The inhibitory actions of prostaglandins on respiratory smooth muscle

Prostaglandin E₁, in concentrations as low as 1 ng/ml., relaxed isolated tracheal muscle from cat, monkey, rabbit, guinea-pig and ferret. Tracheal muscle from the cat, monkey and rabbit did not exhibit inherent tone and the effect of prostaglandin E₁ on these preparations was seen only after a sustained contraction had been produced by a previous dose of acetylcholine or of another agonist. Prostaglandins E₂, E₃ and F_1α also relaxed isolated cat tracheal muscle which had been stimulated by acetylcholine: their activities relative to that of prostaglandin E₁ were, respectively, 1.0, 0.2 and 0.002. In the anaesthetized cat prostaglandin E₁ increased lung “resistance to inflation” (presumably comparable to bronchial resistance) and the heart rate. In the anaesthetized rabbit and guinea-pig, prostaglandin E₁ antagonized the rise in resistance to inflation of the lungs obtained after vagal stimulation or after the intravenous injection of histamine; it sometimes lowered the resistance to inflation in these species. The possibility that prostaglandin may have a local physiological role in the control of bronchial smooth muscle tone is discussed.