MECHANICAL AND ENERGY CHARACTERISTICS DURING SHORTENING IN ISOLATED TYPE-1 MUSCLE-FIBERS FROM XENOPUS-LAEVIS STUDIED AT MAXIMAL AND SUBMAXIMAL ACTIVATION

The mechanical and energy characteristics of isolated fast-twitch musc le fibres (type 1) of Xenopus laevis in isometric-and isovelocity cont ractions were measured at 20 degrees C. The fibres were stimulated at either 60 Hz or 20 Hz to produce contractions at different levels of a ctivation. The high stimulation frequency gave fused contractions, whi le at the low stimulation frequency tension fluctuated. When maximum i sometric force had been reached, the fibres were shortened by 10% of t he fibre length at different velocities. At 60 Hz stimulation during s hortening the rate of heat production increased above the isometric ra te of heat production. At 20 Hz stimulation during shortening, however , the rate of heat production was not different from the isometric rat e of heat production. Mechanical efficiency was the same at the high a nd low level of activation. The actomyosin efficiency (i.e. the mechan ical efficiency corrected for ''activation heat'') was highest at the low level of activation. We conclude that in fast-twitch muscle fibres from X. laevis, actomyosin efficiency is highest for partially activa ted muscle. From a comparison of the present results with those obtain ed from a study of slow-twitch muscle fibres presented earlier, it is concluded that fast-twitch muscle fibres are less efficient than slow- twitch muscle fibres.