Fig. 1

Effects of MgADP or Pi on the rate of rise of active force in rabbit psoas muscle fibers; pCa 4.5, SL 2.0 μm. a Typical chart recordings showing active force responses in the absence and presence of MgADP (top) or Pi (bottom). Data obtained from the same preparation for either MgADP or Pi. Black arrowheads and double arrowheads indicate the points at which the solution was switched from low-EGTA (0.5 mM) relaxation to contraction and from contraction to high-EGTA (10 mM) relaxation, respectively. Because of a change in surface tension, a hump was caused upon solution switch from low-EGTA (0.5 mM) relaxation to contraction and from contraction to high-EGTA (10 mM) relaxation. The time to half-maximal activation (50%) was measured as indicated by α and β, and the relative value, i.e., β/α, was obtained for each preparation and defined as t _1/2 [8]. Red arrow in each record, 50% of maximal force. b Graph summarizing the effects of various concentrations of MgADP (left) or Pi (right) on t _1/2. *P < 0.05 compared with 0 mM MgADP or Pi. Note that t _1/2 is ~1.0 in the absence of MgADP or Pi, indicating reproducibility of the rate of rise of active force (as in [8]); n = 6 for both MgADP and Pi