Study on muscle fiber composition and biochemical properties of the adductor mandibulae in fishes showing different feeding activities

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Lactate dehydrogenase (LDH) isozyme distributions in the jaw-closing and -opening muscles, myotome muscle, heart, and eye of the parr-type and smolt-type fish in a primitive teleostean, Oncorhynchus masou (O.masou),which have adapted to the rapid current at the upper reach of a river and in the mild current at the middle reach,respectively, were investigated by horizontal agarose gel electrophoresis. Both jaw muscles of the parr-type fish had following three isozymes: the slowest anodal migrative LDHA_4, the fastest anodal migrative LDHB_4, and an isozyme migrating between them. The major isozyme was LDHA_4, amounting over 80% of total LDH activity. The white portion of the myotome muscle which is the dominant part and contracts fast and forcefully, and the heart muscle which contracts slowly and continuously had only one isozyme LDHA_4 or LDHB_4, respectively. The red portion of the myotome muscle had three isozymes, though LDHB_4 one was predominant. These facts suggest that jaw-clos … More ing and-opening muscles and myotome muscle are well adapted for swift feeding behavior in the rapid current, because forceful and rapid contractions of those muscles would be needed to catch the insects floating down in the rapid current. With growth of length of the fish, LDHB_4 isozyme activity significantly increased in the jaw-closing muscle, but not in the jaw-opening muscle. Furthermore, the smolt-type fish which lives in open-habitats of rivers and lakes and feed mainly on other species of fish, had a higher level of LDHB_4 isozyme in the jaw-closing muscle than the parr-type fish.Thus, the higher levels of LDHB_4 isozyme in the jaw-closing muscles of the larger parr-type fish and the smolt-type fish, and appearance of LDHB_4 isozyme in the white portion of the myotome muscle of the smolt-type fish, suggest that the function of the muscles may change with alternation of food habits and locomotion manners respectively, in this species. Moreover, these data may be very important for explanation of the physiological meaning of each type of isozyme of LDH in the muscles. Less