K+ channels which contribute to the acetylcholine-induced hyperpolarization in smooth muscle of the guinea-pig submucosal arteriole

Role of Pericytes in the Initiation and Propagation of Spontaneous Activity in the Microvasculature

The microvasculature is composed of arterioles, capillaries and venules. Spontaneous arteriolar constrictions reduce effective vascular resistance to enhance tissue perfusion, while spontaneous venular constrictions facilitate the drainage of tissue metabolites by pumping blood. In the venules of visceral organs, mural cells, i.e. smooth muscle cells (SMCs) or pericytes, periodically generate spontaneous phasic constrictions, Ca²⁺ transients and transient depolarisations. These events arise from spontaneous Ca²⁺ release from the sarco-endoplasmic reticulum (SR/ER) and the subsequent opening of Ca²⁺-activated chloride channels (CaCCs). CaCC-dependent depolarisation further activates L-type voltage-dependent Ca²⁺ channels (LVDCCs) that play a critical role in maintaining the synchrony amongst mural cells. Mural cells in arterioles or capillaries are also capable of developing spontaneous activity. Non-contractile capillary pericytes generate spontaneous Ca²⁺ transients primarily relying on SR/ER Ca²⁺ release. Synchrony amongst capillary pericytes depends on gap junction-mediated spread of depolarisations resulting from the opening of either CaCCs or T-type VDCCs (TVDCCs) in a microvascular bed-dependent manner. The propagation of capillary Ca²⁺ transients into arterioles requires the opening of either L- or TVDCCs again depending on the microvascular bed. Since the blockade of gap junctions or CaCCs prevents spontaneous Ca²⁺ transients in arterioles and venules but not capillaries, capillary pericytes appear to play a primary role in generating spontaneous activity of the microvasculature unit. Pericytes in capillaries where the interchange of substances between tissues and the circulation takes place may provide the fundamental drive for upstream arterioles and downstream venules so that the microvasculature network functions as an integrated unit.

Role of K+ channels in maintaining the synchrony of spontaneous Ca2+ transients in the mural cells of rat rectal submucosal arterioles

Mural cells in precapillary arterioles (PCAs) generate spontaneous Ca²⁺ transients primarily arising from the periodic release of Ca²⁺ from sarcoendoplasmic reticulum (SR/ER). The Ca²⁺ release induces Ca²⁺-activated chloride channel (CaCC)-dependent depolarisations that spread to neighbouring mural cells to develop the synchrony of their Ca²⁺ transients. Here, we explored the roles of K⁺ channels in maintaining the synchrony of spontaneous Ca²⁺ transients. Intracellular Ca²⁺ dynamics in mural cells were visualised by Cal-520 fluorescence Ca²⁺ imaging in the submucosal PCAs of rat rectum. Increasing extracellular K⁺ concentration ([K⁺]_o) from 5.9 to 29.7 mM converted synchronous spontaneous Ca²⁺ transients into asynchronous, high-frequency Ca²⁺ transients. Similarly, the blockade of inward rectifier K⁺ (K_ir) channels with Ba²⁺ (50 μM) or K_v7 voltage-dependent K⁺ (K_v7) channels with XE 991 (10 μM) disrupted the synchrony of spontaneous Ca²⁺ transients, while the blockers for large-, intermediate- or small-conductance Ca²⁺-activated K⁺ channels had no effect. K_ir2.1 immunoreactivity was detected in the arteriolar endothelium but not mural cells. In the PCAs that had been pretreated with XE 991 or Ba²⁺, nifedipine (1 μM) attenuated the asynchronous Ca²⁺ transients but failed to restore their synchrony. In contrast, levcromakalim, an ATP-sensitive K⁺ channel opener, restored the synchronous Ca²⁺ transients. Thus, constitutively active K_v7 and K_ir channels appear to be involved in maintaining the relatively hyperpolarised membrane of mural cells. The hyperpolarised membrane prevents depolarisation-induced 'premature' Ca²⁺ transients to ensure sufficient SR/ER Ca²⁺ refilling that is required for regenerative Ca²⁺ release resulting in synchronous Ca²⁺ transients amongst the mural cells.

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.

Nonlinear effects of potassium channel blockers on endothelium-dependent hyperpolarization

In a number of published studies on endothelium-dependent hyperpolarization and relaxation, the results of the effects of K⁺ blockers have been difficult to interpret. When the effects of two blockers have been studied, often either blocker by itself had little effect, whereas the two blockers combined tended to abolish the responses. Explanations suggested in the literature include an unusual pharmacology of the K⁺ channels, and possible blocker binding interactions. In contrast, when we applied the same blockers to segments of small blood vessels under voltage clamp, the blockers reduced the endothelium-dependent K⁺ current in a linearly additive manner. Resolution of these contrasting results is important as endothelium-derived hyperpolarization (EDH) makes its greatest contribution to vasorelaxation in arterioles and small resistance arteries, where it can exert a significant role in tissue perfusion and blood pressure regulation. Furthermore, EDH is impaired in various diseases. Here, we consider why the voltage-clamp results differ from earlier free-running membrane potential and contractility studies. We fitted voltage-clamp-derived current-voltage relationships with mathematical functions and considered theoretically the effects of partial and total block of endothelium-derived K⁺ -currents on the membrane potential of small blood vessels. When the K⁺ -conductance was partially reduced, equivalent to applying a single blocker, the effect on EDH was small. Total block of the endothelium-dependent K⁺ conductance abolished the hyperpolarization, in agreement with various published studies. We conclude that nonlinear summation of the hyperpolarizing response evoked by endothelial stimulation can explain the variable effectiveness of individual K⁺ channel blockers on endothelium-dependent hyperpolarization and resulting relaxation.

A1-adenosine acute withdrawal response and cholecystokinin-8 induced contractures are regulated by Ca(2+)- and ATP-activated K(+) channels

In isolated guinea-pig ileum (GPI), the A1-adenosine acute withdrawal response is under the control of several neuronal signalling systems, including the μ/κ-opioid and the cannabinoid CB1 systems. It is now well established that after the stimulation of the A1-adenosine system, the indirect activation of both μ/κ-opioid and CB1 systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated the involvement of the Ca(2+)/ATP-activated K(+) channels in the regulation of both acute A1-withdrawal and CCk-8-induced contractures in the GPI preparation. Interestingly, we found that: (a) the A1-withdrawal contracture is inhibited by voltage dependent Ca(2+)-activated K(+) channels, Kv, while it is enhanced by the voltage independent Ca(2+)-activated K(+) channels, SKCa; (b) in the presence of CCk-8, the inhibitory effect of the A1 agonist, CPA, on the peptide induced contracture is significantly enhanced by the voltage independent Ca(2+)-activated K(+) channel, SKCa; and (c) the A1-withdrawal contracture precipitated in the presence of CCk-8 is controlled by the ATP-sensitive potassium channels, KATP. Our data suggest, for the first time, that both Ca(2+)- and ATP-activated K(+) channels are involved in the regulation of both A1-withdrawal precipitated and CCk-8 induced contractures.

