ADAPTIVE TRANSFORMATION OF RAT SOLEUS MOTOR UNITS DURING GROWTH - HISTOCHEMISTRY AND CONTRACTION SPEED

Soleus motor units of 5 and 34 wk-old rats, weighing 150 and 520 g, respectively, were examined for contraction time at 36.degree. C, innervation ratio as well as cross-sectional area, ATPase and succinate dehydrogenase (SDH) activity of fibers. The muscle was continuously adjusted to the growth of the animal. Between 5 and 34 wk the proportion of Type II fiber units with 15-26 ms contraction time decreased from 33 to 10%, and Type I fiber units with 27-40 ms contraction time thus increased from 67 to 90%. This was parallelled by approximately the same relative changes in number of Type II and Type I fibers. The average innervation ratio of Type I fiber units appeared unchanged between 5-34 wk, 83 .+-. 16, respectively, 83 .+-. 17/unit (N = 18) and about 99% of the fibers of the units were histochemically uniform. At 5 wk there occurred continuous transitions between Type II and Type I fiber units as regards all tested markers including contraction time, intensity of alkali-, formaldehyde- and acid-stable ATPase, SDH activity and fiber area. The increase in Type I fiber units with growth results from transformation in toto of Type II fiber units to Type I owing to changing properties of their motoneurones from phasic to tonic. Thus one and the same motoneurone determines not only the histochemical and related functional characteristics of its muscle fibers at a specific level but may also alter them qualitatively. With increasing contraction time of units transformation of fibers was manifested first by an increase in intensity of acid-stable ATPase, followed by a decrease in formaldehyde-stable and later by a decrease in alkali-stable ATPase. This led to the following sequence of relative intensities of ATPase at pH 9.4 and 4.35, respectively: dark-light (Type II A).fwdarw.dark intermediate (Type II C).fwdarw.(dark-dark).fwdarw.intermediate-dark.fwdarw.light-dark (Type I). Fibers under transformation (ATPase intermediate fibers) were retarded in growth and showed within the unit variations in intensity of ATPase reactions which to some extent were topographically orientated. Transformation from Type II to Type I was complemented at a slower rate by a decrease in SDH activity, which is opposite to that in larger mammals, indicating that myofibrillar ATPase and mitochondria-bound oxidative enzymes are independently controlled by the motoneurone.