The electrical response of cultured guinea-pig coronary endothelial cells to endothelium-dependent vasodilators

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

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

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

Mechanism of C-type natriuretic peptide-induced endothelial cell hyperpolarization

C-type natriuretic peptide (CNP) has a demonstrated hyperpolarizing effect on vascular smooth muscle cells. However, its autocrine function, including its electrophysiological effect on endothelial cells, is not known. Here, we report the effect of CNP on the membrane potential (E(m)) of pulmonary microvascular endothelial cells and describe its target receptors, second messengers, and ion channels. We measured changes in E(m) using fluorescence imaging and perforated patch-clamping techniques. In imaging experiments, samples were preincubated in the potentiometric dye DiBAC(4)(3), and subsequently exposed to CNP in the presence of selective inhibitors of ion channels or second messengers. CNP exposure induced a dose-dependent decrease in fluorescence, indicating that CNP induces endothelial cell hyperpolarization. CNP-induced hyperpolarization was inhibited by the K(+) channel blockers, tetraethylammonium or iberiotoxin, the nonspecific cation channel blocker, La(3+), or by depletion or repletion of extracellular Ca(2+) or K(+), respectively. CNP-induced hyperpolarization was also blocked by pharmacological inhibition of PKG or by small interfering RNA (siRNA)-mediated knockdown of natriuretic peptide receptor-B (NPR-B). CNP-induced hyperpolarization was mimicked by the PKG agonist, 8-bromo-cGMP, and attenuated by both the endothelial nitric oxide synthase (eNOS) inhibitor, N(omega)-nitro-l-arginine methyl ester (l-NAME), and the soluble guanylyl cyclase (sGC) inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. Presence of iberiotoxin-sensitive, CNP-induced outward current was confirmed by perforated patch-clamping experiments. We conclude that CNP hyperpolarizes pulmonary microvascular endothelial cells by activating large-conductance calcium-activated potassium channels mediated by the activation of NPR-B, PKG, eNOS, and sGC.

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

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

The cellular mechanisms by which adenosine evokes release of nitric oxide from rat aortic endothelium

Adenosine and nitric oxide (NO) are important local mediators of vasodilatation. The aim of this study was to elucidate the mechanisms underlying adenosine receptor-mediated NO release from the endothelium. In studies on freshly excised rat aorta, second-messenger systems were pharmacologically modulated by appropriate antagonists while a NO-sensitive electrode was used to measure adenosine-evoked NO release from the endothelium. We showed that A1-mediated NO release requires extracellular Ca2+, phospholipase A2 (PLA2) and ATP-sensitive K+ (KATP) channel activation whereas A2A-mediated NO release requires extracellular Ca2+ and Ca2+-activated K+ (KCa) channels. Since our previous study showed that A1- and A2A-receptor-mediated NO release requires activation of adenylate cyclase (AC), we propose the following novel pathways. The K+ efflux resulting from A1-receptor-coupled KATP-channel activation facilitates Ca2+ influx which may cause some stimulation of endothelial NO synthase (eNOS). However, the increase in [Ca2+]i also stimulates PLA2 to liberate arachidonic acid and stimulate cyclooxygenase to generate prostacyclin (PGI2). PGI2 acts on its endothelial receptors to increase cAMP, so activating protein kinase A (PKA) to phosphorylate and activate eNOS resulting in NO release. By contrast, the K+ efflux resulting from A2A-coupled KCa channels facilitates Ca2+ influx, thereby activating eNOS and NO release. This process may be facilitated by phosphorylation of eNOS by PKA via the action of A2A-receptor-mediated stimulation of AC increasing cAMP. These pathways may be important in mediating vasodilatation during exercise and systemic hypoxia when adenosine acting in an endothelium- and NO-dependent manner has been shown to be important.

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

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

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

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

Altered calcium dynamics do not account for attenuation of endothelium-derived hyperpolarizing factor-mediated dilations in the female middle cerebral artery

BACKGROUND AND PURPOSE

The contribution of endothelium-derived hyperpolarizing factor (EDHF) to ATP-mediated dilations is significantly attenuated in the rat middle cerebral artery of intact and estrogen-treated ovariectomized (OVX) females compared with males and vehicle-treated OVX females. Since an increase in endothelial calcium appears to be a critical prerequisite in the EDHF response, we tested the hypothesis that endothelial cell intracellular calcium ([Ca(2+)](i)) fails to reach sufficient levels to elicit robust EDHF-mediated dilations in females and that this effect is mediated by estrogen.

METHODS

Vascular diameter and [Ca(2+)](i) were measured concomitantly in perfused middle cerebral artery segments with the use of videomicroscopy and fura 2 fluorescence, respectively.

RESULTS

In the presence of N(G)-nitro-L-arginine methyl ester and indomethacin, the dilation to 10(-5) mol/L ATP was significantly reduced (P<0.05) in intact females (42+/-8%; n=6) and estrogen-treated OVX females (25+/-6%; n=9) compared with intact males (89+/-5%; n=6) and vehicle-treated OVX females (92+/-2%; n=7). Contrary to our initial hypothesis, endothelial cell [Ca(2+)](i) increased to comparable levels in intact females (461+/-116 nmol/L), estrogen-treated OVX females (417+/-50 nmol/L), intact males (421+/-77 nmol/L), and vehicle-treated OVX females (530+/-92 nmol/L). In response to luminal ATP (10(-5) mol/L), smooth muscle cell [Ca(2+)](i) decreased to a greater degree in males (37+/-4%; n=8) compared with females (21+/-5%; n=7) and in vehicle-treated OVX females (18+/-7%; n=7) compared with estrogen-treated OVX females (3+/-5%; n=9).

CONCLUSIONS

Our data suggest that loss of a factor coupling EDHF to reduction of ionized smooth muscle cell [Ca(2+)](i) accounts for the attenuated EDHF-mediated dilations in the female middle cerebral artery.

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

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

Lipopolysaccharide reduces intercellular coupling in vitro and arteriolar conducted response in vivo

Our recent in vitro study (Lidington et al. J Cell Physiol 185: 117-125, 2000) suggested that lipopolysaccharide (LPS) reduces communication along blood vessels. The present investigation extended this study to determine whether any effect of LPS and/or inflammatory cytokines [tumor necrosis factor-alpha, interleukin (IL)-1beta, and IL-6] on endothelial cell coupling in vitro could also be demonstrated for an arteriolar conducted response in vivo. Using an electrophysiological approach in monolayers of microvascular endothelial cells, we found that LPS (10 microg/ml) but not these cytokines reduced intercellular conductance (c(i)) (an index of cell communication) and that LPS together with these cytokines did not further reduce c(i). Also, c(i) was restored after LPS washout, and the LPS-induced reduction was prevented by protein tyrosine kinase (PTK) inhibitors (1.5 microM Tyr A9 and 10 nM PP-2). In our in vivo experiments in arterioles of the mouse cremaster muscle, local electrical stimulation evoked vasoconstriction that conducted along arterioles. LPS in the muscle superfusate did not alter local vasoconstriction but reduced the conducted response. Washout of LPS restored the conducted response, whereas PTK inhibitors prevented the effect of LPS. On the basis of a newly developed mathematical model, the LPS-induced reduction in conducted response was predicted to reduce the arteriolar ability to increase resistance to blood flow. We conclude that LPS can reduce communication in in vitro and in vivo systems comparably in a reversible and tyrosine kinase-dependent manner. Based on literature and present results, we suggest that LPS may compromise microvascular hemodynamics at both the arteriolar responsiveness and the conduction levels.

