ENERGETICS OF SHORTENING DEPEND ON STIMULATION FREQUENCY IN SINGLE MUSCLE-FIBERS FROM XENOPUS-LAEVIS AT 20-DEGREES-C

Single intact slow-twitch (type 3) muscle fibres from the iliofibulari s muscle of Xenopus laevis were shortened at a constant velocity (0.4 L(0)/s, where L(0) is the initial length at different levels of activa tion (40, 15, 12.5, and 10 Hz). A stimulation frequency of 40 Hz gave fused tetanic records. At this frequency the mean heat production rate during shortening (0.38 +/- 0.05 W/g dry weight) was slightly higher than the isometric heat production rate (0.33 +/- 0.03 W/g dry weight) . The lower stimulation frequencies gave unfused tetanic contractions, the average isometric force of which was 40 +/- 3% of the isometric f orce at 40 Hz. In these unfused tetani during shortening the heat prod uction rate (0.18 +/- 0.02 W/g dry weight) significantly decreased bel ow the isometric heat production rate (0.25 +/- 0.02 W/g dry weight). At full activation the rate of total energy production (mechanical pow er plus heat production rate) during shortening was 1.88 +/- 0.32 time s the isometric total energy production rate. This effect. i.e. an inc rease in energy turnover with shortening, is known as the Fenn effect. At sub-maximal stimulation the energy output during shortening was on ly 1.07 +/- 0.08 times the isometric value. These results show that th e Fenn effect is dependent on the level of activation. The efficiency (ratio of mechanical power to total energy output) was independent of the stimulation frequency (0.37 +/- 0.06).