The effects of aerosols of carbamylcholine, serotonin and propranolol on the ventilatory response to CO2 in guinea pigs and comparison with the effects of histamine and sulfuric acid

Carbon dioxide accumulation during small animal, whole body plethysmography: effects on ventilation, indices of airway function, and aerosol deposition

Barometric (whole body) plethysmography is used to examine changes in ventilation and breathing pattern in unrestrained animals during exposure to therapeutic or toxic aerosols. Whole body plethysmographs (WBP) may be operated with a bias flow in order to maintain an adequate supply of oxygen and remove expired CO(2). However, some aerosol generation and delivery methods may require operation of the WBP without bias flow, which would artificially deplete aerosol concentration. Under these conditions, expired CO(2) accumulates in the plethysmograph and stimulates ventilation, increasing total aerosol deposition, shifting regional deposition, and significantly altering some airway function indices. We characterized these effects in guinea pigs using a commercially available 4.5-L WBP, with and without a 1 L/min bias flow. CO(2)-induced changes in breathing frequency (f), tidal volume (Vt), minute ventilation (Ve), and indices of airway function -- including enhanced pause (penh), flow derived parameter (FDP), and respiratory duty cycle -- were measured. Without bias flow, CO(2) in the plethysmograph increased steadily to 5.4% after 30 min compared to a steady state 0.9% with bias flow. This resulted in a moderate suppression of f, and significant increases in Vt and Ve by factors of 1.5 and 1.4, respectively. Changes in regional deposition were stimulated for 300 mg/m(3) polydisperse aerosols with mass median aerodynamic diameters of 0.3, 1, 3, or 7 microm and geometric standard deviations of 1.7. Percent increase in aerosol deposition from CO(2) inhalation ranged from 24% to 90%, by mass, depending on aerosol size distribution and respiratory tract region. In addition, fractional deposition shifted toward the pulmonary region. Empirical indices of airway constriction, penh and FDP, also were increased significantly to 1.7 and 1.3 times their respective baseline values. The study quantifies the effect of inadvertent coexposure to CO(2) on ventilation, aerosol deposition, and airway function in WBP evaluation of aerosol effects in airway function.

Characterisation of a novel airway late phase model in the sensitized guinea pig which uses silica and Bordetella pertussis as adjuvant for sensitization

The objective of the present investigation was to validate a novel model of allergic late phase reaction in the airways of conscious guinea pigs by monitoring airway function with CO2-forced respiration. In addition airway inflammation as one possible cause for the development of airway late phase reaction was characterized by a novel technique which consists of bronchoalveolar lavage via the orotracheal route. Guinea pigs were sensitized twice at 2-week intervals with ovalbumin in silica and Bordetella pertussis. Two weeks after the booster sensitization all guinea pigs showed an acute decrease of tidal volume under CO2-forced respiration 5-15 min after antigen challenge. In contrast 42 out of 68 (= 62%) screened guinea pigs exhibited airway late phase response between 4-10 h after aerosol antigen challenge. During a subsequent cross-over study methylprednisolone (twice at 16 and 1 h before ovalbumin) did not significantly interfere with the acute response. In contrast the airway late phase response as well the associated eosinophil influx into the bronchoalveolar lavage were attenuated by the steroid. In conclusion, the sensitization procedure in combination with the novel method for monitoring airway function allowed measurement of a reproducible airway late phase response in about 60% of sensitized guinea pigs. The sensitivity of exclusively the late phase response and eosinophil influx to treatment with a glucocorticoid not only correlates this model with clinical pharmacotherapy but also strengthens the inflammatory nature of this model.

Pharmacological characterisation of a new model of antigen-induced pulmonary late-phase reaction in the conscious guinea pig which uses additional polymyxin B inhalation

The aim of the present study was to develop a new model of allergic late-phase reaction in the airways of conscious guinea pigs (GPs) and to characterise it by pharmacological intervention. GPs were pretreated with cyclophosphamide and sensitized with ovalbumin (OA) in Al(OH)3. Weekly inhalations of polymyxin B were performed before and during sensitization and continued throughout the study period. Under cover of 10 mg/kg i.p. mepyramine all GPs still exhibited a pronounced immediate reaction (IR), peaking during the first 15 min after OA. Nine out of 15 GPs demonstrated, during screening, a reproducible (twice) second phase (late phase reaction (LPR)] of decreased airflow and tidal volume (TV), peaking 4-8 h after OA. In a cross over study, methylprednisolone (MP) at 30 mg/kg p.o. (16 h and 1 h before OA) significantly inhibited the LPR at its peak (4-8 h) (peak decrease of TV to % of basal: control 49.4 +/- 3.7; MP 78.9 +/- 7.5; p < 0.01: n = 7). After another booster sensitization with 2 micrograms OA/GP under the same conditions, the Paf-antagonist WEB 2347 at 3 mg/kg p.o. (1 h before OA) inhibited the LPR at its peak again (peak decrease of TV to % of basal: control 57.3 +/- 3.5; WEB 2347 74.8 +/- 7.6: p < 0.01; n = 6). In conclusion more than 50% of repeatedly ovalbumin sensitized (and polymyxin B-treated) unanaesthetized GPs developed a reproducible pulmonary late phase reaction (LPR). The LPR peaked at 4-8 h after antigen-exposure. The inhibitory effect by a glucocorticoid and the Paf-antagonist WEB 2347 suggests the inflammatory nature of the LPR and the involvement of platelet-activating factor (Paf) in this model.

