INTRODUCTION

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The possibility of an abnormal penetration of inhaled agents into airway tissue attracts interest as a pathogenetic mechanism in asthma and rhinitis (1, 2). Such a hyperpermeability paradigm would seem to agree well with the occurrence of epithelial disruption. Epithelial eosinophilia, damage, and shedding are a hallmark of asthma (3, 4); significant epithelial disturbance may also occur in the eosinophilic, allergic nose (5, 6). A number of studies have reported that persons with asthma (7) or allergic rhinitis (8) exhibit abnormally high rates of absorption of tracer molecules across the bronchial and nasal mucosa, respectively. As a corollary mechanisms of increased permeability in epithelial cell culture models have become an advanced area of in vitro research. However, several investigators, who have carried out physiologic in vivo studies, have not been able to corroborate the presence of airway hyperpermeability, either in asthma (11, 12) or in rhinitis (13). Preliminary in vivo data (11, 13, 15, 16) even suggest that inflamed and hyperreactive airways may be abnormally tight and absorb solutes less well than the airways of healthy subjects. This latter possibility may appear paradoxical, and it remains unrecognized. However, an increased tightness could be compatible with inflammation and increased epithelial shedding, i.e., if restitution of an epithelial barrier structure with large, flat "repair" cells (11, 17) is prompt and efficient.

This study examined nasal absorption of a peptide in patients with seasonal allergic rhinitis. Nontraumatic and relatively noninvasive techniques (18) have been employed to repeatedly assess the absorption rate across the same airway mucosal surface area. By determination of absorption outside and inside the Swedish birch pollen season, this study further explores the possibility that an airway disease characterized by hyperresponsiveness and exudative, eosinophilic inflammation (19, 20) may be associated with an improved mucosal barrier function.

	METHODS

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Nasal absorption of desmopressin (1-deamino-8-D-arginine vasopressin) was examined in 12 patients 21 to 27 yr of age with a history of strictly birch-pollen-allergic rhinitis verified by a positive skin prick test (Aquagen SQ; ALK, Copenhagen, Denmark). The two examination time points prior to and late into the natural birch pollen season (study Day 32) (Figure 1) were separated by about 3 mo. The patients with allergic rhinitis were instructed to register nasal symptoms (sneezes, blockage, and rhinorrhea) and overall eye symptoms, each on a four-graded scale each day during the pollen season. The symptoms were scored as 0: no, 1: mild, 2: moderate, and 3: severe symptoms, yielding a maximal total score of 12. They were allowed occasional use of antihistamines as rescue medication (no steroids). Ten healthy subjects 22 to 27 yr of age participated as control subjects to the patients when the latter were free from symptoms. The healthy subjects were thus examined only once (together with the patients prior to the pollen season). These subjects had no history of allergic rhinitis and a negative skin prick test. The study was performed after approval by the local ethics committee and according to the declaration of Helsinki. Informed consent was obtained from the subjects.

View larger version (24K): [in this window] [in a new window]   Figure 1.   Symptom scores recorded in patients with allergic rhinitis during the birch pollen season. The symptoms were significantly increased from study Day 10 to and including study Day 32 when the nasal absorption ability was examined.

A nasal pool device (compressible plastic container with a nasal orifice adapter) was used to gently fill the unilateral nasal cavity with a solution of desmopressin (20 µg/ml) in isotonic saline (18). Prior to instillation the solution was kept at room temperature (20 to 22° C). By keeping the device compressed and the head flexed forward 60 degrees from the upright position, the tracer solution was maintained in contact with a large and almost constant nasal mucosal surface area for 15 min. The instillate was then drained back into the device. The mucosal surface was irrigated twice immediately after the completed tracer instillation in order to minimize uptake of the tracer beyond the 15 min of exposure. These latter irrigations were carried out with isotonic saline using two nasal pool devices, each containing 15 ml of fluid and each left in contact with the nasal mucosa for 30 s. Elimination of absorbed desmopressin is largely by the renal route (21, 22), 65% of an intravenously administered dose being excreted in the urine (22). Urine was collected for 24 h after nasal administration, and the concentration of desmopressin (coded samples) was analyzed by a specific radioimmunoassay (22). The total amount excreted during the 24-h period was calculated. The cross-reactivity with arginine vasopressin and oxytocin was less than 0.01%. The detection limit of desmopressin was 5.0 pM. Friedman's test and Wilcoxon's signed rank test were used for comparison within the group of patients with allergic rhinitis. The Mann Whitney U test was used for comparisons between the group of patients with allergic rhinitis and the group of healthy subjects. A p value less than 0.05 was considered significant. Data are presented as mean ± SEM.

