The Effect of Low-Level Activation on the Mechanical Properties of Isolated Frog Muscle Fibers

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Abstract
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/cm(2) (mean +/- SD) at 2.2 microm sarcomere length.(1) The slope of the tension curve (DeltaP/DeltaL) 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 DeltaP/DeltaL 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 DeltaP/DeltaL 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 DeltaP/DeltaL. 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 DeltaP/DeltaL. The results suggest that stiffness and tension development are not related in a simple way.
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