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SKELETAL MUSCLE AND EXERCISE |
1 Muscle Contraction Group, Department of Physiology, School of Medical Sciences, University of Bristol, Bristol BS8 1TD, UK
We examined the tension change induced by a rapid temperature jump (T-jump) in shortening and lengthening active muscle fibres. Experiments were done on segments of permeabilized single fibres (length (L0) 
2 mm, sarcomere length 2.5 µm) from rabbit psoas muscle; [MgATP] was 4.6 mM, pH 7.1, ionic strength 200 mM and temperature 9°C. A fibre was maximally Ca2+-activated in the isometric state and a 3°C, rapid (< 0.2 ms), laser T-jump applied when the tension was approximately steady in the isometric state, or during ramp shortening or ramp lengthening at a limited range of velocities (0–0.2 L0 s–1). The tension increased to 2- to 3 x P0 (isometric force) during ramp lengthening at velocities > 0.05 L0 s–1, whereas the tension decreased to about < 0.5 x P0 during shortening at 0.1–0.2 L0 s–1; the unloaded shortening velocity was 1 L0 s–1 and the curvature of the force–shortening velocity relation was high (a/P0 ratio from Hill's equation of 0.05). In isometric state, a T-jump induced a tension rise of 15–20% to a new steady state; by curve fitting, the tension rise could be resolved into a fast (phase 2b, 40–50 s–1) and a slow (phase 3, 5–10 s–1) exponential component (as previously reported). During steady lengthening, a T-jump induced a small instantaneous drop in tension, followed by recovery, so that the final tension recorded with and without a T-jump was not significantly different; thus, a T-jump did not lead to a net increase of tension. During steady shortening, the T-jump induced a pronounced tension rise and both its amplitude and the rate (from a single exponential fit) increased with shortening velocity; at 0.1–0.2 L0 s–1, the extent of fibre shortening during the T-jump tension rise was estimated to be 1.2% L0 and it was shorter at lower velocities. At a given shortening velocity and over the temperature range of 8–30°C, the rate of T-jump tension rise increased with warming (Q10 
2.7), similar to phase 2b (endothermic force generation) in isometric muscle. Results are discussed in relation to the previous findings in isometric muscle fibres which showed that a T-jump promotes an early step in the crossbridge–ATPase cycle that generates force. In general, the finding that the T-jump effect on active muscle tension is pronounced during shortening, but is depressed/inhibited during lengthening, is consistent with the expectations from the Fenn effect that energy liberation (and acto-myosin ATPase rate) in muscle are increased during shortening and depressed/inhibited during lengthening.
(Received 1 August 2007;
accepted after revision 1 October 2007;
first published online 4 October 2007)
Corresponding author K. W. Ranatunga: Muscle Contraction Group, Department of Physiology, School of Medical Sciences, University of Bristol, Bristol BS8 1TD, UK. Email: k.w.ranatunga{at}bristol.ac.uk
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  B. Colombini, M. Nocella, G. Benelli, G. Cecchi, and M. A. Bagni Effect of temperature on cross-bridge properties in intact frog muscle fibers Am J Physiol Cell Physiol, April 1, 2008; 294(4): C1113 - C1117. [Abstract] [Full Text] [PDF]
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