Substance P-induced histamine release in human cutaneous mast cells.

Substance P is an undecapeptide found in multiple sites throughout the central and peripheral nervous systems including small unmyelinated (type C) cutaneous nerve fibers. Previous studies demonstrated that antidromic stimulation results in substance P (SP) release from nerve endings, SP stimulates histamine release (HR) from rat mast cells in vitro, and intradermal SP in humans produces wheals identical to those induced by histamine. These studies suggest a possible role for SP as a link between neurologic events and cutaneous mast cell-mediated reactions. We therefore investigated SP-induced HR in an in vitro preparation of human skin mast cells. Human foreskin sections were incubated with varying concentrations of SP. Histamine was assayed using automated fluorimetry and release was calculated as a percentage of total tissue histamine. Substance P caused dose-dependent HR over a range from 10(-5) M (1.3%) to 5 X 10(-4) M (25.1%). Histamine release was optimal at 3 mM calcium and was blocked by pretreatment with calcium chelation. Naloxone failed to block HR. These studies suggest that HR from skin mast cells by SP may play a role in neural modulation of poorly understood inflammatory skin conditions.

Mediators, initiating the inflammatory response, released in organ culture by full-thickness human skin explants exposed to the irritant, sulfur mustard.

Mediators released from injured human skin that initiate the inflammatory response have not been adequately identified. Organ culture of full-thickness skin explants enables us to do so, because injury to the skin can be made in vitro, eliminating the rapid leakage of serum and infiltration of leukocytes that occur in vivo. In our studies, the military vesicant sulfur mustard (SM) (10 microliters of a 0.01 to 1.0% dilution) was topically applied to injure the epidermis of the explant. Then, the explants were cultured in small Petri dishes, usually for 18 h at 36 degrees C, and the organ-culture fluids were assayed for various inflammatory mediators. We found that the culture fluids from SM-exposed and control explants contained similar amounts of angiotensin-converting enzyme, trypsin-like and chymotrypsin-like proteases, acid phosphatase, beta-glucuronidase, beta-galactosidase, lysozyme, deoxyribonuclease, ribonuclease, interleukin 1, and lactic dehydrogenase. However, the culture fluids from SM-exposed explants contained increased amounts of histamine and plasminogen-activating activity, and often prostaglandin E2, when compared to culture fluids from control explants. After 3 to 4 d in culture, full-thickness human skin explants, when exposed to 0.2% SM (but not when exposed to 1.0% SM), sometimes showed separation of the epidermis and increased collagenase activity (i.e., hydroxyproline release). Thus, histamine (from local mast cells), and prostaglandin E2 and plasminogen-activating activity (probably from both mast cells and epidermal cells) are apparently involved in early mediation of the inflammatory response.

Impaired activation of adenylyl cyclase in lung of the Basenji-greyhound model of airway hyperresponsiveness: decreased numbers of high affinity beta-adrenoceptors.

1. To evaluate mechanisms involved in the impaired beta-adrenoceptor stimulation of adenylyl cyclase in tissues from the Basenji-greyhound (BG) dog model of airway hyperresponsiveness, we compared agonist and antagonist binding affinity of beta-adrenoceptors, beta-adrenoceptor subtypes, percentage of beta-adrenoceptors sequestered, and coupling of the beta-adrenoceptor to Gs alpha in lung membranes from BG and control mongrel dogs. We found that lung membranes from the BG dog had higher total numbers of beta-adrenoceptors with a greater percentage of receptors of the beta 2 subtype as compared to mongrel lung membranes. 2. Agonist and antagonist binding affinity and the percentage of beta-adrenoceptors sequestered were not different in BG and mongrel dog lung membranes. However, the percentage of beta-adrenoceptors in the high affinity state for agonist was decreased in BG lung membranes suggesting an uncoupling of the receptor from Gs alpha. 3. Impaired coupling between the beta-adrenoceptor and G protein documented by the decreased numbers of beta-adrenoceptors in the high affinity state in BG lung membranes, is a plausible explanation for the reduced stimulation of adenylyl cyclase and the resultant reduction in airway smooth muscle relaxation in this model.

Dyphylline aerosol attenuates antigen-induced bronchoconstriction in experimental canine asthma.

