Martin H, Barsotti R J
Bockus Research Institute, Graduate Hospital, Philadelphia, Pennsylvania 19146.
Biophys J. 1994 Nov;67(5):1933-41. doi: 10.1016/S0006-3495(94)80676-9.
The kinetics of force production in chemically skinned trabeculae from the guinea pig were studied by laser photolysis of caged ATP in the presence of Ca2+. Preincubation of the tissue during rigor with the enzyme apyrase was used to reduce the population of MgADP-bound cross-bridges (Martin and Barsotti, 1994). In untreated tissue, tension remained constant or dipped slightly below the rigor level immediately after ATP release, before increasing to the maximum measured in pCa 4.5 and 5 mM MgATP. The in-phase component stiffness, which is a measure of cross-bridge attachment, exhibited a large decrease before increasing to 55% of that measured in rigor. Neither the rate of the decline nor of the rise in tension was sensitive to the concentration of photolytically released ATP. The rate of the decline in stiffness was found to be dependent on [ATP]: 1.8 x 10(4) M-1/s-1, a value more than four times higher than that previously measured in similar experiments in the absence of Ca2+. The rate of tension development averaged 14.9 +/- 2.5 s-1. Preincubation with apyrase altered the mechanical characteristics of the early phase of the contraction. The rate and amplitude of the initial drop in both tension and stiffness after caged ATP photolysis increased and became dependent on [ATP]. The second-order rate constants measured for the initial drop in tension and stiffness were 8.4 x 10(4) M-1 s-1 and 1.5 x 10(5) M-1 s-1. These rates are more than two times faster than those previously measured in the absence of Ca2+. The effects of apyrase incubation on the time course of tension and stiffness were consistent with the hypothesis that during rigor, skinned trabeculae retain a significant population of MgADP-bound cross-bridges. These in turn act to attenuate the initial drop in tension after caged ATP photolysis and slow the apparent rate of rigor cross-bridge detachment. The results also show that Ca2+ increases the rate of cross-bridge detachment in both untreated and apyrase-treated tissue, but the effect is larger in untreated tissue. This suggests that in cardiac muscle Ca2+ modulates the rate of cross-bridge detachment.
通过在钙离子存在的情况下对笼锁ATP进行激光光解，研究了豚鼠化学去皮小梁中力产生的动力学。在强直期用腺苷三磷酸双磷酸酶对组织进行预孵育，以减少与MgADP结合的横桥数量（Martin和Barsotti，1994）。在未处理的组织中，ATP释放后张力立即保持恒定或略低于强直水平，然后增加到在pCa 4.5和5 mM MgATP条件下测量的最大值。同相分量刚度是横桥附着的一种度量，在增加到强直时测量值的55%之前出现大幅下降。张力下降和上升的速率对光解释放的ATP浓度均不敏感。发现刚度下降速率取决于[ATP]：1.8×10⁴ M⁻¹/s⁻¹，该值比之前在无钙离子的类似实验中测量的值高出四倍多。张力发展速率平均为14.9±2.5 s⁻¹。用腺苷三磷酸双磷酸酶预孵育改变了收缩早期的力学特性。笼锁ATP光解后张力和刚度的初始下降速率和幅度增加，并变得取决于[ATP]。测量得到的张力和刚度初始下降的二级速率常数分别为8.4×10⁴ M⁻¹ s⁻¹和1.5×10⁵ M⁻¹ s⁻¹。这些速率比之前在无钙离子的情况下测量的速率快两倍多。腺苷三磷酸双磷酸酶孵育对张力和刚度时间进程的影响与以下假设一致：在强直期，去皮小梁保留了大量与MgADP结合的横桥。这些横桥反过来作用于减弱笼锁ATP光解后张力的初始下降，并减缓强直横桥解离的表观速率。结果还表明，钙离子增加了未处理和经腺苷三磷酸双磷酸酶处理的组织中横桥解离的速率，但在未处理的组织中作用更大。这表明在心肌中钙离子调节横桥解离的速率。
