Individual, age and sex differences in fiber type composition of slow and fast muscles of adult Lewis rats: comparison with other rat strains.

We analyzed fiber type composition of soleus and extensor digitorum longus (EDL) muscles of 3- to 19-month-old male and female inbred Lewis rats using histochemical demonstration of mATPase activity. The rats were divided into four groups of the mean age of 3, 6, 9 and 14 months. We found that the soleus muscle of 3-month-old rats contained significantly more of fast 2A fibers and less of slow type 1 fibers compared to older rats, while no significant difference was found between female and male rats at any age group. In contrast, we found no significant difference in the EDL fiber type composition among the age groups, but we found that the EDL muscle of female rats contained significantly less 2A fibers and more 2B fibers than that of male animals. Our results thus revealed an age difference in the soleus muscle and a sex difference in the EDL muscle among postnatal Lewis rats. The number of slow type 1 fibers in the soleus muscle varied between 87 and 100% and that of 2A fibers between 13 and 0%. In the EDL the percentage of type 1 fibers varied between 2.6 and 8.7%, that of 2A fibers between 12.6 and 25.8% and that of 2B fibers between 70.4 and 81.6%. Both muscles thus exhibited a considerable degree of variability among individual animals even in the same age group. Furthermore, a comparison of the Lewis rats with literature data of other rat strains showed that the number of fast 2A fibers in the soleus muscle of 4-month-old and older animals decreased in this order: SHR>Lister Hooded>Fisher 344>Sprague-Dawley>Wistar>WBN/Kob>Lewis strain, being almost 20% in the SHR and less than 2% in the Lewis rats. In contrast, the "fastest" composition (judged according to the percentage of the fastest 2B fibers) of the EDL muscle was demonstrated by Lewis, Wistar and Fisher 344 rats (about 75%), while Sprague-Dawley and WBN/Kob rats contained only about 50% of 2B fibers. The percentage of slow type 1 fibers in the EDL was low in all strains (about 5%). Our results thus show that the individual, age and sex as well as inter-strain differences in muscle fiber type composition should not be ignored when comparing results of different studies. We also demonstrated that the inbred Lewis strain appears to have more "specialized" muscle composition, as its soleus is the "slowest" and its EDL is the "fastest" among the routinely used rat strains.