Toxicology of Industrial Compounds

132 PULMONARY HYPERREACTIVITY TO INDUSTRIAL POLLUTANTS
For challenge exposures it appears to be advantageous to use the free
chemical in slightly irritant concentrations rather than the proteinconjugate
of the hapten. It is believed that the in vitro synthesis of the
hapten-protein conjugate may not necessarily result in immunologically
identical conjugates when compared with those produced under in vivo
conditions. Also standardized procedures to synthesize and characterize
hapten-protein conjugates of multifunctional, highly reactive chemicals are
not yet established. On the other hand, an essential prerequisite for
challenge exposures with the free chemical is the evaluation of the irritant
threshold concentration of the hapten under investigation. The importance
of concentration in distinguishing irritation from sensitization cannot be
overstated and is one of the most critical determinants of this animal
model.
For volatile, irritant haptens the characteristic feature of upper
respiratory tract irritation is the reflexively induced decrease in respiratory
rate which is a common finding in laboratory rodents (Figure 9.1).
Consistent with this approach, naive mice, rats and guinea-pigs were
exposed for 45 min to slightly irritant concentrations of phenyl isocyanate
(PI). As evident from Figure 9.1, the exposure to ca. 5 mg PI m −3 air
provoked a decrease in respiratory rate of approximately 25–45%. The
observation that remarkable differences in response patterns between mice,
rats and guinea-pigs did not occur demon strate that irritant threshold
concentrations obtained in mice may also be valid for guinea-pigs. Mainly
for volatile chemicals attempts have been made to establish methods for the
measurement and analysis of the irritant-induced changes in respiratory
pattern in mice (Vijayaraghavan et al., 1993) and to understand the
mechanisms of the irritant receptor stimulation (Nielsen, 1991). For
volatile irritant haptens, such as PI, an unequivocal respiratory
hypersensitivity response is characterized by a shallow rapid breathing
pattern, i.e. a response opposite to that occurring as a result of upper
respiratory tract irritation. For volatile irritant haptens this type of
breathing pattern, however, can only be obtained when using the proteinconjugate
of the hapten.
The interpretation of changes in respiratory pattern induced by irritant
particulates is less predictable because of the size-dependent deposition of
particles within the respiratory tract. Irritant aerosols that evoke bronchial
or pulmonary irritation may produce changes similar to those occurring
following immediate-onset responses. Therefore, the selection of adequate
haptenchallenge concentrations as well as the measurement of several
breathing parameters is of primary importance. For such chemicals,
currently the relative effectiveness of the acute high-concentration
inhalation (single inhalation exposure of 15 min) and the high-dose
intradermal route for sensitization of guinea-pigs had been investigated
(Pauluhn and Eben, 1991; Pauluhn and Mohr, 1994). The airway function