Metamorphosis of the American eel, Anguilla rostrata LeSeur: III. Contractile characteristics of skeletal muscle.

Previous work has suggested a common migratory strategy among fishes may involve changes in recruitment pattern of skeletal muscle types, allowing fast muscle to function continuously. In this study no evidence was found for changes in adenine nucleotide metabolism, thought to be important in fatigued muscle, with metamorphosis from the nonmigratory yellow to migratory silver eel in either slow or fast muscle tissue. Myofibrillar ATPase activity was found to be lower than reported values for other teleosts, around 0.075 and 0.17 microM inorganic phosphate mg-1 min-1 for slow and fast muscle, respectively. No change was found in the Ca++-kinetics of the enzyme within either muscle type. Likewise, no change in the contractile performance of fast muscle was evident, arguing against changes in activity pattern. In contrast to mammalian endurance exercise training where major changes in aerobic capacity occur in fast muscle, migratory pre-adaptation in eel appears to be restricted to changes in slow muscle performance. A displacement of the slow muscle force-velocity curve to the right upon metamorphosis results in 30% increase in the tension developed at maximal power output from 2.4 to 3.2 N cm-2. The difference in migratory potential between yellow and silver eels was shown previously to involve an increased aerobic capacity. The change in contractile characteristics may further improve endurance by permitting a portion of the tissue to periodically replenish endogenous energy stores.