Effects of initial length on intrinsic tone in guinea pig tracheal smooth muscle

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 l ength and load. In the guinea pig, IT is entirely suppressed by the cycloox ygenase inhibitor indomethacin. IT could be measured as the difference betw een resting tone before and after indomethacin addition. We examined, in el ectrically stimulated TSM strips (n = 9), the influence of initial muscle l ength (L-i) on IT, the relationship between IT and the maximum extent of re laxation (Delta F-1), and the influence of indomethacin on active isometric force. When L-i decreased from 100 to 75% of optimal L-i, there was a sign ificant decrease in IT (from 12.0 +/- 0.2 to 5.3 +/- 0.1 mN; P < 0.001). Ov er the range of L-i studied, Delta F-1 underestimated the amount of IT, but there was a close linear relationship between Delta F-1 and IT (r = 0.9). Compared with the basal state, indomethacin increased active isometric forc e (from 9.5 +/- 1.0 to 19.7 +/- 2.0 mN at optimal L-i; P < 0.001) and induc ed its length dependency. In guinea pig TSM, L-i was an important determina nt of IT.