Tension responses to joule temperature jump in skinned rabbit muscle fibres.

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Abstract
1. Joule temperature jumps (T‐jumps) from 5‐9 degrees C up to 40 degrees C were used to study the cross‐bridge kinetics and thermodynamics in skinned rabbit muscle fibres. To produce a T‐jump, an alternating current pulse was passed through a fibre 5 s after removing the activating solution (pCa congruent to 4.5) from the experimental trough. The pulse frequency was congruent to 30 kHz, amplitude less than or equal to 3 kV, and duration 0.2 ms. The pulse energy liberated in the fibre was calculated using a special analog circuit and then used for estimation of the T‐jump amplitude. 2. The T‐jump induced a tri‐exponential tension transient. Phases 1 and 2 had rate constants k1 = 450‐1750 s‐1 and k2 = 60‐250 s‐1 respectively, characterizing the tension rise, whereas phase 3 had a rate constant k3 = 5‐10 s‐1 representing tension recovery due to the fibre cooling. 3. An increase from 13 to 40 degrees C for the final temperature achieved by the T‐jump led to an increase in the amplitudes of phases 1 and 2. After T‐jumps to 30‐40 degrees C during phase 1, tension increased by 50‐80%. During phase 2 an approximately 2‐fold tension increase continued. Rate constants k1 and k2 increased with temperature and temperature coefficients (Q10) were 1.6 and 1.7, respectively. 4. To study which processes in the cross‐bridges are involved in phases 1 and 2, a series of experiments were made where step length changes of ‐9 to +3 nm (hs)‐1 (nanometres per half‐sarcomere length) were applied to the fibre 4 ms before the T‐jump. 5. After the step shortening, the rate constant of phase 1 increased, whereas its amplitude decreased compared to those without a length change. This indicates that phase 1 is determined by some force‐generating process in the cross‐bridges attached to the thin filaments. This process is, most probably, the same as that producing the early tension recovery following the length change. The enthalpy change (delta H) associated with the reaction controlling this process was estimated to be positive (15‐30 kJ mol‐1). 6. Both the rate constant k2 and the maximal tension achieved at the end of phase 2 were practically independent of the preceding length changes. This means that phase 2 is accompanied by the cross‐bridge detachment and reattachment to new sites on the thin filaments.(ABSTRACT TRUNCATED AT 400 WORDS)