Effects of gap junction inhibition on contraction waves in the murine small intestine in relation to coupled oscillator theory

Waves of contraction in the small intestine correlate with slow waves generated by the myenteric network of interstitial cells of Cajal. Coupled oscillator theory has been used to explain steplike gradients in the frequency (frequency plateaux) of contraction waves along the length of the small intestine. Inhibition of gap junction coupling between oscillators should lead to predictable effects on these plateaux and the wave dislocation (wave drop) phenomena associated with their boundaries. It is these predictions that we wished to test. We used a novel multicamera diameter-mapping system to measure contraction along 25- to 30-cm lengths of murine small intestine. There were typically two to three plateaux per length of intestine. Dislocations could be limited to the wavefronts immediately about the terminated wave, giving the appearance of a three-pronged fork, i.e., a fork dislocation; additionally, localized decreases in velocity developed across a number of wavefronts, ending with the terminated wave, which could appear as a fork, i.e., slip dislocations. The gap junction inhibitor carbenoxolone increased the number of plateaux and dislocations and decreased contraction wave velocity. In some cases, the usual frequency gradient was reversed, with a plateau at a higher frequency than its proximal neighbor; thus fork dislocations were inverted, and the direction of propagation was reversed. Heptanol had no effect on the frequency or velocity of contractions but did reduce their amplitude. To understand intestinal motor patterns, the pacemaker network of the interstitial cells of Cajal is best evaluated as a system of coupled oscillators.

Protection of coronary endothelial function during cardiac surgery: potential of targeting endothelial ion channels in cardioprotection

Vascular endothelium plays a critical role in the control of blood flow by producing vasoactive factors to regulate vascular tone. Ion channels, in particular, K⁺ channels and Ca²⁺-permeable channels in endothelial cells, are essential to the production and function of endothelium-derived vasoactive factors. Impairment of coronary endothelial function occurs in open heart surgery that may result in reduction of coronary blood flow and thus in an inadequate myocardial perfusion. Hyperkalemic exposure and concurrent ischemia-reperfusion during cardioplegic intervention compromise NO and EDHF-mediated function and the impairment involves alterations of K⁺ channels, that is, K_ATP and K_Ca, and Ca²⁺-permeable TRP channels in endothelial cells. Pharmacological modulation of these channels during ischemia-reperfusion and hyperkalemic exposure show promising results on the preservation of NO and EDHF-mediated endothelial function, which suggests the potential of targeting endothelial K⁺ and TRP channels for myocardial protection during cardiac surgery.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Neurohumoral regulation of spontaneous constrictions in suburothelial venules of the rat urinary bladder

Venules of the bladder suburothelium develop spontaneous phasic constrictions that may play a critical role in maintaining venular drainage of tissue metabolites. We aimed to investigate neurohumoral regulation of the spontaneous venular constrictions (SVCs). Changes in venular diameter of the rat bladder suburothelium were monitored using a video tracking system, whilst the effects of electrical field stimulation (EFS) and bath-applied bioactive substances were investigated. The innervation of the suburothelial microvasculature was examined by immunohistochemistry. EFS (10Hz for 30s) induced an increase in the frequency of SVCs that was prevented by phentolamine (1μM). In phentolamine-pretreated venules, EFS suppressed SVCs with a venular dilatation in a manner attenuated by propranolol (1μM) or l-nitro arginine (LNA, 10μM). BRL37344 (1μM), a β3 adrenoceptor agonist, dilated venules and reduced the frequency of SVCs in an LNA-sensitive manner. ACh (1-10μM) increased the frequency of SVCs. ATP (1μM) transiently constricted venules and then caused LNA-sensitive cessation of SVCs associated with a dilatation. Substance P (100nM) caused a venular constriction, whilst calcitonin gene related peptide (CGRP, 100nM) caused a dilatation of venules and suppression of SVCs that were not inhibited by LNA. Immunohistochemical staining demonstrated sympathetic as well as substance P- and CGRP-containing nerves running along the venules. Spontaneous constrictions of suburothelial venules are accelerated by sympathetic α-adrenergic stimulation, but suppressed upon β-adrenergic stimulation. In addition, suburothelial venular constrictions appear to be modulated by several bioactive substances that could be released from urothelium or suburothelial sensory nerves.

Calcium-activated potassium channels in vasculature in response to ischemia-reperfusion

Based on the genetic relationship, single-channel conductance, and gating mechanisms, calcium-activated potassium (KCa) channels identified in vasculature can be divided into 3 groups including large-conductance KCa, small, and intermediate conductance KCa. KCa channels in smooth muscle and endothelial cells are essential for the regulation of vascular tone. Vascular dysfunction under ischemia-reperfusion (I-R) or hypoxia-reoxygenation (H-R) conditions is associated with modulations of KCa channels that are attributable to multiple mechanisms. Most studies in this regard relied on the change of relaxation components sensitive to certain channel blockers to indicate the alteration of KCa channels under I-R conditions, which however provided conflicting results for the effect of I-R. The possible mechanisms involved in KCa channel modulation under I-R/H-R include overproduction of reactive oxygen species such as superoxide anion, hydrogen peroxide, and peroxynitrite, increase of intracellular H ion, and lactate accumulation, etc. However, more studies are necessary to further understand the discrepancies in the sensitivity of KCa channels to I-R injury in different vascular beds.

Mechanisms of relaxing response induced by rat/mouse hemokinin-1 in porcine coronary arteries: roles of potassium ion and nitric oxide

Rat and mouse hemokinin-1(r/m hemokinin-1) is a recently described member of the tachykinin family whose cardiovascular functions are not fully understood. In this study, we investigated the mechanisms of the relaxing response induced by r/m hemokinin-1 in isolated porcine coronary arteries by using a specific antagonist of tachykinin NK(1) receptor (SR140333), a nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (L-NNA), and 1H-[1,2,4] Oxadiazolo [4,3-a] quinoxalin-1-one (ODQ), a blocker of cGMP production. r/m Hemokinin-1 (10(-12)-10(-6 )M) evoked a marked endothelium-dependent vasodilatation (E(max)=121.12+/-10.6% and 91.79+/-2.39% in 10(-6) M PGF(2)alpha and 30 mM KCl precontracted arterial rings, respectively) of coronary arteries mediated by activation of endothelial tachykinin NK(1) receptors. Two components contributed to this r/m hemokinin-1-elicited vasodilatation, the first of which was endothelium-derived hyperpolarizing factor (EDHF), which played a major role. This EDHF was identified as a potassium current through certain kinds of potassium channels on the endothelial cell membrane of porcine coronary arteries. Specific antagonists of Ca(2+)-activated K(+) channels (dequalinium and clotrimazole) did not have an inhibitory effect on the r/m hemokinin-1-induced vasodilatation, whereas they did on the substance P-induced vasodilatation. When potassium ion efflux was impaired by a high K(+) concentration (30 mM) or removal of K(+) from the surroundings, NO synthesis was triggered by r/m hemokinin-1 to produce an equivalent EDHF (K(+))-independent vasorelaxation as a compensatory mechanism.

C-type natriuretic peptide: a new endothelium-derived hyperpolarizing factor?

Vascular relaxation mediated by endothelium-derived hyperpolarizing factor (EDHF) is important for resistance artery function and is underpinned by hyperpolarization of the smooth muscle cells of the blood vessel wall. Debate surrounds the identity of EDHF and its mechanism of action, with the consensus being that there is no universal EDHF. Regional differences in vascular function reflect the complex mechanisms of EDHF. Two primary mechanistic pathways are implicated: (i) myoendothelial gap junctions mediating the spread of endothelial cell hyperpolarization or small signaling molecules (or both) to the smooth muscle; and (ii) diffusible mediators released from the endothelium, including K+ and epoxyeicosatrienoic acids. Here, we discuss the evidence for and against C-type natriuretic peptide (CNP), the latest candidate for a diffusible mediator.

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.