This study compared the bronchodilator effect in experimental canine asthma of dyphylline administered by aerosol and intravenous routes in doses producing equivalent concentrations of the drug in the plasma. Pulmonary resistance (RL) was calculated from simultaneous measurements of pressure and flow during fixed-volume controlled ventilation at the same peak flow and corrected for elastic recoil pressure. Dynamic compliance (Cdyn) was calculated by dividing tidal volume by the change in pressure measured between points of zero flow. Concentrations of dyphylline in the plasma were measured using high-performance liquid chromatographic techniques. Rates of infusion of dyphylline were determined from values for clearance observed in preliminary experiments with intravenous injection. Prior to exposure to antigen, RL and Cdyn were not significantly different in control and dyphylline-treated dogs. Following challenge, with antigen RL increased by 8.3 +/- 2.6 times (mean +/- SE) in untreated dogs but only by 2.4 +/- 0.4 times in dyphylline treated dogs. Levels of dyphylline in the plasma averaged 4.2 micrograms/ml +/- 0.6 micrograms/ml at the end of the ten-minute period of aerosol administration and remained at that level for 60 minutes. At equivalent plasma levels (4.3 micrograms/ml +/- 0.3 micrograms/ml), infusion of dyphylline did not significantly after the response to Ascaris antigen, whereas dyphylline administered by the aerosol route markedly attenuated the response.

Antigen sensitization and methacholine responses in dogs with hyperreactive airways.

Antigen sensitization was induced in six Basenji-Greyhound (BG) dogs by weekly aerosol exposure to Ascaris suum. The effects on airway responsiveness to inhaled methacholine were studied before and at least 2 wk following Ascaris sensitization. All dogs developed detectable serum levels of Ascaris-specific immunoglobulin E (IgE), and five out of six dogs developed airway responsiveness to antigen over the 4- to 6-mo period. This was accompanied by a decrease rather than an increase in airway responsiveness to inhaled methacholine. When dogs were challenged with methacholine 30 min after Ascaris antigen aerosol challenge, however, dogs reactive to Ascaris became hyperresponsive to methacholine. The magnitude of the response to antigen correlated (r = 0.85) inversely with the dose of methacholine increasing pulmonary resistance 200%. These data show that in BG dogs airway responsiveness to methacholine is increased by acute antigen exposure but that sensitization of previously unsensitized dogs does not increase nonspecific airway responsiveness.

Calcium chelators increase airway responsiveness.

To test the effect of calcium chelation on airway responsiveness to methacholine, purebred Basenji dogs were pretreated with a calcium-chelating aerosol (edetate disodium, Na2EDTA) or a placebo aerosol (saline or CaNa2-EDTA) and then challenged with methacholine bromide aerosols. The lowest dose of methacholine (0.15 mg/ml) produced no change in pulmonary resistance (RL) following pretreatment with the placebo aerosols, but RL increased (P less than 0.05) by 5.1 +/- 1.2 (SE) cmH2O X l-1 X s following pretreatment with Na2EDTA. The highest dose of methacholine (1.5 mg/ml) increased RL in all animals, but the increase was greater (P less than 0.01) following pretreatment with Na2EDTA (9.5 +/- 1.9 cm H2O X l-1 X s) than following pretreatment with a placebo aerosol (6.4 +/- 1.5 cmH2O X l-1 X s). These studies show that calcium-chelating aerosols significantly increase airway responsiveness and suggest that a localized calcium deficit may contribute to hyperresponsive airway disease.

Direct effects of inhaled nitric oxide on canine peripheral airways.

The effects of nitric oxide on peripheral airways in vivo, and whether these effects occur via direct or indirect mechanisms, are unknown. We studied effects of inhaled nitric oxide on histamine-constricted canine peripheral airways in the presence or absence of atropine and an inhibitor of guanylyl cyclase, methylene blue. Peripheral resistance (Rp) was measured by using a wedged-bronchoscope technique in anesthetized dogs. A stable baseline Rp was established. Histamine was infused intravenously, and increasing concentrations of nitric oxide (50-500 ppm) were delivered through the bronchoscope. In separate experiments, histamine was infused intravenously in the presence or absence of atropine (0.2 mg/kg iv) or methylene blue (20 mg/min iv). When Rp stabilized, nitric oxide (500 ppm) was delivered. Nitric oxide partially reversed histamine-induced bronchoconstriction in a dose-dependent fashion (maximum of 42 +/- 3% reduction at 500 ppm; n = 5; P < 0.01) that did not differ in the presence or absence of atropine. Methylene blue blocked the effect of nitric oxide on histamine-induced constriction (n = 6; P = 0.45). These findings suggest that high concentrations of nitric oxide produce small but significant bronchodilation of peripheral airways through a mechanism independent of the cholinergic neural pathway. The mechanism of action appears to involve activation of guanylyl cyclase.

