Depolarization-mediated inhibition of Ca(2+) entry in endothelial cells

Membrane depolarization is required for pressure-dependent pulmonary arterial tone but not enhanced vasoconstriction to endothelin-1 following chronic hypoxia

Enhanced vasoconstriction is increasingly identified as an important contributor to the development of pulmonary hypertension. Chronic hypoxia results in enhanced Rho kinase mediated Ca²⁺ sensitization contributing to pressure-dependent pulmonary arterial tone as well as augmented vasoconstriction to endothelin-1 and depolarizing stimuli. We sought to investigate the interaction between these vasoconstrictor stimuli in isolated, pressurized, pulmonary arteries. We used the K⁺ ionophore, valinomycin, to clamp membrane potential (V_m) to investigate the role of membrane depolarization in endothelin-1 and pressure-dependent constriction, and endothelin-1 receptor inhibitors to determine whether membrane depolarization or stretch signal through endothelin-1 receptors. Clamping V_m prevented pressure-dependent tone, but not enhanced vasoconstriction to endothelin-1 following chronic hypoxia. Furthermore, endothelin-1 receptor inhibition had no effect on either pressure-dependent tone or vasoconstriction to KCl. As Src kinases contribute to both pressure-dependent tone and enhanced endothelin-1 vasoconstriction following chronic hypoxia, we further investigated their role in depolarization-induced vasoconstriction. Inhibition of Src kinases attenuated enhanced vasoconstriction to KCl. We conclude that membrane depolarization contributes to pressure-dependent tone but not enhanced vasoconstriction to ET-1, and that Src kinases serve as upstream mediators facilitating enhanced Rho kinase-dependent vasoconstriction following chronic hypoxia.

Ion channel signaling influences cellular proliferation and phagocyte activity during axolotl tail regeneration

Little is known about the potential for ion channels to regulate cellular behaviors during tissue regeneration. Here, we utilized an amphibian tail regeneration assay coupled with a chemical genetic screen to identify ion channel antagonists that altered critical cellular processes during regeneration. Inhibition of multiple ion channels either partially (anoctamin1/Tmem16a, anoctamin2/Tmem16b, KV2.1, KV2.2, L-type CaV channels and H/K ATPases) or completely (GlyR, GABAAR, KV1.5 and SERCA pumps) inhibited tail regeneration. Partial inhibition of tail regeneration by blocking the calcium activated chloride channels, anoctamin1&2, was associated with a reduction of cellular proliferation in tail muscle and mesenchymal regions. Inhibition of anoctamin 1/2 also altered the post-amputation transcriptional response of p44/42 MAPK signaling pathway genes, including decreased expression of erk1/erk2. We also found that complete inhibition via voltage gated K+ channel blockade was associated with diminished phagocyte recruitment to the amputation site. The identification of H+ pumps as required for axolotl tail regeneration supports findings in Xenopus and Planaria models, and more generally, the conservation of ion channels as regulators of tissue regeneration. This study provides a preliminary framework for an in-depth investigation of the mechanistic role of ion channels and their potential involvement in regulating cellular proliferation and other processes essential to wound healing, appendage regeneration, and tissue repair.

Involvement of ion channels and transporters in carcinoma angiogenesis and metastasis

Angiogenesis is a finely tuned process, which is the result of the equilibrium between pro- and antiangiogenic factors. In solid tumor angiogenesis, the balance is highly in favor of the production of new, but poorly functional blood vessels, initially intended to provide growing tumors with nutrients and oxygen. Among the numerous proteins involved in tumor development, several types of ion channels are overexpressed in tumor cells, as well as in stromal and endothelial cells. Ion channels thus actively participate in the different hallmarks of cancer, especially in tumor angiogenesis and metastasis. Indeed, from their strategic localization in the plasma membrane, ion channels are key operators of cell signaling, as they sense and respond to environmental changes. This review aims to decipher how ion channels of different families are intricately involved in the fundamental angiogenesis and metastasis hallmarks, which lead from a nascent tumor to systemic dissemination. An overview of the possible use of ion channels as therapeutic targets will also be given, showing that ion channel inhibitors or specific antibodies may provide effective tools, in the near future, in the treatment of carcinomas.

