In vivo effect of cimetidine on canine pulmonary responsiveness to aerosol histamine

Bronchopulmonary disease in the cat: historical, physical, radiographic, clinicopathologic, and pulmonary functional evaluation of 24 affected and 15 healthy cats

The results of clinical and pulmonary functional evaluation of 24 cats with bronchopulmonary disease and 15 healthy cats are presented. Affected cats had historical evidence of excessive reflexes (coughing, sneezing); physical evidence of airway secretions (crackles), obstruction (wheezing), and increased tracheal sensitivity; radiographic evidence of bronchial and interstitial lung disease; and cytological evidence of airway inflammation or mucous secretions. Bacterial isolates from healthy and affected cats were predominantly Gram-negative rods, indicating that bronchi of cats are not always sterile and that normal flora should be considered in interpreting cultures from cats with suspected bronchopulmonary disease. Cats were grouped according to relative disease severity based on scored historical, physical, and radiographic abnormalities. The mean (+/- standard deviation) baseline lung resistance measurement in healthy cats was 28.9 cm H2O/L/s (+/- 6.2 cm H2O/L/s), whereas in mildly, moderately, and severely affected cats it was 38.3 cm H2O/L/s (+/- 21.5 cm H2O/L/s), 44.8 cmH2O/L/s (+/- 7.7 cm H2O/L/s), and 105.2 cm H2O/L/s (+/- 66.9 cm H2O/L/s), respectively. In healthy cats, dynamic lung compliance was 19.8 (+/- 7.4), whereas in mildly, moderately, and severely affected cats it was 14.7 mL/cm H2O (+/- 3.8 mL/cm H2O), 17.7 mL/cm H2O (+/- 6.9 mL/cm H2O), and 13.0 mL/cm H2O (+/- 7.9 mL/cm H2O), respectively. Thus, airway obstruction was present in many of the affected cats. Based on acute response to the bronchodilator, terbutaline, airway obstruction was partially reversible in many affected cats, although the degree of reversibility varied. Furthermore, based on bronchoprovocation testing, 6 (of 7) affected cats evaluated also had increased airway responsiveness to aerosolized methacholine.

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.

A new role for histamine H2-receptors in asthmatic airways

Histamine tachyphylaxis develops with repeated inhalation of histamine in asthmatic subjects, and this appears to be due to the release of inhibitory prostaglandins. The purpose of this study was to determine whether histamine H2-receptors are also involved in the development of this protective effect in the airways. Seven subjects with mild asthma were studied on 2 days separated by at least 1 wk. On both days, three histamine inhalation tests were performed, separated by 1 h. The response was expressed as the provocative concentration of histamine causing a 20% decrease in FEV1 (histamine PC20). Before each study day subjects were pretreated with placebo or cimetidine (300 mg twice a day) for 3 days in a double-blind, randomized, crossover study. Cimetidine pretreatment had no effect on either baseline FEV1 or on baseline histamine PC20 (p = 0.461). After pretreatment with placebo, histamine tachyphylaxis occurred in all subjects; the mean PC20 increased from 3.01 mg/ml (%SD, 1.44) to 4.88 mg/ml (%SD, 1.68) and to 6.84 mg/ml (%SD, 1.68). Cimetidine pretreatment prevented histamine tachyphylaxis; the mean PC20 was 2.72 mg/ml (%SD, 1.77), 3.03 mg/ml (%SD, 1.73), and 3.56 mg/ml (%SD, 1.59) with repeated tests. These values differed significantly from placebo for both the second (p = 0.014) and third (p = 0.001) tests. This study demonstrated that cimetidine pretreatment prevents histamine tachyphylaxis and suggests that histamine tachyphylaxis occurs through stimulation of histamine H2-receptors in the airway.

The response of cat airways to histamine in vivo and in vitro

The effects of histamine have been examined in anaesthetized cats and on cat cat isolated lung parenchyma strip. Histamine infused intravenously for 2 min produced a small and inconsistent effect on central airways and a small but consistent constriction of peripheral airways. Histamine bronchoconstriction of the central airways was unmasked by non-selective and beta 2-adrenoceptor blockade but not by beta 1-adrenoceptor blockade. This bronchoconstriction was antagonized by atropine but not by cimetidine or prazosin. Bronchoconstriction of the peripheral airways was not affected in a dose-related manner by beta-adrenoceptor blockade. The bronchoconstriction was antagonized by mepyramine but not by atropine or prazosin. beta-Adrenoceptor antagonists produced a bell-shaped dose-response curve on histamine contractions in cat isolated lung parenchyma strip. Strips of lung parenchyma obtained from reserpine-treated cats produced a larger contraction to histamine which was not potentiated by propranolol. It is concluded that in the central airways, histamine bronchoconstriction produced by an action on irritant receptors is masked by an action on beta 2-adrenoceptors of catecholamines released locally and from the adrenal glands. In the peripheral airways, histamine bronchoconstriction is mediated by H1-receptors and beta 2-adrenoceptor blockade may either potentiate or antagonize the histamine response depending on the concentration.

