VIP and airway hyperresponsiveness: The development of airway hyperresponsiveness was apparent from the significantly greater responses to MCh in the rats in Group HC (p < 0.05 and p < 0.001, at the second and third doses of MCh vs. Group C, respectively), whereas a significant reduction in the lung responsiveness to MCh was obvious in the hyperoxic rats treated with sildenafil (Group HS, P < 0.001 at the third dose of MCh vs. Group C) or VIP (Group HV, p < 0.001 at third dose of MCh vs. Group C).