Endothelial coordination of cerebral vasomotion via myoendothelial gap junctions containing connexins 37 and 40

Control of cerebral vasculature differs from that of systemic vessels outside the blood-brain barrier. The hypothesis that the endothelium modulates vasomotion via direct myoendothelial coupling was investigated in a small vessel of the cerebral circulation. In the primary branch of the rat basilar artery, membrane potential, diameter, and calcium dynamics associated with vasomotion were examined using selective inhibitors of endothelial function in intact and endothelium-denuded arteries. Vessel anatomy, protein, and mRNA expression were studied using conventional electron microscopy high-resolution ultrastructural and confocal immunohistochemistry and quantitative PCR. Membrane potential oscillations were present in both endothelial cells and smooth muscle cells (SMCs), and these preceded rhythmical contractions during which adjacent SMC intracellular calcium concentration ([Ca(2+)](i)) waves were synchronized. Endothelium removal abolished vasomotion and desynchronized adjacent smooth muscle cell [Ca(2+)](i) waves. N(G)-nitro-l-arginine methyl ester (10 microM) did not mimic this effect, and dibutyryl cGMP (300 muM) failed to resynchronize [Ca(2+)](i) waves in endothelium-denuded arteries. Combined charybdotoxin and apamin abolished vasomotion and depolarized and constricted vessels, even in absence of endothelium. Separately, (37,43)Gap27 and (40)Gap27 abolished vasomotion. Extensive myoendothelial gap junctions (3 per endothelial cell) composed of connexins 37 and 40 connected the endothelial cell and SMC layers. Synchronized vasomotion in rat basilar artery is endothelium dependent, with [Ca(2+)](i) waves generated within SMCs being coordinated by electrical coupling via myoendothelial gap junctions.

Dependency of endothelial cell function on vascular smooth muscle cells in guinea-pig mesenteric arteries and arterioles

Using guinea-pig mesenteric arteries and arterioles, we investigated the membrane potential of endothelial cells at rest and during application of acetylcholine (ACh) with and without the smooth muscle layers attached. When smooth muscle and endothelial layers were in close apposition, the resting membrane potentials of the two types of cells were closely related and were slightly more negative in the smooth muscle cells than in the endothelial cells. Once the endothelial layer was separated from the smooth muscle layer, the endothelial cells depolarized (the average, -4.2 mV). In the isolated endothelial layer, ACh did not induce a membrane hyperpolarization as expected, but did induce a quick depolarization soon after conventional whole-cell recording was started. However, as the pipette solution (high K+) gradually diffused into the endothelial layer, the membrane response to ACh gradually changed toward hyperpolarization. ACh-induced hyperpolarization was also observed after incubating preparations in a high-potassium bath solution. Our results indicate that vascular smooth muscle cells and endothelial cells are influencing each other as a functional unit and that the endothelial cells rely on the smooth muscle cells for their intracellular ionic composition and resting membrane potential.

Rhythmicity in arterial smooth muscle

Many arteries and arterioles exhibit rhythmical contractions which are synchronous over considerable distances. This vasomotion is likely to assist in tissue perfusion especially during periods of altered metabolism or perfusion pressure. While the mechanism underlying vascular rhythmicity has been investigated for many years, it has only been recently, with the advent of imaging techniques for visualizing intracellular calcium release, that significant advances have been made. These methods, when combined with mechanical and electrophysiological recordings, have demonstrated that the rhythm depends critically on calcium released from intracellular stores within the smooth muscle cells and on cell coupling via gap junctions to synchronize oscillations in calcium release amongst adjacent cells. While these factors are common to all vessels studied to date, the contribution of voltage-dependent channels and the endothelium varies amongst different vessels. The basic mechanism for rhythmical activity in arteries thus differs from its counterpart in non-vascular smooth muscle, where specific networks of pacemaker cells generate electrical potentials which drive activity within the otherwise quiescent muscle cells.

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.

Factors, fiction and endothelium-derived hyperpolarizing factor

1. The principal mediators of vascular tone are neural, endothelial and physical stimuli that result in the initiation of dilator and constrictor responses to facilitate the control of blood pressure. Two primary vasodilatory stimuli produced by the endothelium are nitric oxide (NO) and prostaglandins. An additional endothelium-dependent vasodilatory mechanism is characterized as the hyperpolarization-mediated relaxation that remains after the inhibition of the synthesis of NO and prostaglandins. This mechanism is due to the action of a so-called endothelium-derived hyperpolarizing factor (EDHF) and is dependent on either the release of diffusible factor(s) and/or to a direct contact-mediated mechanism. 2. Most evidence supports the concept that 'EDHF' activity is dependent on contact-mediated mechanisms. This involves the transfer of an endothelium-derived electrical current, as an endothelium-derived hyperpolarization (EDH), through direct heterocellular coupling of endothelial cells and smooth muscle cells via myoendothelial gap junctions (MEGJ). However, there is a lack of consensus with regard to the nature and mechanism of action of EDHF/EDH (EDH(F)), which has been shown to vary within and between vascular beds, as well as among species, strains, sex and during development, ageing and disease. 3. In addition to actual heterogeneity in EDH(F), further heterogeneity has resulted from the less-than-optimal design, analysis and interpretation of data in some key papers in the EDHF literature; with such views being perpetuated in the subsequent literature. 4. The focus of the present brief review is to examine what factors are proposed as EDH(F) and highlight the correlative structural and functional studies from our laboratory that demonstrate an integral role for MEGJ in the conduction of EDH, which account for the heterogeneity in EDH(F), while incorporating the reported diffusible mechanisms in the regulation of this activity. Furthermore, in addition to the reported heterogeneity in the nature and mechanism of action of EDH(F), the contribution of experimental design and technique to this heterogeneity will be examined.

Effects of insulin on the acetylcholine-induced hyperpolarization in the guinea pig mesenteric arterioles

BACKGROUND

Insulin induces endothelium-dependent vasodilatation, which may be casually related to the insulin resistance and hypertension. Endothelium-derived nitric oxide (NO) is the most important mechanism of insulin-induced vasodilatation, and a possible contribution of endothelium-derived hyperpolarizing factor (EDHF) is also considered. Attempts were made to observe the effects of insulin on acetylcholine (ACh)-induced hyperpolarization in the submucosal arteriole of the guinea pig ileum, the objective being to investigate possible involvement of EDHF in the actions of insulin.

METHODS

Conventional microelectrode techniques were applied to measure the membrane potential of smooth muscle cells in the submucosal arteriole. EDHF-induced hyperpolarization was elicited by ACh in the presence of both N(omega)-nitro-L-arginine (L-NNA) (100 microM) and diclofenac (1 microM).

RESULTS

The resting membrane potential was -70.9 mV, and Ba(2+) (0.5 mM) depolarized the membrane to -33.0 mV. Insulin (10 microU/ml to 100 mU/ml) did not change the membrane potential in the absence or presence of Ba(2+). In the presence of Ba(2+), ACh (3 microM) hyperpolarized the membrane with two components, an initial large hyperpolarization followed by a slow and small one. Low concentration of insulin (100 microU/ml) did not alter the ACh-induced hyperpolarization. High concentration of insulin (100 mU/ml) shortened the time required to reach the peak amplitude and tended to increase the peak amplitude of the ACh-induced hyperpolarization.

CONCLUSIONS

The data show that insulin enhances the ACh-induced hyperpolarization in the submucosal arterioles of the guinea pig ileum. The results suggested that EDHF also accounts for one of the endothelial factors involved in the insulin-induced vasodilatation.

