Chloride ion transport and overexpression of TMEM16A in a guinea-pig asthma model.

BACKGROUND: TMEM16A, a Ca-activated Cl channel, regulates various physiological functions such as mucin secretion. However, the role of TMEM16A in hyper-secretion in asthma is not fully understood. OBJECTIVE: The aim of this study is to evaluate Cl ion transport via TMEM16A and determine the localization of TMEM16A in a guinea-pig asthma model. METHODS: Guinea-pigs were sensitized with ovalbumin (OVA) i.p. on Days 1 and 8. On Day 22, we assessed OVA challenge-induced Cl ion transport in the sensitized tracheas ex vivo in an Ussing chamber, compared with the non-sensitized tracheas. We then examined the effect of T16Ainh-A01, a TMEM16A inhibitor, on the increase in Cl ion transport. The tracheal epithelium was immunostained with an anti-TMEM16A antibody. Epithelial cells from guinea-pig tracheas were cultured at the air-liquid interface in the presence of IL-13 for in vitro study. We studied the effect of TMEM16A inhibitors on Ca-dependent agonist, uridine triphosphate (UTP)-induced increases in Cl ion transport in the cultured cells. The cells were immunostained with an anti-TMEM16A antibody, an anti-MUC5AC antibody and an anti-α-tubulin antibody. RESULTS: OVA challenge induced an increase in short circuit current within 1 min in the OVA-sensitized tracheas but not in the non-sensitized tracheas, which was inhibited by pretreatment of T16Ainh-A01. Sensitized tracheas showed goblet cell metaplasia with more positive TMEM16A immunostaining, particularly in the apical portion compared with the non-sensitized tracheas. The in vitro UTP-induced increase in Cl ion transport was strongly inhibited by pretreatment with T16Ainh-A01, benzbromarone, and niflumic acid. TMEM16A was positively immunostained at the apical portion and in the MUC5AC-positive area in IL-13-induced goblet cell metaplasia. CONCLUSIONS: Antigen challenge and Ca-dependent agonist treatment increased Cl ion transport via the overexpression of TMEM16A in goblet cell metaplasia in a guinea-pig asthma model. TMEM16A inhibitors may be useful for the treatment of hyper-secretion in asthma.

Role of Na(+)-K(+)-ATPase in sodium nitroprusside-induced relaxation of pulmonary artery under hypoxia.

BACKGROUND: The sodium pump (Na(+)-K(+)-ATPase) plays a part in the regulation of smooth muscle contractility, and alterations of enzyme activity by hypoxia could contribute to the mechanism of hypoxic pulmonary vasoconstriction. OBJECTIVE: To determine the role of Na(+)-K(+)-ATPase in the sodium nitroprusside (SNP)-induced relaxation of pulmonary artery in hypoxia. METHODS: Using isolated canine pulmonary arterial rings, we measured the relaxant responses of KCI-contracted tissues to SNP under hyperoxic (95% O2, 5% O2) and hypoxic conditions (5% O2, 5% CO2, 90% N2 in vitro. Na(+)-K(+)-ATPase activity was assessed by measuring ouabain-sensitive (86)Rb uptake. RESULTS: The SNP-induced relaxation was reduced under hypoxia, so that the maximal relaxation decreased from 80.1 +/- 8.6 to 57.8 +/- 6.8% (p < 0.01) and the concentration of SNP required to produce 50% relaxation increased from 1.9 +/- 0.4 x 10(-6) to 2.6 +/- 0.6 x 10(-5) M (p < 0.01). Addition of ouabain, an Na(+)-K(+)-ATPase inhibitor, attenuated the relaxant response to SNP and this inhibition was still observed under hypoxia. Incubation of endothelium-denuded rings with SNP caused dose-dependent increases in intracellular cGMP levels and ouabain-sensitive (86)Rb uptake, and these effects were not significantly altered by hypoxia. CONCLUSION: These results suggest that sarcolemmal Na(+)-K(+)-ATPase activity may be implicated in the mechanism of nitrovasodilator-induced vasodilation of pulmonary artery and may still be functioning under hypoxia.

Effect of suplatast tosilate, a Th2 cytokine inhibitor, on steroid-dependent asthma: a double-blind randomised study. Tokyo Joshi-Idai Asthma Research Group.