Sildenafil inhibits chronically hypoxic upregulation of canonical transient receptor potential expression in rat pulmonary arterial smooth muscle

In pulmonary arterial smooth muscle cells (PASMCs), Ca2+ influx through store-operated Ca2+ channels thought to be composed of canonical transient receptor potential (TRPC) proteins is an important determinant of intracellular free calcium concentration ([Ca2+](i)) and pulmonary vascular tone. Sildenafil, a type V phosphodiesterase inhibitor that increases cellular cGMP, is recently identified as a promising agent for treatment of pulmonary hypertension. We previously demonstrated that chronic hypoxia elevated basal [Ca2+](i) in PASMCs due in large part to enhanced store-operated Ca2+ entry (SOCE); moreover, ex vivo exposure to prolonged hypoxia (4% O2 for 60 h) upregulated TRPC1 and TRPC6 expression in PASMCs. We examined the effect of sildenafil on basal [Ca2+](i), SOCE, and the expression of TRPC in PASMCs under prolonged hypoxia exposure. We also examined the effect of sildenafil on TRPC1 and TRPC6 expression in pulmonary arterial smooth muscle (PA) from rats that developed chronically hypoxic pulmonary hypertension (CHPH). Compared with vehicle control, treatment with sildenafil (300 nM) inhibited prolonged hypoxia induced increases of 1) basal [Ca2+](i), 2) SOCE, and 3) mRNA and protein expression of TRPC in PASMCs. Moreover, sildenafil (50 mg . kg(-1) . day(-1)) inhibited mRNA and protein expression of TRPC1 and TRPC6 in PA from chronically hypoxic (10% O2 for 21 days) rats, which was associated with decreased right ventricular pressure and right ventricular hypertrophy. Furthermore, we found, in PASMCs exposed to prolonged hypoxia, that knockdown of TRPC1 or TRPC6 by their specific small interference RNA attenuated the hypoxic increases of SOCE and basal [Ca2+]i, suggesting a cause and effect link between increases of TRPC1 and TRPC6 expression and the hypoxic increases of SOCE and basal [Ca2+]i. These results suggest that sildenafil may alter basal [Ca2+](i) in PASMCs by decreasing SOCE through downregulation of TRPC1 and TRPC6 expression, thereby contributing to decreased vascular tone of pulmonary arteries during the development of CHPH.

Reduced store-operated Ca2+ entry in pulmonary endothelial cells from chronically hypoxic rats

Chronic hypoxia (CH)-induced pulmonary hypertension may influence basal endothelial cell (EC) intracellular Ca(2+) concentration ([Ca(2+)](i)). We hypothesized that CH decreases EC [Ca(2+)](i) associated with membrane depolarization and reduced Ca(2+) entry. To test this hypothesis, we assessed 1) basal endothelial Ca(2+) in pressurized pulmonary arteries and freshly isolated ECs, 2) EC membrane potential (E(m)), 3) store-operated Ca(2+) current (I(SOC)), and 4) store-operated Ca(2+) (SOC) entry in arteries from control and CH rats. We found that basal EC Ca(2+) was significantly lower in pressurized pulmonary arteries and freshly isolated ECs from CH rats compared with controls. Similarly, ECs in intact arteries from CH rats were depolarized compared with controls, although no differences were observed between groups in isolated cells. I(SOC) activation by 1 muM thapsigargin displayed diminished inward current and a reversal potential closer to 0 mV in cells from CH rats compared with controls. In addition, SOC entry determined by fura 2 fluorescence and Mn(2+) quenching revealed a parallel reduction in Ca(2+) entry following CH. We conclude that differences in the magnitude of SOC entry exist between freshly dispersed ECs from CH and control rats and correlates with the decrease in basal EC [Ca(2+)](i). In contrast, basal EC Ca(2+) influx is unaffected and membrane depolarization is limited to intact arteries, suggesting that E(m) may not play a major role in determining basal EC [Ca(2+)](i) following CH.

