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¹ From the Nobel Institute for Neurophysiology, Karolinska Institutet, S-104 01 Stockholm 60, Sweden

The mechanical properties, as revealed by minute length changes, of isolated twitch fibers of the frog have been studied at rest and during low-level activation. Resting tension is 77 ± 23 mN/cm² (mean ± SD) at 2.2 µm sarcomere length.¹ The slope of the tension curve (P/L) recorded during a constant-speed length change of a resting fiber is initially large. At length changes exceeding about 0.18 % of the initial length of the fiber P/L falls abruptly and remains close to zero during the rest of the length change. The amplitude of the tension response is reduced after a length change and returns to normal in about 3 min. Hypertonic sucrose-Ringer solutions cause a small, maintained rise in tension up to 1.4–1.6 times normal osmotic strength. Higher sucrose concentrations cause relatively large, transient tension responses. The initial P/L is increased in moderately hypertonic solutions; it may be reduced in more strongly hypertonic solutions. Elevated [K]_o (range 10–17.5 mM) causes a marked reduction in P/L. In this range of [K]_o the reduction is not accompanied by changes in resting tension. Addition of 1–1.5 mM caffeine to the Ringer solution affects the resting tension very little but also reduces P/L. The results suggest that stiffness and tension development are not related in a simple way.

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