Dissociation of airway responsiveness and bronchoalveolar lavage (BAL) cell composition in sensitized guinea-pigs after daily inhalation of ovalbumin

The association between inflammatory cell influx, cell activation status and change of airway responsiveness to acetylcholine (ACh) after daily inhalation of ovalbumin (OA) in sensitized guinea-pigs was investigated. Starting 3 weeks after sensitization (OA at 50 mg/kg s.c.+i.p.) guinea-pigs were exposed daily to 2% OA (10 min; under cover of 0.5 mg/kg mepyramine i.p. 15 min before OA) for 2 weeks. Concentration-response curves (CRCs) for inhaled ACh were performed 24 h after the last OA-challenge and 24 h after another single OA-inhalation 1 week later. CRCs for inhaled ACh were neither affected 24 h after the last OA challenge (daily for two weeks) nor 24 h after another OA-inhalation one week later. In contrast, bronchoalveolar lavage (BAL) from repeatedly OA- sensitized/-challenged guinea-pigs immediately after the last CRC showed a significant increase of total cell count by about tenfold and increases in eosinophils by about 20-fold, neutrophils by 30-fold, macrophages by about fivefold and lymphocytes by about tenfold (P < 0.05, multiple Wilcoxon-test). In contrast, markers of cell activation (EPO, MPO) were significantly decreased (P < 0.05). Methylprednisolone almost completely prevented these changes in increased cell numbers and decreased cell activation (vs OA contr., P < 0.05). The lack of increased airway hyperresponsiveness despite a massive inflammatory cell influx suggests other factors controlling airway responsiveness than inflammation.

Characteristic modifications of the breathing pattern of mice to evaluate the effects of airborne chemicals on the respiratory tract

A system was developed for exposure of unanesthetized mice to airborne chemicals and for continuous measurement of their breathing pattern prior to, during and following exposure. By measuring inspiratory and expiratory airflows (VI and VE), and integration with time to yield tidal volume (VT), we obtained characteristic modifications to the normal breathing pattern. These permitted recognition that a specific portion of the respiratory tract was affected by the selected airborne chemicals. Following recognition, we also quantitated the degree of effect using one specific measurement in each case. An effect on the upper respiratory tract, induced by the sensory irritant, 2-chlorobenzylchloride, was quantitated by measuring a decrease in respiratory frequency. An effect on the conducting airways, induced by the airway constrictor, carbamylcholine, was quantitated by a decrease in VE at the mid-point of VT. An effect at the alveolar level, induced either by the vagal nerve ending stimulant, propranolol, or by the pulmonary irritant, machining fluid G, was quantitated by an increase in the length of a pause induced at the end of expiration. The system is easy to construct and operate and can be used to rapidly evaluate the effects of airborne chemicals on the respiratory tract.

Respiratory effects of trimellitic anhydride aerosols in mice

Groups of mice were exposed to trimellitic anhydride (TMA) aerosols for 30 min at concentrations ranging from approximately 2 to 150 mg/m3. The sensory and pulmonary irritating properties of TMA were assessed during these exposures. Sensory irritation, as evidenced by a lenghthening of the time of expiration (TE), was not evoked in any exposure. However, TMA aerosols evoked other rapid, reversible alterations of respiratory cycle timing, suggestive of pulmonary irritation. The time of inspiration (TI) and expiration (TE) decreased as TMA exposure concentration was increased. Also, characteristic pauses (i.e., apneic periods) occurred between breaths. The length of these pauses (TP) increased as TMA exposure concentration was raised. Respiratory frequency (f) was dependent upon the magnitude of increase in TP. At lower TMA concentrations, TP was short and f was elevated, whereas at higher concentrations, TP was long and f was reduced. Interestingly, there was no other evidence of pulmonary irritation besides the alterations in respiratory cycle timing. Histopathological evaluation of the respiratory tracts of TMA-exposed animals revealed no changes from those of controls. Finally, there was no evidence of cumulative or delayed pulmonary effects in mice repeatedly exposed to TMA aerosols. These acute changes in respiratory cycle timing may have occurred as a result of stimulation of vagal nerve endings along the conducting airways and in the deep lung, directly produced by TMA aerosols.