Decreased ion transport by tracheal epithelium of the basenji-greyhound dog.

The Basenji-Greyhound (BG) dog model shows altered beta-adrenergic function in both airway smooth muscle and leukocytes. To investigate a possible beta-adrenergic pathway defect in airway epithelial cells of BG dogs, we studied the electrophysiological behavior of tracheal epithelia in vitro and measured tracheal electrical potentials in vivo. Baseline short-circuit currents of isolated tracheal epithelia from BG (n = 6) and mongrel control dogs (n = 7) were 18.7 +/- 3.4 and 43.7 +/- 4.2 microA/cm2, respectively (P = 0.001). Significant differences between short-circuit currents of BG and control epithelia persisted after inhibition of Cl- secretion by indomethacin or stimulation by isoproterenol or dibutyryl adenosine 3',5'-cyclic monophosphate. In vivo tracheal potentials were also significantly less (P = 0.01) in BG dogs (-22.3 +/- 2.5 mV; n = 12) than in control dogs (-32.5 +/- 2.6 mV; n = 10), and intravenous indomethacin reduced the tracheal potential of BG dogs but had no effect in control animals. There was no correlation in BG dogs between tracheal potential and the dose of methacholine required to double total lung resistance. These data suggest that ion transport by tracheal epithelium is decreased in BG dogs, that this difference is not due to diminished beta-adrenergic activity, and that cyclooxygenase products are important in maintaining tracheal potential in vivo in this model.

Role of the epithelium in airway smooth muscle responses to relaxant agonists.

We studied the role of the guinea pig tracheal epithelium in modulating tracheal smooth muscle responses to the relaxant agonists albuterol, sodium nitroprusside, and theophylline. We used an in vitro preparation that allowed separation of the fluids bathing the luminal (internal) and serosal (external) surfaces of the trachea, and bronchodilators were administered to either surface of carbachol-contracted tracheae. All three drugs produced dose-dependent relaxation. However, albuterol and nitroprusside were less potent (concentration that produced half-maximal effect increased by 100- and 32-fold, respectively) when given to the epithelial side with the epithelium intact compared with the epithelium denuded or compared with serosal administration with the epithelium intact. These differences were not observed for theophylline, where smooth muscle responses were independent of either the side of stimulation or of the presence or absence of the epithelium. Direct measurements of the diffusion of theophylline across the tracheal wall in the presence or absence of epithelium showed that after 5 h of incubation with a fixed luminal concentration of theophylline, only 1.7% had diffused across the tracheal wall with the epithelium intact. This increased to only approximately 3.3% when the epithelium was denuded. These results suggest that the epithelial is a relatively weak barrier for lipophilic agents but has a major role as a diffusion barrier to hydrophilic substances.

Calcium channel blockers modulate airway constriction in the canine lung periphery.

We studied the effect of two voltage-sensitive calcium channel blockers on Na2EDTA-induced bronchoconstriction in the canine lung periphery. A wedged bronchoscope technique was used to measure collateral system resistance before and after challenges with aerosolized Na2EDTA, hypocapnia, aerosolized acetylcholine, and increased flow of dry air in anesthetized mongrel dogs. Nifedipine, a dihydropyridine calcium channel blocker, reduced hypocapnia-induced bronchoconstriction by 88 +/- 6% (SE) but did not alter Na2EDTA-induced constriction. Verapamil, a phenylalkylamine calcium channel blocker, attenuated hypocapnia- and Na2EDTA-induced bronchoconstriction by 69 +/- 6 and 44 +/- 7%, respectively, but did not significantly alter responses to either acetylcholine or dry air challenge. We conclude that calcium influx through voltage-sensitive calcium channels, perhaps of the T subtype, has a limited role in the initiation of Na2EDTA-induced bronchoconstriction in the canine lung periphery.

Large-volume ventilation results in bronchoconstriction of Basenji-Greyhound dogs.

