Studies on the effects of adenosine A3 receptor stimulation on human eosinophils isolated from non-asthmatic or asthmatic donors

OBJECTIVE

This study was designed to demonstrate the presence of adenosine A3 receptors on human peripheral blood eosinophils, and to investigate the effect of A3 receptor stimulation on eosinophil function.

MATERIAL

Eosinophils from either non-asthmatic or asthmatic donors.

METHODS

Eosinophils were isolated from peripheral venous blood by discontinuous gradient centrifugation and negative immunoselection. Receptor localisation was investigated by immunoblotting and by immunocytochemistry using a novel antibody specific for the human A3 receptor. Two pharmacological responses were studied: elevation of intracellular calcium in single eosinophils, measured by microfluorimetry, and hydrogen peroxide generation in cell suspensions.

RESULTS

The expression of A3 receptors by eosinophils was confirmed using the selective antibody. Addition of the A3 receptor selective agonist, IB-MECA (100 nM), produced increases in intracellular calcium in less than 10% of the eosinophils isolated from non-asthmatic donors. These responses were only partially attenuated with the A3 receptor antagonist, I-ABOPX. IB-MECA (0.001-1000 nM) did not stimulate hydrogen peroxide (H2O2) generation, nor did it enhance fMLP- or C5a-stimulated generation of H2O2. In fact high concentrations of IB-MECA inhibited the generation of H2O2 (when stimulated by fMLP or C5a), an effect probably mediated by A2 receptors. Similar results were obtained using eosinophils from asthmatic donors.

CONCLUSIONS

Stimulation of adenosine A3 receptors does not appear to be a prime mechanism for free radical generation by human peripheral blood eosinophils.

The discovery and synthesis of highly potent, A2a receptor agonists

A series of N6,2-disubstituted adenosine analogues have been synthesized and their functional activity measured against A2a and A1 receptors. Examples of compounds with both a lipophilic N6-substituent and amino-functionalized 2-position were highly active at the A2a receptor on the human neutrophil.

Formoterol on airway smooth muscle and human lung mast cells: a comparison with salbutamol and salmeterol

Formoterol, like salbutamol and salmeterol, relaxed isolated preparations of guinea-pig trachea and human bronchus, and inhibited antigen-induced mediator release from human lung fragments in a concentration-related fashion. In each case, these actions were mediated through beta 2-adrenoceptors, with formoterol being 50-120-fold more potent than salbutamol, and 2-27-fold more potent than salmeterol. The duration of action of formoterol was longer than that of salbutamol in all preparations, but was markedly shorter than that of salmeterol, whose actions persisted for many hours despite continuous or extensive washing of the tissues. In conscious guinea-pigs, inhaled formoterol, salbutamol and salmeterol all caused dose-related inhibition of histamine-induced bronchoconstriction. Formoterol was again more potent (10-20-fold) than either salbutamol or salmeterol. However, while the actions of a threshold-effective dose of formoterol persisted for less than 3 h, somewhat longer than those of salbutamol (< 1.5 h), an equivalent dose of salmeterol was active for at least 6 h. Therefore, while formoterol is a potent beta 2-adrenoceptor agonist in vitro and in vivo, and is consistently longer-acting than salbutamol, its duration of action is markedly shorter than that of salmeterol.

The pharmacology of salmeterol

Salmeterol was developed to provide prolonged bronchodilatation to control nocturnal symptoms and improve maintenance therapy in asthmatic patients. Salmeterol is > 10,000 times more lipophilic than salbutamol and has greater affinity for the beta 2-adrenoceptor. Membrane binding is non-competitive and dissociation is slow so that its effects last for many hours. Despite this, salmeterol does not accumulate in tissues. Its mechanism of action can be explained by binding to a specific exo-site domain of the beta 2-receptor protein to produce continuous stimulation of the active site of the receptor, which gives salmeterol a profile of pharmacological activity unlike that of other beta 2-agonists. Due to its potent and prolonged activation of beta 2-adrenoceptors in airway smooth muscle cells, endothelial cells, mast cells and epithelial cells, salmeterol induces prolonged bronchodilatation, reduced vascular permeability, inhibition of inflammatory mediators, stimulation of ciliary function and modulation of ion and water transport across the bronchial mucosa.

