Kinetic analysis of drug-receptor interactions of long-acting beta2 sympathomimetics in isolated receptor membranes: evidence against prolonged effects of salmeterol and formoterol on receptor-coupled adenylyl cyclase.
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The long-acting beta2 sympathomimetics salmeterol and formoterol have been presumed to exert their prolonged action either by binding to an accessory binding site ("exo-site") near the beta2 adrenoceptor or by their high affinity for beta2 adrenoceptors and correspondingly slow dissociation. Whereas most studies with salmeterol had been done in intact tissues, which have slow diffusion and compartmentation of drugs in lipophilic phases, that restrict drug access to the receptor biophase, we used purified receptor membranes from rat lung and disaggregated calf tracheal myocytes as model systems. Binding experiments were designed to measure the slow dissociation of agonists by means of delayed association of (-)-[125I]iodopindolol. Rat lung membranes were pretreated with high concentrations of agonists (salmeterol, formoterol, isoprenaline) before dissociation was induced by 50-fold dilution. Half-times of association of (-)-[125I]iodopindolol remained unchanged compared with untreated controls, indicating that dissociation of agonists occurred in less than 2 min. Adenylyl cyclase experiments were designed to determine the on and off kinetics of agonists to beta2 adrenoceptors by measuring the rate of receptor-induced cyclic AMP (cAMP) formation. Experiments were performed in tracheal membranes characterized by high Vmax values of cAMP formation. Adenylyl cyclase activation occurred simultaneously with the addition of the agonist, continued linearly with time for 60 min, and ceased immediately after the antagonist was added. Similarly, when receptor membranes were preincubated in a small volume with high salmeterol concentrations, there was a linear increase in cAMP formation, which was immediately interrupted by a 100-fold dilution of the reaction mixture. This militates against the exo-site hypothesis. On the other hand, dissociation by dilution was much less when membranes were preincubated with a large volume of salmeterol at the same concentration, indicating that physicochemical effects, and not exo-site binding, underlie its prolonged mode of action. (+info)
Mechanisms of prostaglandin E2 release by intact cells expressing cyclooxygenase-2: evidence for a 'two-component' model.
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Prostaglandin (PG) release in cells expressing constitutive cyclooxygenase-1 is known to be regulated by liberation of arachidonic acid by phospholipase A2 followed by metabolism by cyclooxygenase. However, the relative contribution of phospholipase A2 to the release of PGs in cells expressing cyclooxygenase-2 is not clear. We addressed this question by using radioimmunoassay to measure PGE2 release by human cells (A549) induced to express cyclooxygenase-2 (measured by Western blot analysis) by interleukin-1beta. Cells were either unstimulated or stimulated with agents known to activate phospholipase A2 (bradykinin, Des-Arg10-kallidin, or the calcium ionophore A23187) or treated with exogenous arachidonic acid. When cells were treated to express cyclooxygenase-2, the levels of PGE2 released over 15 min were undetectable; however, in the same cells stimulated with bradykinin, A23187, or arachidonic acid, large amounts of prostanoid were produced. Using selective inhibitors/antagonists, we found that the effects of bradykinin were mediated by B2 receptor activation and that prostanoid release was due to cyclooxygenase-2, and not cyclooxygenase-1, activity. In addition, we show that the release of PGE2 stimulated by either bradykinin, A23187, or arachidonic acid was inhibited by the phospholipase A2 inhibitor arachidonate trifluoromethyl ketone. Hence, we have demonstrated that PGE2 is released by two components: induction of cyclooxygenase-2 and supply of substrate, probably via activation of phospholipase A2. This is illustrated in A549 cells by a clear synergy between the cytokine interleukin-1beta and the kinin bradykinin. (+info)
Evidence for beta3-adrenoceptor subtypes in relaxation of the human urinary bladder detrusor: analysis by molecular biological and pharmacological methods.
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The purpose of the present study was to confirm the presence of beta3-adrenoceptor subtype in the relaxation of human urinary bladder detrusor tissue by reverse transcription-polymerase chain reaction (PCR); direct sequencing of the PCR product, in situ hybridization; and isometric contraction. Using reverse transcription-PCR, the mRNAs of three receptor subtypes (beta1, beta2, and beta3) were expressed in the human urinary bladder detrusor tissue. Direct sequencing of the PCR product of the above beta3-adrenoceptor revealed no mutation in the amplified regions. In situ hybridization with digoxygenin-labeled oligonucleotide probe revealed the presence of the mRNA of beta3-adrenoceptor subtype in the smooth muscle of the urinary bladder. The relaxant effects of isoproterenol (a nonselective beta-adrenoceptor agonist); ZD7114, BRL37344, and CGP12177A (putative selective beta3-adrenoceptor agonists); and SR59230A (a putative selective beta3-adrenoceptor antagonist) were tested using an isometric contraction technique. Isoproterenol in either the presence or absence of both atenolol (a beta1-adrenoceptor-selective antagonist) and butoxamine (a beta2-adrenoceptor-selective antagonist) revealed a relaxant effect on the carbachol-induced contraction of the human urinary bladder detrusor. Both BRL37344 and CGP12177A also revealed relaxant effects on the human urinary bladder detrusor, but ZD7114 did not elicit any relaxation. These results suggest that beta3-adrenoceptor may have some role in urine storage in the human urinary bladder. (+info)
Beta2-adrenoceptor polymorphism and bronchoprotective sensitivity with regular short- and long-acting beta2-agonist therapy.