Histamine receptor blocking effects of cimetidine in the airways

We investigated the modification of histamine-induced bronchoconstriction by the H2-antagonist cimetidine in conscious sheep. One hundred breaths of 5% histamine aerosol increased mean (SD) pulmonary resistance (RL) by 5.6 (1.4) cmH2O/l/sec. This increase in RL was completely blocked by intravenous clemastine (0.5 mg), a specific H1-antagonist, indicating that the histamine-induced bronchoconstriction was mediated by H1-receptors. Intravenous cimetidine caused a dose-dependent enhancement of the histamine response between 1 and 1000 mg with a mean peak delta RL of 15.3 (5) cmH2O/l/sec (p less than 0.05) at the 1000 mg dose, while it blocked the histamine response at a dose of 2400 mg [delta RL = 1.9 (2) cmH2O/l/sec, p = NS]. This paradoxic effect was not related to an anticholinergic mechanism as intravenous cimetidine (2400 mg) failed to block carbachol-induced (25 breaths of 1% solution) bronchoconstriction. We conclude that in the ovine airway, cimetidine is a selective H2-histamine receptor blocker at lower tissue concentrations, and a combined H2- and H1-histamine receptor blocker at high tissue concentrations.

A species comparison of the histamine H2-receptor on mast cells and basophils

The histamine H2-agonist dimaprit was employed in experiments to investigate the role of H2-receptors in mediator release systems from the rat, guinea-pig and man. Whereas inhibition of histamine release by dimaprit was observed in all 3 species, this effect appeared not always to be related to H2-receptor occupancy. Although the data in general support previous evidence for the presence of H2-receptors on human basophils and guinea-pig mast cells, the use of dimaprit as a pure H2-agonist in these studies is questioned. No evidence for H2-receptors on rat mast cells was obtained.

The modification by ketotifen of respiratory responses to histamine and antigen in guinea-pigs

Aerosolized solutions of histamine or ovalbumin were administered to control or ovalbumin-sensitized guinea-pigs. The time to onset of respiratory distress (preconvulsion time) during challenge with these agents was measured using a force-displacement transducer on the animal's back. The preconvulsion time for each guinea-pig was compared with and without ketotifen pretreatment. Ketotifen administered both by aerosol and intraperitoneal injection (i.p.) significantly protected (p less than 0.001) each guinea-pig from the effects of aerosol challenge with histamine. Ovalbumin-sensitized animals were also protected from the effects of antigen aerosol challenge by ketotifen pretreatment. Sodium cromoglycate (DSCG), administered as an aerosol, did not protect sensitized animals from ovalbumin challenge. The Konzett-Rössler technique was used to measure the response to the intravenous injection (i.v.) of histamine in anaesthetized artificially ventilated guinea-pigs. Ketotifen was three times more potent than mepyramine in inhibiting the histamine-induced increase in air overflow volume.

Histamine1--histamine2 antagonism: effect of combined clemastine and cimetidine pretreatment on allergen and histamine-induced reactions of the guinea pig lung in vivo and in vitro

In a preliminary study high doses of the H1-antagonist clemastine (clem) and the H2-antagonist cimetidine (cim) were used in order to detect the side effects of both drugs on allergic reactions. After pretreatment with clem or clem/cim different guinea pigs were challenged with either OA or histamine. Bronchial obstruction was measured by body plethysmography using a new parameter ('compressed air'). Pretreatment with clem/cim in high doses protected guinea pigs against OA-induced bronchial obstruction much more effectively than H1-receptor antagonism alone; lower cim doses produced insignificant effects. In histamine-challenged animals cim showed no protective effects. In vitro clem/cim caused a marked reduction of histamine release measured in perfused lungs from 16.9 +/- 4.2 ng/ml (eight control cases) to 2.8 +/- 1.7 ng/ml (n = 8). Our preliminary data suggest that high doses of clem/cim can protect sensitized guinea pigs against allergen-induced bronchial obstruction by inhibiting histamine release.