Slow motor units in female rat soleus are slower and weaker than their male counterparts

Abstract

The aim of the study was to investigate sex-related differences in contractile properties, parameters of action potentials, and mechanisms of force regulation of motor units in the rat soleus muscle, which is a frequent experimental model in animal research. It was revealed that the mean mass of the muscle in males was bigger than in females, by approximately 80%. However, the relation of the muscle mass to the body mass was not significantly different. These results correspond to approximately twice as much tetanic force per motor unit in male rats, and higher maximal contractile output, reflected by the force-time area per stimulus pulse. On the other hand, no differences were observed with respect to twitch forces of motor units. Thus the twitch-to-tetanus ratio was significantly higher in females. Additionally, the contraction and the half-relaxation times were significantly longer in female motor units, which might be due to differences in muscle architecture. The force-frequency curve in males was shifted rightwards with respect to females, indicating that the same relative level of tetanic force could be achieved at considerably lower stimulation frequency in females. The analysis of motor unit action potentials revealed about four times higher amplitudes in male rats, whereas the time parameters of action potentials were similar. The motor units in male and female rat soleus are considerably different and these observations should be taken in the consideration in various experiments on the muscle.

Decomposition of motor unit tetanic contractions of rat soleus muscle: Differences between males and females

“…The ratios of values of the force and the force-time area for the strongest decomposed twitch to the weakest one were significantly higher for male MUs. The above results well reflect sex differences in the twitch-to-tetanus ratios between male and female soleus MUs, as this parameter has been found to be almost twice lower for males in relation to females (0.11 and 0.19, respectively), while no sex differences have been observed in twitch forces between male and female soleus MUs (Drzymała-Celichowska and Krutki, 2015). This difference suggests that in male soleus MUs, the rate coding plays a bigger role in force regulation processes than in females.…”

“…One of the most obvious differences between slow MUs of these two muscles is that the values of the twitch contraction and relaxation times as well as of the twitch force are substantially higher in soleus MUs than in medial gastrocnemius MUs (Drzymała-Celichowska and Krutki, 2015). These differences should be related to differences in the innervation ratio, muscle fiber diameter and density within a MU (Mierzejewska-Krzyżowska et al, 2011) as well as in the architecture of the two muscles -the soleus is a pennate muscle with a smaller pennation angle and shorter muscle fibers (Eng et al, 2008).…”

“…Such an example is the soleus, a homogeneous, slow-twitch muscle, composed predominantly of slow MUs (Kugelberg, 1973;Burke et al, 1974). Although the soleus and medial gastrocnemius muscles contain slow MUs, it has been shown that basic contractile properties of slow MUs of these two muscles are considerably different, as they are active in different motor tasks (Burke and Tsairis, 1973;Burke et al, 1974;Kanda and Hashizume, 1992;Drzymała-Celichowska and Krutki, 2015). Moreover, previous studies of rat soleus MUs have revealed that this muscle is also sexually dimorphic, and properties of its MUs are different for males and females (Drzymała-Celichowska and Krutki, 2015).…”

“…Although the soleus and medial gastrocnemius muscles contain slow MUs, it has been shown that basic contractile properties of slow MUs of these two muscles are considerably different, as they are active in different motor tasks (Burke and Tsairis, 1973;Burke et al, 1974;Kanda and Hashizume, 1992;Drzymała-Celichowska and Krutki, 2015). Moreover, previous studies of rat soleus MUs have revealed that this muscle is also sexually dimorphic, and properties of its MUs are different for males and females (Drzymała-Celichowska and Krutki, 2015). MUs of the male soleus have shorter contraction times and generate higher tetanic forces, whereas the twitch forces are similar for animals of both sexes; consequently, the twitch-to-tetanus force ratios are considerably (about twice) higher for females.…”

Prolonged activity evokes potentiation and the “sag” phenomenon in slow motor units of rat soleus

“…In most of physiological studies performed on MUs of cat and rat hindlimb muscles the division of MUs into fast (F) and slow (S) has been based on the sag profile, a rapid increase of tetanic tension followed by a decrease of tension present in unfused tetani of fast units (Burke et al 1973) whereas this phenomenon has never been observed in slow MUs. In previous papers we have revealed that basic contractile properties of MUs in rat soleus muscle are sex-related, and male MUs are faster (Drzymała-Celichowska and Krutki 2015), consistent with higher contents of IIa myosin in the latter (Eason et al 2000. It is known that a substantial portion of diversity of physiological properties can be attributed to the presence of distinct sets of myofibrillar proteins that define general classes of muscle fibers (Pette and Staron 1990, Stephenson et al 1998, Carp et al 1999.…”