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The aim of the present study was to investigate bronchoprotective sensitivity in patients receiving regular treatment with short- and long-acting beta2-agonists and to evaluate any possible association with genetic polymorphism. Thirty-eight patients with stable mild to moderate asthma and receiving inhaled corticosteroids were randomized in a parallel group, double-blind, double-dummy fashion to receive 2 weeks of treatment with either formoterol (12 microg once daily, 6 microg twice daily or 24 microg twice daily) or terbutaline (500 microg four times daily). Bronchoprotection against methacholine challenge (as a provocative dose to produce a 20% fall in forced expiratory volume in 1.0 s: PD20) was measured at baseline (unprotected) after an initial 1 week run-in without beta2-agonist, and at 1 h after the first and last doses of each treatment. The PD20 values were log-transformed and calculated as change from baseline. Percentage desensitization of log PD20 for first- versus last-dose bronchoprotection was calculated and analysed according to effects of treatment and beta2-adrenoceptor polymorphism at codon 16 or 27. The mean degree of desensitization for bronchoprotection was comparable with all four treatments and there were no significant differences in absolute PD20 values after 2 weeks of chronic dosing. The PD20 values were (as microg of methacholine, geometric means+/-S. E.M.): formoterol, 12 microg once daily, 99+/-42 microg; formoterol, 6 microg twice daily, 107+/-44 microg; formoterol, 24 microg twice daily, 108+/-45 microg; terbutaline, 500 microg four times daily, 88+/-37 microg. All patients receiving formoterol, 24 microg twice daily, exhibited a loss of protection greater than 30% which was unrelated to polymorphism at codon 16 or 27. For codon 16, the use of lower doses of formoterol (12 microg once daily or 6 microg twice daily) showed wider variability in the propensity for protection loss in patients who were heterozygous, in contrast to a more uniform protection loss seen with homozygous glycine patients. The amount of protection loss was not significantly related to polymorphism at codon 16 or 27, expressed as values (mean+/-S.E.M.) for percentage desensitization according to each genotype (pooled treatments): Gly-16, 66+/-11%; Het-16, 53+/-8%; Arg-16, 69+/-18%; Glu-27, 68+/-12%; Het-27, 58+/-8%; Gln-27, 52+/-12%. The results of this preliminary study showed that bronchoprotective desensitization occurred readily in response to short- or long-acting beta2-agonist exposure irrespective of beta2-adrenoceptor polymorphism at codon 16 or 27. Further studies with larger patient numbers are required to further evaluate the effects of polymorphisms with lower doses of regular formoterol. (+info)
The contribution of extraneuronal uptake to the trachea-blood vessel selectivity of beta-adrenoceptor stimulants in vitro in guinea-pigs.
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1 The potencies relative to isoprenaline of isoetharine, tertiary butyl noradrenaline, salbutamol, orciprenaline, Me 506, rimiterol, fenoterol, carbuterol and terbutaline on isolated preparations of guinea-pig trachea and blood vessels (perfused hind limb) were determined. All the compounds were selective for trachea and selectivity values, i.e. relative potency on trachea divided by relative potency on hind limb, ranged from 2.3 to 21.4. 2 Responses to isoprenaline (the reference compound), tertiary butyl noradrenaline and isoetharine were potentiated on trachea by 50 muM phenoxybenzamine (PHB) and by other inhibitors of extraneuronal uptake (ENU). Under these conditions the selectivity values of all the compounds was close to unity. 3 Selectivity values were also close to unity if they were calculated from data obtained without ENU inhibition, provided that only those compounds not potentiated by PHB on trachea were used. 4 It is proposed that the trachea-blood vessel selectivity shown by beta-adrenoceptor stimulants can be caused by the influence of ENU upon them, rather than by their ability to distinguish between two beta2-adrenoceptors. 5 The suggestion that differences exist between beta2-adrenoceptors in respiratory and vascular smooth muscle is not supported by the in vitro experiments described. (+info)
The cat lung strip as an in vitro preparation of peripheral airways: a comparison of beta-adrenoceptor agonists, autacoids and anaphylactic challenge on the lung strip and trachea.
