A new role for histamine H2-receptors in asthmatic airways

Low-dose salbutamol suppresses airway responsiveness to histamine but not methacholine in subjects with asthma

BACKGROUND

Airway hyperresponsiveness is a cardinal feature of asthma. Although the modulation of cholinergic neuroeffector transmission may play a role in airway responsiveness, in vivo evidence remains scarce. It is well known that histamine causes bronchoconstriction partly via vagal reflex, whereas methacholine does not. To investigate the significance of modulating neuroeffector transmission, we compared the effect of low-dose salbutamol-a β2-adrenoceptor agonist-on airway responsiveness to histamine with that to methacholine.

METHODS

We enrolled 12 subjects with stable asthma. After screening confirmed that inhalation of low-dose salbutamol (1μg) did not change their basic pulmonary function, subjects underwent measurement of airway responsiveness to inhaled histamine and methacholine with or without pretreatment with low-dose salbutamol, in a randomized, crossover fashion. Airway responsiveness was measured by an astograph by which respiratory conductance (Grs) was assessed by the forced oscillation method during continuous inhalation of histamine or methacholine in stepwise incremental concentrations. Airway responsiveness was calculated as the cumulative dose of bronchoconstrictors that induced a decrease of 35% in Grs.

RESULTS

Inhalation of 1μg of salbutamol significantly attenuated airway responsiveness to histamine but not methacholine. This selective attenuation was observed irrespective of disease severity or phenotype, namely atopy or non-atopy.

CONCLUSION

Low-dose salbutamol suppresses airway responsiveness to histamine but not methacholine in subjects with asthma. The present study may provide a novel insight into the bronchoprotective mechanism of β2-adorenoceptor agonist in clinical settings.

Roles of histamine receptor in a guinea pig asthma model

Histamine plays an important role in bronchoconstriction mediated by histamine receptors which provoke bronchial asthma attack. In this study, we measured H1 and H2 receptors in the guinea pig lung membrane fraction and obtained the following results. The maximum binding (Bmax) of H1 receptors in the guinea pig lung membrane fraction was significantly higher in the OA-sensitized group than that in the non-sensitized group, but affinity (Kd) did not differ between the groups. Otherwise, the maximum binding (Bmax) of H2 receptors in the guinea pig lung membrane fraction was significantly lower in the OA-sensitized group than that in the non-sensitized group. But affinity (Kd) did not differ between the groups. These findings suggest a close association of Histamine receptors both H1 and H2 in the pathology of asthma.

Interaction between prostaglandins and selective phosphodiesterase inhibitors in isolated guinea-pig trachea in vitro

The possible interaction between spontaneously synthesized relaxant prostaglandins and the relaxation produced by three different isoenzyme-selective phosphodiesterase inhibitors was investigated in the isolated guinea-pig trachea in vitro. The relaxant action of siguazodan (phosphodiesterase III inhibitor), rolipram (phosphodiesterase IV inhibitor) and zaprinast (phosphodiesterase V inhibitor) was investigated in preparations with either spontaneously tone alone or in preparations with spontaneous tone and additionally stimulated with histamine (1 microM). In addition, relaxant effects were assessed in preparations without spontaneous tone (inhibited by indomethacin 2 microM) and precontracted with histamine (1 microM) or prostaglandin F2 alpha (10 microM), either alone or in the presence of a non-relaxant concentration (20 nM) of prostaglandin E2. All three phosphodiesterase inhibitors preferentially relaxed preparations with spontaneous tone and showed increased relaxant effects in preparations with spontaneous tone and additionally stimulated with histamine compared to preparations contracted by histamine alone. This enhanced relaxing effect observed in the presence of initial spontaneous tone was mimicked by exogenous application of prostaglandin E2 to indomethacin treated preparations either precontracted by histamine or prostaglandin F2 alpha. Furthermore, the study revealed marked differences in the relaxant profiles of siguazodan, rolipram and zaprinast, differences which most likely are related to the functional importance of the phosphodiesterase isoenzymes inhibited by these drugs. It is concluded that endogenously synthesized relaxant prostaglandins and exogenously applied prostaglandin E2 are capable of enhancing the relaxant action of the phosphodiesterase inhibitors siguazodan, rolipram and zaprinast and that cyclooxygenase inhibition is an important way to avoid this interaction in experimental studies of airway smooth muscle relaxants in isolated guinea-pig trachea in vitro.