Junctional and nonjunctional effects of heptanol and glycyrrhetinic acid derivates in rat mesenteric small arteries

1 Heptanol, 18alpha-glycyrrhetinic acid (18alphaGA) and 18beta-glycyrrhetinic acid (18betaGA) are known blockers of gap junctions, and are often used in vascular studies. However, actions unrelated to gap junction block have been repeatedly suggested in the literature for these compounds. We report here the findings from a comprehensive study of these compounds in the arterial wall. 2 Rat isolated mesenteric small arteries were studied with respect to isometric tension (myography), [Ca2+]i (Ca(2+)-sensitive dyes), membrane potential and--as a measure of intercellular coupling--input resistance (sharp intracellular glass electrodes). Also, membrane currents (patch-clamp) were measured in isolated smooth muscle cells (SMCs). Confocal imaging was used for visualisation of [Ca2+]i events in single SMCs in the arterial wall. 3 Heptanol (150 microm) activated potassium currents, hyperpolarised the membrane, inhibited the Ca2+ current, and reduced [Ca2+]i and tension, but had little effect on input resistance. Only at concentrations above 200 microm did heptanol elevate input resistance, desynchronise SMCs and abolish vasomotion. 4 18betaGA (30 microm) not only increased input resistance and desynchronised SMCs but also had nonjunctional effects on membrane currents. 18alphaGA (100 microm) had no significant effects on tension, [Ca2+]i, total membrane current and synchronisation in vascular smooth muscle. 5 We conclude that in mesenteric small arteries, heptanol and 18betaGA have important nonjunctional effects at concentrations where they have little or no effect on intercellular communication. Thus, the effects of heptanol and 18betaGA on vascular function cannot be interpreted as being caused only by effects on gap junctions. 18alphaGA apparently does not block communication between SMCs in these arteries, although an effect on myoendothelial gap junctions cannot be excluded.

The effect of perfusion with UW solution on the skeletal muscle and vascular endothelial exocrine function in rat hindlimbs

BACKGROUND

The effect of University of Wisconsin (UW) solution perfusion for extremity preservation is still unknown although it is widely used. The purpose of this study is to examine the effect of UW solution perfusion on skeletal muscle preservation in a rat model.

MATERIALS AND METHODS

Rat hindlimbs were amputated and either preserved with UW solution perfusion (UW perfusion group) or given no perfusion (no-perfusion group) for 5 h at 25 degrees C. They were then transplanted to other isogeneic rats. ATP in the muscle and serum creatine phosphokinase were measured after 24 h of reperfusion. The vascular endothelial function of the femoral artery rings was measured before and after 24 h of reperfusion in the presence or absence of indomethacin (cyclooxygenase inhibitor) and L-NMMA (nitric oxide synthase inhibitor). TEA (calcium-activated potassium channel inhibitor) was also used to verify the vasodilator function. Reperfusion blood flow was monitored during the first 2 h of reperfusion.

RESULTS

ATP in the UW perfusion group was significantly decreased after 24 h of reperfusion, while that in the no-perfusion group recovered. Reperfusion blood flow in the UW solution perfusion group was significantly lower than that in the no-perfusion group. Acetylcholine-induced relaxation in the UW perfusion group was significantly reduced before and after 24 h of reperfusion compared to that in the no-perfusion group and was mostly diminished by indomethacin and L-NMMA administration.

CONCLUSIONS

Skeletal muscle injury is augmented by UW solution perfusion, probably due to deterioration of the vascular endothelial function resulting in blood supply diminution.

[Recent advances in the study of endothelium-dependent hyperpolarizing factor (EDHF)]

Recent advances in the properties and physiological functions of endothelium-dependent hyperpolarizing factor (EDHF) in vascular tissues were reviewed briefly. The EDHF-induced hyperpolarization is inhibited by charybdotoxin, indicating that the potential is produced mainly by activation of intermediate conductance Ca-sensitive K-channels. During generation of EDHF responses, endothelial Ca2+ concentration was elevated, suggesting that the activated K-channels were distributed on the endothelial membrane. This was confirmed by direct recording of membrane potentials from endothelial and smooth muscle cells using double patch electrodes. Measurement of the propagation of potentials applied to endothelial or smooth muscle cells to surrounding cells revealed that there were tight electrical connections between endothelial cells much more than between endothelial and smooth muscle cells or between smooth muscle cells, and these observations yielded a possible spread of electrical signal along the endothelial layer first, and then the signals would be conducted to smooth muscle cell layers. These properties of vascular tissues allow speculating that EDHF is an electrical signal propagated from endothelial cells electrotonically through myoendothelial gap junctions. Several candidates have been proposed as EDHF, and possibilities of individual substances for EDHF were discussed. The cellular mechanism of the hyperpolarization-induced vasodilatation remains unclear, and this should be clarified in the future for further understanding of the EDHF-induced vasodilatation.

EDHF, but not NO or prostaglandins, is critical to evoke a conducted dilation upon ACh in hamster arterioles

Vasomotor reactions upon focal stimulation of arterioles have been shown to be conducted along the vascular wall. Such a conduction, which is assumed to reflect the spread of electrical signals, may contribute to coordination of responses within a vascular segment. We aimed to identify which endothelial autacoid(s) act as mediators of the local and conducted dilator responses, respectively. To this end, arterioles in the hamster cremaster microcirculation were locally stimulated with endothelium-dependent [acetylcholine (ACh)] or endothelium-independent dilators [sodium nitroprusside (SNP)], and the resulting changes in diameter were measured using a videomicroscopy technique at the site of application and up to 1.4 mm upstream at distant sites. Experiments were also performed after blockade of nitric oxide (NO) synthase, cyclooxygenase, P-450 monooxygenase, or K(+) channels. Dilations upon ACh (71 +/- 3%) were conducted rapidly (<1 s) to upstream sites (at 1.4 mm: 37 +/- 5%). Although the NO donor SNP induced a similar local dilation (71 +/- 7%), this response was not conducted. Maximal amplitudes of ACh-induced dilations were not attenuated after inhibition of NO synthase and cyclooxygenase at the local and remote sites. However, additional treatment with a P-450 monooxygenase blocker (sulfaphenazole) strongly attenuated the local response (from 62 +/- 9 to 17 +/- 5%) and abrogated dilations at distant sites (at 0.67 mm: from 23 +/- 4% to 4 +/- 3%). Likewise, 17-octadecynoic acid strongly attenuated local and remote responses. Blockers of Ca(2+)-dependent K(+) channels (charybdotoxin or iberiotoxin) attenuated dilations at the local and remote sites after focal application at the ACh stimulation site. In marked contrast, treatment of the upstream site with these blockers was without any effect. We conclude that upon local stimulation with ACh, a cytochrome P-450 monooxygenase product is generated that induces local dilation via the activation of Ca(2+)-dependent K(+) channels and initiates conduction of the dilation. In contrast to the local site, neither activation of these K(+) channels nor the synthesis of NO or prostaglandins is necessary to dilate the arterioles at remote, distant sites. This suggests that endothelium-derived hyperpolarizing factor serves as an important mediator to initiate conducted dilations and, by doing so, may act as a key player in the coordination of arteriolar behavior in the microcirculatory network.

Inhibitory effects of potassium channel blockers on tetramethylpyrazine-induced relaxation of rat aortic strip in vitro

Tetramethylpyrazine (TMP) is one of the active principles contained in Ligusticum chuanxiong Hort. (Umbelliferae), a herb that has been widely used to treat vascular disorders in China. In the present study, role of potassium channel in the vasodilatation of TMP was investigated using the effect of potassium channel blocker on TMP induced relaxation in isolated aortic rings from Wistar rats. TMP produced a concentration-dependent relaxation in the aortic rings precontracted with vasopressin or phenylephrine. Similar effect of TMP on vasoconstrictions by phenylephrine and vasopressin, induced through two different receptors, indicating the direct vasodilatation of TMP. Specific inhibitors for potassium channel were used to characterize the role of potassium channel in this action of TMP. Only the inhibitors specific to small conductance calcium-activated potassium (SK(Ca)) channel or ATP-sensitive potassium (K(ATP)) channel inhibited the action of TMP. Also, the TMP-induced relaxation was reversed by the inhibitor of soluble guanylyl cyclase in a way similar to that of K(ATP) channel blockade. The obtained results indicated that vasodilatation induced by TMP is related to the opening of SK(Ca) and K(ATP) channels.

