Antigen challenge of sensitized rats increases airway responsiveness to methacholine.

We measured airway responsiveness to methacholine (MCh) of highly inbred rats before and after six inhalational challenges with antigen. Ten Brown-Norway rats (130-216 g) that were actively sensitized to ovalbumin (OA) received six challenges with OA at 5-day intervals beginning 19 days after sensitization. An aerosol of OA (5% wt/vol) was inhaled for 1, 2, 5, and 10 min or until pulmonary resistance (RL) increased by at least 50%. Challenges with aerosolized MCh were performed immediately before and 14 days after sensitization, 2 days after the 3rd OA exposure, and 2, 7, 12, and 17 days after the 6th OA challenge. Four unsensitized rats underwent inhalational challenges with MCh over an equivalent time period. Responsiveness to MCh was calculated as the concentration of MCh required to increase RL to 200% of the control value (EC200RL). Seven out of 10 rats in the experimental group reacted to the first OA challenge with an immediate increase in RL of greater than 50% of control (range 70-550%). Three animals were unreactive to OA. Base-line EC200RL for all rats undergoing sensitization was 2.13 mg/ml (geometric mean), and it did not change significantly after sensitization (2.05 mg/ml). However, EC200RL of the rats that reacted to OA (n = 7) decreased significantly after 3 (1.11 mg/ml; P less than 0.005) and 6 OA exposures (0.96 mg/ml; P less than 0.005). The increase in responsiveness to inhaled MCh was present 17 days after the last OA exposure (EC200RL = 1.40 mg/ml; P less than 0.05). EC200RL of neither the unreactive sensitized rats (n = 3) nor the control rats (n = 4) changed after OA challenges.(ABSTRACT TRUNCATED AT 250 WORDS)