Concentrations of whole-cell cAMP in human airway smooth muscle cells treated with the indicated concentrations of PAH for 24 hours. The data represent pmol of cAMP/mg protein measured in cell lysates after 24-hour treatment with PAHs or vehicle (Control). Control cells were treated with the same concentration of vehicle as PAH cells (3%, 5%, or 10% DMSO in complete medium for 1.86, 2.80, and 5.59 ng/ml PAHs, respectively; n = 6 dishes/condition). *P = 0.05, versus same dilution of vehicle-treated control samples. **P = 0.001, versus same dilution of vehicle-treated control samples.