Multiple mechanisms of vascular smooth muscle relaxation by the activation of proteinase-activated receptor 2 in mouse mesenteric arterioles

1. Activation of PAR2 in second-order mesenteric arteriole (MA) rings from C57BL/6J, NOS3 (-/-) and PAR2 (-/-) mice was assessed for the contributions of NO, cyclo-oxygenases, guanylyl cyclase, adenylyl cyclase, and of K(+) channel activation to vascular smooth muscle relaxation. 2. PAR2 agonist, SLIGRL-NH(2) (0.1 to 30 microM), induced relaxation of cirazoline-precontracted MA from C57BL/6J and NOS3 (-/-), but not PAR2 (-/-) mice. Maximal relaxation (E(max)) was partially reduced by a combination of L-(G)N-nitroarginine methyl ester (L-NAME), 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and indomethacin. An ODQ/L-NAME/indomethacin resistant relaxation was also caused by trypsin (30 nM) in PAR2 (+/+), but not in PAR2 (-/-) mice. Relaxation was endothelium-dependent and inhibited by either 30 mM KCl-precontraction, or pretreatment with apamin, charybdotoxin, and their combination; iberiotoxin did not substitute for charybdotoxin nor did scyllatoxin substitute fully for apamin. 3. Tetraethylammonium (TEA), glibenclamide, tetrodotoxin, 17-octadecynoic acid, carboxy-2-phenyl-4,4,5,5,-tetramethyl-imidazoline-1-oxyl-3-oxide, SQ22536, carbenoxolone, arachidonyl trifluoromethyl ketone, 7-nitroindazole, N-(3-(aminomethyl)benzyl)acetamidine (1400W), N-(2-cyclohexyloxy-4-nitrophenyl)-methanesulfonamide (NS-398) and propanolol did not inhibit relaxation. 4-aminopyridine significantly increased the potency of SLIGRL-NH(2). A combination of 30 microM BaCl(2) and 10 microM ouabain significantly reduced the potency for relaxation, and in the presence of L-NAME, ODQ and indomethacin, E(max) was reduced. 4. We conclude PAR2-mediated relaxation of mouse MA utilizes multiple mechanisms that are both NO-cGMP-dependent, and -independent. The data are also consistent with a role for endothelium-dependent hyperpolarization of vascular smooth muscle that involves the activation of an apamin/charybdotoxin-sensitive K(+) channel(s) and, in part, may be mediated by K(+).

Endothelium-derived relaxing factors: A focus on endothelium-derived hyperpolarizing factor(s)

Endothelium-derived hyperpolarizing factor (EDHF) is defined as the non-nitric oxide (NO) and non-prostacyclin (PGI2) substance that mediates endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells (VSMC). Although both NO and PGI2 have been demonstrated to hyperpolarize VSMC by cGMP- and cAMP-dependent mechanisms, respectively, and in the case of NO by cGMP-independent mechanisms, a considerable body of evidence suggests that an additional cellular mechanism must exist that mediates EDH. Despite intensive investigation, there is no agreement as to the nature of the cellular processes that mediates the non-NO/PGI2 mediated hyperpolarization. Epoxyeicosatrienoic acids (EET), an endogenous anandamide, a small increase in the extracellular concentration of K+, and electronic coupling via myoendothelial cell gap junctions have all been hypothesized as contributors to EDH. An attractive hypothesis is that EDH is mediated via both chemical and electrical transmissions, however, the contribution from chemical mediators versus electrical transmission varies in a tissue- and species-dependent manner, suggesting vessel-specific specialization. If this hypothesis proves to be correct then the potential exists for the development of vessel and organ-selective vasodilators. Because endothelium-dependent vasodilatation is dysfunctional in disease states (i.e., atherosclerosis), selective vasodilators may prove to be important therapeutic agents.Key words: endothelium, nitric oxide, potassium channels, hyperpolarization, gap junctions.

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.

Hyperpolarization-induced dilatation of submucosal arterioles in the guinea-pig ileum

1. The effects of inhibition of acetylcholine (ACh)-induced hyperpolarization on dilatation of submucosal arterioles were investigated in the guinea-pig ileum. 2. In smooth muscles of the arterioles depolarized by Ba(2+) (0.5 mM) to about -40 mV, ACh (3 microM) repolarized the membrane to about -65 mV (hyperpolarization), irrespective of the absence or presence of L-N(omega)-nitroarginine (L-NOARG, 0.1 mM) and diclofenac (1 microM), and increased the diameter (dilatation). 3. Combined application of charybdotoxin (CTX, 50 nM) and apamin (0.1 microM), inhibitors of some types of K(+)-channels, abolished the ACh-induced hyperpolarization and dilatation. 4. 18 beta-Glycerrhetinic acid (18 beta-GA, 30 microM), a known inhibitor of gap junctions, depolarized the membrane to about -36 mV, either in the absence or in the presence of Ba(2+), with no associated contraction of the arterioles. In the presence of 18 beta-GA, ACh-induced hyperpolarization was abolished, however the dilatation was inhibited only partially, with associated inhibition of constriction produced by Ba(2+) and NA. 5. 18 beta-GA inhibited the dilatation produced by sodium nitroprusside, an NO donor. 6. The ACh-induced hyperpolarization and dilatation were abolished in the presence of 2-aminoethoxydiphenyl borate (30 microM), an inhibitory modulator of inositol trisphosphate receptor-mediated Ca(2+) release from intracellular stores. 7. It is concluded that in submucosal arterioles, hyperpolarizations produced by ACh have causal relationship to the arteriolar dilatation. 18 beta-GA did not induce parallel relationship between hyperpolarization and dilatation produced by ACh. 18 beta-GA may have unidentified inhibitory effects on agonist-mediated actions, in addition to the inhibition of gap junctions.

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.

Sodium-potassium-ATPase electrogenicity in cerebral precapillary arterioles

Electrogenicity of the Na(+)/K(+) pump has the capability to generate a large negative membrane potential independently of ion-channel current. The high background membrane resistance of arterioles may make them susceptible to such an effect. Pump current was detected by patch-clamp recording from smooth muscle cells in fragments of arterioles (diameter 24-58 microm) isolated from pial membrane of rabbit cerebral cortex. The current was 20 pA at -60 mV, and the extrapolated zero current potential was -160 mV. Two methods of estimating the effect of pump electrogenicity on resting potential indicated an average contribution of -35 mV. In 20% of the recordings, block of inward rectifier K(+) channels by 10-100 microM Ba(2+) led to a small depolarization, but hyperpolarization was a more common response. Ba(2+) also inhibited depolarization evoked by 20 mM K(+). In arterioles within intact pial membrane, Ba(2+) failed to evoke constriction but inhibited K(+)-induced constriction. The data suggest that cerebral arterioles are vulnerable to the hyperpolarizing effect of the Na(+)/K(+) pump, excessive effects of which are prevented by depolarizing inward rectifier K(+) current

Ion channels and vascular tone

Ion channels in the plasma membrane of vascular muscle cells that form the walls of resistance arteries and arterioles play a central role in the regulation of vascular tone. Current evidence indicates that vascular smooth muscle cells express at least 4 different types of K(+) channels, 1 to 2 types of voltage-gated Ca(2+) channels, >/=2 types of Cl(-) channels, store-operated Ca(+) (SOC) channels, and stretch-activated cation (SAC) channels in their plasma membranes, all of which may be involved in the regulation of vascular tone. Calcium influx through voltage-gated Ca(2+), SOC, and SAC channels provides a major source of activator Ca(2+) used by resistance arteries and arterioles. In addition, K(+) and Cl(-) channels and the Ca(2+) channels mentioned previously all are involved in the determination of the membrane potential of these cells. Membrane potential is a key variable that not only regulates Ca(+2) influx through voltage-gated Ca(2+) channels, but also influences release of Ca(2+) from internal stores and Ca(2+)- sensitivity of the contractile apparatus. By controlling Ca(2+) delivery and membrane potential, ion channels are involved in all aspects of the generation and regulation of vascular tone.