BACKGROUND: Th2 cytokines play an important part in the pathogenesis of asthma. Our aim was to study the effect of suplatast tosilate, a selective Th2 cytokine inhibitor, on asthma control and asthma exacerbations during reduction of inhaled corticosteroid dose in patients with steroid-dependent asthma. METHODS: 85 patients with moderate to severe asthma taking high doses (> or = 1500 microg per day) of inhaled beclometasone dipropionate, were assigned suplatast tosilate (100 mg three times daily) or placebo for 8 weeks in a double-blind, randomised, parallel-group, multicentre trial. During the first 4 weeks, other medications remained unchanged (add-on phase); during the next 4 weeks, the doses of beclometasone were halved (steroid-reduction phase). Main outcome measures were pulmonary function, asthma symptoms, and use of beta2-agonists. FINDINGS: Data were available from 77 patients. During the add-on phase, suplatast tosilate treatment, compared with placebo, was associated with higher forced expiratory volume in 1 s (mean difference between groups for changes from baseline at week 4, 0.20 L [95% CI 0.16-0.24], p=0.043), morning peak expiratory flow (18.6 L/min [14.1-23.1], p=0.037), and less diurnal variation in peak expiratory flow rate, asthma symptom scores (7.1 [6.6-7.6], p=0.029), and serum concentrations of eosinophil cationic protein and IgE. In the steroid-reduction phase, pulmonary function, asthma symptoms, and use of beta2-agonist deteriorated significantly more in the placebo group than in the suplatast group. INTERPRETATION: Treatment with a Th2 cytokine inhibitor in steroid-dependent asthma improves pulmonary function and symptom control, and allows a decrease in dose of inhaled corticosteroid without significant side-effects. Some improvements in pharmacokinetics are, however, needed.

Ginsenoside-induced relaxation of human bronchial smooth muscle via release of nitric oxide.

Ginsenoside, an extract of Panax ginseng, is an essential constituent of anti-asthmatic Chinese herbal medicine. To elucidate whether ginsenoside affects airway smooth muscle tone and, if so, what the mechanism of action is, we studied relaxant responses of human bronchial strips under isometric condition in vitro, and directly measured the release of nitric oxide (NO) by an amperometric sensor for this molecule. Addition of ginsenoside relaxed the tissues precontracted with acetylcholine in a dose-dependent manner, the maximal relaxation and the ginsenoside concentration required to produce 50% relaxation being 67+/-8% and 210+/-29 microg ml(-1), respectively. The relaxant responses to ginsenoside were inhibited by N(G)-nitro-L-arginine methylester (L-NAME) and removal of the epithelium, but not by N(G)-nitro-D-arginine methylester (D-NAME) or tetrodotoxin. This inhibitory effect of L-NAME was reversed by L-arginine but not by D-arginine. Addition of ginsenoside to the medium containing bronchial tissues dose-dependently increased NO-selective electrical current, and this effect was greatly attenuated by the epithelial removal or Ca(2+)-free medium. Ginsenoside also increased tissue cyclic GMP contents, an effect that was abolished in the presence of L-NAME. It is concluded that ginsenoside induces relaxation of human bronchial smooth muscle via stimulation of NO generation predominantly from airway epithelium and cyclic GMP synthesis. This action might account for the anti-asthmatic effect of Panax ginseng.

Macrolide antibiotics inhibit nitric oxide generation by rat pulmonary alveolar macrophages.

There is evidence that macrolide antibiotics are effective in the treatment of chronic airway inflammatory diseases, probably through actions other than their antibacterial properties. In order to determine whether macrolides affect the nitric oxide-generating system in the respiratory tract, rat pulmonary alveolar macrophages (PAMs) were studied in vitro. The release of NO was assessed by direct measurement with a specific amperometric sensor for this molecule, and the expression of type II NO synthase (NOS) messenger ribonucleic acid (mRNA) was determined by Northern blotting. Incubation of PAMs with lipopolysaccharide from Escherichia coli and recombinant human interferon-gamma caused release of NO, which was accompanied by induction of type II NOS mRNA. The release of NO was reduced by coincubation of cells with the macrolides erythromycin, clarithromycin and josamycin in a concentration-dependent manner, the maximal inhibition being 73+/-10, 81+/-6 and 84+/-9%, respectively, but was not altered by amoxycillin or cefaclor. These macrolides likewise inhibited the induction of type II NOS mRNA, whereas no inhibitory effects were observed with amoxycillin or cefaclor. These results suggest that macrolide antibiotics specifically inhibit type II NO synthase gene expression and consequently reduce NO production by rat pulmonary alveolar macrophages, which might result in attenuation of airway inflammation.