We compared the effects of large-volume ventilation on airway responses to aerosolized histamine in anesthetized mongrel dogs with its effects in Basenji-Greyhound crossbred (B-G) dogs. Before bronchoconstriction, large inflations resulted in only small changes of dynamic compliance (Cdyn) and pulmonary resistance (RL) in both groups of dogs. After the induction of a moderate degree of bronchoconstriction with aerosolized histamine, large inflations had a more substantial effect; Cdyn increased by 7.5 +/- 2.3% (mean +/- SE; P less than 0.05), and RL decreased by 32 +/- 3.4% (P less than 0.001) in the mongrel dogs. In the B-G group, Cdyn increased by only 0.2 +/- 1.8% (NS), and RL increased by 29.3 +/- 9.2% (P less than 0.05); these changes differed significantly (P less than 0.05) from those observed in the mongrel dogs. Large-volume ventilation following the administration of indomethacin (10 mg/kg iv) and histamine increased Cdyn by 11.4 +/- 1.8% (NS vs. without indomethacin) and decreased RL by 43.9 +/- 3.4% (P less than 0.05) in the mongrel group. In the B-G group large-volume ventilation increased Cdyn by 7.6 +/- 1.7% (P less than 0.01) and decreased RL by 15.7 +/- 8.1% (P less than 0.05). Thus indomethacin enhanced the bronchodilator effects of large-volume ventilation in mongrel dogs and reversed the bronchoconstrictor effect of this maneuver on RL in B-G dogs.

Effect of magnesium sulfate on bronchoconstriction in the lung periphery.

Magnesium sulfate has been shown to be effective clinically as a bronchodilator, but its mechanism of action is unknown. We used a wedged bronchoscope technique to study the ability of MgSO4 at clinically relevant concentrations to attenuate hypocapnia-, acetylcholine- (ACh), and dry air-induced bronchoconstriction in the canine lung periphery. Control experiments demonstrated that consecutive challenges of either hypocapnia or ACh resulted in greater collateral system resistance (Rcs) after the second challenge compared with the first. Intravenous infusion of MgSO4 diminished the maximum response to a second hypocapnic challenge (Rcs = 1.59 +/- 0.29 cmH2O.ml-1.s prechallenge vs. 1.12 +/- 0.20 postchallenge) but had no effect on either ACh- or dry air-induced bronchoconstriction. Serum magnesium levels before MgSO4 administration were 1.59 +/- 0.04 meq/l and rose to 6.20 +/- 0.13 during the infusion. Previous studies demonstrated that nifedipine, like MgSO4 in this study, attenuates hypocapnia-induced bronchoconstriction in the canine lung periphery but has no effect on ACh- or dry air-induced bronchoconstriction. We conclude that these results are consistent with the idea that, like nifedipine, magnesium acts in the airway as a voltage-sensitive calcium channel blocker.

Methylprednisolone restores sensitivity to beta-adrenergic agonists in Basenji-Greyhound dogs.

This study investigated the effect of chronic methylprednisolone treatment on the ability of albuterol and aminophylline to inhibit methacholine-induced airway constriction in Basenji-Greyhound (BG) dogs in vivo. Pulmonary responsiveness to methacholine was measured in five untreated BG dogs and in the same dogs pretreated with albuterol or aminophylline (which has been shown in this model to release endogenous catecholamines). Each dog was studied before, during, and after daily subcutaneous methylprednisolone for 6 wk. Changes in pulmonary resistance and dynamic compliance with methacholine aerosol challenge were measured. Neither baseline pulmonary function nor pulmonary responsiveness to aerosolized methacholine was significantly altered by albuterol, aminophylline, or chronic methylprednisolone administration alone. However, pretreatment with albuterol or aminophylline significantly attenuated airway responses to methacholine in BG dogs chronically receiving methylprednisolone. Because the reduced sensitivity to albuterol and aminophylline was restored by chronic methylprednisolone treatment, we conclude that at least part of the beneficial effects of corticosteroids on airways in BG dogs is through modulation of beta-adrenergic function.

Corticosteroids decrease airway hyperresponsiveness in the Basenji-Greyhound dog model of asthma.

Methacholine and citric acid responses were assessed before, during, and after 6 wk of oral treatment with either placebo or methylprednisolone (2 mg.kg-1.day-1) in 12 Basenji-Greyhound dogs. Bronchoalveolar lavage was performed in three dogs in each group before, during, and after pretreatment. Base-line airway resistance and dynamic compliance did not change with treatment in any of the groups. Placebo treatment had no demonstrable effect on methacholine and citric acid responsiveness. Methylprednisolone treatment abolished the constrictor response to citric acid during the 4th and 6th wk of treatment and significantly reduced methacholine responsiveness during the 3rd and 5th wk of treatment. Methylprednisolone treatment was associated with a marked reduction in the percent of eosinophils, but not mast cells, in the bronchoalveolar lavage fluid during the 7th wk. Blood eosinophil counts were also markedly reduced in the methylprednisolone-treated group compared with the placebo-treated group during the 7th wk. The decrease in numbers of eosinophils in blood and bronchoalveolar lavage fluid suggests interference with the inflammatory process as a possible mechanism for the observed reduction in airway hyperresponsiveness in the Basenji-Greyhound dog.

