The effects ofdenervation on isotonic shortening velocity ofrat fast and slow muscle BY A. J. BAKER and D. M. LEWIS. Department of Physiology, University of Bristol, Medical School, Bristol BS8 1TD The motoneurone exerts long-term effects onmanymuscle properties. Measurements obtained after denervation provide information on what aspects of muscle physiology are normally under neural control. Isotonic recordings were made in vivo at 37 0C from normal and from 10 to 40 day denervated fast twitch extensor digitorum longus (EDL) muscle and slow twitch soleus muscle of the rat. It was intended to test whether alterations seen in isometric myograms from denervated muscle (Finol, Lewis & Owens, 1981) might be explained by mechanisms other than an alteration in the contractile proteins. Tetanic isotonic shortening, being independent of both the series elasticity and the time course of the activation events provides a more direct measure of contractile protein behaviour than do isometric contractions. Recording techniques were similar to those described by Close (1964). Initial denervation (under aseptic conditions) and surgical and recording procedures were carried out under sodium pentobarbitone anaesthesia. Shortening velocity was measured under a series of loads from near zero to near tetanic tension, and force: velocity data were fitted to a linearized form of Hill's equation (Hill, 1938) and extrapolated to find the theoretical maximum velocity of shortening (Vmax). Sarcomere lengths were measured using a laser diffraction method and, with fibre length, velocity was calculated per sarcomere in /tm/s (Vsax.) In EDL muscles denervated for 10 days, VUsax fell by only 12 5% from 49-0+ 495 #m/s (n = 8) to 42-8+ 2-8 #m/s (n = 4) (mean+ S.D.) (t = 2-29 P < 0-05) despite a 76 % increase in the time-to-peak of the isometric twitch. No further change in Vsax had occurred at 40 days denervation (Vs ax = 42-8 + 4-85 jtm/s, n = 5). Soleus muscles, however, showed a larger decline in Vs ax from 27-47 + 1-47 ,um/s (n = 5) to 14-84 + 1-73 jm/s (n = 6) after 20 days denervation (t = 12-88 P < 0-001). The fall of 46% is sinilar in magnitude to changes in some of the isometric parameters. The results indicate that the removal of neural influence does affect the isotonic properties and presumably, therefore, the contractile proteins of rat soleus. The effect is not as clear in fast twitch muscle in which only small changes were found and, moreover, these occur early after nerve section.