Limits to shortening in smooth muscle tissues.

The extent of shortening in smooth muscle tissues is limited by a number of internal and external factors. In this study, continuous measurements of the stiffness of active muscle were made to characterize the mechanical forces acting to limit shortening. Rabbit ovarian ligament and mesotubarium superius muscles were allowed to shorten as far as possible under light afterloads; under these conditions a stiffness increase was observed that was closely related to the instantaneous muscle length and that was unaffected by other factors influencing the degree of shortening (afterload, time and intensity of activation, temperature, etc.). The results are considered in terms of a hypothesis relating the tissue-based constraints on radial expansion at short lengths to an additional load on the contractile apparatus, an internal force that is externally manifested as an increase in axial stiffness. Changing the cellular volume by varying the tonicity of the bathing medium provided tentative confirmation of the hypothesis.