Levcromakalim causes indirect endothelial hyperpolarization via a myo-endothelial pathway

1. Effects of K+ channel opener, levcromakalim, on vascular endothelial cells were examined. Under voltage- and current-clamp conditions, application of acetylcholine to dispersed endothelial cells isolated from rabbit superior mesenteric artery (dispersed RMAECs) produced hyperpolarization and outward currents. On the other hand, dispersed RMAECs did not respond to levcromakalim. 2. When membrane potential was recorded from endothelium in a mesenteric arterial segment, exposure to levcromakalim in a concentration range of 0.1 to 3 microM caused concentration-dependent hyperpolarization. The hyperpolarization was observed in the absence of external Ca2+ and was inhibited by 10 microM glibenclamide. 3. The presence of 1 mM heptanol did not affect the levcromakalin-induced hyperpolarization, whereas treatment of the mesenteric arterial segment with 20 microM 18 beta-glycyrrhetinic acid significantly reduced the hyperpolarization. The response to acetylcholine of RMAECs in an arterial segment with 18 beta-glycyrrhetinic acid was, however, similar to that without 18 beta-glycyrrhetinic acid. 4. These suggest that although RMAECs themselves are functionally insensitive to levcromakalim, those in an arterial segment are hyperpolarized by levcromakalim via myo-endothelial electrical communication.

Membrane properties and the excitatory junction potentials in smooth muscle cells of cochlear spiral modiolar artery in guinea pigs

Blood circulation changes in the inner ear play an important role in many physiological and pathological conditions of hearing function. The spiral modiolar artery (SMA) is the terminal artery to the cochlea. It was surrounded with nerve fibers immunostained by an antibody for tyrosine hydroxylase. By using intracellular recording techniques on the acutely isolated SMA, membrane properties of the smooth muscle cells and the neuromuscular transmission in this preparation were investigated. With minimum tension and normal extracellular K(+) concentration (5 mM), the majority of muscle cells showed a resting potential near -80 mV and an input resistance of about 8 MOmega. V/I plot showed an inward rectification in these cells. Barium (50-500 microM) caused strong depolarization and an increase in input resistance. Transmural electrical stimulation evoked stimulation intensity-dependent depolarizations (2-31 mV) following a short latency ( approximately 20 ms). The evoked potential by a low intensity stimulus was completely blocked by 1 microM tetrodotoxin. The potential and a depolarization induced by norepinephrine (10 microM) was usually partially (40-90%) blocked by alpha-receptor antagonists prazosin and/or idazoxan with concentrations up to 1 microM. Action potentials were observed when the depolarization was more than -40 mV. It is concluded that SMA smooth muscle cells, similar to those in other brain small arteries, highly express inward rectifying potassium channels; the cells receive catecholaminergic innervation, and stimulation of the nerves elicited an excitatory junction potential that is partially mediated by adrenergic receptors.

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.

Effects of K+Channel blockers on acetylcholine-induced vasodilation in guinea-pig choroid

The purpose of this study is to clarify which K+channels contribute to the acetylcholine (ACh)-induced vasodilation from the diameter changes in arterioles of the guinea-pig choroid. The choroid was isolated from the guinea-pig eyeball, pinned flat on a silicone rubber plate and superfused with warmed oxygenated (35 degrees C) Krebs solution. Diameters of choroidal arterioles were measured using video microscopy and a computer program for analysis. The effects of K+channel inhibitors (glibenclamide, tetraethylammonium [TEA], apamin and charybdotoxin [ChTX]) on the ACh-induced vasodilation were examined in arterioles which had been constricted by either norepinephrine (NE) or high K+solution. In NE (10(-5)m)-constricted arterioles, the combination of nitroarginine (10(-4)m) and indomethacin (10(-5)m) reduced ACh (10(-6)m)-induced vasodilatation by 24%. When high K+solution was used to constrict the arterioles, ACh-induced vasodilation was abolished by nitroarginine and indomethacin. In the presence of nitroarginine and indomethacin, the ACh-induced dilatation of NE-constricted arterioles was attenuated by TEA (10(-3)m), apamin (10(-7)m), and ChTX (10(-7)m) but not by glibenclamide (2x10(-5)m). Simultaneous application of apamin and ChTX inhibited the ACh (10(-6)m)-induced dilatation by 85%. In arterioles of guinea pig-choroid, nitric oxide and prostacyclin are not main mediators in ACh-induced vasodilation. Simultaneous activation of a set of Ca2+-sensitive K+channels may take most part of ACh-induced vasodilation.

Tension oscillation in arteries and its abnormality in hypertensive animals

1. The mechanisms of oscillatory contraction of arterial smooth muscle in vitro are discussed. 2. The membrane potential and cytoplasmic free Ca2+ concentration in smooth muscle cells oscillate in the presence of agonists. 3. The oscillatory change in the membrane potential of smooth muscle cells is related to Ca2+ release from intracellular stores. 4. Gap junctions between smooth muscle cells play important roles in the synchronized oscillation of the cytoplasmic free Ca2+ concentration in this population of cells. 5. Endothelial cells may increase or decrease the tension oscillation of smooth muscle cells. 6. In arteries from hypertensive rats, an increase in membrane excitability and the number of gap junctions between smooth muscle cells and impaired endothelial function are the main factors responsible for the modulation of tension oscillation.

Mechanisms underlying ACh induced modulation of neurogenic and applied ATP constrictions in the submucosal arterioles of the guinea-pig small intestine

1. Role of the vascular endothelium in acetylcholine (ACh) induced modulation of neurogenic and applied ATP (adenosine 5'-triphosphate) constrictions of intestinal submucosal arterioles was investigated. 2. Arteriole constrictions, induced either by exogenous ATP or evoked by perivascular nerve stimulation, were attenuated in the presence of ACh. 100 nM ACh almost completely abolished neurogenic constrictions whereas up to 10 microM ACh reduced constrictions to exogenous ATP by only about 60%. 3. Treatment of the arterioles with 100 microM Nomega-nitro-L-arginine (NOLA) and 5 microM indomethacin, to block respectively nitric oxide (NO) and prostanoid release from the endothelium, had no effect on the ACh induced inhibition of neurogenic constrictions but significantly attenuated the inhibitory effects of ACh on constrictions to exogenous ATP. 4. Disruption of the vascular endothelium had no effect on the ACh induced inhibition of neurogenic constrictions but attenuated the inhibitory effects of ACh on applied ATP constrictions to the same extent as after treatment with NOLA and indomethacin. In comparison, endothelial disruption completely abolished the inhibitory effect of substance P (SP) on exogenously applied ATP constrictions. 5. 50 nM ACh significantly attenuated the amplitude of neurally evoked excitatory junction potentials (ejps) recorded from the vascular smooth muscle without altering the time constant of decay (taudecay) of the ejps. 6. It is concluded that ACh inhibits neurogenic constrictions by prejunctional modulation of transmitter release from the perivascular sympathetic nerves with no major role for endothelial paracrine factors. 7. Endothelial NO and/or prostanoids mediate some of the ACh induced inhibition of constrictions to exogenous ATP whereas the endothelium independent inhibitory effects of ACh are attributed to a direct action of ACh on the vascular smooth muscle. However, an indirect effect resulting from activation of vasodilator nerves cannot be ruled out.

