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 mm and 2.95 mm, and subsequently stretched or shortened (5%SL at 1.0 SL.s21) to reach a final SL of 2.80 mm. 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 = 0.2260.02 Po; L1 = 5.2660.24 nm.HS.1) than in pCa2+4.5 (P1 = 0.1560.01 Po; L1 = 4.4860.25 nm.HS.1). L1 was also increased by MgADP activation during shortening. Force enhancement after stretch was lower in pC