Lu Z, Moss R L, Walker J W
Department of Physiology, University of Wisconsin, Madison 53706.
J Gen Physiol. 1993 Jun;101(6):867-88. doi: 10.1085/jgp.101.6.867.
Addition of MgADP to skinned skeletal muscle fibers causes a rise in Ca(2+)-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, kADP, of 9.6 +/- 0.3 s-1 (mean +/- SE, n = 28) and an amplitude, PADP, of 4.9 +/- 0.3% Po 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). PADP depended upon the concentration of photoreleased MgADP as well as the concentration of MgATP. A plot of 1/PADP 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, kADP, also depended upon the concentration of MgADP and MgATP. At both 4 and 1 mM MgATP, kADP 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. kADP 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 Pi concentration from 0.5 to 30 mM caused acceleration of kADP from 10.2 +/- 0.5 to 27.8 +/- 1.8 s-1, indicating that the tension rise involved crossbridge flux through the Pi dissociation step of the cycle.
向去膜骨骼肌纤维中添加MgADP会导致Ca(2+)激活的等长张力升高。通过在兔腰大肌去膜单纤维内快速光生成ADP，对这种张力增加的潜在机制进行了研究。笼形ADP（P2-1(2-硝基苯基)乙基腺苷5'-二磷酸）的光解导致等长张力呈指数增加，在标准条件下（0.5 mM光释放的MgADP、4 mM MgATP、pH 7.0、pCa 4.5、0.18 M离子强度、15℃），表观速率常数kADP为9.6±0.3 s-1（平均值±标准误，n = 28），幅度PADP为4.9±0.3%Po。PADP取决于光释放的MgADP浓度以及MgATP浓度。在三种MgATP浓度下绘制的1/PADP对1/[MgADP]的曲线与MgADP和MgATP在横桥上的同一位点竞争一致。瞬态速率kADP也取决于MgADP和MgATP的浓度。在4 mM和1 mM MgATP时，在光释放0.1 - 0.5 mM MgADP后，kADP没有显著差异，但在光释放0.5 mM MgADP之前添加3.5 mM MgADP时，kADP降低了28 - 40%。当MgATP从0.5 mM变化到8 mM时，kADP加速了约两倍。MgATP和MgADP的这些作用不能轻易地用ADP解离过程逆转导致的高力产生状态的群体来解释。相反，结果表明在循环结束时MgADP和MgATP在横桥上的竞争减缓了解离，导致产生力的横桥积累。将稳态Pi浓度从0.5 mM提高到30 mM导致kADP从10.2±0.5加速到27.8±1.8 s-1，表明张力升高涉及通过循环中Pi解离步骤的横桥通量。
