Lung injury from intratracheal and inhalation exposures to residual oil fly ash in a rat model of monocrotaline-induced pulmonary hypertension

A rat model of monocrotaline (MCT)-induced pulmonary injury/hypertension ha s been recently used in particulate matter (PM) health effects studies, how ever, results have been equivocal. Neither the mechanism by which mortality occurs in this model nor the variation in response due to differences in P M exposure protocols (i.e., a bolus dose delivered intratracheally versus a similar cumulative dose inhaled over three days) have been fully investiga ted. Sprague Dawley rats (SD, 60 d old; 250-300 g) were injected with eithe r saline (healthy) or MCT, 60 mg/kg, i.p. (to induce pulmonary injury/hyper tension). Ten days later they were exposed to residual oil fly ash (ROFA), either intratracheally (IT; saline, 0.83 or 3.33 mg/kg) or by nose-only inh alation (15 mg/m(3) x 6 h/d x 3 d). Lung histology, pulmonary cytokine gene expression (0 and 18 h post-inhalation), and bronchoalveolar lavage fluid (BALF) markers of injury were analyzed (24 and 96 h post-IT; or 18 h post-i nhalation). Data comparisons examined three primary aspects, 1) ROFA IT ver sus inhalation effects in healthy rats; 2) pulmonary injury caused by MCT; and 3) exacerbation of ROFA effects in MCT rats. In the first aspect, pulmo nary histological lesions following ROFA inhalation in healthy rats were ch aracterized by edema, inflammatory cell infiltration, and thickening of alv eolar walls. Increases in BALF markers of lung injury and inflammation were apparent in ROFA-IT or nose-only exposed healthy rats. Increased IL-6, and MIP-2 expression were also apparent in healthy rats following ROFA inhalat ion. In regards to the second aspect, MCT rats exposed to saline or air sho wed perivascular inflammatory cell infiltrates, increased presence of large macrophages, and alveolar thickening. Consistently, BALF protein, and infl ammatory markers (macrophage and neutrophil counts) were elevated indicatin g pulmonary injury. In regards to the third aspect, 58% of MCT rats exposed to ROFA IT died within 96 h regardless of the dose. No mortality was obser ved using the inhalation protocol. ROFA inhalation in MCT rats caused exace rbation of lung lesions such as increased edema, alveolar wall thickening, and inflammatory cell infiltration. This exacerbation was also evident in t erms of additive or more than additive increases in BALF neutrophils, macro phages and eosinophils. IL-6 but not MIP-2 expression was more than additiv e in MCT rats, and persisted over 18 h following ROFA. IL-10 and cellular f ibronectin expression was only increased in MCT rats exposed to ROFA. In su mmary, only the bolus IT ROFA caused mortality in the rat model of lung inj ury/hypertension. Exacerbation of histological lesions and cytokine mRNA ex pression were most reflective of increased ROFA susceptibility in this mode l.