Divergent biological actions of coronary endothelial nitric oxide during progression of cardiac hypertrophy

Coronary endothelial NO synthase expression and NO bioactivity were investigated at sequential stages during the progression of left ventricular hypertrophy. Male guinea pigs underwent abdominal aortic banding or sham operation. Left ventricular contractile function was quantified in isolated ejecting hearts. Coronary endothelial and vasodilator function were assessed in isolated isovolumic hearts in response to boluses of bradykinin (0.001 to 10 micromol/L), substance P (0.01 to 100 micromol/L), diethylamine NONOate (DEA-NO) (0.1 to 1000 micromol/L), N(G)-monomethyl-L-arginine monoacetate (L-NMMA) (10 mmol/L), and adenosine (10 mmol/L). At a stage of compensated left ventricular hypertrophy (3 weeks), left ventricular endothelial NO synthase protein expression was unaltered (Western blot and immunocytochemistry). Vasoconstriction in response to L-NMMA was increased in banded animals compared with sham-operated animals (13.8+/-2.1% versus 6.2+/-1.3%, n=10; P<0.05), but agonist- and DEA-NO-induced vasodilation was similar in the 2 groups. At a stage of decompensated left ventricular hypertrophy (8 to 10 weeks), left ventricular endothelial NO synthase protein expression was significantly lower in banded animals (on Western analysis: banded animals, 7.8+/-0.4 densitometric units; sham-operated animals, 12.2+/-1.7 densitometric units; n=5; P<0.05). At this time point, vasoconstriction in response to L-NMMA was similar in the 2 groups, but vasodilatation in response to bradykinin (30.9+/-2.4% versus 39.7+/-2.2%, n=10; P<0.05), DEA-NO (26.2+/-1.8% versus 34.6+/-1.8%, n=10; P<0.05), and adenosine (24.3+/-2.0% versus 35.7+/-2.0%, n=10; P<0.01) was attenuated in banded animals. These findings indicate that there is an increase in the basal activity of NO (without a significant change in endothelial NO synthase expression) in early compensated left ventricular hypertrophy, followed by a decrease in both endothelial NO synthase expression and NO bioactivity during the transition to myocardial failure.

Rat Coronary Endothelial Cell Membrane Potential Responses During Hypertension

-The purpose of this study was to provide the first membrane potential profile in coronary endothelial cells from normotensive sham-operated control and 1-kidney, 1-clip renal hypertensive rats. Dilator responses were assessed in cannulated coronary arteries from control and 1-kidney, 1-clip rats, and the perforated patch-clamp method was used to compare membrane potential responses between the intact endothelial cells. Under these conditions, acetylcholine (100 pmol/L to 10 µmol/L) induced similar large dilations of coronary arteries from control and 1-kidney, 1-clip rats that were associated with endothelial cell hyperpolarizing responses of 16+/-3 and 18+/-2 mV, respectively. Substance P (10 fmol/L to 1 nmol/L) and bradykinin (100 fmol/L to 10 nmol/L) also substantially dilated coronary arteries from control rats but only induced small (2 to 4 mV) endothelial cell hyperpolarizing responses. These dilations, which appeared independent of membrane potential changes, were highly blunted or absent in arteries from 1-kidney, 1-clip rats. Thus, dilator responses to acetylcholine that are associated with large endothelial hyperpolarizing responses are normal in the small coronary arteries of 1-kidney, 1-clip rats. However, dilator response to substance P and bradykinin, which apparently are not heavily dependent on endothelial cell hyperpolarizations, are selectively targeted for impairment in the coronary arteries of this model of hypertension

Involvement of Ca2+ -activated K+ channels in ginsenosides-induced aortic relaxation in rats

Ginsenosides (GS), an extract of Panax ginseng, have been reported to be effective in inducing vascular relaxation mediated by nitric oxide (NO) release. The present experiments were designed to determine whether this GS-induced vasorelaxation also involves Ca2+ -activated K+ (KCa) channels in vascular smooth muscle cells (VSMC) in addition to endothelium-derived NO. GS induced vasorelaxation in rat aortic rings, which had been precontracted with phenylephrine, in a concentration-dependent manner. This GS-induced relaxation was partially reversed by tetraethylammonium (TEA), an inhibitor of KCa channels; methylene blue (MB), an inhibitor of soluble guanylate cyclase; as well as Nomega-nitro-L-arginine (L-NNA), but not by glybenclamide. In cultured VSMC and endothelial cells, KCa channels were activated by GS. This action was abolished by TEA, but was not blocked by glybenclamide. In addition, the GS-induced activity of KCa channels was partially inhibited by MB or H-8. These results indicate that the activation of KCa channels involved, at least in part, the GS-induced vasorelaxation of rat aorta.

A new in vitro model for agonist-induced communication between microvascular endothelial cells

Microvascular endothelial cells (MECs) grown in Matrigel form capillary-like structures. We hypothesized that these "capillaries" better mimic communication properties of microvessels than conventional cell monolayers. MECs were isolated from the rat hindlimb skeletal muscle. Functional communication was tested by visualizing the spread of microinjected 6-carboxyfluorescein (CF) dye and by measuring a conducted change of membrane potential after micropipette application of 500 mM KCl or 10 mM adenosine triphosphate (ATP) on the capillary and monolayer. MECs grown under both conditions were dye-coupled, as demonstrated by the spread of CF injected into a single cell. The membrane potential of cells grown in capillaries (-59 +/- 5 mV) was significantly greater than that of cells grown in monolayers (-24 +/- 2 mV). KCl and ATP caused local depolarization (18 +/- 3 mV) and hyperpolarization (21 +/- 3 mV) in capillaries that yielded conducted 13 +/- 3 mV depolarization and 15 +/- 5 mV hyperpolarization at a 300-microm distal site, respectively. In monolayers, local and distal responses to agonists were 3- to 6-fold and 9- to 10-fold less, respectively, than the corresponding responses in capillaries. Cells grown under both conditions expressed connexin 43, as demonstrated by immunohistochemistry and Western blotting. We conclude that cells grown in capillaries yield substantially larger local and communicated responses than cells in monolayers and thus offer a more sensitive model for mechanistic studies of MEC communication.

Endotoxin increases intercellular resistance in microvascular endothelial cells by a tyrosine kinase pathway

Gap junction communication between microvascular endothelial cells has been proposed to contribute to the coordination of microvascular function. Septic shock may attenuate microvascular cell-to-cell communication. We hypothesized that lipopolysaccharide (LPS) attenuates communication between microvascular endothelial cells derived from rat hindlimb skeletal muscle. Endothelial cells grown in monolayers expressed mRNA for connexin 37, 40, and 43. The expression of connexin 43 protein was confirmed, but connexin 40 protein was not detected by immunocytochemistry or immunoblot analysis. Intercellular resistance between cells of the monolayer, calculated using a Bessel function model, was increased from 3.3 to 5.3 MOmega by LPS. The effect was seen after 1 h of exposure and required a minimum concentration of 10 ng/ml. Intercellular resistance returned to normal 1 h following removal of LPS. Neither the response to LPS, nor its reversal, was blocked by the protein synthesis inhibitor cycloheximide (10 microg/ml). Pretreatment of monolayers with the tyrosine kinase inhibitors PP-2 (10 nM), lavendustin-C (1 microM), and geldanamycin (200 nM) prevented this LPS response; geldanamycin was also able to reverse the response. Inhibitors of MAP kinases, PD 98059 (5 microM) and SB 202190 (5 microM), and PKC (500 nM bisindolylmaleimide I) were unable to block the LPS response. We propose that LPS attenuates cell-to-cell communication through a signaling pathway that is tyrosine kinase dependent.

