264 K. W. Ranatunga and others J Physiol 585.1 but their exact correspondence, and the structural basis of the T-jump force response, remain unresolved (see Bershitsky & Tsaturyan, 2002; Coupland et al. 2005; Ferenczi et al. 2005). In isometric muscle ﬁbres, the T-jump force generation has been identiﬁed as a molecular step before the release of inorganic phosphate (Pi) by cycling crossbridges, i.e. a transition between two AM.ADP.Pi states (Ranatunga, 1999). Interestingly, other studies on muscle ﬁbres (Fortune et al. 1991; Kawai & Halvorson, 1991; Dantzig et al. 1992; Ranatunga et al. 2002) and on myoﬁbrils (Tesi et al. 2000) and also mechano-kinetic modelling (Smith & Sleep, 2004) also have led to a similar conclusion that the force generation occurs prior to Pi release; effects of increased [MgADP] on muscle force could also be accommodated on such a scheme (Coupland et al. 2005). Apart from a brief report on shortening muscle ﬁbres (Bershitsky & Tsaturyan, 1990), all the T-jump studies summarized above have been on isometric muscle and there have been no studies in lengthening muscle. It is well known that the force that a muscle develops varies with the velocity of ﬁlament sliding, i.e. during muscle shortening and lengthening. With an increase of shortening velocity, force declines below isometric force (P0) and reaches zero at the maximum velocity (V max); conversely, an active muscle develops ∼2 × P0 as lengthening velocity is increased to 1–2 L0 (muscle ﬁbre length) s−1 (Katz, 1939; Lombardi & Piazzesi, 1990). Moreover, the energy production and the acto-myosin ATPase rate in muscle are increased with shortening and decreased with lengthening, a cardinal principle of muscle contraction that is commonly referred to as the Fenn effect (Fenn, 1924; Curtin & Davies, 1973; He et al. 1999; Linari et al. 2003b). On the basis of the above, it would be of interest to examine the T-jump-induced force generation in shortening and lengthening muscle ﬁbres; this was the aim of our study. Thus, we have examined in maximally activated single psoas ﬁbres the force responses induced by a small, rapid (∼3◦C in < 0.2 ms) T-jump at 8–9◦C when the ﬁbre is shortening or lengthening at different velocities. Our results show that, compared with that in isometric muscle, the T-jump force rise is enhanced during shortening and depressed during lengthening, basically demonstrating the Fenn effect. Preliminary data from this study have been reported to the European Muscle Conference and published in abstract form (Ranatunga et al. 2006). Methods Fibre preparation and buffer solutions Adult male rabbits were killed by an intravenous injection of an overdose of sodium pentobarbitone and other tissues were harvested for different experiments by other researchers. Fibre bundles from the psoas muscle were prepared and chemically skinned using 0.5% Brij 58, as previously described (Fortune et al. 1989). The buffer solutions contained 10 mm glycerol-2-phosphate (a temperature-insensitive pH buffer; pH = 7.1), 4.6 mm MgATP, 12 mm creatine phosphate, 15 mm EGTA (relaxing solution), CaEGTA (activating solution) or HDTA (pre-activating solution), 9 mm glutathione and ∼50 mm potassium acetate. The solution compositions were calculated using a computer program (provided for us by Professor Mike Ferenczi, Imperial College, London) for solving multi-equilibria, maintaining ionic strength (at 200 mm) and a free [Ca2+] in the activating solutions of ∼0.032 mm. Creatine kinase (1–2 mg ml−1; ∼300 units mg−1, Sigma) was added to buffer solutions before use to decrease [MgADP]. All solutions also contained 4% Dextran (mol. mass ∼500 kDa) in order to compress the ﬁlament lattice spacing to normal dimensions (Maughan & Godt, 1979). Apparatus and laser T-jump technique Details of the trough assembly, design of the force transducer (natural resonant frequency, 14 kHz) and other aspects of the experimental apparatus have been described in detail previously (Ranatunga, 1996, 1999; Coupland et al. 2001). Brieﬂy, the trough assembly was mounted on the stage of an optical microscope and the ﬁbre could be moved and immersed in the different trough solutions (relaxing, pre-activating and activating solutions), by means of a lever mechanism. Using thermo-electric modules (Peltier units) ﬁxed on the back wall, and with feedback from a small thermistor, the solution temperature in the front experimental trough could be changed and/or clamped at a desired pre-T-jump temperature and was monitored with a thermocouple. In order to maintain ﬁbre stability, the temperature in the other troughs containing relaxing and pre-activating solutions was kept below 5◦C by cooling ﬂuid circulating through the trough assembly. The details of the technique of laser pulse-induced T-jumps in muscle ﬁbres have been previously described (Ranatunga, 1996, 1999). In most of the present experiments, the temperature of the front experimental trough was clamped at 8–9◦C and a T-jump was induced by a 0.2 ms laser pulse radiation (λ = 1.32 μm) from a Nd–YAG laser (Schwartz Electro-Optics) that entered the experimental trough through the front glass window. A laser pulse heated the buffer solution in the trough and the muscle ﬁbre bathed in it; the heated ﬁbre/heated ﬂuid volume ratio was > 2500. A ﬁne thermocouple placed near the ﬁbre showed that, after a laser pulse, the elevated solution temperature remained constant for at least ∼500 ms and decreased only slowly (half-time longer than 5 s) afterwards (see Fig. 1 of Ranatunga, 1996). C 2007 The Authors. Journal compilation C 2007 The Physiological Society