Calcium responses induced by acetylcholine in submucosal arterioles of the guinea-pig small intestine

1. Calcium responses induced by brief stimulation with acetylcholine (ACh) were assessed from the fluorescence changes in fura-2 loaded submucosal arterioles of the guinea-pig small intestine. 2. Initially, 1-1.5 h after loading with fura-2 (fresh tissues), ACh increased [Ca2+]i in a concentration-dependent manner. This response diminished with time, and finally disappeared in 2-3 h (old tissues). 3. Ba2+ elevated [Ca2+]i to a similar extent in both fresh and old tissues. ACh further increased the Ba2+-elevated [Ca2+]i in fresh tissues, but reduced it in old tissues. Responses were not affected by either indomethacin or nitroarginine. 4. In fresh mesenteric arteries, mechanical removal of endothelial cells abolished the ACh-induced increase in [Ca2+]i, with no alteration of [Ca2+]i at rest and during elevation with Ba2+. 5. In the presence of indomethacin and nitroarginine, high-K+ solution elevated [Ca2+]i in both fresh and old tissues. Subsequent addition of ACh further increased [Ca2+]i in fresh tissues without changing it in old tissues. 6. Proadifen, an inhibitor of the enzyme cytochrome P450 mono-oxygenase, inhibited the ACh-induced changes in [Ca2+]i in both fresh and Ba2+-stimulated old tissues. It also inhibited the ACh-induced hyperpolarization. 7. In fresh tissues, the ACh-induced Ca2+ response was not changed by apamin, charybdotoxin (CTX), 4-aminopyridine (4-AP) or glibenclamide. In old tissues in which [Ca2+]i had previously been elevated with Ba2+, the ACh-induced Ca2+ response was inhibited by CTX but not by apamin, 4-AP or glibenclamide. 8. It is concluded that in submucosal arterioles, ACh elevates endothelial [Ca2+]i and reduces muscular [Ca2+]i, probably through the hyperpolarization of endothelial or smooth muscle membrane by activating CTX-sensitive K+ channels.

Endothelium-derived relaxing, contracting and hyperpolarizing factors of mesenteric arteries of hypertensive and normotensive rats

Differences in the acetylcholine (ACh)-induced endothelium-dependent relaxation and hyperpolarization of the mesenteric arteries of Wistar Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) were studied. Relaxation was impaired in preparations from SHRSP and tendency to reverse the relaxation was observed at high concentrations of ACh in these preparations. Relaxation was partly blocked by NG-nitro-L-arginine (L-NOARG, 100 microM) and, in the presence of L-NOARG, tendency to reverse the relaxation was observed in response to higher concentrations of ACh, even in preparations from WKY. The relaxation remaining in the presence of L-NOARG was also smaller in preparations from SHRSP. The tendency to reverse the relaxation observed at higher concentrations of ACh in preparations from SHRSP or WKY in the presence of L-NOARG were abolished by indomethacin (10 microM). Elevating the K+ concentration of the incubation medium decreased relaxation in the presence of both indomethacin and L-NOARG. Relaxation in the presence of L-NOARG and indomethacin was reduced by the application of both apamin (5 microM) and charybdotoxin (0.1 microM). This suggests that the relaxation induced by ACh is brought about by both endothelium-derived relaxing factor (EDRF, nitric oxide (NO)) and hyperpolarizing factor (EDHF), which activates Ca2+-sensitive K+ channels. Electrophysiological measurement revealed that ACh induced endothelium-dependent hyperpolarization of the smooth muscle of both preparations in the presence of L-NOARG and indomethacin; the hyperpolarization being smaller in the preparation from SHRSP than that from WKY. These results suggest that the release of both NO and EDHF is reduced in preparations from SHRSP. In addition, indomethacin-sensitive endothelium-derived contracting factor (EDCF) is released from both preparations; the release being increased in preparations from SHRSP.

Endothelium-dependent hyperpolarization and intercellular electrical coupling in guinea-pig mesenteric arterioles

1. Using the conventional whole-cell clamp method, the electrical responses of individual smooth muscle and endothelial cells to acetylcholine (ACh) were observed in multicellular preparations where the two types of cells remained in close apposition. 2. In both types of cells, ACh induced similar hyperpolarizing responses which, when recorded in current clamp mode, had two phases (an initial fast and a second slower phase). 3. After blocking gap junctions, including myoendothelial junctions, with 18beta-glycyrrhetinic acid, ACh induced an outward current with two phases in voltage-clamped endothelial cells. The outward current appeared around -90 mV and increased linearly with the membrane depolarization. 4. In smooth muscle cells, ACh failed to induce a membrane current after gap junctions had been blocked with 18beta-glycyrrhetinic acid. The inhibition of ACh-induced response by 18beta-glycyrrhetinic acid was observed using either sharp or patch electrodes. 5. Nominally Ca2+-free solution reduced the initial phase and abolished the second phase of ACh-induced responses of endothelial cells. Both phases were also reduced by charybdotoxin (CTX). 6. Our results indicate that in guinea-pig mesenteric arterioles, ACh hyperpolarizes endothelial cells by activating Ca2+-activated K+ channels which are sensitive to CTX. On the other hand, hyperpolarizing responses detected in smooth muscle cells seem to originate in endothelial cells and conduct to the muscle layer via myoendothelial gap junctions.

Inhibition of endothelium-dependent hyperpolarization by endothelial prostanoids in guinea-pig coronary artery

1. In smooth muscle of the circumflex coronary artery of guinea-pig, acetylcholine (ACh, 10(-6) M) produced an endothelium-dependent hyperpolarization consisting of two components. An initial component that occurs in the presence of ACh and a slow component that developed after ACh had been withdrawn. Each component of the hyperpolarization was accompanied by an increase in membrane conductance. 2. Indomethacin (5 x 10(-6) M) or diclofenac (10(-6) M), both inhibitors of cyclooxygenase, abolished only the slow hyperpolarization. The initial hyperpolarization was not inhibited by diclofenac nor by nitroarginine, an inhibitor of nitric oxide synthase. 3. Both components of the ACh-induced hyperpolarization were abolished in the presence of atropine (10(-6) M) or high-K solution ([K+]0 = 29.4 mM). 4. The interval between ACh-stimulation required to generate an initial hyperpolarization of reproducible amplitude was 20 min or greater, but it was reduced to less than 5 min after inhibiting cyclooxygenase activity. Conditioning stimulation of the artery with substance P (10(-7) M) also caused a long duration (about 20 min) inhibition of the ACh-response. 5. The amplitude of the hyperpolarization generated by Y-26763, a K+-channel opener, was reproducible within 10 min after withdrawal of ACh. 6. Exogenously applied prostacyclin (PGI2) hyperpolarized the membrane and reduced membrane resistance in concentrations over 2.8 x 10(-9)M. 7. At concentrations below threshold for hyperpolarization and when no alteration of membrane resistance occurred, PGI2 inhibited the initial component of the ACh-induced hyperpolarization. 8. It is concluded that endothelial prostanoids, possibly PGI2, have an inhibitory action on the release of endothelium-derived hyperpolarizing factor.

