Himpens B, Matthijs G, Somlyo A V, Butler T M, Somlyo A P
Pennsylvania Muscle Institute, University of Pennsylvania, School of Medicine, Philadelphia 19104-6083.
J Gen Physiol. 1988 Dec;92(6):713-29. doi: 10.1085/jgp.92.6.713.
The time course of [Ca2+]i, tension, and myosin light chain phosphorylation were determined during prolonged depolarization with high K+ in intact tonic (rabbit pulmonary artery) and phasic (longitudinal layer of guinea pig ileum) smooth muscles. [Ca2+]i was monitored with the 340 nm/380 nm signal ratio of the fluorescent indicator fura-2. The fluorescence ratio had a similar time course in both muscle types during depolarization with 109 mM [K+]o; after a transient peak, there was a decline to 70% of its peak value in tonic smooth muscle, and to 60% in phasic smooth muscle. Tension, however, continued to increase in the pulmonary artery, while in the ileum it declined in parallel with the [Ca2+]i. On changing [K+]o from 109 to 20 mM, tension and [Ca2+]i either remained unchanged or declined in parallel in the pulmonary artery. Phosphorylation of the 20-kD myosin light chain, measured during stimulation of muscle strips with 109 mM [K+]o in another set of experiments, increased from 3% to a peak of 50% in the intact pulmonary artery, and then declined to a steady state value of 23%. In the intact ileum, a very rapid, early transient phosphorylation (up to 50%) at 2-3 s was seen. This transient declined by 30 s to a value that was close to the resting level (7%), while tension remained at 55% of its peak force. A quick release during maintained stimulation induced no detectable change in the [Ca2+]i in either type of smooth muscle. We discuss the possibility that the slowly rising tonic tension in pulmonary artery could be due to cooperativity between phosphorylated and nonphosphorylated crossbridges.
在完整的张力型（兔肺动脉）和平滑型（豚鼠回肠纵行肌层）平滑肌中，用高钾进行长时间去极化时，测定了细胞内钙离子浓度（[Ca2+]i）、张力和肌球蛋白轻链磷酸化的时间进程。用荧光指示剂fura-2的340nm/380nm信号比值监测[Ca2+]i。在109mM细胞外钾浓度（[K+]o）去极化过程中，两种肌肉类型的荧光比值具有相似的时间进程；在短暂峰值后，张力型平滑肌中荧光比值下降至峰值的70%，而在平滑肌中下降至60%。然而，肺动脉中的张力持续增加，而回肠中的张力则与[Ca2+]i平行下降。当[K+]o从109mM变为20mM时，肺动脉中的张力和[Ca2+]i要么保持不变，要么平行下降。在另一组实验中，用109mM[K+]o刺激肌条时测定的20-kD肌球蛋白轻链磷酸化，在完整的肺动脉中从3%增加到峰值50%，然后下降到稳定状态值23%。在完整的回肠中，在2-3秒时观察到非常快速的早期瞬时磷酸化（高达50%）。这种瞬时磷酸化在30秒时下降到接近静息水平（7%）的值，而张力保持在峰值力的55%。在持续刺激期间的快速释放未在任何一种平滑肌中引起可检测到的[Ca2+]i变化。我们讨论了肺动脉中张力缓慢上升可能是由于磷酸化和非磷酸化横桥之间协同作用的可能性。