Succinylcholine potentiates responses to intravenous acetylcholine in the canine lung periphery.

Using the wedged bronchoscope technique to measure collateral resistance (Rcs), we evaluated the effect of succinylcholine (SCh) on the response to acetylcholine (ACh) and methacholine (MCh) in the lung periphery in six mongrel dogs. Dogs were anesthetized, intubated, and mechanically ventilated. After a stable baseline Rcs was obtained, responses to intravenous ACh (25-200 micrograms), intravenous MCh (3-30 micrograms), and aerosolized ACh (30-100 micrograms/ml for 15 s) were measured. We compared the Rcs responses with 1) ACh alone, 2) ACh 2 min after SCh (0.5 mg/kg), 3) ACh 2 min after SCh and during hexamethonium infusion (5 mg/kg + 10 mg.kg-1.h-1), 4) MCh 2 min after SCh, and 5) ACh aerosol 2 min after SCh. SCh did not significantly alter baseline airway tone. SCh increased the Rcs response to ACh by 48 +/- 17% (SE) (P less than 0.01). SCh in the presence of hexamethonium increased the Rcs response by 10 +/- 3% (P less than 0.05), while hexamethonium itself increased the response to ACh by 69 +/- 27%. Because SCh did not increase the Rcs response to intravenous MCh or to aerosolized ACh, SCh probably enhances airway reactivity to intravenous ACh by competing for pseudocholinesterase in plasma. We conclude that the level of muscle relaxant must be taken into account in interpreting studies of airway reactivity when intravenous ACh is employed.

Functional antagonism of airway constriction in the canine lung periphery.

We studied the ability of a beta-adrenergic agonist (albuterol) to attenuate calcium chelator- and acetylcholine-induced airway constriction in the lung periphery of anesthetized mongrel and Basenji-Greyhound (BG) dogs. A wedged bronchoscope technique was used to measure collateral system resistance before and after challenges with aerosolized Na2EDTA and acetylcholine. Time course of the response to Na2EDTA differed significantly between mongrel and BG dogs. Peak response to challenge with 4% Na2EDTA occurred within 2 min for mongrel dogs and at 5 min for BG dogs. Albuterol (1 microgram/kg iv) significantly attenuated Na2EDTA-induced bronchoconstriction in both groups of animals (P less than 0.01, each group). Albuterol (1 microgram/kg iv) significantly attenuated acetylcholine-induced bronchoconstriction in mongrel (P less than 0.01) but not in BG dogs. We conclude that a qualitative difference exists in the mechanism of Na2EDTA-induced constriction in the lung periphery of BG compared with mongrel dogs. In addition, the lung periphery of BG dogs demonstrates reduced beta-adrenergic sensitivity with respect to a cholinergic challenge compared with mongrels, suggesting enhanced cholinergic inhibition of the beta-adrenergic system.

Reduced sensitivity to beta-adrenergic agonists in Basenji-Greyhound dogs.

To investigate the inhibitory effects of beta-adrenergic agonists and aminophylline on pulmonary responsiveness, we evaluated the ability of albuterol and aminophylline to attenuate pulmonary responses to aerosol challenge with methacholine and histamine in intact Basenji-Greyhound (BG) and selected mongrel dogs. Pulmonary responses were measured in untreated dogs and in dogs pretreated with albuterol (1 and 2.5 micrograms/kg) or aminophylline. Before aerosol challenge, baseline pulmonary resistance (RL) and dynamic compliance (Cdyn) were not significantly different between the BGs and the mongrels. In the untreated dogs, pulmonary responses to methacholine and histamine aerosols were not different between the BGs and the mongrels. Pretreatment with albuterol (1 microgram/kg) or aminophylline significantly attenuated the pulmonary response to methacholine in the mongrels but was without effect in the BGs. Albuterol (2.5 micrograms/kg) significantly attenuated the pulmonary response to methacholine in the BGs and the mongrels; however, this attenuation was significantly greater (P less than 0.05) in the mongrels than in the BGs. In response to histamine challenge, no differences were seen between the BGs and the mongrels in the control state (no pretreatment) or after pretreatment with albuterol or aminophylline. This study demonstrates that in BGs pulmonary responsiveness to methacholine but not histamine is resistant to inhibition by beta-adrenergic agonists. This may result from a qualitative or quantitative defect in either the cholinergic or beta-adrenergic receptor or to an abnormality distal to the receptors in the signal transduction mechanism at a site where the two signals interact.

