We investigate the role of resting tension on thrombin (THR) induced relaxation of guinea-pig tracheas precontracted with acetylcholine (ACh).Isometric contractions of isolated guinea-pig tracheas were recorded at 4 and 6 g resting tension; and ACh dose-response curves were performed.THR relaxed ACh-precontracted tracheas and this effect was mimicked by the type 2 protease activating receptor agonist peptide (PAR-2 AP) and trypsin.The relaxant effect of 3 U ml K1 THR and 100 nmol ml K1 PAR-2 AP was prevented at 4 g by preincubation with the nitric oxide synthase (NOS) inhibitor L-NAME and at 6 g resting tension by ibuprofen and diclofenac. However, adenosine trisphospahate (ATP) relaxation was totally prevented by cyclooxygenase (COX) inhibitors but not by NOS inhibitors at both resting tensions.Resting tension influenced the effect of PGE 2 on contractile tone of isolated guinea-pig tracheas, the maximal relaxation being K11.1G 2.97 and K2.0G0.46 mg mg K1 tissue wet weight at 6 and 4 g, respectively. Moreover, 30 nmol ml K1 PGE 2 can relax ACh-precontracted tracheas, being the effect up to 91 and 30% at 6 and 4 g, respectively.These data demonstrate that trachea responsiveness is highly dependent on the smooth muscle length, revealing new aspects of stretchactivated receptors that can influence trachea responsiveness in vivo. q