FORCE AND MYOSIN CONTENT VARIATION IN ISOLATED INTACT SINGLE MUSCLE-FIBERS FROM RANA-TEMPORARIA

We studied the relation between force normalized by dry mass per unit length and the myosin fraction of muscle dry mass, The two tibialis an terior muscles were dissected from 12 frogs (Rana temporaria). Then. f rom one muscle, two single fast-twitch fibres were isolated. Each fibr e was mounted isometrically in Ringer's solution, and electrically sti mulated using a standardized protocol, Peak force production, normaliz ed by the fibre's dry mass per unit length, varied by a factor of 1.4. Little variation in normalized force was measured between Fibres from the same animal, whereas between animals a significant difference was found (P<0.05), The contralateral muscle was used to determine the my osin fraction of the dry mass. The relationship between the fraction m yosin of the dry mass and force normalized by dry mass per unit length showed a high correlation (r = 0.81; n = 12). From this we conclude t hat variation in normalized tetanic force is determined greatly (65%) by variations in myosin content.