Isotonic contraction of temp-step activated muscle fibers with varied tonicity: effects of cell volume and the degree of activation.

These studies on intact fibers describe the effects of calcium, ionic strength and volume on the contraction properties. The results provide firm evidence that cell volume affects the speed but not the force. On the other hand, sarcoplasmic ionic strength affects the force development, with no effect on unloaded speed of shortening. These results suggest that there are essential differences in the rate limiting steps for isometric and isotonic properties of the cross-bridge mechanism. The studies at various degrees of activation indicate that Ca acts as a simple "on-off" switch for cross-bridge activation, in intact fibers.