Gretchen Meyer and Richard L. Lieber
Differences in passive muscle mechanical properties between amphibians and mammals have led to differing hypotheses on the functional role of titin in skeletal muscle. Early studies of frog muscle clearly demonstrated intracellular load-bearing by titin, but more recent structural and biological studies in mice have shown that titin may serve other functions. Here we present biomechanical studies of isolated frog and mouse fibers and fiber bundles to compare the relative importance of intracellular vs. extracellular load bearing in these species. Mouse bundles exhibited increased modulus compared with fibers on the descending limb of the length-tension curve, reaching a 2.4-fold elevation at the longest sarcomere lengths. By contrast, frog fibers and bundles had approximately the same modulus at all sarcomere lengths tested. These findings suggest that, in contrast to frog muscle, titin does not bear a significant amount of the whole muscle passive tension in mammals.