Hypertonicity and force development in frog skeletal muscle fibres

Mechanical experiments were performed on isolated fibre bundles of frog semitendinosus muscle, using solutions made hypertonic by the addition of sucrose or NaC1. Muscle stiffness was measured with 1-ms step changes of length, from the plateau of an isometric tetanus. Within 20 min in hypertonic Ringer, isometric tetanus tension fell by 50% or more. The rate of tension redevelopment following stepwise shortening was reduced in hypertonic solution. Point voltage clamp studies showed that even when isometric tension had fallen to this extent, the stimulus strength--duration relation for mechanical activation was identical to that for controls. Point voltage clamp was used to intermittently repolarize fibres depolarized in solutions of high ionic strength, reactivating the excitation contraction coupling mechanism. Subsequent depolarization of the fibres allowed a visual test to be made of the fibres' ability to contract. The period during which contraction could be seen grew shorter as the ionic strength of the bathing medium was raised. The results of these different types of mechanical experiments suggested that the increased intracellular ionic strength caused by osmotic shrinkage in hypertonic Ringer's solution slows the rate at which individual cross bridges can form and develop tension, as well as reducing the maximum amount of tension they can generate. Muscle stiffness did not remain at control level in hypertonic solution; nor did it fall as much as did developed tension. The most obvious, although not the only, interpretation of this observation was that cross bridges are less stretched when isometric force is reduced and the length-tension relation of the elastic element is nonlinear.