New approach to the mechanism of antiasthmatic action of Tranilast.

The mechanism proposed to explain the antiasthmatic antiallergic action of Tranilast is the inhibition of chemical mediator release from mast cells and leukocytes as well as the antagonism of smooth muscle contracting activity of leukotrienes. It has been shown that this drug inhibits platelet aggregation induced "in vitro" by different stimuli. We investigated the effect of Tranilast on the release of thromboxane from guinea pig isolated lungs stimulated by the calcium ionophore A 23187 and the contraction of smooth muscle induced by this eicosanoid. Tranilast did not inhibit the release of arachidonic acid metabolites from the lungs but it prevented the contraction of the rat aorta induced by thromboxane released from lungs. Moreover, the drug antagonized the contraction of rat and rabbit isolated aortas stimulated by the thromboxane/endoperoxide mimetic U 46619. These effects might be mediated by a blockade of calcium uptake, since the drug was able to induce the relaxation of rabbit aortas previously contracted by potassium. Calcium ions are involved in the activation of mast cells, leukocytes, platelets and smooth muscle; therefore, the inhibition of calcium uptake might mediate the pharmacological properties of Tranilast.