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1 A new in vitro preparation, the isolated lung strip of the cat, is described for investigating the direct effect of drugs on the smooth muscle of the peripheral airways of the lung. The preparation comprises a thin strip of lung parenchyma which can be mounted in a conventional organ bath for isometric tension recording. Its pharmacological responses have been characterized and compared with the isolated tracheal preparation of the cat. 2 The lung strip exhibited an intrinsic tone which was relaxed by catecholamines, aminophylline and flufenamate. It was contracted strongly by histamine, prostaglandin F2alpha, acetylcholine, compound 48/80, potassium depolarizing solution and alternating current field stimulation. In contrast, the cat trachea was unresponsive to histamine and prostaglandin F2alpha and did not exhibit an intrinsic tone. 3 (-)-Isoprenaline and (-)-adrenaline were much more potent in relaxing the lung strip than the trachea. The potency order of relaxation responses to isoprenaline, adrenaline and (+/-)-noradrenaline in the lung strip was isoprenaline greater than adrenaline greater than noradrenaline but in the trachea was isoprenaline greater than noradrenaline greater than or equal to adrenaline. 4 beta2-Adrenoceptor selective agonists salbutamol and terbutaline were more potent in the lung strip than the trachea, suggesting beta2-adrenoceptors predominated in the lung strip. Propranolol was equipotent in inhibiting isoprenaline relexations of the lung strip and trachea, whereas practolol was much less effective in inhibiting lung strip than trachea, further supporting a predominance of beta2-adrenoceptors in lung strip and beta1-adrenoceptors in trachea. 5 Strong Schultz-Dale type contractions were elicited in both lung strips and trachea by Ascaris lumbricoides antigen in actively sensitized cats. The initial phase of the contractile response of the lung strip following challenge was shown to be due to histamine release and was absent in the trachea. The delayed phase of the contraction which took several minutes to develop in both the mepyramine-treated lung strip and trachea was not due to prostaglandins E1, F2alpha or bradykinin, the probable mediator being slow reacting substance of anaphylaxis (SRS-A). 6 It is concluded that the isolated lung strip of the cat is useful as an in vitro model for investigating the effect of drugs on the smooth muscle of the peripheral airways of the lungs. (+info)
Reversal of severe pulmonary hypertension with beta blockade in a patient with end stage left ventricular failure.
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A 52 year old man with severe chronic left ventricular failure (New York Heart Association class IV) was considered unsuitable for cardiac transplantation because of high and irreversible pulmonary vascular resistance (PVR). In an attempt to produce symptomatic improvement, metoprolol was cautiously introduced, initially at 6.25 mg twice daily. This was slowly increased to 50 mg twice daily over a two month period and continued thereafter. After four months of treatment the patient's symptoms had improved dramatically. His exercise tolerance had increased and diuretic requirements reduced to frusemide 160 mg/day only. Assessment of right heart pressures was repeated and, other than a drop in resting heart rate, there was little change in his pulmonary artery pressure or PVR. His right heart pressures were reassessed showing a pronounced reduction in pulmonary artery pressure and a significant reduction in PVR, which fell further with inhaled oxygen and sublingual nitrates. He was then accepted onto the active waiting list for cardiac transplantation. A possible mechanism of action was investigated by assessing responses to beta agonists during treatment. Not only was there pronounced improvement in PVR but it was also demonstrated that beta receptor subtype cross-regulation may have contributed to the mechanism of benefit. (+info)
As-required versus regular nebulized salbutamol for the treatment of acute severe asthma.
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Current British guidelines for the administration of beta2-agonists in acute severe asthma recommend regular nebulized therapy in hospitalized patients, followed by as-required (p.r.n.) use via hand-held devices after discharge. Since beta2-agonists do not possess anti-inflammatory activity in vivo, and are thus unlikely to influence the rate of recovery from an asthma exacerbation, it was hypothesized that patients given the short-acting beta2-agonist salbutamol on an as-required basis after admission to hospital would recover as quickly as those on regular treatment, but with potential reductions in the total dose delivered. Forty-six patients with acute severe asthma were randomly assigned to either regular prescriptions of nebulized salbutamol or to usage on a p.r.n. basis, from 24 h after hospital admission. The primary outcome measures were length of hospital stay, time to recovery, and frequency of salbutamol nebulization from 24 h after admission to discharge. Secondary outcome measures were treatment side-effects (tremor, palpitations), and patient satisfaction. Length of hospital stay was reduced in those patients allocated to p.r.n. salbutamol (geometric mean (GM) 3.7 days) versus regular salbutamol (GM 4.7 days). Time taken for peak expiratory flow to reach 75% of recent best was the same in both groups. There was a highly significant reduction in the number of times nebulized therapy was delivered to the p.r.n. group (GM 7.0, range 1-30) compared with the regular treatment group (GM 14.0, range 4-57; p=0.003; 95% confidence interval for ratio of GMs 1.29-3.09). In addition, patients reported less tremor (p=0.062) and fewer palpitations (p=0.049) in the p.r.n. group. Of the patients in the p.r.n. group who had received regular nebulized therapy on previous admissions (n=12), all preferred the p.r.n. regimen. Prescribing beta2-agonists on a p.r.n. basis from 24 h after hospital admission is associated with reduced amount of drug delivered, incidence of side-effects, and possibly length of hospital stay. This has implications for the efficient use of healthcare resources. (+info)