Effects of Ca2+ Concentration and MgADP on the Forces Produced After Stretch and Shortening of Skeletal Muscle Fibers

Abstract

The force produced by skeletal muscles increased after stretch and decreased after shortening, when compared to isometric contractions developed at corresponding lengths. The aim of this study was to compare the effects of Ca2+-induced activation and MgADP-induced activation on the force produced after stretching or shortening. MgADP was used to assess if cross-bridges play a role in force changes, as it induces strong cross-bridge attachment to the thin filament. Two sets of experiments were performed with permeabilized fibres isolated from the psoas muscle: (i) fibers were activated at pCa2+ of 4.5 and 6.0, and (ii) fibres were activated at pCa2+ 4.5 before and after administration of 5 mM MgADP. In all experiments, fibres were initially activated to produce isometric contractions at nominal sarcomere lengths (SL) of 2.7, 2.85 and 2.6 μm. When fibers were activated at 2.6 and 2.85 μm (after full force development was obtained) they were stretched or shortened (5%SL at a speed of 1.0 SL•s−1), to reach a final SL of 2.7μm. In all conditions, the force after stretch was higher than the isometric force at a corresponding length. The level of force enhancement was lower at pCa2+ 4.5 (14.9 + 5.4%) than at pCa2+ 6.0 (38.8 + 7.5%). Force did not decrease significantly after shortening at pCa 4.5, but it decreased by 17.2 + 5.6% at pCa2+ 6.0. The levels of force enhancement and force depression were not changed when fibres were activated with MgADP, suggesting that Ca2+ concentration modulates the forces produced after length changes by mechanisms that are independent of cross-bridges activation of the thin filament.