ACh-induced hyperpolarizations in EC and had only small effects on SMC. ACh-induced dilations were strongly attenuated by iberiotoxin but only slightly by apamin. We conclude that myoendothelial coupling in arterioles in vivo in the murine cremaster is weak, as EC and SMC behaved electrically different. Small conductance KCa mediate endothelial hyperpolarization in response to ACh, whereas large conductance KCa are important in SMC. Because tight myoendothelial coupling was found in vitro in previous studies, we suggest that it is differentially regulated between vascular beds and/or by mechanisms acting in vivo.

Connexin45 Cannot Replace the Function of Connexin40 in Conducting Endothelium-Dependent Dilations Along Arterioles

Intercellular communication through gap junctions coordinates vascular tone by the conduction of vasomotor responses along the vessel wall. Gap junctions in arterioles are composed of different connexins (Cxs) (Cx40, Cx37, Cx45, Cx43), but it is unknown whether Cxs are interchangeable. We used mice with a targeted replacement of Cx40 by Cx45 (Cx40KI45) to explore whether Cx45 can functionally replace Cx40 in arterioles. Arterioles were locally stimulated using acetylcholine, bradykinin, adenosine, and K+ in the cremaster of Cx40KI45, Cx40-deficient (Cx40ko), and wild-type mice, and diameter changes were assessed by intravital microscopy. Additionally, arterial pressure was measured by telemetry and Cx expression verified by immunofluorescence. Acetylcholine initiated a local dilation of a similar amplitude in all genotypes (≈50%), which was rapidly conducted to upstream sites (1200 &mgr;m distance) without attenuation in wild type. In marked contrast, the remote dilation was significantly reduced in Cx40ko (25±3%) and Cx40KI45 (24±2%). Likewise, dilations initiated by bradykinin application were conducted without attenuation up to 1200 &mgr;m in wild type but not in Cx40ko and Cx40KI45. Adenosine-induced dilations and K+-induced constrictions were conducted similarly with decaying amplitude in all genotypes. Arterial pressure was strongly elevated in Cx40ko (161±1 versus 116±2 mm Hg) but only moderately in Cx40KI45 (133±8 mm Hg). This demonstrates that Cx40 function is critical for the conduction of acetylcholine and bradykinin dilations and cannot be substituted by Cx45. Therefore, unique properties of Cx40 are required for endothelial signal conduction, whereas nonspecific restoration of communication maintains additional functions related to blood pressure control.

Intact Endothelium-Dependent Dilation and Conducted Responses in Resistance Vessels of Hypercholesterolemic Mice in vivo

Atherosclerosis and hyperlipidemia impair endothelium-dependent nitric oxide (NO)-mediated dilations in conducting arteries. In addition to NO, the endothelium releases an endothelium-derived hyperpolarizing factor (EDHF) in response to acetylcholine (ACh), which is particularly important in microvessels and initiates a dilation that conducts along the vessel through gap junctional communication. The expression of connexins is, however, altered by hypercholesterolemia. Therefore, we studied endothelium-dependent dilations and their conduction in murine hypercholesterolemic models. Dilations were assessed by intravital microscopy in arterioles with a diameter of ∼35 µm in ApoE and LDL receptor (LDLR–/–)-deficient mice after superfusion or locally confined application of ACh. ACh induced comparable concentration-dependent dilations in wild-type, LDLR–/–, and ApoE–/– mice fed a normal or high-cholesterol diet, however EC50 was slightly higher in ApoE–/– mice. Furthermore, the NO donor sodium-nitroprusside dilated arterioles to a similar extent (∼60%). Locally initiated ACh dilations (∼68%) conducted up to a distance of 1,100 µm without significant attenuation even under severe hypercholesterolemic conditions. Since ACh dilation in the arterioles of mice is mainly mediated via EDHF, we conclude that hypercholesterolemia does not alter EDHF release and efficacy. This conclusion is confirmed by an intact conducted response since EDHF is a prerequisite for this response. The intact conduction also suggests that gap-junctional communication is functionally preserved in these models.

Prominent role of KCa3.1 in endothelium-derived hyperpolarizing factor-type dilations and conducted responses in the microcirculation in vivo