Inhibition of airway smooth muscle tone by Chinese herbal medicines.

The aim of the present study was to elucidate whether Chinese traditional herbal drugs, Gorei-San (TJ-17) and Toki-Shakuyaku-San (TJ-23), affect airway smooth muscle tone and, if so, to determine what the mechanism of action is. Rabbit tracheal segments were isolated and the contractile responses to electrical field stimulation and acetylcholine were measured before and after the application of TJ-17 or TJ-23 under isometric conditions in vitro. Ouabain-sensitive rubidium-86 (86Rb) uptake by tissues in response to each drug was also measured. Each herbal medicine attenuated the contractile responses to electrical field stimulation and acetylcholine in a concentration-dependent manner, the maximal inhibition of acetylcholine-induced contraction being 37.5+/-4.9% for TJ-17 and 42.4+/-5.3% for TJ-23 (p<0.05 for each). These effects were not altered by mechanical removal of the epithelium, indomethacin, the nitric oxide synthase inhibitor NG -nitro-L-arginine methyl ester, the cyclic adenosine monophosphate (cAMP)-dependent protein kinase inhibitor adenosine 3'5'-cyclic monophosphorothioate (Rp-cAMPS), the cyclic guanosine monophosphate (cGMP)-dependent protein kinase inhibitor KT5823, or the calcium (Ca2+)-activated potassium (K+) channel inhibitor charybdotoxin, but were greatly inhibited in the presence of the sodium (Na+)-K+ adenosine triphosphatase (ATPase) inhibitor ouabain. Incubation of tissues with TJ-17 and TJ-23 dose dependently increased ouabain-sensitive 86Rb uptake. The results of the study suggest that both Gorei-San and Toki-Shakuyaku-San reduce airway smooth muscle tone via a postjunctional mechanism probably through stimulation of the sodium pump and the subsequent hyperpolarization/repolarization of the cell membrane. These effects may contribute to the antiasthmatic properties of these herbal medicines.

Atypical adrenoceptor-mediated relaxation of canine pulmonary artery through a cyclic adenosine monophosphate-dependent pathway.

To determine whether functional atypical beta-adrenoceptors (beta(3)-adrenoceptors) are present in pulmonary vascular smooth muscle, we studied isolated canine pulmonary arterial rings under isometric conditions in vitro. Addition of beta-adrenoceptor agonists produced a concentration-dependent relaxation of noradrenaline-precontracted tissues, a rank order potency being isoproterenol (1) > salbutamol (0.95) > selective beta(3)-adrenoceptor agonists, CL 316243 (0.85), and BRL 37344 (0. 83). A marked desensitization to salbutamol occurred by pretreatment with salbutamol but not with CL 316243. When beta(1)-adrenoceptors had been blocked, the relaxant responses to salbutamol were competitively antagonized by the beta(2)-adrenoceptor antagonist ICI 118551 with a pA(2) value of 7.67 +/- 0.21 (mean +/- S.E.), but the response to CL 316243 was weekly antagonized by ICI 118551 only at a high concentration of 10(-5) M, where an apparent pA(2) value was 5. 24. In contrast, cyanopindolol, a nonselective beta-adrenoceptor antagonist, antagonized CL 316243-induced relaxation in a competitive manner with a pA(2) of 6.10 +/- 0.11. This pA(2) value was lower than that when salbutamol was used as an agonist (6.69 +/- 0.14, p < 0.01). Intracellular 3',5'-cyclic adenosine monophosphate (cAMP) levels were increased by CL 316243 in a concentration-dependent fashion, an effect that was not altered by ICI 118551. These results suggest that beta(3)-adrenoceptors may exist in canine pulmonary artery smooth muscle and that stimulation of this atypical receptor causes vasodilation through a cAMP-dependent pathway.

Macrolide antibiotics protect against immune complex-induced lung injury in rats: role of nitric oxide from alveolar macrophages.