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

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

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

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

Combination of Ca2+ -activated K+ channel blockers inhibits acetylcholine-evoked nitric oxide release in rat superior mesenteric artery

BACKGROUND AND PURPOSE

The present study investigated whether calcium-activated K+ channels are involved in acetylcholine-evoked nitric oxide (NO) release and relaxation.

EXPERIMENTAL APPROACH

Simultaneous measurements of NO concentration and relaxation were performed in rat superior mesenteric artery and endothelial cell membrane potential and intracellular calcium ([Ca2+]i) were measured.

KEY RESULTS

A combination of apamin plus charybotoxin, which are, respectively, blockers of small-conductance and of intermediate- and large-conductance Ca2+ -activated K channels abolished acetylcholine (10 microM)-evoked hyperpolarization of endothelial cell membrane potential. Acetylcholine-evoked NO release was reduced by 68% in high K+ (80 mM) and by 85% in the presence of apamin plus charybdotoxin. In noradrenaline-contracted arteries, asymmetric dimethylarginine (ADMA), an inhibitor of NO synthase inhibited acetylcholine-evoked NO release and relaxation. However, only further addition of oxyhaemoglobin or apamin plus charybdotoxin eliminated the residual acetylcholine-evoked NO release and relaxation. Removal of extracellular calcium or an inhibitor of calcium influx channels, SKF96365, abolished acetylcholine-evoked increase in NO concentration and [Ca2+]i. Cyclopiazonic acid (CPA, 30 microM), an inhibitor of sarcoplasmic Ca2+ -ATPase, caused a sustained NO release in the presence, but only a transient increase in the absence, of extracellular calcium. Incubation with apamin and charybdotoxin did not change acetylcholine or CPA-induced increases in [Ca2+]i, but inhibited the sustained NO release induced by CPA.

CONCLUSIONS AND IMPLICATIONS

Acetylcholine increases endothelial cell [Ca2+]i by release of stored calcium and calcium influx resulting in activation of apamin and charybdotoxin-sensitive K channels, hyperpolarization and release of NO in the rat superior mesenteric artery.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

GABA release and uptake regulate neuronal precursor migration in the postnatal subventricular zone

In the postnatal subventricular zone (SVZ), astrocyte-like cells tightly encapsulate chains of migrating neuronal precursors, although an influence of the astrocyte-like cells on precursor migration has not yet been demonstrated. Cell migration was studied in acute sagittal brain slices to determine whether GABA signaling between astrocyte-like cells and neuronal precursors controls the speed of neuronal precursor migration in the anterior SVZ and rostral migratory stream of juvenile and adult mice. Application of GABA at 10 microm, a nondesensitizing concentration for GABA(A) receptors (GABA(A)Rs), reduced the rate (mean of approximately 50 microm/hr) of cell migration by 21% via GABA(A)R activation. Application of the GABA(A)R antagonist bicuculline enhanced the migration rate by 30%, suggesting that endogenous GABA tonically reduces the speed of cell migration via GABA(A)R activation. Using immunohistochemistry, we found that astrocyte-like cells express the high-affinity GABA transporter subtype GAT4 on processes ensheathing neuronal precursors that contain GABA. Inhibition of GABA uptake into astrocyte-like cells or enhancement of GABA release from neuronal precursors during high K(+) application further reduced the migration rate by increasing ambient GABA levels. GABA altered the migration speed by interfering with intracellular Ca(2+) signaling independently of cell depolarization, because high K(+) application did not alter the speed of cell migration in the presence of bicuculline. These data indicate that astrocyte-like cells create a microenvironment in which their uniquely positioned GABA transporters control the degree of GABA(A)R activation and the migration of neuronal precursors.

