Sub-maximally Activated Rat Soleus Fibers Exhibit Stretch Activation

Abstract

Stretch activation occurs when a muscle develops force after an imposed stretch. The effect is large in some types of insect flight muscle and is also thought to contribute to cardiac function. The potential role of stretch activation in mammalian skeletal muscle is less clear. We activated chemically permeabilized rat soleus fibers in solutions with pCa values (=log10[Ca2+]) ranging from 9.0 to 4.5 and imposed ramp and hold length changes of different sizes (0.002 to 0.07 muscle lengths) and different stretch velocities (0.07 to 1.40 muscle lengths per second). All experiments were performed at 22C. When fibers were stretched in pCa 9.0 solution, force increased linearly during the stretch and decayed during the hold portion of the trial. This is consistent with the behavior of a simple visco-elastic system. Fibers immersed in solutions with pCa values between 6.6 and 6.0 exhibited more complex properties. During the stretch, the force response was biphasic due to the muscle's short-range stiffness. Immediately after the stretch, force dropped transiently, and then rose to a peak value of up to 130% of the pre-stretch isometric force. This is stretch activation. The peak stretch activated force increased with the magnitude of the stretch but was not markedly changed by stretch velocity. Fibers immersed in solutions with pCa values less than 6.0 exhibited a short-range stiffness but force declined during the hold portion of the trial and did not exhibit a delayed activation. Our measurements demonstrate that mammalian skeletal muscle can exhibit prominent stretch activation in solutions with sub-maximal calcium concentrations. To investigate the mechanisms responsible for stretch activation, we ran preliminary simulations using the MyoSim model of dynamically coupled myofilaments. These calculations suggest that force-dependent recruitment from the myosin OFF state may contribute to stretch activation.