Eicosanoids and asthma: an update

There have been significant advances in our understanding of the role of eicosanoids as mediators in inflammation since their discovery over 50 years ago. Our more recent understanding of asthma as an inflammatory disease has led to the appreciation of eicosanoids potentially being pivotal mediators in promoting some of the changes in asthma. Of particular importance are the cysteinyl LTs in producing bronchospasm and bronchial hyperresponsivenss, and PGE2 in modulating the bronchospastic and inflammatory response. Evidence from clinical studies suggests that other eicosanoids may also contribute, but their importance is secondary and their relative contributions vary between individuals. The development of new drugs based on our partial understanding of the role that eicosanoid mediators may play in asthma promises new approaches to the treatment of this common chronic inflammatory condition.

Epithelium-derived inhibitory prostaglandins modulate human bronchial smooth muscle responses to histamine

The role of the bronchial epithelium and inhibitory prostaglandins in the induction of histamine tachyphylaxis in human isolated bronchial smooth muscle was investigated using bronchi obtained from 14 patients who had been treated with non-steroidal anti-inflammatory drugs (NSAID) for > 2 months and from 14 untreated patients. Epithelium-intact bronchial strips from untreated patients demonstrated tachyphylaxis to histamine with the maximum response (Emax) reduced by 30 +/- 5% (P < 0.02) and the EC50 increased 1.86-fold (P < 0.02). Tachyphylaxis was not observed in epithelium-denuded strips. In epithelium-intact bronchial preparations from NSAID treated patients, the mean initial maximum tension generated in response to histamine was significantly greater than that for bronchial preparations from untreated patients (P < 0.05). In NSAID-treated patients, both epithelium-intact and denuded preparations failed to demonstrate tachyphylaxis. The generation of prostaglandin E2 and prostacyclin was assessed by radio-immunoassay using bronchi from untreated patients (n = 8). In epithelium-intact bronchial preparations, the generation of both prostaglandin E2 and prostacyclin was significantly increased by histamine exposure (P < 0.05) and was completely inhibited by indomethacin. However, the selective histamine H2 receptor antagonist, ranitidine, selectively inhibited the synthesis of prostaglandin E2 alone. Production of both prostaglandins was not altered by exposure to acetylcholine. These results suggest that prostaglandin E2 and prostacyclin are released primarily from the epithelium in response to histamine and may be specifically involved in inhibiting human bronchial smooth muscle responsiveness to this mediator. Significantly, the release of prostaglandin E2 appears to be selectively controlled by histamine H2 receptors, resident on the epithelium.

Time course of histamine-induced bronchoconstriction and its adrenergic and H2 modulation

We characterized the complete time course of histamine-induced bronchoconstriction and its modulation via the release of endogenous catecholamines and by its actions on H2-receptors in anesthetized, tracheostomized, paralyzed, and artificially ventilated mongrel dogs. Respiratory resistance (R) and elastance (E) were estimated continuously with a recursive least squares estimator. Three protocols were followed in which multiple histamine bolus injections were given 1 h apart. We found that the time courses of E and R had consistent patterns (transient peak that returned to baseline within 1000 sec) even in cases of low mean arterial pressure (MAP). Indomethacin pre-treatments prevented tachyphylaxis to repeated i.v. challenges. beta-blockade produced a mild increase in baseline and a potentiation of the histamine-induced response in E and these effects were not altered with further alpha-or H2-blockade. Blockade of alpha-receptors increased the time to recovery in both E and MAP presumably by decreasing blood flow. Finally, we suggest that preventing the H2-receptor induced increase in bronchial blood flow may have increased the time to maximal E without affecting the recovery time.