	RESULTS

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The regional birch pollen counts (data not shown) suggested that the season started study Day 5 and went on for more than 4 wk. The symptom scores demonstrated moderate symptoms of allergic rhinitis that were significantly increased study Days 12, 16, 17, 19, and 23 to 32 compared with study Day zero (Figure 1). According to diary cards only antihistamine eye drops were used as rescue medication. The amount of desmopressin excreted in urine was reduced in patients with allergic rhinitis late into the pollen season as compared with outside the season (Wilcoxon's test; p < 0.05) (Figure 2). There was no significant difference in the excretion of desmopressin between patients with allergic rhinitis examined outside the pollen season and healthy subjects (Mann Whitney; p = 0.3). There were no significant differences between the volumes of the 24-h urine samples within the group of patients with allergic rhinitis (Wilcoxon's test; p = 0.5), or between patients with allergic rhinitis examined outside the pollen season and healthy subjects (Mann Whitney U test; p = 0.5).

View larger version (37K): [in this window] [in a new window]   Figure 2.   Excretion of desmopressin in 24-h urine samples after its nasal administration. In patients with allergic rhinitis absorption was significantly reduced compared with outside the season (*p < 0.05). Absorption did not differ (p > 0.05) between healthy subjects and patients with allergic rhinitis outside the pollen season.

	DISCUSSION

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The present study involving subjects with seasonal allergic rhinitis demonstrated a reduced nasal absorption of a tracer peptide during the active allergic disease. Moreover, the airway absorption ability in the patients outside the season was not different from that recorded (this study) in a group of healthy subjects. These results suggest that several weeks of symptom-producing allergic airway inflammation may produce a tightness of the airway mucosa, reducing penetration of topically deposited molecules. The present observations may have implications for our understanding of airway disease mechanisms.

The human nose offers experimental advantages, compared with human bronchi in vivo, of importance for assessment of the mucosal absorption ability (19). By use of the present pool device (18), a large and reproducible area of the mucosal surface could be atraumatically exposed to the absorption tracer at different experimental days. Airway secretions and exudations may affect absorption, but a major role of this additional barrier is unlikely in the present study where mucosal surface liquids were allowed to mix with the tracer-containing pool fluid during the course of absorption. Further, the exposed mucosa was selectively lavaged to remove lumenal tracer molecules, thus practically ending the absorption after a defined period of time. Because the nose is so accessible these experimental aspects could be satisfied without intubations or other interventions that would distort the baseline integrity of the sensitive airway mucosa. Indeed, similarly controlled conditions may not be attainable in human bronchi in vivo. The present pool technique would also minimize any displacement of the tracer to the gut, in part because mucociliary transport would be severely impeded in the fluid-filled nasal cavity. If a small portion, nevertheless, was to be swallowed this would not distort the nasal absorption data because of efficient catabolism of ingested desmopressin (21, 22). Taken together these methodologic aspects support the possibility that the present measurements reflect the absorption capacity of the human airway mucosa in vivo. An additional aspect is that the present absorption data would reflect the ability of the mucosa to reabsorb a wide range of endogenous peptides proteins that may abound on the airway mucosa, particularly in inflammatory conditions (19, 20).