Sodium-calcium exchanger contributes to membrane hyperpolarization of intact endothelial cells from rat aorta during acetylcholine stimulation

1. The role of sodium-calcium exchanger in acetylcholine (Ach)-induced hyperpolarization of intact endothelial cells was studied in excised rat aorta. The membrane potential was recorded using perforated patch-clamp technique. 2. The mean resting potential of endothelial cells was -44.1+/-1.4 mV. A selective inhibitor of sodium-calcium exchanger benzamil (100 microm) had no significant effect on resting membrane potential, but reversibly decreased the amplitude of sustained Ach-induced endothelial hyperpolarization from 20.9+/-1.4 to 5.7+/-1.1 mV when applied during the plateau phase. 3. The blocker of reversed mode of the exchanger KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate, 20 microm) reversibly decreased the amplitude of sustained Ach-induced hyperpolarization from 20.5+/-2.9 to 7.5+/-1.8 mV. 4. Introduction of tetraethylammonium (10 mm) in the continuous presence of Ach decreased the sustained phase of hyperpolarization from 17.9+/-1.5 by 12.9+/-0.9 mV. Subsequent addition of 20 microm KB-R7943 further depolarized endothelial cells by 4.8+/-1.1 mV. 5. Substituting external sodium with N-methyl d-glucamine during the plateau phase of Ach-evoked hyperpolarization reversibly decreased the hyperpolarization from -61.8+/-2.7 to -54.2+/-1.9 mV. In the majority of preparations, the initial response to removal of external sodium was a transient further rise in the membrane potential of several mV. Sodium ionophore monensin hyperpolarized endothelium by 10.3+/-0.7 mV. 6. The inhibitory effect of benzamil on Ach-induced endothelial sustained hyperpolarization was observed in endothelium mechanically isolated from smooth muscle. 7. These results suggest that the sodium-calcium exchanger of intact endothelial cells is able to operate in reverse following stimulation by Ach, contributing to sustained hyperpolarization. Myoendothelial electrical communications do not mediate the effect of blockers of sodium-calcium exchanger.

Inhibition of endothelium-dependent vasorelaxation by extracellular K(+): a novel controlling signal for vascular contractility

The effects of extracellular K+ on endothelium-dependent relaxation (EDR) and on intracellular Ca2+ concentration ([Ca2+]i) were examined in mouse aorta, mouse aorta endothelial cells (MAEC), and human umbilical vein endothelial cells (HUVEC). In mouse aortic rings precontracted with prostaglandin F2alpha or norepinephrine, an increase in extracellular K+ concentration ([K+]o) from 6 to 12 mM inhibited EDR concentration dependently. In endothelial cells, an increase in [K+]o inhibited the agonist-induced [Ca2+]i increase concentration dependently. Similar to K+, Cs+ also inhibited EDR and the increase in [Ca2+]i concentration dependently. In current-clamped HUVEC, increasing [K+]o from 6 to 12 mM depolarized membrane potential from -32.8 +/- 2.7 to -8.6 +/- 4.9 mV (n = 8). In voltage-clamped HUVEC, depolarizing the holding potential from -50 to -25 mV decreased [Ca2+]i significantly from 0.95 +/- 0.03 to 0.88 +/- 0.03 microM (n = 11, P < 0.01) and further decreased [Ca2+]i to 0.47 +/- 0.04 microM by depolarizing the holding potential from -25 to 0 mV (n = 11, P < 0.001). Tetraethylammonium (1 mM) inhibited EDR and the ATP-induced [Ca2+]i increase in voltage-clamped MAEC. The intermediate-conductance Ca2+-activated K+ channel openers 1-ethyl-2-benzimidazolinone, chlorozoxazone, and zoxazolamine reversed the K+-induced inhibition of EDR and increase in [Ca2+]i. The K+-induced inhibition of EDR and increase in [Ca2+]i was abolished by the Na+-K+ pump inhibitor ouabain (10 microM). These results indicate that an increase of [K+]o in the physiological range (6-12 mM) inhibits [Ca2+]i increase in endothelial cells and diminishes EDR by depolarizing the membrane potential, decreasing K+ efflux, and activating the Na+-K+ pump, thereby modulating the release of endothelium-derived vasoactive factors from endothelial cells and vasomotor tone.