Exercise-induced refractoriness in asthmatic subjects involves leukotriene and prostaglandin interdependent mechanisms

Exercise-induced bronchoconstriction is caused, in part, by leukotriene (LT)D4 release in asthmatic airways. Asthmatics become refractory to exercise bronchoconstriction with repeated challenges, due to inhibitory prostaglandin release. The purpose of this study was to test the hypothesis that exercise refractoriness is caused by LTD4-induced inhibitory prostaglandin release. Fourteen stable asthmatic subjects with exercise-induced bronchoconstriction were studied. On the first 2 days, subjects underwent two challenges, 1 h apart, with either exercise or inhaled LTD4. Eight subjects then took part in three double-blind, randomized, placebo-controlled, crossover studies with flurbiprofen, a prostaglandin synthetase inhibitor, to determine whether cross refractoriness occurs between exercise and LTD4, whether flurbiprofen attenuates this effect, and whether flurbiprofen attenuates LTD4 tachyphylaxis. There was a reduction in the intensity of bronchoconstriction to the second challenge both with exercise (refractoriness) and with LTD4 (tachyphylaxis). The degrees of refractoriness and tachyphylaxis were correlated (r = 0.72, p = 0.005). Flurbiprofen attenuated LTD4 tachyphylaxis. Cross refractoriness occurred between exercise and LTD4, and flurbiprofen treatment also attenuated this effect. One hour after LTD4 challenge, the mean fall in FEV1 after exercise was 12.3% (%SEM 2.3) on placebo and 17.1% (%SEM 3.8) on flurbiprofen (p = 0.027). Similarly, 1 h after exercise, the LTD4 PC20 increased to 0.73 (%SEM 1.4) microgram/ml on placebo and 0.30 (%SEM 1.8) microgram/ml on flurbiprofen (p = 0.026). These results suggest that LTD4 released in asthmatic airways as a result of exercise stimulates inhibitory prostaglandin release, resulting in exercise refractoriness.

Histamine tachyphylaxis in human airway smooth muscle. The role of H2-receptors and the bronchial epithelium

The role of airway epithelium and H2-receptors in the development of histamine tachyphylaxis was studied using human isolated bronchial smooth muscle strips obtained from 18 patients undergoing thoracotomy. In epithelium-intact strips, a 38% reduction in the maximal contractile response (Emax) (p less than 0.002) and a 2.14-fold increase in the EC50 (p less than 0.02; n = 18) was observed after three separate histamine cumulative concentration effect curves (CCEC). In contrast, significant differences were not seen for either Emax (p greater than 0.4; n = 10) or EC50 (p greater than 0.26; n = 10) in epithelium-denuded strips. In separate experiments, both intact and denuded muscle strips were treated with the H2-receptor antagonist ranitidine (60 microM), either 30 min prior to the first or 30 min prior to the second histamine CCEC. In epithelium-intact strips, pretreatment with ranitidine caused a 1.8-fold increase in Emax in the initial CCEC (p less than 0.02), and both ranitidine schedules prevented tachyphylaxis (n = 8). In epithelium-denuded preparations, ranitidine did not enhance the responsiveness to histamine beyond that seen in untreated epithelium-denuded strips (n = 6). These data suggest that histamine-induced tachyphylaxis occurs in human airway smooth muscle and is mediated, at least in part, via H2-receptors resident on airway epithelium. In vivo, this may function as a protective mechanism, but damage to the epithelium and loss of H2-receptors may be significant in the development of histamine bronchial hyperreactivity as seen in asthma.

Histamine antagonists in the treatment of acute allergic reactions

STUDY OBJECTIVE

We compared the efficacies of cimetidine (an H2-receptor antagonist) and diphenhydramine (an H1-receptor antagonist) alone and in combination for alleviation of symptoms of acute allergic reactions. STUDY DESIGN AND INTERVENTIONS: In this prospective, randomized, double-blind study, patients and examiners assessed the severity of symptoms and signs of acute allergic reactions using a visual-analog scale before treatment and 30 minutes after treatment with 300 mg IV cimetidine and placebo, 50 mg IV diphenhydramine and placebo, or diphenhydramine plus cimetidine.

SETTING AND PARTICIPANTS

Thirty-nine patients with acute allergic reactions presenting to two emergency departments of teaching hospitals.

