Force Transients during Shortening of Activated Skeletal Muscle Fibers

Abstract

During a ramp shortening, the force response of an active muscle shows an early change in slope (P1 transition) followed by a gradual change in slope (P2 transition), that happen at given sarcomere length (SL) extension (SL1 and SL2, respectively). In this study, we investigated the mechanisms of forces produced during an imposed shortening to active muscles, by altering either the number of crossbridges attached to actin or the crossbridge state prior to shortening. Three sets of experiments were performed with permeabilized psoas muscle fibers: (i) fibers were activated and shortened by 10% Lo, at 1.0 Lo•s−1 in different pCa2+ (4.5, 5.0, 5.5, 6.0), (ii) fibers were activated and shortened by 10% Lo at 1.0 Lo•s−1 in pCa2+ 4.5 containing either 5µM of blebbistatin (+/-) or its inactive isomer (+/+), or (iii) fibers were activated then shotened by 10% at speeds ranging from 0.125 to 2.0 Lo•s−1 in pCa2+ 4.5 or pCa2+ 6.0 and 5µM blebbistatin (+/-). All shortenings started at a SL 2.5µm. P1, P2 SL1 and SL2 were not affected by different Ca2+ concentrations; the pooled experiments provided P1 and P2 of 0.79 + 0.003 and 0.27 + 0.01 times Po, respectively, and SL1 and SL2 of 4.62 + 0.16 and 24.17+ 0.20 nm•HS−1 respectively. P1 and P2 diminished, and SL1 and SL2 augmented with increasing shortening velocity. Blebbistatin (+/-) decreased P1 (0.74 ± 0.04 times Po) when compared to control fibers (0.81 ± 0.02 times Po), but it did not change the P2. Blebbistatin did not change SL1 and SL2. The results suggest that pre-powerstroke crossbridges contributes to changes in P1, as they may be pushed into the powerstroke during shortening. Such crossbridges detach from actin at the same SL extension.