1Unite ́ de Pneumologie, Service de Médecine Interne, and 3Service de Physiologie

initial length on intrinsic tone in guinea pig tracheal smooth muscle. Am. J. Physiol. 275 (Lung Cell. Mol. Physiol. 19): L1026–L1030, 1998.—In the guinea pig, tracheal smooth muscle (TSM) exhibits intrinsic tone (IT). The active nature of IT suggests that it could be influenced by muscle length and load. In the guinea pig, IT is entirely suppressed by the cyclooxygenase inhibitor indomethacin. IT could be measured as the difference between resting tone before and after indomethacin addition. We examined, in electrically stimu-lated TSM strips (n 5 9), the influence of initial muscle length (Li) on IT, the relationship between IT and the maximum extent of relaxation (DF1), and the influence of indomethacin on active isometric force. When Li decreased from 100 to 75% of optimal Li, there was a significant decrease in IT (from 12.0 6 0.2 to 5.3 6 0.1 mN; P, 0.001). Over the range of L