RESULTS

Of the 35 patients with pruritus, all 12 receiving diphenhydramine placebo had clinically significant relief compared with six of ten (60%) receiving cimetidine plus placebo (P = .03). Twelve of 13 (92%) receiving diphenhydramine plus cimetidine had relief, which was not a significant difference from the single drugs. Comparison of mean differences in pretreatment and post-treatment symptom scores (relief scores) among groups of patients with pruritus detected significantly more relief in the group receiving diphenhydramine plus placebo (80.3 +/- 7.4) than in those receiving cimetidine plus placebo (48.8 +/- 13.4) (P = .022). Of the 33 patients with urticaria, five of 11 (46%) receiving diphenhydramine plus placebo had clinically significant relief compared with eight of ten (80%) receiving cimetidine plus placebo (P = .18). Eleven of 12 patients (92%) receiving diphenhydramine plus cimetidine had relief, which is a significant difference from those receiving diphenhydramine plus placebo (P = .027). Comparison of mean relief scores in patients with urticaria detected significantly more relief in the group receiving diphenhydramine plus cimetidine (55.3 +/- 6.5) than in the group receiving diphenhydramine plus placebo (30.7 +/- 6.1) (P = .006).

CONCLUSION

For treatment of pruritus from acute allergic reactions, diphenhydramine is more effective than cimetidine, and the combination offers no additional benefit. For treatment of acute urticaria, the combination of cimetidine and diphenhydramine is more effective than diphenhydramine alone.

Histamine receptors in the lung

Histamine, which has long been implicated as a mediator of allergic airway disease, has multiple effects on airways which are mediated by at least three histamine receptors. H1-receptors mediate bronchoconstriction, vasoconstriction and dilatation, microvascular leak and activation of sensory nerves. H2-receptors mediate vasodilatation in some species and mucus secretion. H3-receptors mediate vasodilatation in some species and mucus secretion. H3-receptors modulate cholinergic neurotransmission, the release of neuropeptides from sensory nerves and allergen-induced bronchoconstriction.

Repeated provocation tests in asthmatic children for testing tachyphylaxis to histamine

Tachyphylaxis to histamine was investigated in 16 children, aged 7-15 years, with mild asthma. Three consecutive histamine challenges were performed at intervals of 24 hours and 1 hour, respectively. No significant differences in IVC, FEV1, and PC20-histamine values between the three measurements were observed. After a 24 hour interval there was no difference in percentage fall of FEV1, but there was a slight (not significant) decrease in fall of FEV1 after a 1 hour interval. The PC20-histamine values showed good reproducibility with a 24 hour as well as with a 1-hour period between the tests (geometric mean PC20, 2.04 mg/mL +/- 3.50 %SD, 1.96 mg/mL +/- 4.37 %SD, 2.17 mg/mL +/- 4.12 %SD; correlation coefficients for a 24 hour interval, r = 0.87 and for a one-hour interval, r = 0.94 (P less than 0.01]. We conclude that in children there is no strong evidence for tachyphylaxis to histamine. Our results differ from studies on tachyphylaxis in adult asthmatics. Possibly different mechanisms exist in children and in adults.

The effect of oral prostaglandin E1 on airway responsiveness in asthmatic subjects

Prostaglandin E1 (PGE1) is a bronchodilator in humans, but the effect of this prostaglandin on airway responsiveness in asthmatic subjects is not known. The purpose of this study was to examine the effect of oral PGE1 treatment on airway responsiveness to inhaled histamine and methacholine in asthmatic subjects. The subjects had inhalation tests performed on separate study days over a period of 2 weeks. PGE1 (200 micrograms 4 times daily) or placebo was taken in a double-blind fashion for 4 consecutive days. Each week, after three days of treatment, a histamine or methacholine inhalation test was performed on one day, followed by an inhalation test with the alternative agonist the following day. Baseline forced expired volume in 1 s (FEV1) values were similar before each inhalation test on each study day (P = 0.81). However, both histamine and methacholine airway responsiveness decreased following PGE1 treatment. The mean provocative concentration of histamine causing a 20% fall in FEV1 (PC20 histamine) on the placebo day was 1.47 mg/ml (%SD 3.05) and was 2.35 mg/ml (%SD 2.72) (p less than 0.005) during PGE1 treatment period. Similarly the PC20 methacholine was 1.84 mg/ml (%SD 2.30) on the placebo day and was 2.53 mg/ml (%SD 2.06) during PGE1 treatment period (p = 0.025). This study demonstrates that oral PGE1, administered in doses which did not change airway caliber, can reduce airway responsiveness in mild asthmatics.