The effects of imidazole on contractility and cyclic AMP levels of guinea-pig tracheal smooth muscle.

1. Imidazole contracted guinea-pig isolated tracheal smooth muscle, and at low concentrations (50 mumol/l to 2 mmol/l) also enhanced contractions induced by histamine. 2. Imidazole had no effect on contractile responses to potassium chloride, acetylcholine, prostaglandin F2 alpha, or the ionophore A23187; nor did it affect anaphylactic contractions in tracheal strips from guniea-pigs sensitized to ovalbumin. 3. Imidazole had no effect on the content on cyclic AMP in guinea-pig tracheal smooth muscle, nor did it affect the accumulation of cyclic AMP induced by histamine, isoprenaline, theophylline plus isoprenaline, or prostaglandin E1. 4. Imidazole-induced contractions were partially antagonized by atropine, mepyramine maleate and the omission of extracellular calcium ions. 5. Imidazole-induced hyperreactivity to histamine was not affected by atropine or indomethacin, and was more pronounced in the absence of extracellular calcium ions. 6. It is concluded that imidazole does not affect cyclic nucleotide phosphodiesterase activity in guinea-pig trachea, and its effects on contractile responses of the smooth muscle are unrelated to any action of cyclic AMP metabolism. The potentiating effect of imidazole may involve inhibition of histaminase, or the facilitation of calcium ion release from intracellular storage sites.