THE EFFECTS OF REPEATED ACTIVE STRETCHES ON TENSION GENERATION AND MYOPLASMIC CALCIUM IN FROG SINGLE MUSCLE-FIBERS

1. A series of contractions with stretches (eccentric contractions) be yond the optimal length for tension generation (optimum) were shown to induce a shift in that optimum in single muscle fibres of frog, as ha s been previously reported for whole muscles. Shifts averaging 0 . 129 mu m (sarcomere)(-4) 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-sen sitive fluorescent dyes indicated that such contractions did not reduc e the amplitude of the intracellular calcium transient, but did increa se its duration. A rise in resting [Ca2+] was found to accompany damag e, but not necessarily the shift in optimum. 4. The twitch potentiator nitrate was shown to increase myoplasmic [Ca2+] during twitch and tet ani, but not to reverse the shift in optimum length due to eccentric c ontractions. Both eccentric contractions and twitch potentiation reduc ed the maximum stimulation rate to which a fibre could respond with pr opagated action potentials. 5. These results exclude reduced myoplasmi c [Ca2+] as the cause of the shift in optimum length in this preparati on.