Rhee A Y, Brozovich F V
Department of Physiology and Biophysics, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106-4970 USA.
Biophys J. 2000 Sep;79(3):1511-23. doi: 10.1016/S0006-3495(00)76402-2.
The mechanical characteristics of smooth muscle can be broadly defined as either phasic, or fast contracting, and tonic, or slow contracting (, Pharmacol. Rev. 20:197-272). To determine if differences in the cross-bridge cycle and/or distribution of the cross-bridge states could contribute to differences in the mechanical properties of smooth muscle, we determined force and stiffness as a function of frequency in Triton-permeabilized strips of rabbit portal vein (phasic) and aorta (tonic). Permeabilized muscle strips were mounted between a piezoelectric length driver and a piezoresistive force transducer. Muscle length was oscillated from 1 to 100 Hz, and the stiffness was determined as a function of frequency from the resulting force response. During calcium activation (pCa 4, 5 mM MgATP), force and stiffness increased to steady-state levels consistent with the attachment of actively cycling cross-bridges. In smooth muscle, because the cross-bridge states involved in force production have yet to be elucidated, the effects of elevation of inorganic phosphate (P(i)) and MgADP on steady-state force and stiffness were examined. When portal vein strips were transferred from activating solution (pCa 4, 5 mM MgATP) to activating solution with 12 mM P(i), force and stiffness decreased proportionally, suggesting that cross-bridge attachment is associated with P(i) release. For the aorta, elevating P(i) decreased force more than stiffness, suggesting the existence of an attached, low-force actin-myosin-ADP- P(i) state. When portal vein strips were transferred from activating solution (pCa 4, 5 mM MgATP) to activating solution with 5 mM MgADP, force remained relatively constant, while stiffness decreased approximately 50%. For the aorta, elevating MgADP decreased force and stiffness proportionally, suggesting for tonic smooth muscle that a significant portion of force production is associated with ADP release. These data suggest that in the portal vein, force is produced either concurrently with or after P(i) release but before MgADP release, whereas in aorta, MgADP release is associated with a portion of the cross-bridge powerstroke. These differences in cross-bridge properties could contribute to the mechanical differences in properties of phasic and tonic smooth muscle.
一类是相位性的，即快速收缩型；另一类是紧张性的，即缓慢收缩型（《药理学评论》20:197 - 272）。为了确定横桥循环的差异和/或横桥状态的分布是否会导致平滑肌力学特性的差异，我们测定了兔门静脉（相位性）和主动脉（紧张性）经曲通（Triton）处理后的肌条在不同频率下的力和刚度。将处理后的肌条安装在压电长度驱动器和压阻式力传感器之间。使肌肉长度在1至100赫兹之间振荡，并根据产生的力响应测定刚度随频率的变化。在钙激活过程中（pCa 4，5 mM MgATP），力和刚度增加到与活跃循环横桥附着一致的稳态水平。在平滑肌中，由于产生力所涉及的横桥状态尚未阐明，因此研究了无机磷酸盐（P(i)）和MgADP升高对稳态力和刚度的影响。当门静脉肌条从激活溶液（pCa 4，5 mM MgATP）转移到含有12 mM P(i)的激活溶液中时，力和刚度成比例下降，这表明横桥附着与P(i)释放有关。对于主动脉，升高P(i)时力的下降幅度大于刚度，这表明存在一种附着的、低力的肌动蛋白 - 肌球蛋白 - ADP - P(i)状态。当门静脉肌条从激活溶液（pCa 4，5 mM MgATP）转移到含有5 mM MgADP的激活溶液中时，力保持相对恒定，而刚度下降约50%。对于主动脉，升高MgADP会使力和刚度成比例下降，这表明对于紧张性平滑肌，相当一部分力的产生与ADP释放有关。这些数据表明，在门静脉中，力在P(i)释放的同时或之后、但在MgADP释放之前产生，而在主动脉中，MgADP释放与一部分横桥动力冲程有关。横桥特性的这些差异可能导致相位性和紧张性平滑肌力学特性的差异。