AIMS The activation of endothelial Ca2+-dependent K+-channels, KCa3.1 (IKCa), and KCa2.3 (SKCa) has been proposed to be a prerequisite for endothelial hyperpolarization, which subsequently hyperpolarizes and relaxes smooth muscle [endothelium-derived hyperpolarizing factor (EDHF)-type dilation] and initiates conducted dilations. Although EDHF is the main mediator of acetylcholine (ACh)-induced dilation in the murine skeletal microcirculation, the differential contribution of KCa3.1 and KCa2.3 is not known. METHODS AND RESULTS We assessed agonist-induced and conducted dilations as well as endothelial hyperpolarization in the cremaster microcirculation of KCa3.1-deficient (KCa3.1-/-) and wild-type mice (wt) in vivo after blockade of NO and prostaglandins. Compared with wt, resting tone was enhanced by approximately 25% in arterioles of KCa3.1-/- mice. ACh-induced dilations in KCa3.1-/- mice were virtually abolished at low and intermediate concentrations and a remaining dilation at 10 micromol/L ACh was abrogated by blockade of KCa2.3 with UCL1684. Sodium nitroprusside- and adenosine-induced dilations were similar in wt and KCa3.1-/-. Focal application of ACh induced dilations at the local site in both genotypes, which conducted along the vessel. However, the amplitude of the dilation decreased with distance only in KCa3.1-/-. Blockade of KCa2.3 in wt did not affect conducted dilations. A KCa3.1 opener induced a conducting dilation in wt but not in KCa3.1-/-. Membrane potential recordings in vivo demonstrated endothelial hyperpolarization in response to ACh in both genotypes; however, the hyperpolarization was severely impaired in KCa3.1-/- (Delta membrane potential: -3 +/- 1 vs. -14 +/- 2 mV). CONCLUSION We conclude that KCa3.1 is of major importance for endothelial hyperpolarization and EDHF-type responses in skeletal muscle arterioles, and its deficiency is not compensated by KCa2.3. Sole activation of KCa3.1 is capable of initiating conducted responses, and KCa3.1 may contribute to the propagation of the signal, although its presence is not mandatory.

Endothelial mediators and communication through vascular gap junctions

Abstract Cellular interaction in vessels is achieved by multiple communication pathways, including gap junctions (GJs). They provide intercellular channels, allowing direct interaction of endothelial and smooth muscle cells and the coordination of cellular behaviour along the vessel. The latter is a prerequisite for large flow increases because an adaptation of resistance along the vessel length is required. Longitudinal communication is studied by confined local stimulation of arterioles and the observation of responses at distant locations. Certain vascular stimuli induce local and concomitant remote responses of a similar type, verifying rapid longitudinal conduction of vasomotor signals, most likely changes in membrane potential. This is achieved for dilatory responses via the endothelium, possibly by an endothelium-derived hyperpolarising factor (EDHF) that induces local hyperpolarisation, which is then transferred to remote sites through GJs. In vessels, GJs are composed of different connexins (Cx), but Cx40 is of special importance because its lack impairs longitudinal conduction of vasodilations. Interestingly, Cx40-deficient mice are hypertensive, suggesting that Cx40-dependent coupling is necessary to regulate vascular behaviour and peripheral resistance. While the role of other connexins is less well established, an abundance of data has proven the necessity of GJ communication to coordinate vascular behaviour during blood flow regulation.

Connexin-dependent communication within the vascular wall: contribution to the control of arteriolar diameter.

Communication between cells is important to the microcirculation and enables the coordination of cellular behavior along the length of the vessel. Arterioles span considerable distances within the microcirculatory network, and thus flow changes require the adaptation of vessel resistance over the whole length of the vessel in order to be effective. Such a task requires communication along the vessel wall, and gap junction channels that connect endothelial as well as smooth muscle cells with each other set the stage for this requirement. Communication along the vessel wall can be tested experimentally by confined local stimulation of arterioles either in vitro or in vivo. Certain vascular stimuli induce both a local response and a concomitant uniform remote response, confirming the rapid conduction of vasomotor stimuli along the vessel wall. Gap junctions in vascular tissue are composed of connexins (Cx) Cx40, Cx43, Cx37 and Cx45. Of these, Cx40 is of special importance: its lack results in a deficient conduction of vasodilator stimuli along the vessel wall. Interestingly, Cx40-deficient mice display an elevated mean arterial pressure, suggesting that Cx40-depending gap junctional coupling is necessary to regulate vascular behavior and peripheral resistance. While the role of other connexins is less well established, an abundance of experimental data has proven the necessity of gap junctional communication to coordinate vascular behavior during adaptive blood flow regulation.

Connexins in the Vasculature

Vascular function requires the highly coordinated behavior of individual cells. The modulation of vascular resistance and blood flow required to match oxygen delivery to a wide dynamic range of tissue needs can be achieved only by coordinated diameter changes over large distances along the vessel. This is accomplished by longitudinal long-distance communication through the vessel wall by gap junctions. The vascular endothelial cells are structurally suited for this task and are coupled extraordinarily well to form a functional unit within the vessel wall. Cx40 is the most abundant connexin in the vascular endothelium. Its loss results in functional deficits, such as hypertension and a lack of coordination of vascular responses. Gap junctions also couple the vascular smooth muscle cells. These intercellular junctions are formed by Cx43, though evidence of Cx45 expression has been provided recently. In addition, gap junctions interconnect endothelial and smooth muscle cells heterocellularly to create short-distance transverse pathways. This theoretically allows reciprocal direct communication pathways between the cells of these two vascular compartments and could potentially provide for the function of the elusive endothelium-derived hyperpolarizing factor.

Tanks to the Reviewers

The Editors and the members of the editorial board would like to thank the following individuals for their expert assistance in acting as reviewers for the Journal of Vascular Research in the period of September 2004 to September 2005. Our aim of establishing the journal as a leading journal in the fi eld of vascular science is to a large extent in the hands of our reviewers and we, together with our authors and readers, much appreciate the time which they have freely given.