Salmeterol: a potent and long-acting inhibitor of inflammatory mediator release from human lung

1. The effects of salmeterol, a novel long-acting beta 2-adrenoceptor agonist, have been investigated on antigen-induced mediator release from passively sensitized fragments of human lung in vitro. 2. Salmeterol was a potent inhibitor of the release of histamine (-log IC50 = 8.54), leukotriene C4 (LTC4)/LTD4 (-log IC50 = 9.07) and prostaglandin D2 (-log IC50 = 8.81). It was slightly less potent (1-3 fold) than isoprenaline, but significantly more potent (10-35 fold) than salbutamol. 3. Propranolol competitively antagonized the inhibitory effects of salmeterol on histamine release (pA2 = 8.41) and LTC4/LTD4 release, (pA2 = 8.40) indicating an action via beta-adrenoceptors. 4. The inhibitory effects of isoprenaline (20 nM) and salbutamol (200 nM) were removed after washing the lung tissue for 2 h and 4 h respectively. In contrast, the inhibitory effects of salmeterol (40 nM) were much longer-lasting, and were still evident after 20 h. 5. Salmeterol therefore exhibits potent and persistent inhibition of anaphylactic mediator release from human lung. This anti-inflammatory effect may be important for the therapeutic potential of salmeterol in the treatment of bronchial asthma.

The effect of prostanoids on the function of human eosinophils

The effects of prostanoids on eosinophil cationic protein (ECP) release and eosinophil migration have been measured. ECP release was inhibited by prostanoids active on DP and EP receptors. In contrast the DP agonist protaglandin D2 enhanced eosinophil migration.

Histamine-containing cells from bronchial lavage of macaque monkeys. Time course and inhibition of anaphylactic histamine release

Bronchial lavage of rhesus and cynomolgus monkeys provided leucocyte suspensions with viable histamine-containing cells (HCC) as 1--8% of the white cell population. These HCC released histamine or challenge with antiserum to human IgE. HCC from 2 monkeys with pulmonary and cutaneous hypersensitivity to Ascaris antigen (AA) released histamine on challenge with AA. The extent of histamine release was related to the concentration of antigen and antiserum, and histamine release was complete within 10 min of challenge. (+/-)Salbutamol and (-)isoprenaline were potent inhibitors of anaphylactic histamine release from HCC, but disodium cromoglycate and AH 9679 were relatively poor inhibitors. The HCC system combines the reproducibility of a cell suspension with a response to drugs similar to that of human lung fragments.

Characterization of the receptor mediating the antianaphylactic effects of beta-adrenoceptor agonists in human lung tissue in vitro

1 The rank order of potency of six beta-adrenoceptor agonists as inhibitors of the anaphylactic release of histamine from fragments of passively sensitized human lung in vitro was (--)-isoprenaline greater than (--) -adrenaline greater than (+/-)-salbutamol greater than (--)-noradrenaline greater than R0363 greater than H133/22. 2 The beta-adrenoceptor antagonists, propranolol, atenolol and H35/25, blocked the response to both (--)-isoprenaline and (+/-)-salbutamol competitively. Each antagonist gave similar pA2 values with both agonists. pA2 values were consistently at the high end of the range expected for interaction at a beta 2-adrenoceptor. 3 Practolol did not antagonize isoprenaline in a competitive manner but was a competitive antagonist of salbutamol with a pA2 at the high end of the range expected for interaction at a beta 2-adrenoceptor. 4 Data obtained with agonists are consistent with the receptor being of the beta 2-subtype. Data obtained with antagonists indicate a consistently higher affinity for the receptor than observed for the beta 2-subtype in other tissues but do not suggest a novel beta-adrenoceptor subtype on the mast cell of the human lung.

A comparison of the anti-anaphylactic activities of salbutamol and disodium cromoglycate in the rat, the rat mast cell and in human lung tissue

1 Salbutamol and disodium cromoglycate were compared for anti-anaphylactic activity against passive anaphylaxis in rat skin and peritoneum in vivo and in rat mast cells and human lung fragments in vitro.2 Salbutamol administered intravenously to rats inhibited cutaneous anaphylaxis, but also inhibited cutaneous responses to histamine and 5-hydroxytryptamine. Salbutamol administered intraperitoneally inhibited the release of slow reacting substance of anaphylaxis (SRS-A) but not the release of histamine in the peritoneum. It was a very weak inhibitor of histamine release from rat mast cells in vitro.3 Disodium cromoglycate administered intravenously to rats inhibited cutaneous anaphylaxis. Disodium cromoglycate administered intraperitoneally to rats inhibited the release of histamine and, to a lesser extent, SRS-A in the peritoneum. It was an effective but short-acting inhibitor of histamine release from rat mast cells in vitro.4 Salbutamol was a potent inhibitor of the anaphylactic release of histamine and SRS-A from fragments of human lung.5 Disodium cromoglycate was a weak inhibitor of the anaphylactic release of histamine and SRS-A from fragments of human lung. The inhibition was variable and not dose-related.6 The concentration of salbutamol required to inhibit anaphylaxis in human lung is of the same order as that required to relax human bronchial muscle. It is suggested that salbutamol may be more effective in allergic asthma if given in a prophylactic regimen.