THE FORCE-VELOCITY RELATION OF THE RABBIT INFERIOR OBLIQUE MUSCLE INFLUENCE OF TEMPERATURE

The contractile properties of the rabbit inferior oblique muscle (IO) were studied in vitro with direct stimulation at temperatures between 20 and 35 degrees C. Isovelocity releases were used to determine the f orce/ velocity relation. Cooling the muscle from 35 degrees C to 20 de grees C increased contraction and half-relaxation times of single twit ches with a temperature coefficient (Q(10)) of 0.4, but did not affect significantly the twitch tension. The tetanic tension increases with increasing temperature (Q(10) = 1.32). Cooling decreased the maximum s hortening velocity of the IO with a Q(10) of 1.6 and the maximum mecha nical power with a Q(10) of 2.3. At 35 degrees C, the maximum speed of shortening of the muscle (19 +/- 2 muscle lengths/s, mean+/-SEM) corr esponded to a maximum shortening velocity of the sarcomeres of 57 +/- 6 mu m/s. This value is similar to data obtained for extraocular muscl es (EOM) of smaller rodents (mice and rats). In comparison with mammal ian limb muscles the isometric and force-velocity properties of mammal ian EOM appear to be virtually independent of the size of the animal. Thus, IO is a fast-twitch muscle endowed with a maximum velocity of sh ortening higher than that of fast-twitch skeletal muscle, but using a tetanic mechanical power lower than that produced by slow-twitch muscl e: the combination of these properties makes it ideally suited to move an ocular globe of low mass at high velocity.