Tension, stiffness, unloaded shortening speed and potentiation of frog muscle fibres at sarcomere lengths below optimum.

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Abstract
Unloaded shortening velocity, stiffness and the effects of potentiators were studied to understand the basis for the shallow ascending limb (1.65-2.0 .mu.m sarcomere length) of the sarcomere length-tension diagram of frog single fibers. The velocity of externally unloaded shortening was constant over most of this range. It is therefore unlikely that this part of the sarcomere length-tension diagram results from an internal force opposing shortening. Stiffness did not vary in proportion with tension between sarcomere lengths of 1.65 and 2.0 .mu.m, nor was it constant between 2.0 and 2.2 .mu.m, where tension is constant. By assuming a small filament compliance, the observations could be adequately modeled on the hypothesis that the variation in tension in the range of sarcomere lengths of 1.65-2.0 .mu.m was caused by variations in the number of attached crossbridges. The twitch potentiators Zn2+, tetraethylammonium (TEA), nitrate and caffeine did not change the shape of the sarcomere length-tension diagram. Potentiation in a tetanus was < 3% in all experiments. Contractures induced by raised [K+] in the bathing solution produced more tension than a tetanus beyond optimum length, insignificantly different tension near optimum length and less tension at sarcomere lengths near 1.7 .mu.m. An explanation is proposed for these results in terms of inhomogeneous activation and internal motion. There is no evidence from this work that a tetanized fiber is other than maximally activated over the range of sarcomere lengths spanned by the shallow ascending limb.