Role of endothelial intermediate conductance KCa channels in cerebral EDHF-mediated dilations

The present study evaluated the role of endothelial intermediate conductance calcium-sensitive potassium channels (IKCa) in the mechanism of endothelium-derived hyperpolarizing factor (EDHF)-mediated dilations in pressurized cerebral arteries. Male rat middle cerebral arteries (MCA) were mounted in an isolated vessel chamber, pressurized (85 mmHg), and luminally perfused (100 microl/min). Artery diameter was measured simultaneously with either endothelial intracellular Ca2+ concentration ([Ca2+]i; fura-2) or changes in endothelial membrane potential [4-[2-[6-(dioctylamino)-2-naphthalenyl]ethenyl]1-(3-sulfopropyl)-pyridinium (di-8-ANEPPS)]. Nitric oxide synthase and cyclooxygenase inhibitors were present throughout. Luminal application of UTP produced EDHF-mediated dilations that correlated with significant endothelial hyperpolarization. The dilation and endothelial hyperpolarization were virtually abolished by inhibitors of IKCa channels but not by selective inhibitors of small or large conductance KCa channels (apamin and iberiotoxin, respectively). Additionally, direct stimulation of endothelial IKCa channels with 1-ethyl-2-benzimidazolinone (1-EBIO) produced endothelial hyperpolarization and vasodilatation that were blocked by inhibitors of IKCa channels. 1-EBIO hyperpolarized the endothelium but did not affect endothelial [Ca2+]i. We conclude that the mechanism of EDHF-mediated dilations in cerebral arteries requires stimulation of endothelial IKCa channels to promote endothelial hyperpolarization and subsequent vasodilatation.

Smooth muscle membrane potential modulates endothelium-dependent relaxation of rat basilar artery via myo-endothelial gap junctions

The release of endothelium-derived relaxing factors, such as nitric oxide (NO), is dependent on an increase in intracellular calcium levels ([Ca(2+)](i)) within endothelial cells. Endothelial cell membrane potential plays a critical role in the regulation of [Ca(2+)](i) in that calcium influx from the extracellular space is dependent on membrane hyperpolarization. In this study, the effect of inhibition of vascular smooth muscle delayed rectifier K(+) (K(DR)) channels by 4-aminopyridine (4-AP) on endothelium-dependent relaxation of rat basilar artery to acetylcholine (ACh) was assessed. ACh-evoked endothelium-dependent relaxations were inhibited by N-(Omega)-nitro-L-arginine (L-NNA) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), confirming a role for NO and guanylyl cyclase. 4-AP (300 microM) also suppressed ACh-induced relaxation, with the maximal response reduced from approximately 92 to approximately 33 % (n = 11; P < 0.01). However, relaxations in response to exogenous NO, applied in the form of authentic NO, sodium nitroprusside or diethylamineNONOate (DEANONOate), were not affected by 4-AP treatment (n = 3-11). These data are not consistent with the view that 4-AP-sensitive K(DR) channels are mediators of vascular hyperpolarization and relaxation in response to endothelium-derived NO. Inhibition of ACh-evoked relaxation by 4-AP was reversed by pinacidil (0.5-1 microM; n = 5) or 18beta-glycyrrhetinic acid (18betaGA; 5 microM; n = 5), indicating that depolarization and electrical coupling of the smooth muscle to the endothelium were involved. 4-AP caused depolarization of both endothelial and vascular smooth muscle cells of isolated segments of basilar artery (mean change 11 +/- 1 and 9 +/- 2 mV, respectively; n = 15). Significantly, 18betaGA almost completely prevented the depolarization of endothelial cells (n = 6), but not smooth muscle cells (n = 6) by 4-AP. ACh-induced hyperpolarization of endothelium and smooth muscle cells was also reduced by 4-AP, but this inhibition was not observed in the combined presence of 4-AP and 18betaGA. These data indicate that 4-AP can induce an indirect inhibition of endothelium-dependent relaxation in the rat basilar artery by electrical coupling of smooth muscle membrane depolarization to the endothelium via myo-endothelial gap junctions.

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

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

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.