Histochemical composition, contraction speed and fatiguability of rat soleus motor units

The relation between the histochemical myosin ATPase and SDH activities and the speed and endurance of contraction at 34.5–35.5°C was studied in 28 rat soleus motor units. 1. (1) Myosin ATPase activity was associated with speed and from the intensity of the reaction it was possible to predict the contraction time of a fast unit with unexpected accuracy. Units with a strong myosin ATPase reaction after formaldehyde fixation and an inhibited reaction after acid preincubation at pH 4.35 had a contraction time of 15–20 msec. Units with moderate staining reactions had a contraction time of 21–23 msec and those with an inhibited reaction after fixation and strong reaction after preincubation had contraction times of 26–44 msec. The fibres of fast and slow units appeared histochemically uniform; fibres of intermediate units revealed subtle variations in the intensity of the ATPase staining. Three units were histochemically transitional between two adjacent groups. 2. (2) The direct correlation between SDH activity and endurance was confirmed. Resistance to fatigue was in addition inversely related to speed. Despite the higher SDH activity in fast and intermediate motor units compared with slow units, the decline in tetanus tension in the three groups was proportionate to their speed of contraction. The greater stability of slow units in a fused tetanus was the result of better matching between the capacity of neuromuscular transmission and tetanic fusion frequency and in a moderate, incomplete tetanus was associated with a larger increase in plateau tetanus-twitch ratio. 3. (3) The intense SDH activity in many fibres with high myosin ATPase activity typical for limb muscles of small animals with great rapidity of limb movements serves to compensate for the higher energy expenditure demanded by their operation at high speed; they therefore combine endurance with speed, particularly apparent in strong repetitive phasic contractions. The independent variability of myosin ATPase and mitochondrial enzymes enables a wide range of specifications of motor units with regard to speed and endurance, required by muscles with different functions.