J Physiol 585.1 (2007) pp 263–277 263 Force generation examined by laser temperature-jumps in shortening and lengthening mammalian (rabbit psoas) muscle ﬁbres K. W. Ranatunga, M. E. Coupland, G. J. Pinniger, H. Roots and G. W. Offer 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 × P0 (isometric force) during ramp lengthening at velocities > 0.05 L0 s−1, whereas the tension decreased to about < 0.5 × 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. (Resubmitted 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@bristol.ac.uk The active force in isometric muscle is temperature sensitive (Hadju, 1951) so that, in mammalian muscle, it increases ∼2-fold when the temperature is raised from ∼10◦C to physiological (> 30◦C) temperatures (Ranatunga & Wylie, 1983; Ranatunga, 1994). Consequently, a rapid temperature jump (T-jump) induces a rise in force to a level as expected from the steady state experiments, in skinned and intact muscle ﬁbres (Ranatunga, 1996; Coupland & Ranatunga, 2003). The T-jump force-rise is bi-exponential (labelled phase 2b and phase 3), where the (faster) phase 2b is identiﬁed as ‘endothermic force generation’ in attached cross- bridges (Davis & Harrington, 1987; Goldman et al. 1987; Bershitsky & Tsaturyan, 1989, 1992; Ranatunga, 1996). Phase 2b has been compared with a slow component of the (phase 2) quick tension recovery (Huxley & Simmons, 1971) obtained in length-release experiments (Davis & Harrington, 1993; Davis & Rodgers, 1995), C 2007 The Authors. Journal compilation C 2007 The Physiological Society DOI: 10.1113/jphysiol.2007.142208