ATP-sensitive potassium channels in capillaries isolated from guinea-pig heart

The full-length cDNAs of two different alpha-subunits (Kir6.1 and Kir6.2) and partial cDNAs of three different beta-subunits (SUR1, SUR2A and SUR2B) of ATP-sensitive potassium (KATP) channels of the guinea-pig (gp) were obtained by screening a cDNA library from the ventricle of guinea-pig heart. Cell-specific reverse-transcriptase PCR with gene-specific intron-spanning primers showed that gpKir6.1, gpKir6.2 and gpSUR2B were expressed in a purified fraction of capillary endothelial cells. In cardiomyocytes, gpKir6.1, gpKir6.2, gpSUR1 and gpSUR2A were detected. Patch-clamp measurements were carried out in isolated capillary fragments consisting of 3-15 endothelial cells. The membrane capacitance measured in the whole-cell mode was 19.9 +/- 1.0 pF and was independent of the length of the capillary fragment, which suggests that the endothelial cells were not electrically coupled under our experimental conditions. The perforated-patch technique was used to measure the steady-state current-voltage relation of capillary endothelial cells. Application of K+ channel openers (rilmakalim or diazoxide) or metabolic inhibition (250 microM 2,4-dinitrophenol plus 10 mM deoxyglucose) induced a current that reversed near the calculated K+ equilibrium potential. Rilmakalim (1 microM), diazoxide (300 microM) and metabolic inhibition increased the slope conductance measured at -55 mV by a factor of 9.0 (+/-1.8), 2.5 (+/-0.2) and 3.9 (+/-1.7), respectively. The effects were reversed by glibenclamide (1 microM). Our results suggest that capillary endothelial cells from guinea-pig heart express KATP channels composed of SUR2B and Kir6.1 and/or Kir6.2 subunits. The hyperpolarization elicited by the opening of KATP channels may lead to an increase in free cytosolic Ca2+, and thus modulate the synthesis of NO and the permeability of the capillary wall.

Adenosine and muscle vasodilatation in acute systemic hypoxia

Adenosine is released by skeletal and cardiac muscles when their metabolism increases: it serves to couple O2 supply with O2 demand by causing vasodilatation. This review argues that adenosine plays a similar role in skeletal muscle in systemic hypoxia. It accounts for approximately 50% of the increase in muscle vascular conductance and, within muscle, it causes dilatation of individual arterioles, thus maximizing the distribution of O2 and allowing O2 consumption to remain constant when O2 delivery is reduced. In vivo and in vitro studies have indicated that adenosine can induce dilatation in several different ways. This review argues that during systemic hypoxia, adenosine is predominantly released from the endothelium and acts on endothelial A1 receptors to produce dilatation in a nitric oxide (NO)-dependent manner. A1 receptor stimulation increases the synthesis of NO by a process initiated by opening of ATP-sensitive K+ (KATP) channels. Moreover, recent findings suggest that prostaglandins also make a major contribution to the hypoxia-induced dilatation, but that the dilator pathways for adenosine, NO and prostaglandins are interdependent. In addition, adenosine released from the skeletal muscle fibres contributes indirectly to the dilatation by stimulating A1 and A2 receptors on the muscle fibres, opening KATP channels and allowing efflux of K+, which is a vasodilator. Finally, by acting on endothelial A1 receptors, adenosine attenuates the vasoconstrictor effects of constant or bursting patterns of sympathetic activity. This limits the extent to which the sympathetic nervous system can reduce O2 delivery to muscle when it is already compromised by systemic hypoxia.

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

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

The endothelial component of cannabinoid-induced relaxation in rabbit mesenteric artery depends on gap junctional communication

1. We have shown that the endocannabinoid anandamide and its stable analogue methanandamide relax rings of rabbit superior mesenteric artery through endothelium-dependent and -independent mechanisms that are unaffected by blockade of NO synthase and cyclooxygenase. 2. The endothelium-dependent component of the responses was attenuated by the gap junction inhibitor 18alpha-glycyrrhetinic acid (18alpha-GA; 50 microM), and a synthetic connexin-mimetic peptide homologous to the extracellular Gap 27 sequence of connexin 43 (43Gap 27, SRPTEKTIFII; 300 microM). By contrast, the corresponding connexin 40 peptide (40Gap 27, SRPTEKNVFIV) was inactive. 3. The cannabinoid CB1 receptor antagonist SR141716A (10 microM) also attenuated endothelium-dependent relaxations but this inhibition was not observed with the CB1 receptor antagonist LY320135 (10 microM). Furthermore, SR141716A mimicked the effects of 43Gap 27 peptide in blocking Lucifer Yellow dye transfer between coupled COS-7 cells (a monkey fibroblast cell line), whereas LY320135 was without effect, thus suggesting that the action of SR141716A was directly attributable to effects on gap junctions. 4. The endothelium-dependent component of cannabinoid-induced relaxation was also attenuated by AM404 (10 microM), an inhibitor of the high-affinity anandamide transporter, which was without effect on dye transfer. 5. Taken together, the findings suggest that cannabinoids derived from arachidonic acid gain access to the endothelial cytosol via a transporter mechanism and subsequently stimulate relaxation by promoting diffusion of an to adjacent smooth muscle cells via gap junctions. 6. Relaxations of endothelium-denuded preparations to anandamide and methanandamide were unaffected by 43Gap 27 peptide, 18alpha-GA, SR141716A, AM404 and indomethacin and their genesis remains to be established.

Cyclic AMP mediates EDHF-type relaxations of rabbit jugular vein

Isolated rings of rabbit jugular vein have been used to test the hypothesis that formation of cAMP within the endothelial cell contributes to relaxations that are attributable to the endothelium-derived hyperpolarizing factor, EDHF. Relaxations induced by acetylcholine under conditions of combined NO synthase and cyclooxygenase blockade were almost abolished by inhibition of adenylate cyclase with the selective P-site agonist 2', 3'-dideoxyadenosine (2',3'-DDA). They were similarly attenuated by the gap junction inhibitors 18alpha-glycyrrhetinic acid (18alpha-GA) and Gap 27 peptide which interrupt direct endothelium-smooth muscle communication without themselves affecting smooth muscle tone. By contrast, stimulation of adenylate cyclase with forskolin promoted gap junction-dependent relaxations, with concentration-relaxation curves to this agent exhibiting an equivalent rightward shift in the presence of 18alpha-GA and following endothelial denudation. The findings suggest that cAMP may cross from the endothelium to smooth muscle via gap junction channels and/or enhance the endothelial hyperpolarization normally associated with agonist stimulation. Both mechanisms may contribute to EDHF/gap junction-dependent relaxations.