The present observations agree well with a previous report on reduced nasal mucosal absorption of Cr⁵¹-EDTA in seasonal allergic rhinitis (15). The agreement strengthens the interpretation that an increased epithelial tightness of the nasal mucosa may explain the present findingsboth the peptide (23, 24) and the Cr⁵¹-EDTA (15) are thus thought to be absorbed by passive diffusion through paracellular epithelial pathways. Also, a postulated catabolism product has not been detected after nasal application of desmopressin in healthy human subjects (25), suggesting that mucosal metabolism may not be an important factor in this study. We cannot exclude the possibility that allergic inflammation has altered the metabolizing capacity of the nasal mucosa. However, it is unlikely that metabolism is a major factor behind the present findings since desmopressin, being a deamination product of the naturally occurring peptide vasopressin, is resistant to the primary peptide-degrading enzymes of absorptive mucosa, the aminopeptidase (26). Taken together the findings on reduced urinary excretion of both desmopressin (this study) and Cr⁵¹-EDTA (15) suggest that the nasal mucosa exhibits a reduced absorption capacity in allergic airway disease. Some caution with regard to this conclusion is warranted since the present study, in addition to demonstrating a difference between allergic disease and allergy without disease, also may have involved a temporal change (whether airway absorption ability may exhibit variations during the year in health or disease now remains unexplored). Another aspect is that the absorption of larger tracers than desmopressin may have to be examined to see whether a reduced absorption in allergic disease may also apply to molecules of the size of common aeroallergens, as, indeed, was reported by Cohen and colleagues (13) in 1930.

In vivo findings in animals and in human subjects suggest that absorption processes may be similarly regulated in the nasal and the tracheobronchial mucosa (for review see Reference 27). There is also a similarity between the nasal and the tracheobronchial airways concerning the arrangement of a profuse microcirculation just beneath a pseudostratified epithelial lining (28). The rich superficial blood supply may provide a surplus flow, making plasma exudation (29) and possibly also absorption processes little dependent of fluctuations in blood flow. Neither the effect of the disease nor the effect of desmopressin on nasal mucosal blood flow (desmopressin may have a weak vasodilator effect in the nose) (30) is, therefore, a likely cause of the present results. Challenge with allergen and histamine-type mediators causes prompt lumenal entry of a bulk plasma exudate by valvelike paracellular epithelial routes across an airway mucosa that maintains its absorption barrier intact (27). Indeed, it was demonstrated in human subjects that even a very large concentration of histamine (2,000 µg/ml), which produces marked exudation and likely affects blood flow, too, did not affect the rate of nasal mucosal absorption of Cr⁵¹-EDTA (31). Considering this lack of effect of histamine on nasal absorption we did not expect any influence of antihistaminic medications, occasionally used in this study, on the absorption of desmopressin. The asymmetry of the airway mucosal barrier, allowing exudation without increasing absorption, makes the present observation of a reduced airway "absorption permeability" in allergic rhinitis compatible with the notion that this, similar to asthma (32), is an exudative airway disease (19, 33). Interestingly, Elwood and colleagues (11), in a pioneering study on tracheobronchial (and alveolar?) absorption permeability in asthma, recorded a mean permeability index that was somewhat (although not significantly) lower in hyperreactive asthmatic subjects than in healthy subjects. A reduced permeability in asthma has also been reported by Halpin and colleagues (16) who examined absorption of inhaled Tc-DTPA and who made corrections for the concomitant elimination of the tracer by mucociliary transport. The present results, obtained under the controlled conditions that are attainable in the human nose, indicate that exudative and eosinophilic airway diseases can have a functionally tight airway mucosa.

Decreased airway absorption in airway diseases may in part be explained by efficient epithelial restitution mechanisms. If epithelial shedding involves only columnar epithelial cells, the remaining layer of cobbled basal cells may promptly change shape into a barrier structure of overlapping flattened cells (34). Patchy denuded areas may also be sealed promptly in vivo by a plasma exudation-derived gel and by instantaneously and rapidly migrating restitution cells (35). The presence of such poorly differentiated, flat cells would reduce the paracellular stretches (and reduce the number of corners where three cells meet, which is considered a potential absorption site) (11), available for absorption of solutes per unit surface area. Awaiting actual studies of absorption specifically across "repair-epithelium" this proposed mechanism remains an attractive conjecture.

In conclusion, an improved functional tightness of the airway mucosa was found in seasonal allergic rhinitis. These data may fit into the picture of an exudative airway disease because lumenal entry of plasma exudates is not necessarily coupled with an increased "absorption permeability" (27). The present observations are further compatible with the occurrence of epithelial disruption because epithelial restitution may be a very speedy and efficient process in vivo (35). For persons suffering from airway diseases an improved absorption barrier should be welcome because inhaled noxious molecules would be less prone to penetrate into the airway tissue to uncontrollably aggravate the disease condition. In addition, the biologically active molecules that are exuded onto the mucosal surface would not readily be reabsorbed to activate mucosal effector cells.