Macrolide antibiotics have unique immunomodulatory actions apart from antimicrobial properties. We studied the effects of macrolides on IgG immune complex (IgG-ICx)-induced lung injury in rats in vivo and in vitro. Intrapulmonary deposition of IgG-ICx produced a time-dependent increase in the concentration of NO in exhaled air. There were corresponding increases in the number of neutrophils accumulated into alveolar spaces, and lung wet-to-dry weight ratio. All of these changes were inhibited by pretreatment with erythromycin or josamycin, but not by amoxicillin or cephaclor. Incubation of cultured pulmonary alveolar macrophages caused up-regulation of NO production and expression of inducible NO synthase mRNA, an effect that was dose dependently inhibited by erythromycin, roxithromycin, or josamycin. The macrolides also reduced IgG-ICx-induced release of IL-1beta and TNF-alpha, but did not alter the release of NO induced by exogenously added IL-1beta and TNF-alpha. These results suggest that macrolide antibiotics specifically inhibit immune complex-induced lung injury presumably by inhibiting cytokine release and the resultant down-regulation of inducible NO synthase gene expression and NO production by rat pulmonary alveolar macrophages.

Effect of azelastine on platelet-activating factor-induced microvascular leakage in rat airways.

To determine the effect of the antiallergic drug azelastine on airway mucosal inflammation, we studied airway microvascular permeability in response to platelet-activating factor (PAF) in pathogen-free rats. Vascular permeability and neutrophil accumulation were assessed by the percent area occupied by Monastral blue-labeled blood vessels and by myeloperoxidase-containing granulocytes, respectively, in whole mounts of the trachea and main bronchus. Intravenous PAF caused dose-dependent increases in the area density of Monastral blue-labeled vessels and neutrophil influx, and the former effect was inhibited by depletion of circulating neutrophils by cyclophosphamide or treatment with the neutrophil elastase inhibitor ONO-5046. Pretreatment with azelastine inhibited PAF-induced vascular leakage without affecting neutrophil accumulation. This inhibitory effect of azelastine was not seen in neutropenic rats and ONO-5046-treated rats. PAF increased neutrophil elastase contents in bronchoalveolar lavage fluid, an effect that was inhibited by azelastine. Therefore, azelastine attenuates PAF-induced airway mucosal microvascular leakage, probably involving inhibition of the release of neutrophil elastase from activated neutrophils.

[Relaxation of canine pulmonary arteries caused by stimulation of atypical beta-adrenergic receptors].

To examine possible contributions of beta(3)-adrenoceptors to catecholamine-induced pulmonary vasodilation, we studied isolated canine pulmonary arterial segments under isometric conditions. Addition of beta-adrenoceptor agonists produced a concentration-dependent relaxation of tissues precontracted with 50 mM KCl; the rank order of potency was isoproterenol (ISO, 1) > salbutamol (SAL, 0.97) > selective beta(3)-adrenoceptor agonists CL 316243 (CL, 0.87) and BRL 37344 (BRL, 0.86). Relaxant responses to SAL were competitively antagonized by the beta(2)-adrenoceptor antagonist ICI 118551 and the pA2 value was 6.67 +/- 0.21 (mean +/- SE), whereas the response to CL was weekly antagonized only by a high concentration of ICI 118551 (10(-5) M) and the apparent pA2 value was 5.24 when alpha-and beta(1)-adrenergic receptors were blocked. By contrast, the atypical beta-adrenoceptor antagonist cyanopindolol antagonized CL-induced relaxation in a competitive manner; the pA2 value was 6.71 +/- 0.12, which was lower than that with salbutamol (p < 0.05). Intracellular cyclic AMP levels were increased in a contraction-dependent manner by CL. These results suggest that beta(3)-adrenoceptors may exist in canine pulmonary arterial smooth muscle and that stimulation of this atypical receptor causes vasodilation through a cyclic AMP-dependent pathway.

Atypical adrenoceptor-mediated relaxation of canine pulmonary artery through a cAMP-dependent pathway.

We studied the existence of beta 3-adrenoceptors in canine pulmonary artery smooth muscle under isometric conditions in vitro. A rank order potency of vascular relaxation was isoproterenol > salbutamol > selective beta 3-adrenoceptor agonists, CL 316243 and BRL 37344. A marked desensitization to salbutamol occurred by pretreatment with salbutamol but not with CL 316243. When beta 1-adrenoceptors were blocked, the relaxant responses to salbutamol were competitively antagonized by the beta 2-adrenoceptor antagonist ICI 118551, whereas the response to CL 316243 was not. Cyanopindolol, a non-selective beta-adrenoceptor antagonist, antagonized CL 316243-induced relaxation in a competitive manner with a pA2 of 6.10, and this value was lower than that when salbutamol was used as an agonist (6.69). Intracellular cAMP levels were increased by CL 316243, an effect that was not altered by ICI 118551. Therefore, beta 3-adrenoceptors may be present and functioning in canine pulmonary artery.