Substance P and bradykinin activate different types of KCa currents to hyperpolarize cultured porcine coronary artery endothelial cells

1. Substance P and bradykinin, endothelium-dependent vasodilators of pig coronary artery, trigger in endothelial cells a rise in cytosolic Ca2+ concentration ([Ca2+]i) and membrane hyperpolarization. The aim of the present study was to determine the type of Ca2+-dependent K+ (KCa) currents underlying the endothelial cell hyperpolarization. 2. The substance P-induced increase in [Ca2+]i was 30 % smaller than that induced by bradykinin, although the two peptides triggered a membrane hyperpolarization of the same amplitude. The two agonists evoked a large outward K+ current of the same conductance at maximal stimulation. Agonists applied together produced the same maximal current amplitude as either one applied alone. 3. Iberiotoxin (50 nM) reduced by about 40 % the K+ current activated by bradykinin without modifying the substance P response. Conversely, apamin (1 microM) inhibited the substance P-induced K+ current by about 65 %, without affecting the bradykinin response. Similar results were obtained on peptide-induced membrane hyperpolarization. 4. Bradykinin-induced, but not substance P-induced, endothelium-dependent relaxation resistant to NG-nitro-L-arginine and indomethacin was partly inhibited by 3 microM 17-octadecynoic acid (17-ODYA), an inhibitor of cytochrome P450 epoxygenase. Similarly, the bradykinin-induced K+ current was reduced by 17-ODYA. 5. Our results show that responses to substance P and bradykinin result in a hyperpolarization due to activation of different KCa currents. A current consistent with the activation of large conductance (BKCa) channels was activated only by bradykinin, whereas a current consistent with the activation of small conductance (SKCa) channels was stimulated only by substance P. The observation that a similar electrical response is produced by different pools of channels implies distinct intracellular pathways leading to KCa current activation.

Voltage-dependent K(+) current in capillary endothelial cells isolated from guinea pig heart

Capillary fragments were isolated from guinea pig hearts, and their electrical properties were studied using the perforated-patch and cell-attached mode of the patch-clamp technique. A voltage-dependent K(+) current was discovered that was activated at potentials positive to -20 mV and showed a sigmoid rising phase. For depolarizing voltage steps from -128 to +52 mV, the time to peak was 71 +/- 5 ms (mean +/- SE) and the amplitude of the current was 3.7 +/- 0.5 pA/pF in the presence of 5 mM external K(+). The time course of inactivation was exponential with a time constant of 7.2 +/- 0.5 s at +52 mV. The current was blocked by tetraethylammonium (inhibitory constant approximately 3 mM) but was not affected by charybdotoxin (1 microM) or apamin (1 microM). In the cell-attached mode, depolarization-activated single-channel currents were found that inactivated completely within 30 s; the single-channel conductance was 12.3 +/- 2.4 pS. The depolarization-activated K(+) current described here may play a role in membrane potential oscillations of the endothelium.

Effects of P2 purinoceptor agonists on membrane potential and intracellular Ca2+ of human cardiac endothelial cells

Vasoactive agonists like adenosine-5'-triphosphate (ATP) increase intracellular Ca2+ ([Ca2+]i) in vascular endothelial cells with an initial peak due to inositol 1,4,5-triphosphate-mediated Ca2+ release from intracellular stores followed by a sustained plateau that is dependent on the presence of extracellular Ca2+, thus leading to an increased synthesis and release of prostacyclin and nitric oxide. We studied the effects of nucleotides on membrane potential and [Ca2+]i in confluent human microvascular cardiac endothelial cells obtained from patients with dilated cardiomyopathy. The whole-cell configuration of the patch-clamp technique and a confocal laser scanning microscope employing fluo-3 as a Ca2+ indicator were used. Both uridine-5'-triphosphate (UTP) and 2-methylthioadenosine-5'-triphosphate (2MeSATP) induced depolarizations in human microvascular cardiac endothelial cells and increased [Ca2+]i with a rank order of potency 2MeSATP>ATP=UTP (EC50 values (in microM) were 0.084 2MeSATP, 0.67 ATP and 1.1 UTP). This suggests that both P2u and P2y purinoceptors are present on human microvascular cardiac endothelial cells. Maximal [Ca2+]i responses of confluent human microvascular cardiac endothelial cell monolayers to UTP were lower when compared to 2MeSATP. Nucleotide-induced increases in [Ca2+]i consisted of a transient peak, which was also observed in the absence of extracellular Ca2+, and a sustained [Ca2+]i plateau. This plateau, which was not observed in all monolayers studied, was not markedly influenced by increasing extracellular [K+]. Previous incubation with thapsigargin abolished ATP-induced increases of [Ca2+]i. It is concluded that human microvascular cardiac endothelial cells express both P2y and P2u purinoceptors. P2 purinoceptor agonists release Ca2+ from intracellular thapsigargin-sensitive stores and stimulate capacitative Ca2+ influx pathways. K+ efflux through Ca2+-dependent K+ (K(Ca)) channels does not play a major role in the regulation of nucleotide-induced Ca2+ influx in human microvascular cardiac endothelial cells, which might be related to an impaired function of the cells.

Ginsenoside Rg3 mediates endothelium-dependent relaxation in response to ginsenosides in rat aorta: role of K+ channels

The aim of the present study was to characterize the endothelium-dependent relaxation elicited by ginsenosides, a mixture of saponin extracted from Panax ginseng, in isolated rat aorta. Relaxations elicited by ginsenosides were mimicked by ginsenoside Rg1 and ginsenoside Rg1, two major ginsenosides of the protopanaxatriol group. Ginsenoside Rg3 was about 100-fold more potent than ginsenoside Rg1. The endothelium-dependent relaxation in response to ginsenoside Rg3 was associated with the formation of cycle GMP. These effects were abolished by N(G)-nitro-L-arginine and methylene blue. Relaxations in response to ginsenoside Rg3 were unaffected by atropine, diphenhydramine, [D-Pro2, D-Trp7,9]substance P, propranolol, nifedipine, verapamil and glibenclamide but were markedly reduced by tetraethylammonium. Tetraethylammonium modestly reduced the relaxation induced by sodium nitroprusside. These findings indicate that ginsenoside Rg3 is a major mediator of the endothelium-dependent nitric oxide-mediated relaxation in response to ginsenosides in isolated rat aorta, possibly via activation of tetraethylammonium-sensitive K+ channels.

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

1. Using a microelectrode technique, acetylcholine (ACh)-induced membrane potential changes were characterized using various types of inhibitors of K+ and Cl- channels in rabbit aortic valve endothelial cells (RAVEC). 2. ACh produced transient then sustained membrane hyperpolarizations. Withdrawal of ACh evoked a transient depolarization. 3. High K+ blocked and low K+ potentiated the two ACh-induced hyperpolarizations. Charybdotoxin (ChTX) attenuated the ACh-induced transient and sustained hyperpolarizations; apamin inhibited only the sustained hyperpolarization. In the combined presence of ChTX and apamin, ACh produced a depolarization. 4. In Ca2+-free solution or in the presence of Co2+ or Ni2+, ACh produced a transient hyperpolarization followed by a depolarization. In BAPTA-AM-treated cells, ACh produced only a depolarization. 5. A low concentration of A23187 attenuated the ACh-induced transient, but not the sustained, hyperpolarization. In the presence of cyclopiazonic acid, the hyperpolarization induced by ACh was maintained after ACh removal; this maintained hyperpolarization was blocked by Co2+. 6. Both NPPB and hypertonic solution inhibited the membrane depolarization seen after ACh washout. Bumetanide also attenuated this depolarization. 7. It is concluded that in RAVEC, ACh produces a two-component hyperpolarization followed by a depolarization. It is suggested that ACh-induced Ca2+ release from the storage sites causes a transient hyperpolarization due to activation of ChTX-sensitive K+ channels and that ACh-activated Ca2+ influx causes a sustained hyperpolarization by activating both ChTX- and apamin-sensitive K+ channels. Both volume-sensitive Cl- channels and the Na+-K+-Cl- cotransporter probably contribute to the ACh-induced depolarization.

