Tesi C, Colomo F, Piroddi N, Poggesi C
Dipartimento di Scienze Fisiologiche, Università degli Studi di Firenze, Viale GB, Morgagni 63, I-50134 Firenze, Italy.
J Physiol. 2002 May 15;541(Pt 1):187-99. doi: 10.1113/jphysiol.2001.013418.
The inhibitory effects of inorganic phosphate (P(i)) on isometric force in striated muscle suggest that in the ATPase reaction P(i) release is coupled to force generation. Whether P(i) release and the power stroke are synchronous events or force is generated by an isomerization of the quaternary complex of actomyosin and ATPase products (AM.ADP.P(i)) prior to the following release of P(i) is still controversial. Examination of the dependence of isometric force on [P(i)] in rabbit fast (psoas; 5-15 degrees C) and slow (soleus; 15-20 degrees C) myofibrils was used to test the two-step hypothesis of force generation and P(i) release. Hyperbolic fits of force-[P(i)] relations obtained in fast and slow myofibrils at 15 degrees C produced an apparent asymptote as [P(i)]-->infinity of 0.07 and 0.44 maximal isometric force (i.e. force in the absence of P(i)) in psoas and soleus myofibrils, respectively, with an apparent K(d) of 4.3 mM in both. In each muscle type, the force-[P(i)] relation was independent of temperature. However, 2,3-butanedione 2-monoxime (BDM) decreased the apparent asymptote of force in both muscle types, as expected from its inhibition of the force-generating isomerization. These data lend strong support to models of cross-bridge action in which force is produced by an isomerization of the AM.ADP.P(i) complex immediately preceding the P(i) release step.
无机磷酸盐(P(i))对横纹肌等长收缩力的抑制作用表明,在ATP酶反应中,P(i)的释放与力的产生相关联。P(i)的释放与动力冲程是否为同步事件,或者力是否由肌动球蛋白和ATP酶产物的四级复合物(AM.ADP.P(i))在随后P(i)释放之前的异构化产生,目前仍存在争议。通过研究兔快肌(腰大肌;5 - 15摄氏度)和慢肌(比目鱼肌;15 - 20摄氏度)肌原纤维中等长收缩力对[P(i)]的依赖性,来检验力产生和P(i)释放的两步假说。在15摄氏度下,快肌和慢肌肌原纤维中力与[P(i)]关系的双曲线拟合显示,随着[P(i)]趋于无穷大,腰大肌和比目鱼肌肌原纤维分别出现明显的渐近线,其最大等长收缩力(即无P(i)时的力)分别为0.07和0.44,两者的表观解离常数(K(d))均为4.3 mM。在每种肌肉类型中,力与[P(i)]的关系均与温度无关。然而,2,3 - 丁二酮单肟(BDM)降低了两种肌肉类型中力的表观渐近线,这与它抑制力产生的异构化作用预期一致。这些数据有力地支持了横桥作用模型,即在P(i)释放步骤之前,力由AM.ADP.P(i)复合物的异构化产生。