1. A series of contractions with stretches (eccentric contractions) beyond the optimal length for tension generation (optimum) were shown to induce a shift in that optimum in single muscle fibres of frog, as has been previously reported for whole muscles. Shifts averaging 0.129 micron (sarcomere)‐1 or 6% were found, without apparent damage to the fibre. 2. The stiffness of fibres was found to fall during a stretch, even though tension was rising. In addition, the isometric stiffness fell as a result of a series of eccentric contractions. 3. Calcium‐sensitive fluorescent dyes indicated that such contractions did not reduce the amplitude of the intracellular calcium transient, but did increase its duration. A rise in resting (Ca2+) was found to accompany damage, but not necessarily the shift in optimum. 4. The twitch potentiator nitrate was shown to increase myoplasmic (Ca2+) during twitch and tetani, but not to reverse the shift in optimum length due to eccentric contractions. Both eccentric contractions and twitch potentiation reduced the maximum stimulation rate to which a fibre could respond with propagated action potentials. 5. These results exclude reduced myoplasmic (Ca2+) as the cause of the shift in optimum length in this preparation.
The Journal of Physiology – Wiley
Published: Dec 15, 1996
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