The Effects of Ca2+ and MgADP on Force Development during and after Muscle Length Changes

The goal of this study was to compare the effects of Ca2+ and MgADP activation on force development in skeletal muscles during and after imposed length changes. Single fibres dissected from the rabbit psoas were (i) activated in pCa2+4.5 and pCa2+6.0, or (ii) activated in pCa2+4.5 before and after administration of 10 mM MgADP. Fibres were activated in sarcomere lengths (SL) of 2.65 µm and 2.95 µm, and subsequently stretched or shortened (5%SL at 1.0 SL.s−1) to reach a final SL of 2.80 µm. The kinetics of force during stretch were not altered by pCa2+ or MgADP, but the fast change in the slope of force development (P1) observed during shortening and the corresponding SL extension required to reach the change (L1) were higher in pCa2+6.0 (P1 = 0.22±0.02 Po; L1 = 5.26±0.24 nm.HS.1) than in pCa2+4.5 (P1 = 0.15±0.01 Po; L1 = 4.48±0.25 nm.HS.1). L1 was also increased by MgADP activation during shortening. Force enhancement after stretch was lower in pCa2+4.5 (14.9±5.4%) than in pCa2+6.0 (38.8±7.5%), while force depression after shortening was similar in both Ca2+ concentrations. The stiffness accompanied the force behavior after length changes in all situations. MgADP did not affect the force behavior after length changes, and stiffness did not accompany the changes in force development after stretch. Altogether, these results suggest that the mechanisms of force generation during and after stretch are different from those obtained during and after shortening.