Calcium chelators induce bronchoconstriction in the canine lung periphery.

We studied the mechanism by which Na2EDTA, a divalent cation chelator, induces bronchoconstriction in the lung periphery of mongrel dogs as a model of nonspecific small airway hyperresponsiveness. Using a wedged bronchoscope technique, we measured collateral system resistance (Rcs) before and after challenges with aerosolized Na2EDTA. An isotonic solution (4% Na2EDTA, 0.28 osmol/kg) increased Rcs 91 +/- 21%. Na2EDTA increased Rcs in a dose-dependent fashion after challenges of increasing concentration (0, 1, 3, and 6%) or duration (15, 30, 60, and 90 s) with 6% Na2EDTA. Atropine (1 mg/kg iv) significantly (P = 0.01) attenuated the response to an aerosol challenge with distilled H2O. Atropine did not significantly (P = 0.35) alter the response to a challenge with 4% Na2EDTA. Challenge with 6% Na2EDTA (0.42 osmol/kg) increased Rcs to a significantly greater (P less than 0.01) extent than did challenge with 6% CaNa2EDTA (0.37 osmol/kg, 250 +/- 55 vs. 29 +/- 11%, respectively). We conclude that Na2EDTA induces bronchoconstriction in the canine lung periphery in a dose-dependent fashion. As suggested by the Na2EDTA-CaNa2EDTA comparison, hyperosmolality of the solution alone cannot explain this phenomenon. The mechanism does not depend on muscarinic activity and appears to involve chelation of calcium.

Differential effects of nitrovasodilators and nitric oxide on porcine tracheal and bronchial muscle in vitro.

Nitrovasodilators and nitric oxide relax airway smooth muscle. The mechanism by which nitrovasodilators are thought to act is by release of nitric oxide, but the importance of nitric oxide in nitrovasodilator-induced airway smooth muscle relaxation is unclear. The aim of this study was to compare the relaxing effects of nitric oxide itself with those of nitrovasodilators in porcine tracheal muscle and intrapulmonary airways and to investigate the mechanisms involved. Strips of porcine tracheal smooth muscle, rings of bronchi, and strips of bronchi from the same animal were suspended in organ chambers in modified Krebs Ringer solution (95% O2-5% CO2, 37 degrees C). Tissues were contracted with carbachol, and concentration-response curves to nitric oxide, sodium nitroprusside, and SIN-1 (an active metabolite of molsidomine) were obtained. All tissues relaxed to sodium nitroprusside, SIN-1, and nitric oxide. The relaxation to nitric oxide but not to SIN-1 or sodium nitroprusside was inhibited by methylene blue. Tissues pretreated with methylene blue that failed to relax to nitric oxide were, however, relaxed by sodium nitroprusside. These results demonstrate that nitrovasodilators relax airways by a mechanism other than by or in addition to the release of nitric oxide.

Use of collateral airways to assess airway reactivity.

We investigated the correlation between collateral airway reactivity and other indexes of lung reactivity in response to aerosol and intravenous (iv) challenges. In four anesthetized mongrel dogs, we measured the peripheral airway resistance (Rp) to gas flow out of a wedged lung segment in different lobes on multiple occasions. We obtained dose-response curves of peripheral airways challenged with iv histamine or aerosols through the bronchoscope. During the same iv bolus challenge, whole lung airway pressure (Paw) responses to histamine were also measured. On separate occasions, changes in lung resistance (RL) were measured after the whole lung was challenged with a histamine aerosol. Reactivity was assessed from the dose-response curves for Rp and RL as the PD50 (dose required to produce a 50% increase); for changes in Paw we calculated the PD15 (dose required to produce a 15% increase over baseline). Results for Rp showed considerably more variability among different lobes in a given animal with the aerosol challenge through the bronchoscope than with the iv challenge. With aerosol challenge there were no significant differences in the mean PD50 for Rp among any of the animals. However, with the iv challenge two of the dogs showed significant differences from the others in reactivity assessed with Rp (P less than 0.01). Moreover, the differences found in the peripheral airways with iv challenge reflected differences found in whole lung reactivity assessed with either iv challenge (Paw vs. Rp, r2 = 0.96) or whole lung aerosol challenge (RL vs. Rp, r2 = 0.84). We conclude that the measurement of the collateral resistance response to iv challenge may provide a sensitive method for assessing airway reactivity.