Cellular mechanisms by which adenosine induces vasodilatation in rat skeletal muscle: significance for systemic hypoxia

1. In anaesthetized rats, we recorded arterial blood pressure (ABP), heart rate (HR), femoral blood flow (FBF) and femoral vascular conductance (FVC). We tested the effects of the nitric oxide (NO) synthesis inhibitor L-NAME (nitro-L-arginine methyl ester), or the ATP-sensitive K+ (KATP) channel inhibitor glibenclamide, on responses evoked by systemic hypoxia (breathing 8% O2 for 5 min) or i.a. infusion for 5 min of adenosine, the NO donor sodium nitroprusside (SNP), the adenosine A1 receptor agonist CCPA (2-chloro-N6-cyclopentyladenosine) or the adenosine A2A receptor agonist CGS 21680 (2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadeno sin e hydrochloride). 2. L-NAME (10 mg kg-1 i.v.) greatly reduced the increase in FVC induced by hypoxia or adenosine, as we have shown before, but had no effect on the increase in FVC evoked by SNP. In addition, L-NAME abolished the increase in FVC evoked by CCPA and greatly reduced that evoked by CGS 21680. These results substantiate the view that muscle vasodilatation induced by systemic hypoxia and infused adenosine are largely NO dependent. They also indicate that muscle dilatation induced by A1 receptor stimulation is entirely NO dependent while that induced by A2A receptors is largely NO dependent; dilatation may also be induced by direct stimulation of A2A receptors on the vascular smooth muscle. 3. Glibenclamide (10 or 20 mg kg-1 i.v.) reduced the increase in FVC induced by hypoxia, preferentially affecting the early part (< 1 min). In addition, glibenclamide greatly reduced the increase in FVC induced by adenosine, but it had no effect on that evoked by SNP. Further, glibenclamide abolished the increase in FVC evoked by CCPA and greatly reduced that evoked by CGS 21680. These results substantiate the view that hypoxia-induced muscle vasodilatation is initiated by KATP channel opening. They also indicate that NO does not induce muscle vasodilatation by opening KATP channels on the vascular smooth muscle, but indicate that the dilatation induced by adenosine and by A2A receptor stimulation is largely dependent on KATP channel opening, while that induced by A1 receptor stimulation is wholly dependent on KATP channel opening. 4. These results, together with previous evidence that hypoxia-induced vasodilatation in skeletal muscle is largely mediated by adenosine acting on A1 receptors, lead us to propose that adenosine is released from endothelium during systemic hypoxia and acts on endothelial A1 receptors to open KATP channels on the endothelial cells and cause synthesis of NO, which then acts on the vascular smooth muscle to cause dilatation. During severe systemic hypoxia we propose that adenosine may also act on A2A receptors on the endothelium to cause dilatation by a similar process and may act on A2A receptors on the vascular smooth muscle to cause dilatation by opening KATP channels.

ACh- and caffeine-induced Ca2+ mobilization and current activation in rabbit arterial endothelial cells

Fura 2 microfluorometry and perforated-patch whole cell recording were carried out simultaneously to investigate the relationship between intracellular free Ca2+ concentration ([Ca2+]i) and membrane current activation in response to ACh and caffeine in freshly dissociated arterial endothelial cells. ACh and caffeine evoked transient increases in [Ca2+]i. The initial increase in [Ca2+]i was accompanied by a transient outward current, which caused membrane hyperpolarization. The amplitudes of the [Ca2+]i transient and outward current were dependent on caffeine concentration (EC50 approximately 1 mM). Cyclopiazonic acid raised resting [Ca2+]i levels by >/=50 nM and failed to completely block caffeine- or ACh-induced [Ca2+]i transients but slowed [Ca2+]i recovery fourfold. The reversal potential of caffeine-induced currents was dependent on external K+ and Cl- concentrations. Caffeine-induced current amplitudes, but not [Ca2+]i responses, were attenuated by external tetraethylammonium, Zn2+, and La3+. A consistent temporal relationship between agonist-activated membrane current and [Ca2+]i increases was not observed, and, in some cases, time differences were greater than expected for simple diffusion of Ca2+ throughout the cell. These results suggest that Ca2+-dependent current activation monitors local [Ca2+]i changes adjacent to the plasmalemma, whereas single-cell photometry provides a measure of global changes in [Ca2+]i.

Functional effects of expression of hslo Ca2+ activated K+ channels in cultured macrovascular endothelial cells

The aim of the present study is to elucidate the effects of the expression of large conductance Ca2+ activated K+ channels (BK[Ca]) in an endothelial cell type normally lacking this channel. The human homologue hslo of BK(Ca) was expressed in cultured bovine pulmonary artery endothelial (CPAE) cells, which have no endogenous BK(Ca). Membrane potential, ionic currents and Ca2+ signals were investigated in non-transfected and transfected cells using a combined patch clamp and Fura-2 fluorescence technique. In non-transfected control CPAE cells, ATP evoked a Ca2+ activated Cl- current (I[Cl,Ca]). The most prominent current component during ATP stimulation in hslo expressing cells was conducted BK(Ca) which resulted in a pronounced transient hyperpolarization. This hyperpolarization, which was absent in non-transfected cells, was enhanced if I(Cl,Ca) was blocked with niflumic acid. The sustained component of the Ca2+ response during ATP stimulation was significantly larger in hslo transfected cells than in non-transfected cells. This plateau level correlated well with the corresponding effects of ATP on the membrane potential, indicating that the expression of cloned BK(Ca) exerts a positive feedback on Ca2+ signals in endothelial cells by counteracting the negative (depolarizing)effect of stimulation of Ca2+-activated Cl- channels.

Functional electrical properties of the endothelium in lymphatic vessels of the guinea-pig mesentery

1. The resting and agonist-stimulated properties of endothelial cells and electrical communication between the endothelium and smooth muscle were investigated in open segments of guinea-pig mesenteric lymphatic vessels using intracellular microelectrodes. 2. Endothelial cells had a mean resting membrane potential (RMP) of -71.5 +/- 0.5 mV (n = 100) which was significantly different from the value of -60.8 +/- 1.1 mV (n = 75) recorded in smooth muscle. 3. Acetylcholine (ACh, 5-10 microM) generally evoked an initial hyperpolarization followed by depolarization (mean 3.4 +/- 0.5 mV and 15.4 +/- 1.0 mV, respectively, n = 75). 4. Ca(2+)-activated K+ channels were likely to underlie the ACh-induced hyperpolarization as this response exhibited an increased in membrane conductance, was larger in 0.5 mM K+ solution and was blocked by charybdotoxin (50 nM). 5. The endothelium did not exhibit a response to nitric oxide (NO) as the NO-donor sodium nitroprusside did not alter the RMP and the electrical responses to ACh were not affected by the NO-synthase inhibitor N omega-nitro L-arginine at a concentration which markedly inhibited smooth muscle hyperpolarization. 6. Electrical coupling between the endothelium and smooth muscle was not functional as there was extremely limited electrical continuity (1 in 12, endothelial/smooth muscle cell simultaneous recordings) and bradykinin, noradrenaline and isoprenaline caused different electrical responses in the two cell types. 7. These results provide the first description of RMP and electrical responses to various agonists in the lymphatic endothelium and its lack of functional electrical coupling with the smooth muscle.

