Hpj. Buschman et al., ENERGETICS OF SHORTENING DEPEND ON STIMULATION FREQUENCY IN SINGLE MUSCLE-FIBERS FROM XENOPUS-LAEVIS AT 20-DEGREES-C, Pflugers Archiv, 430(2), 1995, pp. 160-167
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).