Alteration of respiratory cycle timing by propranolol

The present study was undertaken to explore the effect of propranolol on respiratory cycle timing and breathing pattern using mice and guinea pigs. Propranolol was administered via aerosol or via injection. Mice exposed to propranolol aerosols exhibited a breathing pattern that was characterized by a pause between breaths. The length of such pauses (TP) increased as the propranolol exposure concentration was raised. We also observed at increasing propranolol concentrations that the time of inspiration (TI) and time of expiration (TE) decreased. Thus, the respiratory rate of these animals was seen to increase, remain unchanged, or decrease depending on the balance between TP versus TI and TE. Guinea pigs also exhibited pauses between breaths during exposure to propranolol aerosols, but such pauses were not observed as consistently as in mice. TP again rose while TI and TE fell in guinea pigs exposed to increasing propranolol concentrations. This species was, however, less sensitive than mice. Intraperitoneal injections of propranolol failed to elicit such alterations in respiratory pattern or timing in either species. These changes in respiratory timing obtained with an aerosol of propranolol were most probably due to a local effect on vagal nerve endings or to its bronchoconstricting effect.

Assessment of airway reactivity in guinea pigs: comparison of methods employing whole body plethysmography

Airway reactivity of guinea pigs was assessed while animals were in whole body plethysmographs by measurement of responses to histamine aerosol. Histamine was delivered using 2 protocols: one employed exposure to a single concentration, the other utilized 1.5-fold stepped concentrations. Both protocols were tested while animals inhaled ambient air, and again when the air contained 10% CO2. Exposures to single concentrations of histamine in air produced increases in both respiratory frequency (f) and plethysmograph pressure (delta P). The latter increase was shown to coincide with a decrease in tidal volume and with the onset of airway constriction. The time to reach a doubling in delta P represented a quantitative measure of airway reactivity. Exposures using the single concentration method in conjunction with 10% CO2 also demonstrated a dose-dependent time to reach the endpoint, in this case a 33% decrease in delta P from the maximum CO2-induced increase. The endpoints for the stepped concentration protocols were the concentration required to double delta P (PC2.delta P) or, in the presence of 10% CO2, the concentration that induced a 33% decline from the maximum delta P (PC33 (CO2]. Mean values for PC2.delta P and PC33 (CO2) were 2.1 mg/m3 and 0.58 mg/m3, respectively. These endpoints were comparable based on the dose received as calculated from the minute volume of animals when breathing the above atmospheres. The intersubject variability, assessed using both stepped concentration protocols, was comparable to that reported for PC20 FEV1 in human studies. Both stepped concentration protocols, conducted in air, or the 10% CO2 atmospheres, proved suitable for determination of airway reactivity of the unrestrained, unanesthetized guinea pigs. However, by not requiring use of CO2, the histamine in air approach is easier to perform, requires less time, and provides a clear endpoint representing early airway constriction.

Simple, noninvasive method to measure the antibronchoconstrictor activity of drugs in conscious guinea pigs

A simple, noninvasive method is described for measurement of the oral antibronchoconstrictor activity of drugs in conscious guinea pigs. The guinea pigs were placed in a head-out plethysmograph for measurement of tidal volume (VT), and a head chamber was used to deliver a hypercapnic gas mixture (10% CO2, 69% N2, 21% O2) and histamine aerosol (0.05% for 30 s). Inhalation of CO2 caused an increase in VT that was reduced (61 +/- 3%) by exposure to histamine. The histamine-induced reduction of VT was significantly (p less than 0.05) reduced 1 hr after oral administration with the bronchodilators albuterol (1 and 10 mg/kg) and aminophylline (100 mg/kg), the calcium antagonist nifedipine (30 mg/kg), and the antihistamine chlorpheniramine (0.5 mg/kg). These results demonstrate the oral antibronchoconstrictor activity of drugs in this model. The major advantages of this technique are that it is simple to operate, is noninvasive, and does not require anesthesia.

Pulmonary effects of a polyisocyanate aerosol: hexamethylene diisocyanate trimer (HDIt) or Desmodur-N (DES-N)

Desmodur-N (DES-N) or hexamethylene diisocyanate trimer (HDIt), a biuret structure of hexamethylene diisocyanate, is a viscous liquid used for durable coatings and is applied by brushing or spraying. DES-N aerosol has been shown to be primarily a pulmonary irritant following a single exposure in mice. To explore the pulmonary effects of this agent further, groups of guinea pigs were exposed to concentrations ranging from 8 to 121 mg/m3 of DES-N for 3 hr. Prior to and following exposure, each animal was challenged with 10% CO2 in 20% O2 and 70% N2 to evaluate their pulmonary performance. Following a single exposure, these animals displayed a concentration-dependent increase in respiratory rate and decrease in tidal volume, as well as coughing and apnea. Their ventilatory response to 10% CO2 was abnormal and characteristic of a lung restriction response. Some airflow limitation was seen during expiration but this occurred more often during air breathing than during CO2 challenge. With daily exposures repeated for 11 consecutive days, guinea pigs began to adapt to the exposures as indicated by a return to a normal ventilatory response to CO2. This adaptation occurred within the first 5 days of exposures. From Days 6 to 11, there was a demonstrable effect, but the level of response was much less than that following the first exposure. No cumulative effect could be demonstrated with this polyisocyanate and the effect was found to be different than that for mono- or diisocyanates. Acceptable levels of exposure to this polyisocyanate for industrial workers are suggested.