Hyperpolarization of isolated capillaries from guinea-pig heart induced by K+ channel openers and glucose deprivation

1. The present study was designed to test if microvascular coronary endothelial cells express ATP-sensitive K+ channels (KATP channels). We performed microfluorometric measurements of the membrane potential of freshly isolated guinea-pig coronary capillaries equilibrated with the voltage-sensitive dye bis-oxonol (bis-[1,3-dibutylbarbituric acid] trimethineoxonol, [DiBAC4(3)]). 2. The resting membrane potential of capillaries in physiological salt solution was -46 +/- 4.2 mV (n = 8) at room temperature (22 degrees C) as determined after calibration of the fluorescence using the Na(+)-K+ ionophore gramicidin in the presence of different K+ concentrations. Spontaneous membrane potential fluctuations of 10-20 mV amplitude were often observed. 3. A reversible, sustained hyperpolarization to a new membrane potential close to the K+ equilibrium potential (EK) could be induced by application of the K+ channel openers HOE 234 (100 nM to 1 microM), diazoxide (10 PM to 100 nM) or pinacidil (100 nM). Subsequent addition of glibenclamide (200 nM to 2 microM) reversed this hyperpolarization. 4. A glibenclamide-sensitive hyperpolarization of coronary capillaries to values near EK was also observed upon omission of D-glucose (10 mM) from the superfusing solution or by substituting L-glucose for D-glucose. Maximum hyperpolarization was reached in less than 10 min. 5. Our results suggest that microvascular coronary endothelial cells express KATP channels which may be activated during hypoglycaemia.

Agonist-induced activation of a non-selective ion current in glomerular endothelial cells

The control of intracellular calcium activity ([Ca2+]i) and membrane voltage (Vm) play an important role in regulating functions of glomerular endothelial cells (GEC). We investigated the effect of extracellular ATP on the intracellular [Ca2+]i, Vm and ion conductances in GEC. ATP (100 mumol/liter) induced a rapid increase of [Ca2+]i in GEC from 20 +/- 6 to 442 +/- 84 nmol/liter, which was followed by a sustained Ca2+ plateau of 112 +/- 29 nmol/liter. In a bath solution with a low extracellular Ca2+ concentration the ATP-induced [Ca2+]i peak was still present, but the [Ca2+]i plateau was completely prevented. In 186 experiments with the patch clamp technique the addition of ATP (1 to 100 mumol/liter) to GEC induced a transient small hyperpolarization, which was followed by a depolarization. During the ATP-induced depolarization an increase of the whole cell conductance was found. The Ca2+ ionophore A23187 (10 mumol/liter) mimicked the effect of ATP on Vm. Reduction of the extracellular Ca2+ to 1 mumol/liter itself depolarized GEC reversibly from -88 +/- 2 to -60 +/- 12 mV and increased the ATP-induced depolarization to -18 +/- 3 mV. In the absence of Na+ in the bathing solution (replacement by NMDG+) ATP induced only an attenuated depolarization and no inward current was activated. Flufenamate (100 mumol/liter), a blocker of non-selective ion channels inhibited the ATP-induced depolarization of Vm significantly by 58 +/- 13%, whereas nicardipine (10 mumol/liter) or amiloride (10 mumol/liter) had no effect. Our data indicate that the resting Vm of GEC cells is almost completely dominated by K+ conductances and that ATP activates a Ca2+ dependent non-selective ion conductance in GEC.

Membrane potential changes visualized in complete growth media through confocal laser scanning microscopy of bis-oxonol-loaded cells

Confocal laser scanning microscopy (CLSM) was employed to visualize and measure membrane potential changes in several types of cultured adherent cells, such as human fibroblasts, mouse mammary tumor C127 cells, and human saphenous vein endothelial cells, preloaded with the anionic dye bis-1, 3,-diethylthiobarbituratetrimethineoxonol (bis-oxonol). The fluorescence of cell-associated bis-oxonol was detected in a single confocal plane. An original flow-chamber apparatus was employed to replace the extracellular medium, avoiding alterations of the plane selected for observation. In all the cell types and the experimental situations tested the intracellular distribution of the dye was typical; perinuclear zones accumulated the dye which, conversely, was excluded by the nucleus. Fluorescence was calibrated versus the membrane potential by varying the extracellular concentration of sodium in the presence of gramicidin. With this approach membrane potential was measured (i) in cultured human fibroblasts incubated under anisotonic conditions, (ii) in heterogeneous cell populations which respond unevenly to potential perturbing conditions, and (iii) in human macrovascular endothelial cells maintained in high-serum, complete growth medium. The results obtained indicate that CLSM can be successfully employed to measure changes of membrane potential in single, bis-oxonol-loaded adherent cells under experimental conditions which severely hinder conventional spectrofluorimetric approaches.

Activation of A2-purinoceptors by adenosine stimulates L-arginine transport (system y+) and nitric oxide synthesis in human fetal endothelial cells

1. Human umbilical vein endothelial cells were challenged acutely with adenosine and its analogues to examine whether adenosine modulates L-arginine transport (system y+) and synthesis of nitric oxide (NO) and prostacyclin (PGI2). 2. L-Arginine transport was stimulated by adenosine (10 microM, 2 min) and the A2-receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680; 100 nM), but not by the A1-receptor agonist N6-cyclopentyladenosine (CPA). 3. Activation of L-arginine transport was inhibited by the A2-receptor antagonists ZM-241385 and 3,7-dimethyl-1-propargylxanthine (DMPX), but unaffected by the A1-receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine and 8-phenyltheophylline or the adenosine transport inhibitor nitrobenzylthioinosine. 4. Adenosine and CGS-21680 evoked a rapid membrane hyperpolarization. 5. Adenosine and CGS-21680 induced increases in intracellular cGMP levels, whereas release of PGI2 was unaffected. NG-nitro-L-arginine methyl ester (an NO synthase inhibitor) and the A2-receptor antagonists ZM-241385 and DMPX prevented increases in cGMP accumulation. 6. Our findings provide the first evidence that activation of human fetal endothelial cell A2-purinoceptors, but not A1-purinoceptors, leads to a membrane hyperpolarization and stimulation of basal rates of L-arginine transport and NO biosynthesis.

Membrane depolarization in NRK fibroblasts by bradykinin is mediated by a calcium-dependent chloride conductance

The effects of the phosphoinositide-mobilizing agonist bradykinin (BK) on membrane potential and intracellular calcium in monolayers of normal rat kidney (NRK) fibroblasts were investigated. BK induced a rapid transient depolarization in these cells, which was mimicked by other phosphoinositide-mobilizing factors such as prostaglandin F2alpha (PGF2alpha), lysophosphatidic acid (LPA), platelet-derived growth factor (PDGF-BB), and serum. Depolarization by BK was independent of extracellular Ca2+ or Na+. It was shown using extracellular Cl- substitutions that the depolarization was caused by an increased Cl- conductance. Depolarization was inhibited by 5-nitro-2-3-phenylpropyl(amino)benzoic acid (NPPB), niflumic acid, and flufenamic acid, inhibitors of calcium-dependent chloride channels. The depolarization provoked by BK could be mimicked by raising intracellular calcium with ionomycin or thapsigargin and could be blocked with geneticin, a blocker of phospholipase C. When intracellular calcium was buffered by loading the cells with 1,2-bis(2-aminophenoxy)ethane-NNN'N'-tetra-acetic acid (BAPTA), depolarization was prevented. We conclude that in NRK fibroblasts extracellular stimuli that increase intracellular calcium, depolarize the cells via the activation of a calcium-dependent chloride conductance. In addition to an increase in intracellular calcium, depolarization may be an important effector pathway in response to extracellular stimuli in fibroblasts. It is hypothesized that, in electrically coupled cells such as NRK fibroblasts, intercellular transmission of these depolarizations may represent a mechanism to coordinate uniform multicellular responses to Ca2+-mobilizing agonists.

