Yamakawa M, Harris D E, Fay F S, Warshaw D M
Department of Physiology and Biophysics, University of Vermont, College of Medicine, Burlington 05405.
J Gen Physiol. 1990 Apr;95(4):697-715. doi: 10.1085/jgp.95.4.697.
Smooth muscle's slow, economical contractions may relate to the kinetics of the crossbridge cycle. We characterized the crossbridge cycle in smooth muscle by studying tension recovery in response to a small, rapid length change (i.e., tension transients) in single smooth muscle cells from the toad stomach (Bufo marinus). To confirm that these tension transients reflect crossbridge kinetics, we examined the effect of lowering cell temperature on the tension transient time course. Once this was confirmed, cells were exposed to low extracellular calcium [( Ca2+]o) to determine whether modulation of the cell's shortening velocity by changes in [Ca2+]o reflected the calcium sensitivity of one or more steps in the crossbridge cycle. Single smooth muscle cells were tied between an ultrasensitive force transducer and length displacement device after equilibration in temperature-controlled physiological saline having either a low (0.18 mM) or normal (1.8 mM) calcium concentration. At the peak of isometric force, after electrical stimulation, small, rapid (less than or equal to 1.8% cell length in 3.6 ms) step stretches and releases were imposed. At room temperature (20 degrees C) in normal [Ca2+]o, tension recovery after the length step was described by the sum of two exponentials with rates of 40-90 s-1 for the fast phase and 2-4 s-1 for the slow phase. In normal [Ca2+]o but at low temperature (10 degrees C), the fast tension recovery phase slowed (apparent Q10 = 1.9) for both stretches and releases whereas the slow tension recovery phase for a release was only moderately affected (apparent Q10 = 1.4) while unaffected for a stretch. Dynamic stiffness was determined throughout the time course of the tension transient to help correlate the tension transient phases with specific step(s) in the crossbridge cycle. The dissociation of tension and stiffness, during the fast tension recovery phase after a release, was interpreted as evidence that this recovery phase resulted from both the transition of crossbridges from a low- to high-force producing state as well as a transient detachment of crossbridges. From the temperature studies and dynamic stiffness measurements, the slow tension recovery phase most likely reflects the overall rate of crossbridge cycling. From the tension transient studies, it appears that crossbridges cycle slower and have a longer duty cycle in smooth muscle. In low [Ca2+]o at 20 degrees C, little effect was observed on the form or time course of the tension transients.(ABSTRACT TRUNCATED AT 400 WORDS)
平滑肌缓慢、经济的收缩可能与横桥循环的动力学有关。我们通过研究蟾蜍（海蟾蜍）胃的单个平滑肌细胞对小幅度、快速长度变化（即张力瞬变）的张力恢复情况，来表征平滑肌中的横桥循环。为了证实这些张力瞬变反映了横桥动力学，我们研究了降低细胞温度对张力瞬变时间进程的影响。一旦得到证实，就将细胞暴露于低细胞外钙浓度（[Ca2+]o）下，以确定[Ca2+]o的变化对细胞缩短速度的调节是否反映了横桥循环中一个或多个步骤的钙敏感性。在含有低钙（0.18 mM）或正常钙（1.8 mM）浓度的温度控制生理盐水中平衡后，将单个平滑肌细胞系在超灵敏力传感器和长度位移装置之间。在等长力峰值时，电刺激后，施加小幅度、快速（在3.6毫秒内小于或等于细胞长度的1.8%）的阶跃拉伸和释放。在正常[Ca2+]o的室温（20摄氏度）下，长度阶跃后的张力恢复由两个指数之和描述，快速相的速率为40 - 90 s-1，慢速相的速率为2 - 4 s-1。在正常[Ca2+]o但低温（10摄氏度）下，拉伸和释放的快速张力恢复相均减慢（表观Q10 = 1.9），而释放的慢速张力恢复相仅受到中度影响（表观Q10 = 1.4），拉伸时则不受影响。在张力瞬变的整个时间进程中测定动态刚度，以帮助将张力瞬变相与横桥循环中的特定步骤相关联。释放后快速张力恢复相期间张力和刚度的解离被解释为该恢复相是由横桥从低力产生状态向高力产生状态的转变以及横桥的瞬时脱离共同导致的证据。从温度研究和动态刚度测量结果来看，慢速张力恢复相最有可能反映了横桥循环的总体速率。从张力瞬变研究来看，平滑肌中的横桥循环似乎更慢且占空比更长。在20摄氏度的低[Ca2+]o条件下，未观察到对张力瞬变的形式或时间进程有明显影响。（摘要截断于400字）