Endothelium-dependent hyperpolarization in resting and depolarized mammary and coronary arteries of guinea-pigs

1. The membrane potential responses in guinea-pig coronary and mammary arteries attributable to endothelium-derived nitric oxide (NO), prostaglandin (PG) and hyperpolarizing factor (EDHF), and to exogenous NO and the prostacyclin analogue, iloprost, were compared at rest and when depolarized with the thromboxane analogue, U46619. 2. In the coronary artery, stimulation of the endothelium with acetylcholine (ACh) evoked hyperpolarization attributable to NO and a PG with similar pD2s at rest and in the presence of U46619. However, in depolarized tissues, the pD2 of the response attributed to EDHF required a 10 fold lower concentration of ACh compared with at rest. 3. In the mammary artery, lower concentrations of ACh were required to evoke NO- and EDHF-dependent hyperpolarizations in depolarized mammary artery compared with at rest, while PG-dependent hyperpolarization did not occur until the concentration of ACh was increased some 10 fold both at rest and in U46619. 4. The smooth muscle of the coronary artery of guinea-pigs was some 4 fold more sensitive to exogenous NO and iloprost than was the mammary artery. 5. In conclusion, the membrane potential response in arteries at rest, that is, in the absence of constrictor, may be extrapolated to events in the presence of constrictor when NO and PG are under study. However, the sensitivity to ACh and the magnitude of the hyperpolarization attributed to EDHF obtained in tissues at rest may underestimate these parameters in depolarized tissues.

Blockade by 18beta-glycyrrhetinic acid of intercellular electrical coupling in guinea-pig arterioles

1. Intercellular electrical communication between smooth muscle and endothelial cells was examined in guinea-pig mesenteric arterioles using the whole-cell patch-clamp method. The time course of the current required to impose a 10 mV voltage clamp step was used to determine the extent of electrical coupling between them. Currents recorded from both smooth muscle and endothelial cells relaxed in a multi-exponential manner, indicating the existence of electrical coupling between cells. 2. 18beta-Glycyrrhetinic acid, a gap junction blocker, quickly blocked electrical communication at 40 microM, while neither heptanol nor octanol did so at concentrations of up to 1 mM. 3. In the current clamp mode, repetitive spikes, induced by 10 mM Ba2+ solutions, could be recorded from both kinds of cells. After blocking gap junctions, spikes could only be recorded from the smooth muscle cell layer, indicating that they had been conducted through myoendothelial junctions. 4. In endothelial cells, acetylcholine (ACh, 3 microM) induced hyperpolarizing responses, which had two phases (an initial fast and a second slower phase) in the current clamp condition. This ACh response persisted in the presence of 18beta-glycyrrhetinic acid, although this compound seemed to make the membrane slightly leaky. 5. After blocking gap junctions, the membrane potential of a single cell in a multicellular preparation could be well clamped. Thus, 18beta-glycyrrhetinic acid may be useful in studying the function of both arteriolar smooth muscle and endothelial cells while they remain located within a multicellular preparation.

Functions of large conductance Ca2+-activated (BKCa), delayed rectifier (KV) and background K+ channels in the control of membrane potential in rabbit renal arcuate artery

1. The types of K+ channel which determine the membrane potential of arcuate artery smooth muscle cells were investigated by patch-clamp recording from isolated cells and lumenal diameter measurements from intact pressurized renal arcuate arteries. 2. Single cells had a mean resting potential of -38 mV and were depolarized by 130 mM K+ but not by the Cl- channel blocker 4,4'-diisothiocyanatostilbene-2, 2'-disulphonic acid (DIDS). 3. Iberiotoxin did not affect the resting potential but inhibited spontaneous transient hyperpolarizations. Iberiotoxin or 1 mM tetraethylammonium (TEA+) constricted intact arteries. 3,4-Diaminopyridine (3,4-DAP)-sensitive delayed rectifier K+ (KV) channel current was elicited by depolarization but 3,4-DAP did not affect the resting potential or induce constriction in the intact artery. 4. A voltage-independent K+ current was inhibited by >= 0.1 mM barium (Ba2+) and unaffected by iberiotoxin, glibenclamide, apamin, 3,4-DAP and ouabain. In six out of ten cells, 1 mM Ba2+ depolarized the resting potential, while in the other cells the potential was resistant to all of the K+ channel blockers and ouabain. Ba2+ (0.1-1 mM) constricted the intact artery, but 10 microM Ba2+, 1 microM glibenclamide or 100 nM apamin had no effect. 5. The data suggest that resting potential is determined by background K+ channels, one type being Ba2+ sensitive and voltage independent, and another type being poorly defined due to its resistance to any inhibitor. Large conductance Ca2+-activated K+ (BKCa) and KV channels do not determine the resting potential but have separate functions to underlie transient Ca2+-induced hyperpolarizations and to protect against depolarization past about -30 mV.

Endothelium-derived hyperpolarizing factor--a critical appraisal

Endothelium-derived hyperpolarizing factor is defined as that substance which produces vascular smooth muscle hyperpolarization which cannot be explained by nitric oxide or by a cyclo-oxygenase product such as prostacyclin. The possibility that the factor is an epoxyeicosatrienoic acid or a cannabinoid agonist such as anandamide continues to be investigated, but definitive evidence in favour of either is lacking. The sensitivity of EDHF-mediated responses to charybdotoxin, to apamin or to mixtures of these two toxins may indicate the opening of more than one smooth muscle K-channel, but the possibility that these are located on the vascular endothelium is discussed.

Potassium channels activated in the endothelium-dependent hyperpolarization in guinea-pig coronary artery

1. Properties of endothelium-dependent hyperpolarization evoked by acetylcholine (ACh) in smooth muscle of the guinea-pig coronary artery were investigated using conventional microelectrode techniques. 2. ACh hyperpolarized the membrane in an endothelium-dependent manner. The hyperpolarization comprised two components: an initial and a slow hyperpolarization. The former appeared during application of ACh, while the latter occurred after withdrawal of ACh. 3. Indomethacin and f1p4ofenac, inhibitors of the enzyme cyclo-oxygenase, blocked only the slow hyperpolarization, indicating that this potential was produced by endothelial prostanoids. 4. Clotrimazole and SKF 525a, known inhibitors of the enzyme cytochrome P450, inhibited both the initial and the slow hyperpolarizations, suggesting that these chemicals acted as non-selective inhibitors of arachidonic acid metabolism. Inhibition of the lipoxygenase pathway of arachidonic acid metabolism by nordihydroguaiaretic acid had no effect on either component of the hyperpolarization. 5. The slow hyperpolarization was inhibited by 4-aminopyridine (4-AP; 10(4) 10(-3) M) and glibenclamide (10(-6) M). The initial hyperpolarization was greatly inhibited by charybdotoxin (CTX; 5 x 10(-8) M) and partially inhibited by apamin (10(-7) M), but was not inhibited by glibenclamide (10(-5) M). Ba2+ (10(-4) M) depolarized the membrane and increased the amplitude of both components of the ACh-induced hyperpolarization. 6. Hyperpolarizations produced by Y-26763, a K+ channel opener, were inhibited by glibenclamide, but not by 4-AP. 7. The results indicate that the slow hyperpolarization is produced by endothelial prostanoids through activation of 4-AP-sensitive K+ channels (possibly delayed rectifier type). The initial hyperpolarization is produced mainly through activation of CTX-sensitive K+ channels (possibly Ca(2+)-sensitive type).