Ion channels in vascular endothelium

The functional impact of ion channels in vascular endothelial cells (ECs) is still a matter of controversy. This review describes different types of ion channels in ECs and their role in electrogenesis, Ca2+ signaling, vessel permeability, cell-cell communication, mechano-sensor functions, and pH and volume regulation. One major function of ion channels in ECs is the control of Ca2+ influx either by a direct modulation of the Ca2+ influx pathway or by indirect modulation of K+ and Cl- channels, thereby clamping the membrane at a sufficiently negative potential to provide the necessary driving force for a sustained Ca2+ influx. We discuss various mechanisms of Ca2+ influx stimulation: those that activate nonselective, Ca(2+)-permeable cation channels or those that activate Ca(2+)-selective channels, exclusively or partially operated by the filling state of intracellular Ca2+ stores. We also describe the role of various Ca(2+)- and shear stress-activated K+ channels and different types of Cl- channels for the regulation of the membrane potential.

Extracellular ATP regulates glomerular endothelial cell function

1. Glomerular endothelial cells form the inner part of the filtration barrier and are involved in pathophysiological processes in the glomerulum. New techniques for culturing glomerular endothelial cells have been established recently. The effect of extracellular ATP on membrane voltage and intracellular calcium activity was examined in bovine glomerular endothelial cells (GEC) in culture. 2. Membrane voltage was measured with the patch clamp technique in the fast whole cell configuration. GEC possess a stable membrane voltage of -88 mV. ATP induced a small transient hyperpolarization, which was followed by a depolarization. The ATP-induced depolarization was significantly inhibited by flufenamate, a blocker of non-selective ion channels. 3. The intracellular calcium activity [Ca2+]i was measured in single cells with the fura-2 technique. ATP stimulated an increase of [Ca2+]i. The increase of [Ca2+]i was biphasic with an initial peak followed by a sustained plateau. The [Ca2+]i peak was still present in an extracellular Ca(2+)-free solution, whereas the plateau was inhibited. 4. The order of potency of different purine nucleotides in stimulating [Ca2+]i and inositol formation was UTP = ATP > ATP-gamma-S > 2-methylthio ATP > [alpha,beta-CH2]ATP. 5. The data indicate that ATP regulates membrane voltage and [Ca2+]i in glomerular endothelial cells by a P2y2 receptor.

Potassium channels and the coronary circulation

1. Mechanisms responsible for the regulation of coronary blood flow during physiologically important situations, such as reactive hyperaemia, hypoxia, ischaemia, coronary artery occlusion and increased metabolic demand, have eluded the scientific community. 2. As knowledge regarding potassium channel physiology and biophysics has expanded, the potential role of these channels in regulating coronary blood flow has been studied. 3. Recent data have demonstrated that ATP-sensitive potassium channels (K+[ATP]) play an important role in maintaining basal coronary blood flow, contribute to the regulation of coronary blood flow during hypoxia, acidosis, ischaemia, reactive hyperaemia and ischaemic preconditioning. The role of potassium channels in the regulation of coronary blood flow during increases in metabolic stimulation is controversial. 4. Thus, potassium channels, particularly K+[ATP], appear to play an important role in regulating coronary blood flow during physiologically important stimuli.

Ca(2+)-dependent non-selective cation and potassium channels activated by bradykinin in pig coronary artery endothelial cells

1. Using the cell-attached and inside-out modes of the patch-clamp technique, we studied the Ca(2+)-dependent ionic channels activated by bradykinin in cultured pig coronary artery endothelial cells to further understand electrophysiological events underlying cellular activation. 2. In the cell-attached mode, bradykinin (94 nM) activated two types of Ca(2+)-dependent channels: a high conductance K+ channel (285 pS in high symmetrical K+), whose open state probability was increased by depolarization, and a lower conductance inwardly rectifying non-selective cation channel (44 pS in high symmetrical K+). 3. The 285 pS K+ channel was half-maximally activated by cytosolic Ca2+ levels of 1.6 and 4.5 microM at +10 and -30 mV, respectively. Such local concentrations should be reached in the presence of bradykinin, which induces a mean maximal cytosolic Ca2+ rise of 1.3 microM. 4. The 285 pS K+ channel was inhibited by d-tubocurarine, which acted by reducing the mean open time duration (flickering pattern), finally reducing the channel conductance. 5. Divalent cations such as Ca2+ could flow through the 44 pS non-selective cation channel, with nearly the same permeability (P) as monovalent cations (PK: PNa: PCa = 1:1:0.7). 6. The cation channel appeared to be more sensitive to Ca2+ than the K+ channel, with a half-maximal open probability induced by 0.7 microM Ca2+ on the intracellular side of the membrane. 7. In contrast to the K+ channel, the cation channel mean open time was clearly increased by bradykinin. This effect was delayed compared with the increase in the channel open state probability and was rapidly lost in the inside-out configuration. Caffeine also activated the cation channel but more transiently than bradykinin and without any effect on the open duration. 8. In the absence of extracellular Ca2+, the bradykinin-induced increase in cytosolic free Ca2+ was shortened temporally by 52% and reduced in amplitude by 88%, whereas the bradykinin-induced hyperpolarization was not significantly reduced in amplitude but was shortened by 70%, thus illustrating the major role of Ca2+ influx in endothelial cell activation by bradykinin. 9. We conclude that bradykinin activates two types of Ca(2+)-dependent channels in coronary endothelial cells: a high conductance K+ channel regulated by membrane potential, and an inwardly rectifying cation channel allowing Ca2+ entry, the cation channel being about 6 times more sensitive to Ca2+ than the K+ channel. The increase in cation channel open state probability involves an increase in open number, like the K+ channel, but also involves a rise in channel open duration. Ca2+ entry via cation channels could contribute to increase the cytoplasmic Ca2+ level, activate Ca(2+)-dependent K+ channels, thus triggering membrane hyperpolarization when the endothelial cell is stimulated by a vasoactive agonist such as bradykinin.

Inwardly rectifying K+ channels in freshly dissociated coronary endothelial cells from guinea-pig heart

1. Inwardly rectifying K+ (IK(IR)) currents of freshly dissociated coronary endothelial cells from guinea-pig heart were investigated with the perforated-patch technique. 2. The whole-cell current-voltage relationship of endothelial cells showed strong inward rectification. Increasing the extracellular K+ resulted in an increase of inward currents. The slope conductance of the cells in the potential range negative to the calculated potassium equilibrium potential (EK) with 5, 60 and 150 mM external potassium was 0.18 +/- 0.14, 0.55 +/- 0.50 and 0.63 +/- 0.29 nS (mean +/- S.D.), respectively. 3. To quantify the steepness of inward rectification, the voltage dependence of the chord conductance of the cells was fitted with a Boltzmann function. The slope factor k describing the steepness of the relationship was 6.8 +/- 1.5 mV. 4. Extracellular barium induced a potential- and time-dependent block of inward currents through endothelial KIR channels. Half-maximum inhibition of IK(IR) currents was achieved with < or = 1 microM barium at a membrane potential of -70 mV in a solution containing 60 mM K+. 5. Whole-cell inward currents revealed the opening and closing of single KIR channels. The single-channel conductance was 26 +/- 3 pS with 60 mM external K+ and 33 +/- 6 pS with 150 mM external K+. 6. Our results suggest that the electrical properties of freshly dissociated endothelial cells are to a large extent determined by five to sixty active strong inwardly rectifying K+ (KIR) channels.

