Specific receptors for prostaglandins in airways

The relative bronchomotor activities of prostaglandins (PG) E1, E2, F2 alpha, F2 beta and I2 and of three synthetic E prostaglandin analogues (TR4161, TR4367 and TR4752) were determined on a large number of isolated preparations of guinea-pig trachea and human bronchial muscle. Each prostaglandin was capable of eliciting both contraction and relaxation, the relative incidence of these responses partly depending on concentration. TR4161 was a virtually pure relaxant; TR4367 was virtually devoid of bronchomotor activity; and TR4752 was a potent relaxant, devoid of contractant activity. The results also provided distinct rank orders of approximate potency for contraction and relaxation. Tachyphylaxis to the relaxant activities of PGE1 and TR4752 confirmed the underlying contractant activity of the two natural E prostaglandins. Antagonism with a high dose of indomethacin of the contractant actions of PGE1, PGE2 and PGF2 alpha confirmed the presence of relaxant activities in each. Inhaled aerosols of the same natural and synthetic prostaglandins were evaluated for irritant activity on the airways, using the cough response of the restrained conscious cat. All of them, except TR4161, elicited severe coughing. The rank order of potencies for irritancy differed from those for tracheobronchial contractant and relaxant activities. These findings suggest that the three responses studied arise from the activation of three distinct PG receptors in the airways. We propose the terms chi (contractant), psi (relaxant) and omega (irritant) for these putative receptors for prostaglandins or possibly other prostanoids.