Does Titin Explain The High Passive Force Observed in Frog Tibialis Anterior Muscle?

INTRODUCTION The force‐length relationship of various skeletal muscles indicates that passive force is generally much lower than active force [1]. However, a previous study showed that the whole frog tibialis anterior (TA) muscle has significantly higher passive than active forces within its normal range of motion ( Figure 1) [2]. It is known that titin, the giant protein within the sarcomere, is a main contributor to passive force in skeletal muscle at rest [3]. Furthermore, variations in titin isoforms have been related to different passive forces; the shorter the titin isoform, the higher the passive force [3, 4]. Therefore, the high passive force observed in frog TA muscle could result from the expression of a short titin isoform and thus a high titin‐based passive force. The purpose of this study was to investigate titin isoforms and titin‐based passive force in frog TA muscle. Titin‐based passive force was determined by testing passive force in single skinned muscle fibres in which titin is considered to be the major source of passive force [3]. METHODS Titin isoforms : Titin isoforms in frog TA and rabbit psoas muscles were determined using agarose strengthened 2% acrylamide SDS‐PAGE gels. Skinned fibre experiment: Active and passive force‐length relationships were established by stretching skinned frog TA fibres (n=10) to various sarcomere lengths (2.4, 2.6, 2.8, 3.0, 3.2, 3.4 and 3.6 μm), and isometrically activating them after steady state force had been reached. RESULTS The frog TA expresses two titin isoforms similar to those expressed by rabbit psoas muscle (3.3 and 3.40 MD). The active and passive force‐length relationships in TA skinned fibres indicate that passive force is significantly lower than active force on the descending limb of the force‐length relationship ( Figure 2). DISCUSSION AND CONCLUSIONS Frog TA titin isoforms are not different from those of rabbit psoas, which are known to have relatively low passive [3, 4] compared to active force. Single skinned fibres isolated from frog TA muscle do not produce high passive compared to active forces. Therefore, it seems that titin is not responsible for the high passive force observed at the whole frog TA muscle level. It is likely that the high passive forces produced by the frog TA muscle originate from the extracellular matrix rather than structures within the fibres. The high passive forces found for the entire frog TA muscle might have implications for the function of the muscle during swimming. Since frogs have large webbed feet, a high passive force in the TA allows frogs to use their feet like human swimmers use passive fins to increase the speed and decrease the energy expenditure of swimming. Support or Funding Information CIHR, NSERC, AIHS