A rapid temperature jump (T-jump) induces a biphasic tension rise to a new steady level in maximally Ca2+-activated skinned fibres (Ranatunga, 1996), and in intact fibres during tetanic contraction (Ranatunga & Coupland, 2002). The aim of this study was to examine the effect of rapid T-jumps during twitch contraction in intact fibre bundles from a mammalian muscle.
The foot muscle, flexor hallucis brevis, was isolated from rats killed with an intraperitoneal injection of an overdose of sodium pentobarbitone (Euthatal, >200 mg (kg body mass)-1). Small bundles of ~5-10 fibres (~2 mm fibre length) were set up for isometric tension recording in a 50 µl trough containing oxygenated Ringer solution. A rapid temperature jump of ~4 °C (see Ranatunga, 1996) was induced in the aqueous medium and the fibre bundle by applying an infrared laser pulse (l{special} = 1.32 µm, 0.2 ms duration) at different times during twitch contraction.
Preliminary data (means ± S.E.M.) obtained at a steady temperature of 10 °C showed that the effect of a T-jump was dependent on when it was applied relative to the twitch time course. Applied during the rising phase, a T-jump led to a transient increase of force (31 ± 2 %, n = 6) compared to the steady temperature control and the rate of tension relaxation was increased. The force increase was less marked (~4 %) when a T-jump was placed near the peak of the twitch. Little if any tension change was induced by T-jumps on the relaxation phase. Similar observations were made in three experiments at 25 °C, but the force increase induced by the T-jump was less marked. Interestingly, there was a transient increase (16 ± 1 %, n = 6) of force induced by a T-jump at 10 °C compared to control twitches at the elevated temperature.
Our present observations indicate that an early step in excitation-contraction coupling is perturbed by a T-jump. It is of interest to note that previous studies showed that twitch tension is also potentiated by high pressure (Vawda et al. 1997) and by stretch (Mutungi & Ranatunga, 2001).
We thank the Wellcome Trust for financial support.