A human bronchial epithelial cell line releases arginine vasopressin: involvement of Ca2+ -activated K+ channels.

To determine whether airway epithelium releases arginine vasopressin (AVP) and, if so, what the mechanism of the release is, we studied cultured human bronchial epithelial cell line, 16-HBE cells, in vitro. The cells spontaneously released small but significant amounts of AVP, and this release was dose dependently increased by platelet-activating factor (PAF) or bradykinin (BK). The PAF- and BK-induced AVP release was inhibited by 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate, a phospholipase C inhibitor, and thapsigargin but not by Ca2+ -free solution. Pretreatment with the big conductance Ca2+-activated K+ channel blocker iberiotoxin attenuated the stimulated release of AVP, whereas apamin and glibenclamide were without effect. These results suggest that human bronchial epithelial cells release AVP and that the release may be stimulated by phospholipase C activation, mobilization of Ca2+ from internal stores and the concomitant activation of big conductance Ca2+-activated K+ channels.

Regulation of adrenergic nerve-mediated contraction of canine pulmonary artery by K+ channels.

The aim of the present study was to investigate the role of certain subtypes of K+ channels in nerve-evoked contractions of pulmonary artery in vitro. The lobar or segmental pulmonary arteries were dissected from dogs, cut into ring segments, and the contractile responses to electrical field stimulation (EFS) and noradrenaline were measured under isometric conditions. Addition of iberiotoxin, a big conductance Ca2+-activated K+ channel blocker, and apamin, a small conductance Ca2+-activated K+ channel blocker, did not change the resting tension but augmented the contractile responses to EFS, so that the electric stimulus frequency required to produce a half-maximal contraction (ES50) was decreased from 18.2+/-3.5 to 7.4+/-2.3 Hz (p<0.01) and from 16.8+/-2.2 to 11.4+/-2.0 Hz (p<0.05), respectively, whereas glibenclamide, an adenosine triphosphate (ATP)-sensitive K+ channel blocker, had no effect. In contrast, none of the K+ channel blockers altered the contractile response to noradrenaline. Incubation of tissues with iberiotoxin and apamin increased the release of 3H-noradrenaline evoked by EFS. We conclude that big conductance Ca2+-activated K+ channels and small conductance Ca2+-activated K+ channels may play a role in the regulation of adrenergic neurotransmission in the pulmonary artery, probably by inhibiting the exocytotic release of noradrenaline from the adrenergic nerve terminals.

Role of the sarcolemmal sodium pump in nitroprusside-induced vasodilation of the pulmonary artery.

To elucidate the mechanism of nitrovasodilator-induced pulmonary vasodilation, we examined the role of sodium pump and K+ channels in the relaxant responses of canine pulmonary arterial rings to sodium nitroprusside (SNP) under isometric conditions in vitro. Pretreatment with the sodium pump inhibitor ouabain attenuated the SNP-induced vasodilation of KCI-contracted tissues, so that the maximal relaxation decreased from 90 +/- 7 to 62 +/- 6% (P < 0.01), and the negative logarithm of SNP concentration required to produce a half-maximal effect (pD2) decreased from 5.9 +/- 0.4 to 5.1 +/- 0.4 (P < 0.01). This effect was not altered by mechanical removal of the endothelium. In contrast, pretreatment with K+ channel blockers including iberiotoxin, apamin and glibenclamide did not change the relaxant responses to SNP. Incubation of endothelium-denuded rings with SNP increased ouabain-sensitive 86Rb uptake in a dose-dependent manner, an effect that was inhibited by KT 5823, a cGMP-dependent protein kinase inhibitor. These results suggest that activation of sarcolemmal sodium pump may be involved in the nitrovasodilator-induced cGMP-mediated pulmonary vasodilation, whereas K+ channels may play a less important role in this action.

Role of Na(+)-K+ ATPase in cyclic GMP-mediated relaxation of canine pulmonary artery smooth muscle cells.

