TENSION TRANSIENTS INITIATED BY PHOTOGENERATION OF MGADP IN SKINNED SKELETAL-MUSCLE FIBERS

Addition of MgADP to skinned skeletal muscle fibers causes arise in Ca2+-activated isometric tension. Mechanisms underlying this tension increase have been investigated by rapid photogeneration of ADP within skinned single fibers of rabbit psoas muscle. Photolysis of caged ADP (p2-1(2-nitrophenyl)ethyladenosine 5'-diphosphate) resulted in an exponential increase in isometric tension with an apparent rate constant, k(ADP), of 9.6 +/- 0.3 s-1 (mean +/- SE, n = 28) and an amplitude, P(ADP), of 4.9 +/- 0.3% P(o) under standard conditions (0.5 mM photoreleased MgADP, 4 mM MgATP, pH 7.0, pCa 4.5, 0.18 M ionic strength, 15-degrees-C). P(ADP) depended upon the concentration of photoreleased MgADP as well as the concentration of MgATP. A plot of I/P(ADP) vs. 1/[MgADP] at three MgATP concentrations was consistent with competition between MgADP and MgATP for the same site on the crossbridge. The rate of the transient, k(ADP), also depended upon the concentration of MgADP and MgATP. At both 4 and 1 mM MgATP, k(ADP) was not significantly different after photorelease of 0.1-0.5 mM MgADP, but was reduced by 28-40% when 3.5 mM MgADP was added before photorelease of 0.5 mM MgADP. k(ADP) was accelerated by about twofold when MgATP was varied from 0.5 to 8 mM MgATP. These effects of MgATP and MgADP were not readily accounted for by population of high force-producing states resulting from reversal of the ADP dissociation process. Rather, the results suggest that competition between MgADP and MgATP for crossbridges at the end of the cycle slows detachment leading to accumulation of force-generating crossbridges. Elevation of steady-state P(i) concentration from 0.5 to 30 mM caused acceleration of k(ADP) from 10.2 +/- 0.5 to 27.8 +/- 1.8 s-1, indicating that the tension rise involved crossbridge flux through the P(i) dissociation step of the cycle.