Modulation of cell membrane potential in cultured vascular endothelium

The present study explores the role of different ionic conductances in the regulation of membrane potential under resting conditions and after bradykinin (BK) or thapsigargin (TG) stimulation of cultured bovine aortic endothelial cells. Under resting conditions, the cell membrane potential observed was -62+/- 5 mV. The main conductance under these conditions is an inwardly rectifying potassium (IRK) channel. Application of 50 nM BK induced a transient hyperpolarization to -87 +/- 4 mV followed by sustained depolarization to -35 +/- 5 mV. The transient hyperpolarization was eliminated by 1 microM noxiustoxin, a blocker of calcium-activated postassium channels (K(Ca)). the sustained depolarization induced by BK was prevented by incubating the cells with the calcium channel blocker lanthanum. TG evoked a similar response in membrane potential, with the exception that the onset of the hyperpolarization was slower compared with BK. The results presented here indicate that the cell resting potential is maintained at -62 +/- 2 mV by the IRK channel. BK or TG stimulation induces a transient hyperpolarization of approximately -20 mV produced by activation of a KCa. This hyperpolarization is followed by a sustained depolarization produced by activation of a calcium-selective channel sensitive to lanthanum.

Electrophysiological properties of human coronary endothelial cells

The electrophysiological properties of human coronary endothelial cells (HCEC) of macro- and microvascular origin were studied using the whole-cell configuration of the patch-clamp technique. The membrane potential of confluent HCEC (-41.9 +/- 3.9 mV (mean +/- SEM, n = 32) for macro- and -33.6 +/- 2.6 mV (n = 64) for microvascular cells, respectively) was less negative than the K+ equilibrium potential. Inward currents of isolated cells at potentials below the K+ equilibrium potential were blocked by external Ba2+ (1 mM), inactivated due to time- and voltage-dependent block caused by external Na+, and their amplitudes were enhanced by increasing extracellular [K+]; these currents were identified as inwardly rectifying K+ currents. Some isolated cells displayed outwardly directed K+ currents which were abolished after replacement of Cs+ for K+ on both sides of the membrane. Voltage-dependent Ca2+ currents could not be observed in isolated HCEC. Hyperpolarizations induced by vasoactive agonists have been observed in some endothelial cells from different species. In contrast, extracellularly applied ATP (adenosine-5'-triphosphate) and ADP (adenosine-5'-diphosphate) at micromolar concentrations depolarized confluent HCEC, whereas adenosine had no effect on resting potentials (RP), indicating that the nucleotide-induced depolarizations were mediated via P2- purinoceptors. These depolarizations occurred even after replacement of N-methyl-D-glucamine for extracellular Na+, indicating that Ca(2+)-influx was involved. There were no marked differences in the electrophysiological properties between cells of macro and microvascular origin.

Hyperpolarization induced by vasoactive substances in intact guinea-pig endocardial endothelial cells

1. The responses of guinea-pig endocardial endothelial (EE) cells to various vasoactive substances were investigated in either the small tissue preparation or freshly isolated cells using the patch clamp technique. 2. The mean resting potential of the EE cell was -44 mV in the small tissue preparation, and applications of ATP, ADP, AMP, adenosine, histamine and substance P induced transient hyperpolarizations of -22, -21, -9, -10, -23 and -15 mV, respectively. The membrane potential of EE cells failed to respond to acetylcholine, bradykinin, thrombin, atrial natriuretic peptide, vasopressin and serotonin. 3. The whole-cell voltage clamp of dissociated cells revealed a transient increase of K+ conductance underlying the ATP and histamine responses. The agonist-induced current showed no time-dependent change during voltage steps. The response was showed no time-dependent change during voltage steps. The response was prevented by adding 10 mM EGTA to the pipette solution. 4. In the cell-attached single channel recordings, ATP induced transient K+ channel activities having a slope conductance of 34 pS. In inside-out patches, similar K+ channels were activated by applying Ca2+ of more than 0.1 microM. 5. These findings are consistent with the idea that the Ca(2+)-dependent K+ channel is involved in the hyperpolarizing response of EE cells, as described in vascular endothelial cells.

Acetylcholine-stimulated changes of membrane potential and intracellular Ca2+ concentration recorded in endothelial cells in situ in the isolated rat aorta

The intracellular free Ca2+ concentration and membrane potential changes evoked by acetylcholine were recorded from whole-cell patch-clamped endothelial cells in situ in the isolated rat aorta. The endothelium had a resting membrane potential of -52 +/- 3 mV (SEM, range -35 mV to -76 mV n = 34) and a low input resistance (32 - 54 M omega). The membrane potential hyperpolarised by 3-30 mV on continuous application of acetylcholine at concentrations that produced endothelium-dependent relaxations in isolated rat aortic rings (range 1-500 nM). The response often comprised complex fluctuations of hyperpolarised membrane potential. Calcium concentration was measured with the fluorescent indicator furaptra, which has a wide range and minimises Ca2+ buffering. Acetylcholine evoked an initial rapid elevation of intracellular Ca2+ concentration, peaking in the range 6-35 microM, which declined with a half time of approximately 6 s, followed by repetitive [Ca2+] spikes of amplitude 2-18 microM in 23 of 34 cells. The initial [Ca2+] transient and hyperpolarisation were unaffected by removal of external Ca2+, whilst subsequent [Ca2+] spikes and maintained hyperpolarisations required the presence of external Ca2+. Both the hyperpolarisation and Ca2+ responses elicited by acetylcholine were abolished by atropine (1 microM). These results show that endothelial cells in situ exhibit large, fast repetitive [Ca2+] spikes in response to extracellular acetylcholine.

The role of the membrane potential of endothelial and smooth muscle cells in the regulation of coronary blood flow

In the mammalian heart the supply of oxygen and energy-rich substrates through the coronary arterioles is continuously adapted to the variations of cardiac work. The coronary resistance arteries and the surrounding myocardium form a functional unit with multiple interactions between coronary endothelial cells, smooth muscle cells, perivascular nerves, and cardiac muscle cells. We describe the mechanisms underlying the electrical and chemical communication between the different cell types, the ionic channels contributing to the resting potential of endothelial and smooth muscle cells, and the mechanisms responsible for modulation of the resting potential. The main conclusion of our analysis is that the membrane potential of coronary endothelial and smooth muscle cells is one of the major determinants of coronary blood flow, and that modulation of the membrane potential provides a way to dilate or constrict coronary resistance arteries. It is proposed that the membrane potential of the myo-endothelial regulatory unit, i.e., of the endothelial cells and the underlying smooth muscle cells in the terminal arterioles, may function as an integrator of the numerous local and global vasodilator and constrictor signals that provide for the adaptation of coronary blood flow to the metabolic demands of the heart.