1. Sodium-potassium adenosine triphosphate (Na(+)-K+ ATPase) plays a role in the regulation of vascular tone, but contribution of this enzyme to intravasodilator-induced pulmonary vasodilation remains uncertain. We thus studied the interaction between guanosine 3':5'-cyclic monophosphate (cyclic GMP) and Na(+)-K+ ATPase in smooth muscle cells isolated from canine pulmonary artery. 2. To assess the contractile properties, changes in smooth muscle cell length were determined microscopically. Application of potassium chloride (KCl) shortened the cell length, an effect which was reduced by sodium nitroprusside and 8-bromo-cyclic GMP in a concentration-dependent manner. Pretreatment of cells with the cyclic GMP-dependent kinase inhibitor KT 5823 (2 microM) abolished the effects of sodium nitroprusside and 8-bromo-cyclic GMP. 3. Ouabain (0.3 microM) did not alter the KCl-induced muscle shortening, but inhibited the relevant responses to sodium nitroprusside and 8-bromo-cyclic GMP. 4. Incubation of smooth muscle cells with sodium nitroprusside concentration-dependently increased intracellular cyclic GMP levels and ouabain-sensitive 86Rb uptake, and these values were significantly correlated. In the presence of KT 5823, sodium nitroprusside increased cyclic GMP levels but did not alter ouabain-sensitive 86Rb uptake. 5. These results suggest that there is a link between accumulation of intracellular cyclic AMP and activation of sarcolemmal Na(+)-K+ ATPase in pulmonary artery smooth muscle cells and that this link may be involved in the sodium nitroprusside-induced pulmonary vasodilatation.

Role of Ca(2+)-activated K+ channel in epithelium-dependent relaxation of human bronchial smooth muscle.

1. To elucidate whether K+ channels play a role in the action of epithelium-dependent bronchodilatation, we studied responses in human bronchial strips in the presence of indomethacin and NG-nitro-L-arginine methylester under isometric conditions, in vitro. 2. Mechanical removal of the epithelium increased the contractile responses to acetylcholine; the pD2 values increased from 5.0 +/- 0.2 to 5.9 +/- 0.3 (P < 0.001). This potentiation was abolished by iberiotoxin but not by apamin or glibenclamide. 3. In cascade bioassay, application of the bathing medium from dispersed, bronchial epithelial cells to epithelium-denuded bronchial strips decreased acetylcholine-induced contraction by 44 +/- 6%. This effect was reduced to 10 +/- 3% (P < 0.01) when the epithelial cells were pretreated with iberiotoxin, and to 4 +/- 1% (P < 0.001) when the epithelial cells were incubated with Ca(2+)-free medium containing [1,2-bis(2) aminophenoxy] ethane N,N,N',N'-tetraacetic acid-acetomethoxy ester. 4. In contrast, the bronchodilator effect of the medium bathing epithelial cells was not altered by the direct addition of iberiotoxin to epithelium-denuded tissues. 5. These results suggest that the Ca(2+)-activated K+ channel may play a role in the synthesis and/or release of smooth muscle relaxing factor, which is neither nitric oxide nor a cyclo-oxygenase product, from airway epithelial cells.

Effect of adenosine on adrenergic neurotransmission and modulation by endothelium in canine pulmonary artery.

To determine the effect of adenosine on adrenergic neurotransmission in pulmonary vasculature and its modulation by endothelial cells, we studied canine pulmonary arteries under isometric conditions in vitro. Adenosine decreased the contractile responses to electrical field stimulation but had no effect on those to norepinephrine. This inhibitory effect was concentration dependent, with a rank order of potency of NECA > 2-chloroadenosine > adenosine >> APNEA (an A3-adenosine-receptor agonist) > CGS-21680 (an A2a agonist) > CCPA (an A1 agonist). Adenosine reduced the electrical field stimulation-evoked 3H overflow in superfused pulmonary artery previously soaked in [3H]norepinephrine. Pretreatment with the adenosine uptake blocker dipyridamole or the adenosine deaminase inhibitor deoxycoformycin enhanced the adenosine action, and this enhancement was not observed in the endothelium-denuded tissues. Adenosine deaminase activity was found in endothelial cells. Therefore, adenosine inhibits norepinephrine release via an A2b-receptor mechanism, an effect that may be modulated by uptake and metabolism by endothelial cells.